a self consistent model of melting magma migration and buoyancy driven circulation beneath mid ocean ridges numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle derived magma at mid ocean ridges our model includes the following effects melting due to decompression magma migration by percolation and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge the distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material the calculation of densities is based on a simple petrological model in which garnet spinel lherzolite melts to form a basaltic liquid and a harzburgite residue we find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis faster than the plate velocity is confined laterally by stably stratified residual material beneath the newly formed plates this effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included our results provide a simple explanation for the narrowness of the zone of crustal formation at mid ocean ridges the model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced perhaps giving rise to along axis variations in the character of seafloor spreading the narrow rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus this may allow the liquid at depth to segregate into macroscopic channels which would account for the consensus from experimental petrology that the liquids parental to morb are derived from well below the base of the crust

a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

the history of the major rivers of southern britain during the tertiary the evolution of the drainage system of lowland britain is discussed on the basis of available geological evidence including that from terrestrial sites and that which has more recently become available from offshore exploration of the north sea the english and bristol channels and the irish sea tertiary stratigraphy throws considerable light on landform and river development paleocene destruction of a chalk cover which seems to have been incomplete in western britain was accompanied by basin sedimentation under a tropical climate the major elements the thames solent hampshire proto avon river irish sea river and possibly an early trent river existed almost throughout the cenozoic the influence of atlantic rifting and thermal doming in nw britain appears to have been stronger and more temporally focused than the persistent flexuring that determined and maintained tertiary drainage lines in the se here also the folded mesozoic terrains on the surface contrast with the more dominant block faulted relief of the palaeozoic oldlands the rivers of the se can be shown to have extended or reduced their lengths in response to relative sea level change and gentle warping drainage antecedence the destruction of the solent system and the breaching of the english channel are also evident by contrast the major river systems of the west are now entirely submerged long term stability of the drainage pattern reflects a persistent tectonic regime in the south with a subdued low relief landscape having a weathered regolith and dense vegetation cover meandering river channels and alluvial styles predominated although channel forms varied according to sediment load slope and discharge variability coarse gravel dominated accumulations are rare and localized chemically stable lithologies dominate the clastic component throughout it is apparent that the deeply incised river valleys seen today are related to high predominantly coarse sediment yields encouraged by substantial rapid climate changes in the pleistocene this emphasizes the significance of mechanical compared with chemical weathering for the rate and nature of landscape dissection and the modifications that have arisen as a result of glaciation frost climate weathering rapidly changing climates and sea levels the stratigraphical evidence here reviewed is at variance with older largely geomorphologically based landform evolution models denudational chronology but gives considerable support to the recent proposals emphasizing the significance of paleocene erosion and enduring low relief landscapes and drainage systems evolving alongside fold development during the paleogene given the depobasin evidence now available postulated fluvially active episodes can and must be linked to contemporaneous deposition some at least of the many controversies involving the identity of erosion surfaces the dating of them using only residual deposits and weathering mantles and the selection of particular tertiary episodes as ones of landscape development can now be resolved

short paper the palynology and correlation of the walton crag read crag formation pliocene the basal sands of the red crag formation at walton on the naze essex contain a pollen assemblage dominated by pinus and with pollen of taxodiaceae and thermophilous trees this is comparable with assemblages from the dutch reuverian c late pliocene it is not closely comparable with spectra from other sections previously correlated with the red crag formation at walton which arguably have praetiglian affinities dinoflagellate cysts typical of warm temperate water are also present and compare well with assemblages from the red crag formation in the southern north sea the pre ludhamian and waltonian are therefore separate stages assignable to the pleistocene and pliocene respectively the base of the walton crag the lowest division of the red crag formation at walton on the naze was long considered to be the local base for the pleistocene in the british isles largely because of the presence of supposedly characteristic pleistocene mammal fossils such as mammuthus meridionalis equus sp and hipparion sp boswell 1952 mitchell et al 1973 funnell west 1977 harland et al 1982 other opinion zagwijn 1974 funnell in curry et al 1978 cameron et al 1984 would attribute the red crag formation to the late pliocene on the grounds of mollusc and foraminifer correlation with belgian and dutch pliocene sequences the foraminifers from the red crag formation at walton on the naze are closely comparable with those from the crag in the lower part of the stradbroke borehole beck et al 1972 funnell west 1977 pollen assemblages from the same unit in the stradbroke borehole were dominated by

geomagnetic field intensity and reversals during the past four million years a long and continuous record of the geomagnetic field intensity shows that intensity variations are dominated by two modes major episodes of field regeneration prevail on timescales of a few thousand years immediately after most reversals whereas stable polarity states are characterized by a slow 0 5 myr relaxation process reversals can be seen as the consequence of a progressive degradation of the dipole field

overdeepened quaternary valleys in the eastern danish north sea morphology and origin abstract a system of 100 350 m deep overdeepened and buried valleys has been mapped in the eastern danish north sea individual valleys can be traced for several tens of kilometres and valleys generally begin and terminate abruptly a correlation to buried valleys onshore southwest denmark is possible the valleys post date glaciotectonic structures and are interpreted to be elsterian and possibly saalian in age the origin of valleys in the eastern danish north sea is explained by repeated cycles of sub glacial down cutting of meltwater channels and channel closure due to ice creep eventually producing deeply incised valleys glacial erosion may have been significant in cases where valleys are broad and shallow the origin of overdeepened quaternary valleys in general can be attributed to a combination of three end member processes i steady state drainage of meltwater ii catastrophic outbursts of meltwater and iii glacial erosion the main requirement for sub glacial valley formation is the presence of a melting ice sheet overlying a poorly consolidated substrate permeability variations of the substrate may produce subregional variations in the pattern of valley incision while pre glacial rivers faults or salt structures are of local importance only

the palaeontology and dating of the weybourne crag an important marker horizon in the early pleistocene of the southern north sea basin abstract in the north sea basin the marine bivalve macoma balthica first appears within the early pleistocene weybourne crag which forms an important biostratigraphical datum here we review the fossil assemblages from sites of this age prompted by new discoveries from sidestrand norfolk uk the molluscan assemblages from this horizon are dominated by intertidal species with some colder deeper water taxa and a few temperate non marine species a high boreal low arctic marine environment with reduced salinities is indicated an extensive assemblage of small mammals dominated by voles includes two species mimomys hordijki and ungaromys dehmi previously unknown from the british pleistocene the assemblage can be assigned to tesakov s mammal biozone mnr1 mn17 middle villafranchian which according to current estimates corresponds to a date of 2 2 2 1 ma mis 84 79 it matches another assemblage from 61 m to 65 m in the zuurland 2 borehole in the netherlands and is similar to that from the dutch tiglian type site at tegelen although this has more temperate elements a late tiglian age is consistent with the co occurrence of the marine bivalves macoma balthica mya arenaria and the freshwater gastropod viviparus glacialis in the zuurland 2 borehole and in a north sea borehole bgs 52 02 472 a macoma balthica mya arenaria concurrent range zone is defined for this assemblage which can be traced across the north sea basin amino acid dating provides strong independent support for these correlations and indicates that the baventian cold stage post dates the bramertonian norwich crag it also confirms that early pleistocene molluscan assemblages with m balthica are younger than those without it the correlation of this marine marker horizon with mammal biozone mnr1 provides a secure link between continental and marine sequences during the early pleistocene it also provides a basis for dating events in the pre glacial fluvial drainage history and linking it to the east european mammal zonation

isotope palaeotemperatures from the tjornes beds in iceland evidence of pliocene cooling abstract the mid pliocene warming episode is well established along the margins of the north atlantic and probably affected areas as far north as iceland rich marine mollusc faunas have been described from the 500 m of pliocene siliciclastic deposits on the tjornes peninsula in northeast iceland the faunal compositions indicate a development from warm water to arcto boreal conditions we have analysed the oxygen isotope composition of 96 well preserved specimens of the bivalves arctica islandica and pygocardia rustica and interpreted the data as palaeotemperatures after corrections for the isotopic composition of pliocene seawater the temperatures are in accordance with other palaeotemperature proxies we interpret our data as reflecting a gradual change from warm water conditions summer temperatures between 10 and 15 c during the deposition of the lower part of the tjornes beds tapes zone and lower part of mactra zone to cold water conditions summer temperatures between 5 and 10 c at the top top of mactra zone and serripes zone the coldest interval is found in the middle of the serripes zone where summer temperatures 5 8 c are comparable to those of the present environment off northern iceland the arrival to tjornes of cold water molluscs of pacific origin postdates the cooling of the seas around iceland we suggest that the cold interval in the serripes zone represents a cooling event prior to the final warm period ca 3 3 3 0 ma dowsett et al 1999 middle pliocene paleoenvironmental reconstruction prism2 us geol survey open file rep 99 53 of the mid pliocene warming

the early quaternary north sea basin the onset of the quaternary 2 58 ma corresponds to significant paleo environmental events such as the intensification and southward extension of northern hemisphere glaciation in the north sea basin a significant late cenozoic succession has been identified as a high resolution archive of paleo environmental changes during the pliocene and pleistocene however the identification of the base of the quaternary has been a long standing issue owing to lack of stratigraphic calibration this study incorporates continuous regional 3d seismic data with high quality chronostratigraphic markers to map the base quaternary surface at high resolution across the entire north sea depth conversion backstripping seismic geomorphology and sedimentation rate calculations are integrated to analyse the paleogeographical evolution of the north sea basin and its infill of c 83 10 3 km 3 of northward prograding marine to deltaic sediments the basin is 600 km long from sse to nnw and largely localized above residual topography of the mesozoic graben system during the earliest quaternary 2 58 2 35 ma paleo water depths were c 300 50 m and solid sedimentation rates calculated from 0 porosity c 32 km 3 ka 1 the base quaternary provides an important marker for further studies of the changing environment of the quaternary of nw europe as well as resource and shallow geohazard analysis supplementary material a base quaternary two way travel time structure map is available at https doi org 10 6084 m9 figshare c 3900343

plio pleistocene palaeogeography of the southern north sea basin 3 75 0 60 ma abstract palaeogeographical maps are presented for the southern north sea basin from approximately 1 w to 7 e and 51 55 n for the period from approximately 3 75 0 60 ma reconstructions centre on 3 75 ma coralline cragbrunssumian 2 55 ma red crag brielle ground reuverian 2 40 ma red crag westkapelle ground praetiglian early tiglian 2 00 ma norwich crag smith s knoll and ijmuiden ground middle tiglian 1 75 1 70 ma cromer forest bed paston winterton shoal late tiglian eburonian 1 40 ma kesgrave yarmouth roads waalian 1 15 ma kesgrave yarmouth roads menapian and 0 60 ma cromer forest bed west runton yarmouth roads cromerian complex from 3 75 2 40 ma the basin is mainly a marine area fringed to the east and south by fluviatile deposition and broadly open via the southwest to the atlantic following marked marine regression and cooling at around 2 40 ma the period from 2 00 1 70 ma is mainly one of renewed marine transgression and warmer conditions during which a great european delta built out fed mainly from the east and south into the southern north sea from 1 40 0 60 ma marine waters were almost entirely excluded from the area and temperate intervals alternated with increasingly glacial episodes whilst the great european delta continued to expand northwards into the northern north sea

deep tow and sea beam studies of dueling propagating ridges on the east pacific rise near 20 40 s nontransform ridge axis discontinuities such as propagating rifts overlapping spreading centers oscs and saddle points occur frequently along the mid ocean ridge system we used sea beam and deep tow to study an example of a relatively large nontransform offset near 20 40 s on the east pacific rise epr the two spreading center tips are 15 km apart and separated by a diffuse shear zone a broad v shaped wake of disturbed topography and magnetics occurs on the flanks of the epr near the offset on a fine scale the offset is tectonically much more complex than either the typical osc or propagating rift structures for two reasons first it is a dueling propagator in which structures produced by successive short term episodes of northward and southward propagation have overprinted each other with migration toward the south prevailing over the long term second its offset distance is such that its tectonic behavior may be transitional from that of a propagating rift to that of an osc system the offset s rate of migration to the south has been 20 mm yr over the last 2 m y far less than total opening rate of 162 mm yr distinct propagation events may have been much more rapid however surging north and south at rates of 160 230 mm yr in the process several fossil spreading ridges 30 50 km long have been abandoned and rafted to the side we propose a fine scale tectonic evolution in which a propagating spreading center either curves inside or runs outside of a soon to be abandoned propagating rift tip we call this process self decapitation dueling propagation at 20 40 s may be caused by competing magmatic sources at shallow points along the epr one to the north near 20 s and one to the south near 21 30 s the 20 40 s dueling propagator occurs at a long wavelength low in the axial depth profile between these competing sources in general the division of spreading centers into segments separated by ridge axis discontinuities occurs at low points along the axis and may be caused by the pattern of upper mantle convection and the factors governing flow of magma along the strike of the ridge

compaction and rock properties of mesozoic and cenozoic mudstones and shales northern north sea abstract in this study rock physical properties and their evolution resulting from compaction processes are investigated for mesozoic and cenozoic mudstones and shales located in southern viking graben and adjacent areas within the norwegian north sea the studied sediments are deposited within a progressively subsided sedimentary basin with no major experience of exhumation events a suite of well log data from 43 exploration wells was utilized to study the compaction behaviour of the mesozoic and cenozoic mudstone and shale intervals the gamma ray log derived shale volume vsh was used to define different lithofacies and discriminate between the studied mudstones and shales the rock properties as a function of burial depth were plotted for the identified mudstone and shale intervals the trends could be divided into a mechanical compaction part and a chemical compaction part depending on the prevalent processes controlling the rock properties with burial depth the transition from mechanical compaction domain to the zone of dominant chemical compaction takes place between 70 and 90 c corresponding to a depth of 2 2 5 km the onset of chemical compaction and cementation occurs in the same sediment found at the same depth range almost throughout the study area in spite of variable geothermal gradient indicating a lithological control on the development of chemical compaction the degree of chemical compaction and cementation reflects the initial smectite content and the availability of potassium for the smectite to illite and quartz reaction to take place this study contributes to the understanding of compaction processes in fine grained siliciclastic sediments delineating the controlling factors in a region which can be regarded a natural laboratory to study compaction mechanisms due to being a subsiding basin with the extensive availability of pertrophysical data generated by hydrocarbon exploration and production activities in the area

comparative sclerochronology of modern and mid pliocene c 3 5 ma aequipecten opercularis mollusca bivalvia an insight into past and future climate change in the north east atlantic region records of environment contained within the accretionarily deposited tissues of fossil organisms afford a means of detailed reconstruction of past climates and hence of rigorous testing of numerical climate models we identify the environmental factors controlling oxygen and carbon stable isotopic composition and microgrowth increment size in the shell of modern examples of the queen scallop aequipecten opercularis this understanding is then applied in interpretation of data from mid pliocene a opercularis from eastern england on the basis of oxygen isotope evidence we conclude that winter minimum seafloor temperature was similar to present values typically 6 7 c in the adjacent southern north sea and that summer maximum seafloor temperature was a few degrees lower than present values typically 16 17 c this contrasts with evidence from other proxies that winter and summer temperatures were higher than present the pattern of seasonal variation in microgrowth increment size suggests the existence of intense thermal stratification in summer we therefore conclude that summer surface temperatures were much higher maxima well over 20 c than those recorded isotopically on the seafloor and that the annual range of surface temperature probably over 14 c was greater than now at the times in the mid pliocene when the investigated a opercularis were alive taken in conjunction with other proxy evidence of warmer winters as well as summers the data point to substantial fluctuation up to 10 c in winter minimum temperatures during the mid pliocene in the north east atlantic region this fluctuation may be attributable to variation in the strength of the gulf stream north atlantic drift since the pliocene has been widely used as a test bed for numerical models of a greenhouse earth the results have implications for prediction of future climate in the north east atlantic region under the influence of anthropogenic global warming

pliocene dinoflagellate cyst stratigraphy palaeoecology and sequence stratigraphy of the tunnel canal dock belgium dinoflagellate cysts and sequence stratigraphy are used to date accurately the tunnel canal dock section which contains the most complete record of marine pliocene deposits in the antwerp harbour area the zanclean kattendijk formation was deposited between 5 0 and 4 4 ma during warm temperate conditions on a shelf influenced by open marine waters the overlying lillo formation is divided into four members the lowest is the luchtbal sands member estimated to have been deposited between 3 71 and 3 21 ma under cooler conditions but with an open water influence the oorderen sands kruisschans sands and merksem sands members of the lillo formation are considered a single depositional sequence and biostratigraphically dated between 3 71 and c 2 6 ma with the oorderen sands member no younger than 2 72 2 74 ma warm temperate conditions had returned but a cooling event is noted within the oorderen sands member shoaling of the depositional environment is also evidenced with the transgressive oorderen sands member passing upwards into near coastal high stand deposits of the kruisschans sands and merksem sands members as accommodation space decreased applying sequence stratigraphy to our section implies that the kattendijk lillo formation boundary corresponds to the sequence boundary sb za2 4 04 ma the luchtbal oorderen sands boundary to sb pia1 3 21 ma and the top of the merksem sands to sb pia2 2 76 ma finally the belgian deposits are compared with marine pliocene deposits of eastern england

palaeomagnetic correlation and dating of plio pleistocene sediments at the southern margins of the north sea basin a series of discontinuous sediment sequences of plio pleistocene age occur onshore around the southern north sea margins notably in the east anglian region of britain intensive lithological and palaeontological analyses of these sediments have shown that they record both major and minor oscillations in climate sea level and environmental conditions however significant uncertainties exist regarding the absolute and relative chronostratigraphies of many of these sequences hindering understanding of the relative impacts of climatic eustatic and tectonic changes on the palaeogeographic development of the southern north sea basin here a number of key east anglian plio pleistocene sites are subjected to robust palaeomagnetic and mineralogical examination in order to determine those sediments which display reliable syn depositional magnetic polarities which are thus of use in ascribing a palaeomagnetically determined age from comparison with the geomagnetic polarity timescale gpts based on a range of palaeomagnetic and complementary mineralogical methods reliable palaeomagnetic directions were obtained from eight sites with reversed polarities displayed by sediments from three sites these polarity determinations can be used to infer absolute ages and test published between site correlations

hot spot ridge crest convergence in the northeast pacific evolution of the juan de fuca ridge during the past 7 m y has been reconstructed taking into account both the propagating rift history and migration of the spreading center in the absolute fixed hot spot reference frame northwestward migration of the spreading center at a rate of 30 km m y has resulted in progressive encroachment of the ridge axis on the cobb hot spot and westward jumping of the central third of the ridge axis more recently than 0 5 ma seamounts in the cobb eickelberg chain are predicted to display systematic variations in morphology and petrology and a reduction in the age contrast between the edifice and underlying crust as a result of the ridge axis approach relative seamount volumes also indicate that magmatic output of the hot spot varied during this interval with a reduction in activity between 2 5 and 4 5 ma compared with relatively more robust activity before and after this period spatial relationships determined in this reconstruction allow hypotheses relating hot spot activity and rift propagation to be evaluated in most cases rift propagation has been directed away from the hot spot during the time period considered individual propagators show some reduction in propagation rate as separation between themore propagating rift tip and hot spot increases but cross comparison of multiple propagators does not uniformly display the same relationship no obvious correlation exists between propagation rate and increasing proximity of the hot spot to the ridge axis or increasing hot spot output taken together these observations do not offer compelling support for the concept of hot spot driven rift propagation however short term reversals in propagation direction at the cobb offset coincide with activity of the heckle melting anomaly suggesting that local propagation effects may be related to excess magma supply at the ridge axis less

role of ridge jumps and ridge propagation in the tectonic evolution of the lau back arc basin southwest pacific gloria sidescan imagery of the central lau basin reveals a complex neotectonic framework in which the central lau spreading center is actively propagating southward at the expense of the eastern lau spreading center the latter is located about 75 km closer to the active tofua arc and the transfer of spreading motion across this distance is achieved via a relay spreading ridge segment new magnetic and bathymetric interpretations suggest that the majority of the central and southern basin was formed by the eastern lau spreading center between 5 ma and the present day the processes of arcward ridge migration ridge propagation and basinward arc migration may reflect the complex interplay of the subduction zone and regional tectonics in driving back arc spreading and may be a common feature of this tectonic environment

thames river sediments of the british early and middle pleistocene abstract the thames drainage system contains the most complete record of fluvial sediments in the british quaternary the kesgrave sands and gravels form the main body of this sediment having been deposited over an interval that spans the later part of the early pleistocene and most of the middle pleistocene conventional stages pre pastonian a through to early anglian ca 1 2 ma this deposit provides a stratigraphic context for related bio morpho and pedostratigraphic evidence and for the recognition of pre anglian pre oi stage 12 upland glaciation in the welsh mountains and intense periglaciation in lowland england it also facilitates correlation with the stratigraphy of northern europe and the global pattern of orbitally forced climatic change this paper describes the first catchment wide correlation of thames river sediments and proposes a formal redefinition of the lithostratigraphic units concerned the scheme outlined here is based on two lithologically distinctive formations defined at type areas reflecting significant changes in catchment dimensions the earlier sudbury formation was formed when the thames catchment extended into wales and drained northwards across east anglia the later colchester formation was formed after the upper catchment had been reduced in extent to that of the present river but continued to flow across east anglia the whole sediment body is now named the kesgrave group

physical properties of cenozoic mudstones from the northern north sea impact of clay mineralogy on compaction trends vertical and lateral changes in physical properties in cenozoic mudstones from the northern north sea basin reflect differences in the primary mineralogical composition and burial history which provides information about sedimentary facies and provenance integration of well log data with mineralogical information shows the effect of varying clay mineralogy on compaction curves in mudstones the main controlling factor for the compaction of eocene to early miocene mudstones within the north sea is the smectite content which is derived from volcanic sources located northwest of the north sea mudstones with high smectite content are characterized by low p wave velocities and bulk densities compared to mudstones with other clay mineral assemblages at the same burial depths smectitic clays are important during mechanical compaction because they are less compressible than other types of clay minerals a comparison between well log data and experimental work also shows that smectite may be a controlling factor for overpressure generation in the smectite rich eocene and oligocene sediments at greater burial depths and temperatures 70 80c the dissolution of smectite and precipitation of illite and quartz significantly increases velocities and densities miocene and younger mudstones from the northern north sea have generally low smectite contents and as a result have higher velocities and densities than eocene and oligocene mudstones lateral differences in the compaction trends between the north and south for these sediments also exist which may be related to two different source areas in the pliocene the log derived petrophysical data from the northern north sea basin show that mudstone lithologies have very different compaction trends depending on the primary composition simplified compaction curves may therefore affect the outcomes from basin modeling the amplitude versus offset response of hydrocarbon sands and the seismic signature on seismic sections are also dependent on the petrophysical properties of mudstones and will vary depending on the mineralogical composition

the magnetic properties of quaternary and tertiary sediments in the southern north sea cores from boreholes penetrating late quaternary glacial interglacial and postglacial sediments and the underlying late cenozoic delta complex of the southern north sea have been examined for their magnetic properties a magnetic polarity stratigraphy has been established as an aid to biostratigraphic dating of the sediments the kaena gauss and gauss matuyama transitions and the base and top of the olduvai subchron have been identified the strength and stability of laboratory induced isothermal remanent magnetisation display clear magneto petrological variations which match lithostratigraphic changes in the cores principal component analysis has picked out a basin wide and palaeoenvironmental consistency in the magnetic data large multi domain magnetite grains predominate in the post deltaic and fluvio deltaic sediments whereas smaller greigite or titanomagnetite grains are concentrated in the intertidal and marine deltaic facies since heavy mineral analysis indicates that most of the deltaic detritus derived from common source areas the differences in magnetic mineralogy have probably been caused by the sediment transport processes operating within the delta complex

pliocene climate and seasonality in north atlantic shelf seas this paper reviews north atlantic shelf seas palaeoclimate during the interval 4 3 ma prior to and incorporating the mid pliocene warm period ca 3 29 2 97 ma fossil assemblages and stable isotope data demonstrate northwards extension of subtropical faunas along the coast of the carolinas virginia yorktown and duplin formations relative to the present day suggesting a more vigorous florida current with reduced seasonality and warm water extending north of cape hatteras reconstructed annual range for virginia 12 30 c this interpretation supports conceptual models of increased meridional heat transport for the pliocene sea temperatures for florida lower pinecrest beds were similar to or slightly cooler than summers 25 27 c today and were probably influenced by seasonal upwelling of cold deep water reduced seasonality is also apparent in the coralline crag formation of the southern north sea with ostracods suggesting winter sea temperatures of 10 c modern 4 c however estimates from pliocene bivalves 3 6 16 6 c are similar to or cooler than the present day this mixed signal is problematic given warmer seas in the carolinas virginia and climate model and oceanographic data that show warmer seas in the mid pliocene eastern north atlantic this may be because the coralline crag formation was deposited prior to peak mid pliocene warmth

early and middle pleistocene river systems in eastern england evidence from leet hill southern norfolk england pleistocene sediments at leet hill southern norfolk are examined in terms of their sedimentary structures palaeocurrent indicators clast and heavy mineral lithology and litho and morphostratigraphic position colour of the quartzite and vein quartz clasts is used to differentiate the bytham and the kesgrave sands and gravels with the bytham sands and gravels having a significantly higher proportion of coloured material the kirby cane sands and gravels are the lower sedimentary unit and were deposited by the bytham river which drained a catchment extending into central england at leet hill erosion of the kesgrave sands and gravels by the bytham river has given the kirby cane sands and gravels a distinctive lithological assemblage trace clast lithologies suggest that the kesgrave sands and gravels in the region of leet hill were deposited in a coastal location with an input from northern sources as well as southern and welsh sources diagnostic of the thames catchment the glaciofluvial leet hill sands and gravels were deposited by outwash from the anglian scandinavian ice sheet initially the flow direction of the outwash was determined by the bytham river valley but this changed to a southerly direction once the valley had been infilled this paper provides the first indication of the location of the boundary early pleistocene coastline between the fluvial kesgrave sands and gravels and the marine equivalent reworked by coastal processes and demonstrates the way the pre glacial relief initially controlled patterns of glaciofluvial sedimentation during the early part of the anglian glaciation copyright 1999 john wiley sons ltd

temporal and spatial variability of seafloor spreading processes in the northern south atlantic the paper reports the results of an aeromagnetic study of the mid atlantic ridge in the northern south atlantic the study consists of 38 000 km of low altitude magnetics tracks centered on the ridge covering nearly 700 000 km2 south of ascension island most tracks extend out to crust of anomaly 5b age large areal coverage coupled with high resolution allows comparison of crustal generation processes over space along ridge strike and time across strike temporal heterogeneities revealed in the data include a pattern of change in spreading rate and direction of plate motion the total spreading rate increased sharply at 10 ma and then decreased between 6 ma and the present the time variation of spreading rate is similar to that seen in other areas such as the north atlantic and the southern and eastern pacific the plate boundary has evolved over time in response to the shift in plate motion direction through fragmentation and rotation of ridge segments between fracture zones in several cases the location of the spreading axis has shifted through jumps or propagation across fracture zones the periods of plate boundary modification correlate with the increase and decrease in spreading rate spatial heterogeneities are caused by the influence of the ascension hotspot on the mid atlantic ridge in the northern portion of the study area and include two apparent seamount chains formed by the hotspot ascension island and several of the seamounts appear to have been produced some distance away from the hotspot location presently about 225 km southeast of ascension island by channeled subaxial magma flow beneath the spreading center the seamount chains therefore do not mark hotspot tracks large local variations in spreading rate and degree of asymmetry along strike are also found

overlapping rift zones at the 5 5 s offset of the east pacific rise a seabeam and magnetometer survey of the pacific nazca plate boundary around 5 5 s mapped a nontransform offset whose geology and kinematics seem typical of a whole class of structures formed where fast spreading rises are laterally offset for distances less than the width of a subaxial magma chamber though the spreading center is abruptly displaced by 15 km right laterally there is no trace of strike slip faulting instead the ends of two 100 km long axial volcanic ridges veer 15 toward each other and overlap for more than 20 km in this overlapping region between 5 22 s and 5 32 s accretion of the upper oceanic crust by dike injection and eruption is partitioned between two adjacent and parallel rift zones with oblique and one sided spreading a continuous but dog legged magma chamber is inferred to underlie both rift zones and an intervening 9 km wide volcano studded basin a strong magnetic signature from the crust which accreted around this oblique chamber identifies the trace of the offset on the adjacent rise flank in the past 0 5 106 years the southern axial ridge has propagated north at 40 mm yr 55 of the spreading half rate

periglacial environments during the early pleistocene in the southern netherlands and northern belgium abstract the palaeoenvironment of three early pleistocene glacials in the netherlands and belgium are studied the intra tiglian beerse glacial and the eburonian and menapian cold stages climatic conditions are expressed by specific periglacial phenomena and by palaeobotanical data the three cold periods are characterized by a nearly treeless tundra vegetation and a continuous or discontinuous permafrost mean summer temperatures are estimated around or slightly below 10 c while mean annual temperatures were below 6 to 4 c

dynamical consequences of compositional and thermal density stratification beneath spreading centers extraction of partial melt to form oceanic crust reduces the density of upwelling mantle beneath spreading centers this compositional buoyancy can significantly affect both the rate of upwelling and the pattern of mantle flow the relative importance of buoyant upwelling and upwelling due to plate spreading depends on the mantle viscosity for a low enough mantle viscosity buoyant upwelling can significantly enhance the thickness of the crust relative to that which would be produced by plate spreading alone for a mantle viscosity of 1022 pa s upwelling due to plate spreading is dominant and crustal thickness is predicted to be a strong function of spreading rate however for a mantle viscosity of 1019 pa s buoyancy dominates mantle upwelling and the calculated crustal thickness like the oceanic crust is nearly independent of spreading rate thermal and compositional density variations result in opposing buoyancy forces that can cause time dependent upwelling for a mantle viscosity of 1020 pa s and a low 1 cm yr spreading rate this behavior takes the form of undamped crustal thickness oscillations with a single well defined period for even slightly lower mantle viscosity the frequency increases and crustal thickness oscillations become more complex with two or more frequencies present while numerical experiments have thus far been carried out only for viscosities significantly greater than that likely to characterize the mantle beneath spreading centers we speculate that these crustal thickness oscillations may explain important aspects of seafloor structure

pollen and dinoflagellates from the red crag at walton on the naze essex evidence for a mild climatic phase during the early late pliocene of eastern england spot sampling of the red crag formation at walton on the naze essex walton cray has revealed a diverse record of dinoflagellates pollen and other palynomorphs pollen from a basal horizon is dominated by pinus but also contains high values of sciadopitys and a small but diverse component of deciduous mesothermal trees taxodium is more common at 2 m above the base of the red crag and may represent a different part of a climate cycle a mild to warm temperate climate is inferred for the walton assemblages and a correlation with the reuverian b pollen substage late pliocene 3 0 to 2 6 ma is demonstrated thereby constraining the timing of an important marine transgression in eastern england dinoflagellates from the basal horizon and from 2 m above the base of the red crag generally indi cate a restricted marine environment but outer neritic to oceanic dinoflagellates are occasionally pre sent and imply the penetration of more saline north atlantic water masses into the southern north sea basin mild to warm temperate conditions are indicated by the presence of several thermophilic species the walton assemblages have strong similarities with those of the mid pliocene coralline crag formation of eastern england but differ in the presence of bitectatodinium tepikiense which along with an apparently reduced species richness gives evidence of cooler conditions a number of thermophilic species at walton are not reported from younger pliocene deposits in eastern england perhaps reflecting a less equable climate after northern hemisphere cooling at 2 55 ma

lower pleistocene dinoflagellate cyst foraminiferal and pollen assemblages in four boreholes in the southern north sea neogene and lower pleistocene stratigraphy in the southern north sea has been investigated in four i g s boreholes between east anglia and the netherlands the foraminifera of the red crag formation in borehole 81 51 are closely similar to upper pliocene assemblages in holland the overlying succession is clearly punctuated by unconformities in seismic profiles separating four early pleistocene formations in the boreholes and indicating intervals of significant stratigraphic hiatus offshore the westkapelle ground and smith s knoll formations correlate with thurnian and antian stage deposits in east anglia pollen spectra in the winterton shoal and yarmouth roads formations are similar to baventian and bramertonian assemblages in britain the autochthonous marine and allochthonous terrestrial microfauna and flora provide conflicting evidence of early pleistocene palaeoclimate the dinoflagellate cysts and foraminifera indicate that each formation was deposited in a warm temperate neritic environment the pollen record containing evidence of fluctuation between boreal and mixed coniferous deciduous regional forest cover suggests alternation between cool and warm temperate palaeoclimate

a record of reversed polarity carried by the iron sulphide greigite in british early pleistocene sediments abstract palaeomagnetic measurements were made on samples extracted from a short sequence of early pleistocene estuarine clays now exposed in a sea cliff near sheringham on the north norfolk coast uk on the basis of earlier palynological work these clays had been ascribed a pastonian late tiglian age the clays show marked changes in colour from reddish brown at the top of the unit to blue grey in the middle and grey brown at the base the palaeomagnetic data vary in close association with these colour changes the top and basal brown clays show scattered normal directions of low intensity while the middle blue clays show strongly clustered reversed directions of much higher intensities some samples taken from the boundary between the middle blue and upper red clays show upon demagnetisation a normal overprint on a stable reversed polarity using high gradient magnetic extraction magnetic concentrates have been obtained from the strongly magnetic middle blue clays the presence of iron sulphide minerals in these concentrates was identified using energy dispersive x ray analysis during scanning electron microscopy more specifically x ray diffraction identifies greigite as the only detectable ferrimagnetic mineral in the magnetic concentrates rock magnetic measurements show clear qualitative differences in the magnetic mineralogies of the three clay subunits but absolute identification of the magnetic mineralogy of the weakly magnetic upper and basal brown clays has not yet been possible we interpret the sequence as a primary reversed polarity record this record is carried by the iron sulphide greigite as a chemical remanence acquired during syn depositional reduction of iron via the decomposition of organic material in these anoxic tidal clays subsequently the upper and basal subunits of the clay have been oxidised by permeation of groundwater from the adjacent coarse grained sediments most of the griegite in the oxidised margins of the clay has been altered as a result to a new less efficient magnetic recording material which thus carries a later scattered low intensity normal overprint

palaeogeographical changes in response to glacial interglacial cycles as recorded in middle and late pleistocene seismic stratigraphy southern north sea offshore stratigraphic records from the north sea contain information to reconstruct palaeo ice sheet extent and understand sedimentary processes and landscape response to pleistocene glacial interglacial cycles we document three major middle to late pleistocene stratigraphic packages over a 401 km2 area norfolk vanguard boreas offshore wind farm offshore east anglia uk through the integration of 2d seismic borehole and cone penetration test data the lowermost unit is predominantly fluviatile yarmouth roads formation marine isotope stage mis 19 13 including three northward draining valleys the middle unit swarte bank formation records the southernmost extent of tunnel valley fills in this area of the north sea providing evidence for subglacial conditions most likely during the anglian stage mis 12 glaciation the yarmouth roads and swarte bank deposits are truncated and overlain by low energy estuarine silts and clays brown bank formation mis 5d 4 smaller scale features including dune scale bedforms and abrupt changes in cone penetration test parameters provide evidence for episodic changes in relative sea level within mis 5 the landscape evolution recorded in deposits of mis 19 5 are strongly related to glacial interglacial cycles although a distinctive aspect of this low relief ice marginal setting are opposing sediment transport directions under contrasting sedimentary process regimes

anomalous heat flow and geoid across the cape verde rise evidence for dynamic support from a thermal plume in the mantle summary we describe results from 200 heat flow measurements across the cape verde rise in the north atlantic they show that the heat flow through normal 125 myr old crust is 45 5 3 4mwm 2 close to the prediction for a lithospheric plate model heat flow increases towards the centre of the rise reaching a maximum of 16 4 mw m 2 above the normal oceanic value the concommitant geoid anomaly at the centre of the rise is 7 6 0 3 m and the depth anomaly is 1900 200 m the anomalous heat flow geoid and bathymetric values are used to constrain a variety of theoretical models of hot spot mechanisms lithospheric reheating models alone whether involving transient or sustained thinning of the lithosphere fail to model the broad heat flow anomaly and the long duration of uplift and volcanism observed on the cape verde rise axisymmetric convection models provide a better fit to the observed data and suggest that the swell is a consequence primarily of dynamic uplift generated by an ascending thermal plume in the underlying mantle uplift caused by expansion of the solid lithosphere is of smaller importance the observed geoid bathymetry and heat flow anomalies are reproduced well using convection models with upper mantle viscosities of 2 4 times 1020 pa s and a coefficient of thermal expansion in the range 2 0 2 5 times 10 5 k 1 the effect of melting in the ascending plume is to buffer the heat flow anomalies in the case of the cape verde rise partial melting is minimal but its effect on other hotspots will be to limit observed heat flow anomalies at the surface to less than 20 25 mw m 2

morphology and pattern of quaternary sedimentation in the north sea basin 52 62 n abstract the quaternary north sea basin which extends from northwest mainland europe in the south 52 n to the norwegian sea in the north 62 n contains a thick up to 1 km sedimentary succession that records the changing nature of sediment supply from surrounding land areas during the last c 2 6 ma we use an extensive 2d and 3d seismic database to correlate major quaternary seismo stratigraphic surfaces and units across the north sea and reconstruct the broad scale infill pattern of the entire quaternary north sea basin the total volume of quaternary sediments in the north sea 350 000 km2 is c 140 000 km3 while the area inside the 500 m contour of the base quaternary surface has a volume of 109 000 km3 two largely independent depocentres developed in the north sea basin during the early quaternary the southern and central sub basin was infilled by shelf fluvio deltaic and prodeltaic sediments delivered from the east and south east whilst the northern sub basin was infilled mainly by prograding glacigenic debris flows deposited from an ice mass centred on the norwegian mainland contour currents were an important mechanism of sediment deposition and reworking with water circulation in the basin probably occurring in an anti clockwise direction whilst most of the southern north sea basin was infilled by around 1 6 1 7 ma a depression persisted in the central north sea until around 1 ma the analysis of landforms on 3d seismic data suggests that the fennoscandian ice sheet fis extended intermittently to the palaeo shelf break in the northern north sea during the earliest quaternary and expanded into the central north sea prior to the excavation of the norwegian channel

major river systems of central and southern britain during the early and middle pleistocene central and southern britain was drained by two main river systems during the larger part of the early and middle pleistocene the thames and bytham rivers evidence for these rivers and their quaternary history is represented by their sediments the kesgrave and bytham sands and gravels respectively the geomorphological position of the sediments biostratigraphy and amino acid geochronology evidence from the earlier parts of the early pleistocene tiglian c4b and earlier indicates low energy river systems and marine conditions over much of east anglia for most of the early pleistocene tiglian c4c to the cromerian complex the ancestral thames was the main river with at its maximal extent a catchment that extended into wales and across east anglia and what is now the north sea to join the ancestral rhine during this period glaciers in the uplands of wales and periglacial mass movement elsewhere supplied material to the catchment and it was at this time that the bulk of the sorted quaternary ssediments of lowland britain were deposited the bytham river system has no successor because the landscape now in existence has been fundamentally altered by glacial erosion this catchment drained most of midland england and joined the thames in central east anglia initially the bytham river was a tributary of the thames but over time it extended its catchment and at the beginning of the cromerian complex it became the main river of southern britain with the anglian glaciation 01 stage 121 the bytham river was destroyed and the thames was diverted to its present route through london

numerical models of faulting at oblique spreading centers obliquely spreading mid ocean ridges such as the reykjanes ridge display two distinct fault sets distinguishable by orientation and position on axis faults are oriented oblique to both the trend of the axis and the normal to the relative plate separation vector while faults on the flanks strike approximately parallel to the ridge axis numerical modeling techniques are used here to simulate the development of faulting on the reykjanes ridge stresses acting in a cross section through the lithosphere at a slow spreading ridge are investigated using the fast lagrangian analysis of continua flac explicit difference modeling software the predicted stresses from the cross sectional models are imposed as a condition in boundary element models of fracture propagation and linkage on axis fault simulations run under conditions similar to the reykjanes ridge successfully reproduce the mapped distribution of faults and predict the observed orientation of the axial volcanic ridges simulations of fractures away from the axis show the development of axis parallel faults by the interaction and linkage of fractures which have been rafted off axis also in accord with observations stresses modeled in cross section favor downdip displacement on faults dipping toward the ridge axis

pre anglian fluvial and coastal deposits in eastern england lithostratigraphy and palaeoenvironments abstract clast lithology and derived microflora are used to provide a basis for correlating pre anglian fluvial and coastal deposits in eastern england and to provide a lithostratigraphic framework for these early and early middle pleistocene sediments three fluvial systems are recognised i the thames which drained an area from wales through midland england to the thames basin and southern east anglia the catchment was truncated over the period concerned and the western headwaters diverted to other river systems ii the bytham river which drained midland england and the southern pennines and reached the southern north sea delta in the region of north central east anglia this river extended its catchment over the period concerned iii the ancaster river which drained the southern pennines and received sediment from northeast england each of these catchments contributed sediment to the southern north sea delta estuary and contributed to the lithologies of the red norwich and wroxham crag formations the lithostratigraphic subdivisions are based on the fact that i the pliocene and early pleistocene rivers and the equivalent red and norwich crags are characterised by far travelled microflora which was transported in suspension and a locally derived clast assemblage transported as bedload and ii the later early and early middle pleistocene rivers and the equivalent wroxham crag are characterised by a far travelled microflora and a far travelled clast assemblage variations in the amount and composition of far travelled materials is determined by the relative importance of the three river systems this provides a basis for identifying different formations of the thames river sediments older sudbury formation and younger colchester formation and different members of the wroxham formation dobb s plantation member local material with minor components of thames and ancaster river sediments how hill member local material with major components of thames and ancaster river sediments mundesley member with major components of bytham river sediments these lithostratigraphic units are related to established north european biostratigraphy and oxygen isotope stratigraphy the factors forcing the changes in river energetics are explained in terms of the relative importance of geomorphic processes operating within the river catchments determined by the patterns of milankovitch orbital forcing

continental break up mechanism lessons from intermediate and fast extension settings continental break up mechanisms vary systematically between slow and fast extension systems slow extension break up has been established from studies of the central atlantic european and adria margins this study focuses on the intermediate and fast cases from gabon and east india and draws from the interpretation of reflection seismic gravimetric and magnetic data interpretation indicates continental break up via continental mantle unroofing in all systems with modifications produced by magmatism in faster extension systems break up of the intermediate extension gabon system involves partial upper continental crustal decoupling from continental mantle whereas in the fast east coast india system decoupled and lower crustal regimes underwent upwarping in soggy zones in the footwalls of major normal faults usually upper crustal break up is affected by pre existing anisotropies which form systems of constraining rails for extending continental crust this modifies the local stress regimes they regain a regional character as the function of constraining rails vanishes during progressive unroofing of the upper mantle different regions attain different amounts of upper crustal stretching prior to the break up the break up location is then controlled by the upper crustal energy balance principle of wound linkage by which the minimum physical work is performed for linking upper crustal wounds leading to successful upper crustal break up

3d seismic geomorphology of a large plio pleistocene delta bright spots and contourites in the southern north sea abstract regional 2d seismic interpretation has been integrated with high resolution 3d interpretation in dutch offshore blocks a f in the southern north sea 17 horizons mmu s1 s4 s4 1 s4 2 s5 s5 1 s6 s13 and 14 seismic units were interpreted expanding upon studies by kuhlmann and wong 2008 the study presents detailed and extensive twt structure and thickness maps of key depositional units allowing paleogeographic reconstructions detailed analysis of incisional scour marks and hypothesised iceberg scours and constructional features contourites identified initially by bright spots on seismic data linked to bottom currents was undertaken paleo paleographic reconstructions of water depth emphasise the role of hypothesised tidally generated contour currents in the late stage deltaic infill of an epicontinental basin north sea

a detailed study of the cobb offset of the juan de fuca ridge evolution of a propagating rift the cobb offset on the northern portion of the juan de fuca ridge has been identified as the tip of a northward propagating rift hey and wilson 1982 map compilations of magnetic and seismic data from four new cruises define the present locus of spreading and volcanism on the two ridge segments abutting the offset and permit detailed modeling of the recent evolution within this transform zone the axis of recent spreading on the southern ridge segment bends from the normal ridge trend n20 e to a n s trend north of 47 15 n the spreading axis on the northern ridge segment generally defines a n20 e trend except at the southern terminus where the spreading center is offset slightly to the east the two spreading centers overlap by about 33 km in the offset vicinity and there is evidence of recent volcanism on both segments present ridge axis morphology exhibits a transitional sequence from a symmetrical axial high along the more normal portions of each ridge segment to a grabenlike depression as the tip is approached the magnetic anomaly patterns observed in the cobb offset vicinity are not consistent with the patterns predicted by models of continuous northward propagation the magnetic anomaly patterns of the brunhes epoch require an event of rapid northward propagation about 0 7 m y b p followed by a more gradual southward propagation in the middle brunhes epoch most recently the spreading center on the southern ridge has extended northward to its present configuration prior to the brunhes epoch modeling of the magnetic anomaly patterns does not indicate a unique solution however net propagation has been northward we present alternative models for the period beginning 1 7 m y b p in the first model the cobb offset has evolved by a series of northward and southward events of propagation with net advance to the north in the second model stable asymmetric spreading from overlapping ridge segments has evolved into a transform fault offset by gradual decay of the northern ridge both models indicate that propagation has been discontinuous and relatively rapid compared to spreading rate and that realignment of ridge segments near the propagator tip has been an important component

pleistocene rhine thames landscapes geological background for hominin occupation of the southern north sea region this paper links research questions in quaternary geology with those in palaeolithic archaeology a detailed geological reconstruction of the netherlands south west offshore area provides a stratigraphical context for archaeological and palaeontological finds progressive environmental developments have left a strong imprint on the area s palaeolithic record we highlight aspects of landscape evolution and related taphonomical changes visualized in maps for critical periods of the pleistocene in the wider southern north sea region the middle pleistocene record is divided into two palaeogeographical stages the pre anglian elsterian stage during which a wide land bridge existed between england and belgium even during marine highstands and the anglian elsterian to saalian interglacial with a narrower land bridge lowered by proglacial erosion but not yet fully eroded the late pleistocene landscape was very different with the land bridge fully dissected by an axial rhine thames valley eroded deep enough to fully connect the english channel and the north sea during periods of highstand this tripartite staging implies great differences in i possible migration routes of herds of herbivores as well as hominins preying upon them ii the erosion base of axial and tributary rivers causing an increase in the availability of flint raw materials and iii conditions for loess accumulation in northern france and belgium and the resulting preservation of middle palaeolithic sites 2011 john wiley sons ltd

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

analysis of propagators along the pacific antarctic ridge evidence for triggering by kinematic changes abstract we analyze the structure and evolution of two propagators along the pacific antarctic ridge par that we surveyed during the pacantarctic cruise of the n o l atalante a large propagator at 63 30 s 167 w shows a n50 e trending segment of the par propagating southwestward while the adjacent n45 e trending segment retreats the propagating and doomed ridges are offset by about 43 km they both curve into the offset to define an overlap zone about 25 km long the inner pseudofault is juxtaposed to a series of e w trending ridges inferred to represent the failed axes their direction and arrangement suggest an evolution as an overlapping propagator with cyclic rift failure the pseudofaults are 35 5 oblique to the propagating ridge which implies a rate of propagation of 44 8 mm yr using a 62 mm yr full spreading rate comparable to that of other propagators with similar morphology the age of the initiation of the propagation from the heirtzler fracture zone is estimated to be 5 6 ma which coincides with the age of a clockwise change in spreading direction a second smaller southwestward propagator is observed northeast of the major one at 63 15 s 165 10 w with a morphology very similar to that of the larger one it is inferred to have started near 1 ma again at the time of a clockwise change in spreading direction along the par these two propagators are likely to have evolved from extensional relay zones which developed within the heirtzler transform fault tf valley following clockwise changes in spreading direction the present day axial discontinuity is less than 40 km in offset and may not be a tf anymore the development of propagators in this area of the par appears to be triggered by kinematic changes rather than by thermal gradients along the ridge other propagators that have left similar signatures on the flanks of the par appear to have developed at similar spreading rates near 50 60 mm yr full rate as a result of kinematic changes

the beginning of the ice age in europe and its major subdivisions abstract the time range of the quaternary is discussed in terms of the view that this period is essentially the one in the earth s history in which during certain intervals inland ice developed in the northern hemisphere this process first became evident about 2 3 ma bp since then the quaternary thus defined has witnessed many climatic variations although for practical reasons it is not feasible to assign the status of stage to each distinclty warm or cold phase climatic change is nevertheless the most appropriate basis for world wide correlation it seems possible to use both climatic change and other correlation parameters to arrive at a practicable subdivision of the quaternary period into stages before about 1 ma vegetational succession during interglacials in western europe differed from that seen in later times a possible cause for this divergence is discussed in the later part of the quaternary after about 1 ma two distinct types of warm temperate intervals occurred one associated with high sea levels and an oceanic climate the other showing lower sea levels and a cooler and more continental climate the difference between these two types is shown by comparison of the distribution of abies during these time intervals

compositional variation and 226ra 230th model ages of axial lavas from the southern mid atlantic ridge 8 48 s we present geological observations and geochemical data for the youngest volcanic features on the slow spreading mid atlantic ridge at 8 48 s that shows seismic evidence for a thickened crust and excess magma formation young lava flows with high sonar reflectivity cover about 14 km2 in the axial rift and were probably erupted from two axial volcanic ridges each of about 3 km in length three different lava units occur along an about 11 km long portion of the ridge and lavas from the northern axial volcanic ridge differ from those of the southern axial volcanic ridge and surrounding lava flows basalts from the axial rift flanks and from a pillow mound within the young flows are more incompatible element depleted than those from the young volcanic field lavas from this volcanic area have 226ra 230th disequilibria model ages of 1 000 and 4 000 years whereas the older lavas from the rift flank and the pillow mound but also some of the lava field are older than 8 000 years glasses from the northern and southern ends of the southern lava unit indicate up to 100 c cooler magma temperatures than in the center and increased assimilation of hydrothermally altered material the compositional heterogeneity on a scale of 3 km suggests small magma batches rising vertically from the mantle to the surface without significant lateral flow and mixing the observations on the 8 48 s lava field support the model of low frequency eruptions from single ascending magma batches that has been developed for slow spreading ridges

the history of quaternary sedimentation in the uk sector of the north sea basin reconnaissance seismic and sampling surveys have shown that up to 500 m of lower and early middle pleistocene deltaic sediments are preserved in the uk sector of the north sea the distributions of delta related facies record the progressive expansion of peripheral delta systems northwards across the southern north sea during the early pleistocene a late middle pleistocene unconformity is present throughout the uk sector and sedimentation above the unconformity was dominated by glacial erosional and depositional processes a complex system of deeply incised valleys was eroded during each of three regional middle and upper pleistocene glaciations and the valleys have been filled or partially filled by glaciolacustrine or glaciomarine and interglacial marine sediments the valleys of the earliest elsterian glaciation are up to 400 m deep in the southern north sea the saalian and weichselian valleys are commonly more than 100 m deep they are eroded through their contemporary tills nearshore but occur well beyond the limits of these tills offshore the origins of these valleys are attributed to a combination of fluvial subglacial and periglacial processes

the molluscan fauna of the coralline crag pliocene zanclean at raydon hall suffolk uk palaeoecological significance reassessed abstract this study assesses the environmental implications of a quantitative study of the unusually well preserved molluscan fauna of over 200 species about 47 extinct from six samples from both the ramsholt and sudbourne members of the coralline crag formation at a british geological survey borehole at raydon hall near orford suffolk uk it reveals a shallow shelf fauna locally transported from a variety of habitats dominated by infaunal species spisula turritella abra amyclina from relatively fine sediments plus epifauna especially heteranomia possibly in part originating from nearby bryozoan communities as well as dead bivalve shells there was a small but consistent component of deeper water elements such as limopsis and a lack of strictly littoral species overall a water depth of about 50 m is suggested with bottom temperatures at times perhaps warmer than the present southern north sea but not as high as the modern mediterranean and a more oceanic setting this is in contrast to inferences from previous nineteenth century studies of coralline crag molluscs which viewed it as largely mediterranean in character but understated the regular presence of boreal forms such as arctica current interpretations still have to account for the coexistence of some lusitanian or mediterranean molluscan taxa alongside the cooler water elements

ridge jumps and propagations in the south atlantic ocean the normal pattern of mid ocean ridge divergent plate boundaries comprises straight spreading segments with narrow zones of crustal accretion bounded by transform faults where these segments are offset normally the configuration of spreading segments and transform faults is stable for long periods but in places on fast and medium rate spreading ridges the transform offsets have propagated along the ridge axis as some spreading segments have grown at the expense of adjacent ones1 7 although such behaviour has hitherto not been well documented on slow spreading ridges we have mapped multiple ridge jumps and propagating rifts in two areas of the slow spreading 18 mm yr 1 half rate south atlantic ridge crest we attribute this anomalous behaviour in the south atlantic to the presence of hotspots near our study areas causing elevated asthenospheric mantle temperatures and lateral mantle flow with consequent excess volcanism and disruption of the normally well focused melt delivery processes at the ridge crest

climatic evolution during the middle pleistocene warm period of bilshausen germany compared to the holocene abstract middle pleistocene warm periods are particularly useful for comparison with our present warm period one of them marine isotope stage mis 11 has been considered a potential analogue for the holocene whereas older interglacials e g mis 13 are distinctly different however terrestrial middle pleistocene sites other than the holsteinian and quantitative climate reconstructions for this period are very rare in europe a unique terrestrial site in north central europe is known from bilshausen germany the sediments are varved and indicate that this interglacial lasted 25 000 years more than twice as long as the holocene so far the warm period probably correlates to mis 11 or possibly to mis 13 a new palynological investigation with large pollen counts is presented which forms the basis for quantitative temperature reconstructions based on the probabilistic indicator taxa approach the so called pdf method the range of tree species that formed the vegetation during the bilshausen warm period is reminiscent of the holocene but the vegetational as well as the reconstructed climatic development shows distinct differences no pronounced initial successional phase is recorded in the pollen stratigraphy from bilshausen in addition a mesocratic or optimum phase of forest development as in the holocene is unclear in the bilshausen sequence reconstructed temperature values are several c lower than holocene temperatures for most of the bilshausen interglacial only for several millennia in the late phase of the bilshausen interglacial were july temperatures higher than the holocene july temperatures seasonality was much stronger during the bilshausen interglacial and the temperature trends differ between the two interglacials likely reasons for the pronounced differences are on one hand global temperature insolation changes and atmospheric circulation on the other hand more regional factors such as the configuration of the north sea basin may also be responsible for the colder winter temperatures and stronger seasonality during the bilshausen interglacial compared to the holocene reliable absolute dating of the bilshausen interglacial is needed which would allow a more detailed evaluation of the forcing factors that determined the climate during the bilshausen interglacial

sea level changes river activity soil development and glaciation around the western margins of the southern north sea basin during the early and early middle pleistocene evidence from pakefield suffolk uk this paper outlines evidence from pakefield northern suffolk eastern england for sea level changes river activity soil development and glaciation during the late early and early mid dle pleistocene mis 20 12 within the western margins of the southern north sea basin during this time period the area consisted of a low lying coastal plain and a shallow offshore shelf the area was drained by major river systems including the thames and bytham changes in sea level caused sev eral major transgressive regressive cycles across this low relief region and these changes are identi fied by the stratigraphic relationship between shallow marine wroxham crag formation fluvial cromer forest bed and bytham formations and glacial happisburgh and lowestoft formations sediments two separate glaciations are recognised the happisburgh mis 16 and anglian mis 12 glaciations and these are separated by a high sea level represented by a new member of the wroxham crag formation and several phases of river aggradation and incision the principal driving mechanism behind sea level changes and river terrace development within the region during this time period is solar insolation operating over 100 kyr eccentricity cycles this effect is achieved by the impact of cold climate processes upon coastal river and glacial systems and these climatically forced processes obscure the neotectonic drivers that operated over this period of time british geological survey natural environment research council copyright 2005 reproduced with the permission of bgs nerc published by john wiley sons ltd

depositional and paleohydrogeological controls on the distribution of organic matter and other reactive reductants in aquifer sediments abstract the reactivity of sedimentary reductants is the main control for the natural attenuation of common groundwater contaminants such as nitrate or chlorinated hydrocarbons middle miocene to late pleistocene 0 01 20 my old aquifer sands of marine fluvial fluvio glacial and aeolian origin were characterized to determine the control of sediment depositional environment sediment age and paleohydrology on the distribution and reactivity of sedimentary reductants in addition to the variability in the molecular composition of sedimentary organic matter som the reduction reactivity of these sediments was determined oxygen consumption and carbon dioxide production during sediment incubations indicated that som pyrite and ferrous iron bearing carbonates were the main reductants bulk 13 c org values 25 indicated that terrestrial higher land plants were the main precursor of som regardless of sediment origin however pyrolysis gc ms analysis showed that som in the marine tertiary sands contained lignin with preserved side chains while in the pleistocene fluvial and fluvio aeolian sediments highly degraded lignin and recalcitrant macromolecular aliphatic structures dominated som indicative of aerobic degradation the higher dynamics of these terrestrial depositional environments as compared with their marine counterpart likely allows for increased oxygen exposure times and thus more intense aerobic degradation of som the aquifer sediment that fills a large erosional valley created during the saale ice age likely consists of fluvio glacially reworked marine tertiary sediments as supported by carbonate isotopic evidence the more degraded status and decreased reactivity of som in this sediment than in its precursor is likely due to the increased oxygen exposure during dynamic fluvio glacial reworking despite the highly degraded nature of som and the absence of pyrite in the pleistocene fluvial and fluvio aeolian sediments oxygen consumption rates were high during the incubation of sediments from the shallowest part of the aquifer a reactive ferroan carbonate phase was inferred as the main source of oxidant demand in shallow fluvial and aeolian sands depleted oxygen and carbon isotopes indicated that this phase was groundwater derived aerobic degradation during sediment deposition appears to control the molecular composition and reactivity of som in aquifer sediments and to affect the potential of subsequent diagenetic pyrite formation in addition paleo hydrological conditions may result in the accumulation of a ferrous iron bearing carbonate phase precipitated during the exfiltration of fe ii containing anoxic groundwater

morphodynamic behaviour of a mixed sand gravel ebb tidal delta deben estuary suffolk uk the morphodynamics of inlets and ebb tidal deltas reflect the interaction between wave and tidal current driven sediment transport and significantly influence the behaviour of adjacent shorelines studies of inlet morphodynamics have tended to focus on sand dominated coastlines and reference to gravel dominated or gravel rich inlets is rare this work characterises and conceptualises the morphodynamics of a meso tidal sand gravel inlet at the month of the deben estuary southeast england behaviour of the inlet and ebb tidal delta over the last 200 yr is analysed with respect to planform configuration and bathymetry the estuary inlet is historically dynamic with ebb tidal shoals exhibiting broadly cyclic behaviour oil a 10 to 30 yr timescale quantification of inlet parameters for the most recent cycle 1981 2003 indicate an average ebb delta volume of 1 x 10 6 m 3 and inlet cross sectional area of 775 m 2 bypassing volumes provide a direct indicator of annual longshore sediment transport rate over this most recent cycle of 30 40 x 10 3 m 3 yr 1 short term increases in total ebb tidal delta volume are linked to annual variability in the north to northeasterly wind climate the sediment bypassing mechanism operating in the deben inlet is comparable to the ebb delta breaching model of fitzgerald fitzgerald d m 1988 shoreline erosional depositional processes associated with tidal inlets in aubrey d g weishar l ed hydrodynamics and sediment dynamics of tidal inlets springer verlag inc new york pp 186 225 although the scales and rates of change exhibited are notably different to sand dominated systems a systematic review of empirical models of sand dominated inlet and ebb tidal delta morphodynamics e g those of o brien m p 1931 estuary tidal prisms related to entrance areas civil engineering 1 738 739 walton t l and adams w d 1976 capacity of inlet outer bars to store sand proceedings of 15th coastal engineering conference 1919 1937 gaudiano d j kana t w 2001 shoal bypassing in mixed energy inlets geomorphic variables and empirical predictions for nine south carolina inlets j coast res 17 2 280 291 shows the deben system to be significantly smaller yet characterised by a longer bypassing cycle than would be expected for its tidal prism this is attributed to its coarse grained sedimentology and the lower efficiency of sediment transporting processes c 2005 elsevier b v all rights reserved

crustal magnetization and accretion at the southwest indian ridge near the atlantis ii fracture zone 0 25 ma 1 we analyze geophysical data that extend from 0 to 25 myr old seafloor on both flanks of the southwest indian ridge swir lineated marine magnetic anomalies are consistent and identifiable within the study area even over seafloor lacking a basaltic upper crust the full spreading rate of 14 km myr has remained nearly constant since at least 20 ma but crustal accretion has been highly asymmetric with half rates of 8 5 and 5 5 km myr on the antarctic and african flanks respectively this asymmetry may be unique to a 400 km wide corridor between large offset fracture zones of the swir in contrast to the mid atlantic ridge crustal magnetization amplitudes correlate directly with seafloor topography along the present day rift valleys this pattern appears to be primarily a function of along axis variations in crustal thickness rather than magnetic mineralogy off axis magnetization amplitudes at paleo segment ends are more positive than at paleo segment midpoints suggesting the presence of an induced component of magnetization within the lower crust or serpentinized upper mantle alteration of the magnetic source layer at paleo segment midpoints reduces magnetization amplitudes by 70 80 within 20 myr of accretion magnetic and ocean drilling program odp hole 735b data suggest that the lower crust cooled quickly enough to lock in a primary thermoremanent magnetization that is in phase with that of the overlying upper crust thus magnetic polarity boundaries within the intrusive lower crust may be steeper than envisioned in prior models of ocean crustal magnetization as the crust ages the lower crust becomes increasingly important in preserving marine magnetic stripes

crustal structure of ascension island from wide angle seismic data implications for the formation of near ridge volcanic islands abstract the study of the internal structure of volcanic islands is important for understanding how such islands form and how the lithosphere deforms beneath them studies to date have focused on very large volcanic edifices e g hawaiian islands marquesas but less attention has been paid to smaller islands which are more common ascension island a 4 km high volcanic edifice with a basal diameter of 60 km is located in the equatorial atlantic 8 s 90 km west of the mid atlantic ridge on 7 ma oceanic lithosphere we present results of a wide angle seismic profile crossing the island revealing a crustal thickness of 12 13 km an overthickened layer 3 7 km thick and little evidence of lithospheric flexure together these results suggest ascension island may be older than previously assumed and may have begun forming at an on axis position around 6 7 ma this hypothesis is further supported by the presence of a young 1 4 km high edifice directly adjacent to the mid atlantic ridge with a volume about 1 7 that of ascension island possibly representing the earliest stages of seamount formation excess magmatism appears to be related to the tectonic setting at the ridge fracture zone intersection

fast rift propagation along the east pacific rise near easter island during a 1983 seabeam cruise we surveyed the easter nazca spreading center which forms the eastern boundary of the easter microplate we have analyzed all available bathymetry and magnetic anomaly data to determine the present fine scale tectonic configuration and recent tectonic evolution of this spreading center the narrowing of the axial brunhes anomaly is best explained by a young propagating rift growing northward at an average rate of 150 mm yr assuming an average spreading rate of 120 mm yr there are indications of a short episode of faster propagation 350 mm yr prior to 0 4 ma these are the fastest rates of rift propagation yet documented the tip of the propagating rift is at about 25 s 112 25 w 180 km south of the previously hypothesized position the recent spreading direction is n85 w a large well defined sheared zone to the west of the western pseudofault bounding the crust formed by the propagating rift suggests that the easter pacific spreading center 360 km to the west is being replaced by the fast propagator

crenulated seafloor evidence for spreading rate dependent structure of mantle upwelling and melting beneath a mid oceanic spreading center abstract the availability of dense geosat gm satellite altimetry data south of 30 s 1 provides the first opportunity to compare at 25 km resolution seafloor generated at ridges spreading with a wide range of spreading rates the axis of the slow spreading southwest indian ridge as well as the seafloor that it has generated has a distinctly crenulated appearance this crenulated seafloor shows regularly spaced gravity lineations in the spreading direction crenulations that record stationary centers of mantle upwelling over the life of the southwest indian ridge a similar pattern of crenulation is seen along sections of the slow spreading northern mid atlantic ridge but along the faster intermediate spreading southern mid atlantic ridge while regularly spaced highs are apparent along sections of the spreading axis stationary crenulation does not persist off the axis this pattern implies that the faster spreading southern mid atlantic ridge has variable centers of time dependent upwelling that migrate along the ridge crenulations are not visible near the axis of the fast spreading pacific antarctic ridge crenulation amplitudes decrease strongly with increasing spreading rate the absence of crenulation at high spreading rate could be due to a 3d 2d transition in flow structure or the effect of mechanisms to redistribute crust along axis while crenulations are not ubiquitous even at slowly spreading ridges where they exist they provide a time record to observe the characteristics of mid ocean ridge upwelling melting and melt migration not only beneath active spreading centers but through a much longer spreading history