morphology and filling of incised submarine valleys on the continental shelf near the mouth of the fly river gulf of papua 1 much historical perspective about terrestrial and marine processes can be obtained from examination of fluvial valleys formed during lower sea level on surfaces that are now continental shelves the continental shelf near the mouth of the fly river gulf of papua has three incised valleys which were not extensively modified or filled during the holocene transgression multibeam mapping of the valleys documents their morphology seismic profiling reveals stratigraphy of sediment fill and coring within and near the valleys suggests mechanisms of filling kiwai valley is deep 20 50 m relief narrow 1 km wide steep sided and meandering due to river flows that caused its excavation through competent sedimentary deposits purutu valley is shallow 10 20 m relief and broad 2 km wide umuda valley is widest 14 km and has multiple channels with variable incision depths suggesting more extensive fluvial activity than the other two valleys valley filling has occurred in several ways reflecting valley morphology and location relative to the present river mouth kiwai and umuda valleys reveal three stages of infill 1 hemipelagic sedimentation at distal sites 2 gravity driven flows spreading down the valley and 3 subsequent clinoform progradation that completely fills the valley purutu valley fill is dominated by clinoform progradation clinoform progradation in umuda valley is driven by intense sediment reworking on the surrounding topset regions and its large width allows progradation from the sides as well as down its axis most shelf valleys around the world were filled long ago and available techniques have severe limitations for documenting the details of morphology and the mechanisms of filling the shelf valleys described in this paper provide a unique perspective to terrestrial and marine processes before during and after the holocene transgression

niobium uranium evidence for early formation of the continental crust niobium uranium ratios in greenstone belt basalts and gabbros indicate that parts of the late archean mantle beneath western australia underwent a level of melt extraction resulting in formation of the continental crust comparable to that seen in the present mantle the implication is either that i the amount of continental crust that formed before 2 7 x 10 9 years ago was much greater than generally thought or ii crustal growth occurred by severe depletion of small volumes of the mantle rather than by moderate depletion of a large volume of mantle

sea level change and storm surge deposition in a late holocene florida salt marsh abstract a 300 km length of west florida s coastline is dominated by an open marine coastal marsh system located along the central part of this sediment poor region waccasassa bay is presently a broad shallow embayment rimmed by an expansive juncus roemerianus salt marsh in this system sediments were first deposited in a forested swamp from 4400 to 1800 cal yr bp contemporaneously large oyster reefs grew in the embayment near a paleo river mouth these deposits indicate a period of slow sea level rise during the late holocene that correlates with the establishment or regression of other west florida coastal systems circa 1800 cal yr bp a rapid transgression of 2 4 km led to salt marsh growth over the former swamp and a brackish marsh developed over a previously unflooded upland surface calculated rates of shoreline retreat are 10 to 20 m yr at this time and compare to 3 0 m yr during most of the late holocene timing of the event in waccasassa bay corresponds with transgressive sequences in other atlantic and gulf coast systems and supports a relative sea level fluctuation ca 1800 cal yr bp following this event sediments overstepped during the transgression were reworked onto the low lying tidal marsh rapid accretion along the shore edge outpaced sea level rise and led to the local development of high intertidal and supratidal levees evidence indicates that storm driven surge was the principal mechanism of sediment transport for these deposits in waccasassa bay storm surge deposition has played a significant role in marsh surface accretion distinguishing the system from more typically tide dominated wetlands within the past 150 years most of the supratidal environment became tidally inundated and there was a widespread transition into the modern salt marsh this change and evidence from adjacent portions of the florida coast implicate a rise in sea level within the past several hundred years

constraining sediment transport to deep marine basins through submarine channels the levant margin in the late cenozoic abstract the recent world class gas discoveries in early miocene sand units offshore israel raises the question of their origin apparently the simplest explanation is to relate them to a fluvial system that arrived from arabia at that time this system predated the modern pliocene nile river supply and existed until captured by the dead sea valley interestingly however very little sedimentation occurred along the levant continental margin before the pliocene in spite of its stepped structure that provided much space for accommodation the only way that sediments could have bypassed the continental margin and arrive at the deep basin without being trapped in the middle is through submarine channels that crossed the continental margin here we explore this possibility using 3 d stratigraphic modeling techniques that quantify the sediment load and the water discharge required to fill the basin by pushing enough sediment through submarine channels we show that such a scenario requires a fluvial system in the order of the largest rivers that exist today on earth in terms of drainage area and water discharge alternatively it requires extreme hydraulic conditions in terms of diffusion coefficients and an elevated drainage basin that could not have existed in the study area we therefore challenge the traditional view of arabia as the main source for oligo miocene deposits in the levant basin and suggest that the basin was mainly fed by a proto nile system that transported clastic material to the north african margin and then farther east by ocean currents in a wider view we demonstrate how numerical modeling can constrain sediment transport through submarine channels as a function of basin geometry and hydraulic conditions and how paleogeographic knowledge can be combined with current data on world rivers to evaluate if modeling results are plausible

depositional facies and stratal geometry of an upper carboniferous prograding and aggrading high relief carbonate platform cantabrian mountains n spain seismic scale continuous exposures of an upper carboniferous bashkirian moscovian carbonate platform n spain provide detailed information about the lithofacies and stratal geometries quantified with differential global positioning system measurements of microbial boundstone dominated steep prograding and aggrading platform margins progradational and aggradational platform to slope transects are characterized by distinct lithological features and stratal patterns that can be applied to the understanding of geometrically comparable high relief depositional systems the bashkirian is characterized by rapid progradation at rates of 415 970 m my 1 characteristic outerplatform facies are high energy grainstones with coated intraclasts ooids and pisoids moderate energy algal skeletal grainstones to packstones and lower energy algal packstone and boundstone units the moscovian aggradational phase is characterized by aggradation rates of 108 m my 1 coated grain shoals are less common whereas crinoidal bars nucleated in well circulated settings below wave base boundstones form a belt 30 300 m wide at the platform break and interfinger inwards with massive algal skeletal wackestones mud rich banks the progradational phase has divergent outer platform strata with basinward dips of 12 to 2 steep clinoforms with dips of 20 28 are 650 750 m in relief and possibly sigmoidal to concave in the lower part the basinward dipping outer platform strata might be depositional for less than 6 consistent with lithofacies deepening seaward the basinward dip is attributed to the downward shift of upperslope boundstone forced by late highstand and relative sea level fall and to compaction induced differential subsidence during progradation the aggradational phase is characterized by horizontally layered platform strata clinoforms steepen to 30 45 reaching heights of 850 m and are planar to concave the evolution from progradation to aggradation at the bashkirian moscovian boundary is attributed to increased foreland basin subsidence and decreased boundstone accumulation rates progradation was primarily controlled by boundstone growth rather than by highstand shedding from the platform top within the major phases aggradational progradational increments are produced by third to fourth order relative sea level

comparing the smooth parabolic shapes of interfluves in continental slopes to predictions of diffusion transport models abstract computer models of stratigraphic development at continental margins often include elements in which sediment topography evolves according to a diffusion equation representing effects of down slope sediment transport however there has been little effort to verify the model for example by showing whether deposit geometries are as expected from it or by considering whether transport is consistent with the model s assumptions the issue is important for understanding transport erosion and deposition in continental slopes more generally and the extent to which such processes are recorded in seabed morphology examples of slope topography illustrating diffusive like characteristics are examined here in the uppermost slope of the usa central atlantic margin sandy areas between canyon heads are smoothly convex upwards and almost parabolic a morphology similar to subaerial soil mantled hillslopes known to evolve according to a diffusion equation other data of interfluves in muddy sediment from both atlantic and pacific usa margins also show this smooth convex upwards form in subaerial hill crests the upwards convex parabolic shape arises where steady erosion by streams along channels provide constant flux boundary conditions i e the parabola is the steady state solution to the diffusion equation although submarine processes will likely be different the similar morphology of submarine interfluves prompts the question as to whether erosion along channels and movements of sediment between channels can also sometimes occur to cause a diffusive like evolution of topography the discussion therefore focuses on submarine processes affecting erosion and deposition in particular to address whether the morphology results from true diffusion which requires that sediment moves down slope with a transport flux proportional to local bed gradient or is fortuitous sediment transport is instead unrelated to local bed gradient and the morphology arises from other effects on accumulation rates

two dimensional stratigraphic forward modeling reconstructing high relief clinoforms in the northern taranaki basin clinoforms the basic large scale architectural form within which sediments are stored and eventually fed down depositional dip in clastic wedges exist in many shapes and sizes understanding how they form evolve and degrade is critical to understanding how transport mechanisms affect the shelf margin and sediment partitioning and distribution and their implications on the presence of a working petroleum system the neogene stratigraphic succession of the taranaki basin in new zealand contains clinoform packages that display a variety of architectures well imaged on seismic data quantitative characterization of this interval was performed to unravel the processes by which clinoforms evolve under the influence of tectonic and isostatic driven subsidence sea level change and sediment supply fluctuations nine different clinoform packages were identified on the basis of changes in their seismic stratigraphic characteristics two dimensional stratigraphic forward modeling was used to conduct a sensitivity analysis and determine the relative importance of different geologic controls on their genesis our results show that during the early to late pliocene clinoform architectures were influenced by the opening of a back arc rifting structure in the taranaki basin northern graben which controlled sediment redistribution and partitioning at the same time a drop in global sea level allowed sediment bypass to distal parts of the basin during the late pliocene changes in the australian pacific subduction zone forced rapid uplifting of the southern alps generating a significant increase in sediment supply model simulations suggest that clinoform architectures during the late pliocene were controlled by this increase in sediment supply and associated loading

micropaleontological evidence of large earthquakes in the past 7200 years in southern hawke s bay new zealand abstract foraminiferal and diatom assemblages in 11 cores 3 7 5 m deep of holocene sediment from brackish marine ahuriri inlet in southern hawke s bay new zealand provide a record of 8 5 m of subsidence followed by 1 5 m of uplift in the last 7200 cal years in a region overlying the subduction zone between the australian and pacific plates modern analogue technique was used to estimate paleotidal elevation of the 97 richest foraminiferal assemblages the most precise estimates are for high tidal salt marsh assemblages cored in marginal settings in the north and south of the former inlet the least precise estimates are from low tidal and subtidal assemblages from cores in the middle of the inlet these paleoelevation estimates combined with sediment thicknesses age determinations from tephrostratigraphy and radiocarbon dates the new zealand holocene sea level curve and estimates of compaction identify the holocene land elevation changes and earthquake displacement events in each core the following major earthquake related displacements are inferred ca 7000 cal yr bp 0 6 m displacement ca 5800 cal yr bp 0 5 m ca 4200 cal yr bp ca 1 5 m ca 3000 cal yr bp 1 4 to 1 8 m ca 1600 cal yr bp ca 1 7 m ca 600 cal yr bp ca 1 m 1931ad napier earthquake 1 5 m further smaller events involving regional subsidence or earthquake shake compaction are indicated during the 7000 3000 yr bp interval but cannot be identified precisely the six possibly subduction interface subsidence events in the last 7000 years have had a return time of 1000 1400 years identified displacement events have a range of sedimentary expressions from an eroded and burrowed hiatus surface to an abrupt lithologic switch from mud to sand or peat to shelly mud or in some places no change in sediment character whatsoever

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

formation of an archean tectonic melange in the schreiber hemlo greenstone belt superior province canada implications for archean subduction accretion process the late archean circa 2750 2670 ma schreiber hemlo greenstone belt superior province canada is composed of tectonically juxtaposed fragments of oceanic plateaus circa 2750 2700 ma oceanic island arcs circa 2720 2695 ma and siliciclastic trench turbidites circa 2705 2697 ma following juxtaposition these lithotectonic assemblages were collectively intruded by synkinematic tonalite trondhjemite granodiorite ttg plutons circa 2720 2690 ma and ultramafic to felsic dikes and sills circa 2690 2680 ma with subduction zone geochemical signatures overprinting relations between different sequences of structures suggest that the belt underwent at least three phases of deformation during d1 circa 2695 2685 ma oceanic plateau basalts and associated komatiites arc derived trench turbidites and oceanic island arc sequences were all tectonically juxtaposed as they were incorporated into an accretionary complex fragmentation of these sequences resulted in broken formations and a tectonic melange in the schreiber assemblage of the belt d2 circa 2685 2680 ma is consistent with an intra arc right lateral transpressional deformation fragmentation and mixing of d2 synkinematic dikes and sills suggest that melange formation continued during d2 the d1 to d2 transition is interpreted in terms of a trenchward migration of the magmatic arc axis due to continued accretion and underplating the d2 intra arc strike slip faults may have provided conduits for uprising melts from the descending slab and they may have induced decompressional partial melting in the subarc mantle wedge to yield synkinematic ultramafic to felsic intrusions a similar close relationship between orogen parallel strike slip faulting and magmatism has recently been recognized in several phanerozoic transpressional orogenic belts suggesting that as in phanerozoic counterparts orogen parallel strike slip faulting in the schreiber hemlo greenstone belt played an important role in magma emplacement

oceanic plateau model for continental crustal growth in the archaean a case study from the kostomuksha greenstone belt nw baltic shield abstract field studies combined with chemical and isotope data indicate that the kostomuksha greenstone belt in the nw baltic shield consists of two lithotectonic terranes one mafic igneous and the other sedimentary separated by a major shear zone the former contains submarine komatiite basalt lavas and volcaniclastic lithologies and the latter is composed of shelf type rocks and bif komatiitic and basaltic samples yield sm nd and pb pb isochron ages of 2843 39 and 2813 78 ma respectively their trace element compositions resemble those of recent pacific oceanic flood basalts with primitive mantle normalized nb th of 1 5 2 1 and nb la of 1 0 1 5 this is in sharp contrast with island arc and most continental magmas which are characterized by nb th la n 1 calculated initial nd isotope compositions nd t 2 8 to 3 4 plot close to an evolution line previously inferred for major orogens momo which is also consistent with the compositions of recent oceanic plateaux the high liquidus temperatures of the komatiite magmas 1550 c and their al depleted nature require an unusually hot 1770 c mantle source for the lavas 200 c hotter than the ambient mantle at 2 8 ga and are consistent with their formation in a deep mantle plume in equilibrium with residual garnet this plume had the thermal potential to produce oceanic crust with an average thickness of 30 km underlain by a permanently buoyant refractory lithospheric mantle keel nb u ratios in the komatiites and basalts calculated on the basis of th u pb relationships range from 35 to 47 and are thus similar to those observed in modern morb and oib this implies that some magma source regions of the kostomuksha lavas have undergone a degree of continental material extraction comparable with those found in the modern mantle the mafic terrane is interpreted as a remnant of the upper crustal part of an archaean oceanic plateau when the newly formed plateau reached the active continental margin its upper part collided with the sedimentary terrane but was too buoyant to subduct as a result the volcanic section of the plateau was imbricated and obducted thus becoming a new segment of continental crust the deeper zones were delaminated and tectonically underplated or subducted

carbonate platform flanks slope angle and sediment fabric more than 20 examples of fossil carbonate platform systems were compared for slope angle and sediment fabric plots of slope angle versus sediment fabric show that grainy non cohesive mud free sediments build steeper slopes than muddy cohesive sediments examples near the end members of grainy and muddy carbonate platform flanks are found in the triassic of the dolomites in northern italy and in the bahamas respectively they document the flank geometry and the processes readjusting the slope profile once the limiting slope angle is exceeded the grainy flank sediments in the dolomites modified by shearing and avalanching produce straight slope profiles with declivities up to 35 whereas the muddy bahamian flank sediments modified by large scale creep and rotational to translational sliding and slumping produce a concave upwards slope profile inclined at less than 4 the comparison between slope angle and sediment fabric indicates that the physical behaviour of sediments in the gravity field angle of shearing and mode of readjustment processes is linked to the composition of the slope sediment among the variables such as sea level subsidence climate plate motion and oceanographic setting windward leeward sediment fabric is suggested to be a major if not the major control on slope angle and slope curvature of carbonate platform flanks besides the recently documented tendency of carbonate sediments to build steeper slopes than siliciclastics this proposed relation sheds new light on the analysis and quantification of the variables influencing the geometry and depositional evolution of carbonate systems furthermore it provides an opportunity to deduce sediment composition from seismic lines and predict lithology prior to drilling

plumes and hotspots as sources of greenstone belts abstract greenstone belts that are older than 0 18 ga formed under circumstances that generate a buoyant garnet poor root because they do not generate enough depletion in the lithospheric mantle juvenile arcs involving average temperature early precambrian mantle were not a significant source of early precambrian greenstone belts instead early precambrian greenstone belts originated from mantle plumes and hotspots we focus on the origin of highly deformed morb like greenstone belts that are underlain by tonalitic to trondhjemitic plutons and surrounded by metamorphosed accretionary wedge like sediments these greenstone belts could form as the consequence of subducting a ridge segment within 500 km of a hotspot located beneath the continent as has happened on the taitao peninsula in chile the tonalitic to trondhjemitic plutons result from partial melting of oceanic crust thickened by extra melt derived from the hotspot the morb like rocks result from decompressive partial melting within the slab window produced by the subducting ridge the stratigraphy and tectonic setting of the recently emplaced taitao ophiolite are a possible phanerozoic analog of some morb like archean and early proterozoic greenstone belts

depths of modern coastal sand clinoforms abstract clinoforms are classical geological features examples in the rock record or in seismic reflection data that originally formed in shallow water can potentially help to locate paleo sea level but their precise links with sea level need to be established using modern examples before such inferences can be made accurately some near shore areas of high wave energy and steep coasts host sand bodies with upwards convex surfaces these clinoforms have been suggested to have been created from sand exported from the upper shoreface during stormy conditions and deposited at depths where wave action diminishes below the threshold of motion field and roy 1984 hernandez molina et al 2000 we have compiled the shapes of these bodies in a variety of wave environments in order to test this idea the interpreted depths at which the sand surface steepens i e the clinoform rollover vary from 21 m off southern california to deeper than 50 m off se australia to assess the link with wave action rollover depths were compared with adjacent deep water wave properties over the period 1992 2001 in the era 40 reanalysis dataset rollover bed shear stresses were computed from properties representing average wave conditions and from properties associated with upper percentile conditions stresses were found to reach 0 1 0 5 pa during upper 5 percentile wave conditions exceeding the sediment threshold of motion estimated from reported grain size data sediment to below the rollover is therefore mobile during extreme wave conditions

topographic characteristics of the submarine taiwan orogen a complete digital elevation and bathymetry model of taiwan provides the opportunity to characterize the topography of an emerging mountain belt the orogen appears to form a continuous wedge of constant slope extending from the subaerial peaks to the submarine basin we compare submarine channel systems from the east coast of taiwan with their subaerial counterparts and document a number of fundamental similarities between the two environments the submarine channel systems form a dendritic network with distinct hillslopes and channels there is minimal sediment input from the subaerial landscape and sea level changes are insignificant suggesting that the submarine topography is sculpted by offshore processes alone we implement a range of geomorphic criteria widely applied to subaerial digital elevation models and explore the erosional processes responsible for sculpting the submarine and subaerial environments the headwaters of the submarine channels have steep straight slopes and a low slope area scaling exponent reminiscent of subaerial headwaters that are dominated by bedrock landslides the main trunk streams offshore have concave up longitudinal profiles extensive knickpoints and a slope area scaling exponent similar in form to the onshore fluvial domain we compare the driving mechanisms of the likely offshore erosional processes primarily debris flows and turbidity currents with subaerial fluvial incision the results have important implications for reading the geomorphic signals of the submarine and subaerial landscapes for understanding the links between the onshore and offshore environments and more widely for focusing the future research of the submarine slope

icp ms a powerful tool for high precision trace element analysis in earth sciences evidence from analysis of selected u s g s reference samples abstract inductively coupled plasma mass spectrometry icp ms is a powerful analytical technique with considerable application to geochemistry in this paper we demonstrate the power of icp ms as a tool in petrogenetic studies data for 28 elements in seven u s g s basalt and andesite reference samples agv 1 bcr 1 bhvo 1 bir 1 dnc 1 w 1 and w 2 are presented accuracy and precision standard deviation for 26 of these elements is either excellent a detailed description of the icp ms technique we have developed is given a simple hf hno 3 dissolution in screwtop teflon bombs was used for sample preparation icp ms instrumental sensitivity was measured by external solutions with surrogate calibration for 4 elements matrix correction was made by use of standard addition naturally occurring internal standards rb y ce and pb were used to correct for instrumental drift between sample and spiked sample measurements we report data for the elements li cs rb sr ba zr hf nb ta y sc u th pb and all 14 of the ree in addition the procedure acquired data for be mo w tl and bi many elements in this array are crucial for the evaluation of petrogenetic models and source tracing herein lies the real power of icp ms one technique one dissolution small sample size good limits of detection good to excellent accuracy and good precision for many geochemically important elements

archean plate tectonics what can be learned from continental geology some basic questions about archean plate tectonics can be addressed by examining accretionary archean margins in particular fault zones with significant strike slip components on the canadian shield 1 were the oceanic plates typically rigid like modern plates yes significant lateral viscosity contrasts in the lithosphere between plates and plate boundaries are required for major strike slip faults to exist conversely strike slip faults are a kinematic consequence of rigid plates 2 did large oceanic plates exist in the archean probably first the length and offset of the longest preserved segments of archean faults are similar to modern examples such as in alaska less directly the duration of a period with a consistent sense of strike slip at a point on the continental side of an accretionary margin should be related to the time that a typical oceanic plate remains outboard of the margin this time varies proportionally with size of typical ocean plates and inversely with their velocity the

stratigraphic modeling of the western taiwan foreland basin sediment flux from a growing mountain range and tectonic implications abstract sediment flux signals from source to sink in foreland basins preserve a record of tectonics sea level and climate through erosion and sedimentation however longitudinal sediment transport often occurs in foreland basins thus removing part of the orogenic material flux from foreland basin records here we use mass balance calculation and stratigraphic simulations of sediment fluxes for the taiwan orogen to provide an order of magnitude estimate of how much orogenic material may bypass a foreland basin our results indicate a significant potentially more than 50 mismatch between sediment volume currently preserved in the basin and the amount of material presumably eroded from the orogen since the onset of collision in taiwan this suggests either a significant overestimation of average erosion rates over the period concerned with orogenic development of taiwan or it supports previous paleogeographic work suggesting that longitudinal sediment transport in the paleo taiwan strait served as a major bypass conduit of importance for the establishment of a steady state orogen we identify candidate submarine topography in the south china sea that may preserve taiwan s missing erosional mass

the effects of topography on sedimentation from particle laden turbulent density currents abstract we present results from a series of laboratory experiments that illustrate the influence of changes in channel topography depth or width on the sedimentation patterns produced by steady particle laden currents on a planar surface in a channel with constant width such currents created deposits that thinned exponentially with distance as long as a current was not blocked to produce a bore topographic features consisting of constrictions ridges and sudden openings caused no significant deviation from exponential deposit thinning or discontinuity in thickness even when there was a transition in flow regime caused by the topographic feature changes in the channel width did affect the distance over which the deposit thinned with the deposits associated with wider parts of the channel thinning more rapidly with distance in contrast if the topographic change was large enough to partially reflect the flow producing an upstream propagating bore then the deposit did not thin exponentially the results are consistent with a model in which the current is assumed to be turbulent and well mixed in this case sedimentation occurs at a rate proportional to the channel width the settling speed of the particles and their concentration within the current and inversely proportional to the current discharge such a model predicts that as a flow passes through a topographic control that does not produce a bore the deposit continues to thin exponentially even if the flow undergoes a transition from the subcritical to the supercritical regime the results suggest that in natural systems changes in current speed and flow regime do not in themselves produce changes in deposit thickness or gradient in thickness unless the flow is partially blocked and a fraction of the flow is reflected upstream

archean metavolcanics from the rouyn noranda district abitibi greenstone belt quebec 2 mobility of trace elements and petrogenetic constraints qualitative evaluation of altered mafic pillows from the rouyn noranda district of the abitibi metavolcanic belt reveals that for low grade metamorphism the high field strength elements hfs zr y ti and the rare earth elements ree are immobile however where alteration is extreme the ree elements are leached and redeposited by carbonate rich and k rich metasomatic fluids whilst zr y and zr ti remain unchanged rubidium ba and to a lesser extent sr are mobile at all levels of alteration however provided certain filters are applied the original geochemical values of the lavas for the alkaline elements the hfs elements and the ree can be defined the geochemical variations thus identified for the volcanic series of the rouyn noranda district are ascribed to a combination of contamination and fractional crystallization of a primitive tholeiitic magma

submarine slopes with an exponential curvature abstract the curvature of submarine slopes is a little used source of information on transport processes and sediment composition in a survey of modern submarine slopes selected from all over the world about 15 are characterized by very regular profiles with sharp shelfbreaks and concave upward curvature an exponential function describes this morphology very well these exponential profiles are from three very different depositional settings a continental margin close to a young mountain belt with very high rates of sedimentation high latitude continental margins with evidence of ice cover during quaternary glacial intervals and a carbonate platform margin we propose that the curvature is controlled by the exponential decay of transport capacity with increasing distance from the shelf edge while exponential decrease of transport may be a general property of most slopes the development of a regular exponential profile is commonly disturbed by other factors in particular by the interplay with wave dominated transport at the upper boundary during sea level fluctuations and with turbidite dominated sedimentation at the lower boundary the good exponential fit of the profiles described here results from the fact that vertical shifts of the shelfbreak are very small compared to the horizontal progradation the three depositional settings mentioned above prevent the shelf edge from fluctuations in the vertical high rates of sedimentation and therefore rapid progradation result in minimal movements of the shelfbreak ice covered margins are limited by the grounding level of the ice and margins built by reef organisms are stationary because the hard limestone is relatively resistant to erosion during lowstands of sea level the numerical value of the exponent is distinctly different for sandy and muddy slopes indicating the possibility of using the curvature to deduce sediment composition of submarine slopes

growth of granite greenstone terranes at convergent margins and stabilization of archean cratons abstract archean granite greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events most archean granite greenstone terranes appear to have acquired their first order structural and metamorphic characteristics at convergent plate margins where large accretionary wedges similar in aspect to the chugach makran and altaids grew through offscraping and accretion of oceanic plateaux oceanic crustal fragments juvenile island arcs rifted continental margins and pelagic and terrigenous sediments buoyant slabs of parts of archean oceanic lithosphere may have been underplated beneath these orogens forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere back stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas sanukitoid suite in the mantle wedge whereas other magmas tonalite trondhjemite granodiorite or ttg suite appear to have been generated by melting of hot young subducted slabs eventual collision of these juvenile orogens with other continental blocks formed anatectic granites then thickened the crust beyond its ability to support its own mass which initiated gravitational collapse and decompressional release of syn to late tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs and aided in the cratonization of the granite greenstone terranes deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid and upper crust forming ore deposits and plutons most preserved granite greenstone terranes have been tectonically stable since the archean and form the cratonic interiors of many continents

the formation and fate of large oceanic igneous provinces abstract large igneous provinces are conspicuous features of late phanerozoic geology and include continental flood basal rifted continental margin volcanic sequences and oceanic plateaus the latter are formed in an environment which typically recycles back into the mantle on a time scale of 200 m y but because comparisons have recently been made between oceanic plateaus and precambrian greenstone belt sequences new questions arise about their formation their fate and their preservation here we review some critical aspects of three oceanic plateaus ontong java kerguelen and the caribbean colombian abducted plateau and comment on their make up and the factors governing their preservation with particular relevance to ancient terranes many large igneous provinces can be linked to mantle plumes where plumes ascend beneath spreading ridges their energy is transformed into a large melt volume producing over thickened plateau crust where the spreading rate is low in relation to magma supply the plateau may become subaerial e g iceland but with fast spreading the plateau remains submarine thicker lithosphere may result in plume incubation before magma extrusion and there are many intermediate situations where plumes could readily break through thin lithosphere oceanic or continental because magma supply exceeds extension rate plateaus may be characterised by thick sequences of flows and sills rather than the sheeted dykes typical of phanerozoic ophiolites precambrian greenstones could represent imbricated oceanic plateaus or plumes penetrating thin continental lithosphere the initial high temperature and the buoyant nature of the depleted refractory keel of plateaus contributes to their preservation relative to normal oceanic crust when they collide with active margins they choke the subduction zone causing subduction flip or backstep and the development of extensive talc alkaline arc volcanism on top of the plateau sequences however after x 100 m y they are potentially negatively buoyant so if fluids become available to promote tmnsfotmation of the deeper zones to eclogite they may be able to spontaneously subduct

clinoform development by advection diffusion of suspended sediment modeling and comparison to natural systems clinoforms are the building blocks of prograding stratigraphic sequences these sig moid shaped surfaces can be found forming today on modem deltas sedimentation rate profiles over the clinoform surface of these deltas show low rates of sediment accumulation on both topset and bottomset regions with a maximum accumulation rate on the upper foreset region we pres ent a model for the formation of clinoforms that relies on the interpretation of modem clinoform sedimentation as a result of the distribution of shear stresses at the mouth of a river model clino form surfaces are generated using an equation for the conservation of suspended sediment concentration together with a conservation of fluid equation for simple time averaged flow velocity fields in the model suspended sediment is advected horizontally into a basin and gravitational settling of sediment particles is counteracted by vertical turbulent diffusion in shallow water shear stresses are too large to allow deposition and sediment bypasses the topset region with increasing water depth near bed shear stresses decrease and sediment is allowed to deposit at the foreset region with gradually decreasing rates toward deeper water this sedimentation pattern leads to progradation of the clinoform surfaces through time the clinoform surfaces produced by the model capture the fundamental morphological characteristics of natural clinoforms these include the gradual slope rollover at the topset and bottomset steeper foreset slopes with increased grain size and an increase in foreset slope through time as clinoforms prograde into deeper water because the parameters controlling the model clinoforms have a direct relation to physical quantities that can be measured in natural systems the model is an important step toward unraveling the physical processes associated with these deposits

modeling the rollovers of sandy clinoforms from the gravity effect on wave agitated sand abstract clinoforms on coasts exposed to ocean waves form from sand exported from the shoreface during extreme wave conditions and deposited where wave action diminishes elsewhere it is shown that during upper 5 percentile wave conditions wave induced shear stresses exceed the sediment threshold of motion to below the clinoform rollovers experiments and theory suggest that where bed sediments are agitated by waves the effect of gravity should move particles down slope with a flux proportional to the slope combined with considerations of continuity this implies a diffusion of the sediment topography a property that would explain the smooth seabed morphology found at sandy rollovers where recorded with multibeam sonar for situations where this gravity effect dominates a simple analytical expression developed here shows how the rollover curvature should relate to wave properties and to the offshore component of sediment flux more sharply curved rollovers are expected where waves have short periods or where the sediment flux is large relative sediment fluxes were calculated using the model from rollover curvature and wave properties for sites from california southeast australia and atlantic and mediterranean iberia the relative magnitudes of the fluxes are roughly as would be expected from local physiography and coastal erosion rates for sandy clinoforms developed under mainly wave influences the model could be useful for exploring variations within clinoform datasets e g how varied convexity in an area of uniform wave properties reflects varied sediment flux and for interpreting how varied curvatures of rollovers within seismic stratigraphy reflect how wave climate and flux have varied in the past

large scale stratigraphic architecture eustatic variation and unsteady tectonism a theoretical evaluation we incorporate a process based depositional model with basin subsidence models to predict stratigraphic records this allows us to investigate the importance of subsidence geometry on coastal stratigraphy and thus to characterize and compare the stratigraphic architecture of two categories of tectonic basins the models demonstrate that the correlation of stratigraphic sequences to eustatic cycles is not the same in passive margin basins as in foreland basins and that in a foreland basin the record of episodic tectonism is distinct from that of eustatic sea level change in the model sediment transport is approximated by slope controlled diffusion nonmarine transport is treated as more efficient diffusion than marine transport three different subsidence and sediment supply models are examined a simple passive margin basin a simple foreland basin and then a foreland basin for which vertical motions are driven by thrust shortening that is compensated flexurally and for which sediment supply is related to relief predicted passive margin stratigraphies for cases of varying eustatic sea level are similar to those of natural basins and include progradational packages and subaerial unconformities which are used to define sequence boundaries that form during sea level fall we explore the timing relationships between stratigraphic features and a sinusoidal sea level history showing that the phase relationship depends on subsidence sediment flux efficiency of sediment transport and period and amplitude of sea level when the basic geometry of the basin is inverted placing the sediment supply on the side with maximum subsidence as is the case in foreland basins the sequence character changes markedly subaerial erosion does not generate unconformities in the models of a dynamic foreland basin sediment supply and subsidence are linked to the structure of the flanking thrust belt and are not necessarily constant for steady thrusting and variable sea level unconformities that define sequence boundaries form only on the distal or forebulge side of the basin and the ages of the sequence boundaries correlate to limes of rising sea level in cases of constant sea level but variable thrusting subaerial unconformities are cut locally on both the proximal margin of the basin and the distal margin of the basin yet the ages of the proximal margin and distal margin unconformities are out of phase in the tectonic cycle erosion is most pronounced during quiescence on the proximal side and during thrusting on the distal side

the mechanisms of petrogenesis of the archaean continental crust comparison with modern processes the petrographic and chemical composition of magmatic rocks generated during the archaean appears to be different from that of post archaean rocks komatiites are widespread before 2 5 ga and rarely occur afterwards in addition the archaean continental crust is primarily ttg tonalitic trondhjemitic and granodioritic in composition exhibiting typical trondhjemitic differentiation trends whereas modern equivalents are granodioritic to granitic following classical calc alkaline differentiation trends this distinction becomes more prominent when rare earth elements ree are taken into account archaean ttg are yb poor ybn 8 5 and have high layb ratios 5 layb n 150 in comparison the post 2 5 ga granitoids emplaced in subduction zone geodynamic environments have high yb content 4 5 ybn 20 with very low layb n ratios 20 theoretical calculations and experimental petrology have shown that the ttg can be produced by partial melting of an archaean tholeiite transformed into garnet bearing amphibolite consequently the low heavy ree content of the ttg is explained by the influence of both residual garnet and hornblende in their source after 2 5 ga the role of these minerals in calc alkaline magma genesis becomes progressively less important which is interpreted in terms of a cooling earth model in modern subduction zone environments the subducted oceanic slab is relatively old and cold and the geothermal gradient along the benioff plane in low ca 10 c km consequently the down going lithosphere undergoes dehydration before partial melting is able to occur the liberated fluids are light ree and lile enriched and ascend into the overlying mantle wedge where they induced partial fusion the produced magmas separate from their mantle source region leaving a residue mainly composed of olivine and pyroxenes mantle derived magmas typically exhibit high yb contents due to low kdyb values for olivine and pyroxenes during the archaean the subducted lithosphere was relatively young and hot providing high geothermal gradients along the benioff zone thus partial melting of the subducted slab was possible at lower temperatures before dehydration would take place garnet and hornblende are the main residual phases accounting for the low yb contents of the archaean ttg this model can be tested using a modern analogue of archaean like subduction processes in south chile an oceanic ridge has subducted and all thermodynamic calculations indicate that this creates locally high geothermal gradients along the benioff zone thus in very small areas archaean like environments may be simulated in modern subduction zones the modern andesites produced in this environment show archaean geochemical characteristics with low ybn 5 whereas the majority of andesites along the andean arc have modern patterns with ybn ranging from 8 to more than 17 this conclusion was generalised to all young subducted lithospheres all over the world in conclusion it appears that since the archaean there has been a change in the site of continental crust genesis the location of calc alkaline magma source in subduction zone environments has migrated through time from the subducted slab to the mantle wedge this is a direct consequence of the progressive cooling of the earth

secular variation in greenstone sequence development emphasizing superior province canada abstract evidence from the superior province of the canadian shield suggests the existence of several distinct lithostratigraphic associations in greenstone belts 1 quartz arenite and carbonate bearing sequences 2 mafic ultramafic volcanic sequences 3 mafic to felsic volcanic cycles and 4 timiskaming type sequences of fluviatile sediments and calc alkaline to alkaline volcanics recently recognized quartz arenite bearing sequences in the sachigo and wabigoon subprovinces are from base to top quartz arenite stromatolite bearing carbonate oxide facies iron formation and komatiite or tholeiite primary structures in the quartz arenites indicate shallow water deposition and represent mature sedimentation granitoid provenance of these sediments is postulated based upon petrography of coarse arenites and clasts of associated conglomerates the few dated superior province quartz arenite bearing sequences are 2 85 ga the shallow water origin of the primary structures and biota and the areal extent of the quartz arenite and carbonate sequences invite comparison with platform sediments at passive margins platform sequences unconformably overlie 3 ga granitoid rocks and older mafic volcanics where original lower contacts are preserved in two localities mafic volcanic sequences contain sheet flows iron formations and deep water argillites based on physical volcanology and sedimentology these sequences are compared to modern deep water mafic plain and hawaiian volcanism they generally underlie mafic to felsic volcanic cycles and overlie platform sequences mafic sequences are 2740 2700 ma in age mafic to felsic volcanic cycles consist of deep water tholeiitic flows overlain by shallow water to subaerial intermediate to felsic pyroclastics and flows the facies style of volcanism petrography and geochemistry of the mafic to felsic cycles most closely resemble the characteristics of crust in continental magmatic arc systems this type of lithostratigraphic association varies in age from 2775 to 2700 ma in the superior province with minor exceptions timiskaming type sequences consist of fluviatile conglomerate immature arenites and wackes with subaqueous to subaerial volcanics of variable chemistry longitudinal and lateral variation within fault bounded basins is extreme and the sequences have generally been affected by only late stages of deformation the combination of rock types facies distribution the fault bounded nature of the repositories and the locus of sedimentation along zones of transcurrent displacement suggest timiskaming sequences are best compared with modern pull apart basins superior province timiskaming sequences are within the superior province platform sequences are the earliest postorogenic lithostratigraphic unit in greenstone belts followed by mafic plain sequences arc volcanics and pull apart basins we suggest that platform sequences based upon limited evidence are preceded by an orogeny and that pull apart basins represent a subsequent orogeny the lithostratigraphic units above have distinct metallogenic associations 1 platform sequences based upon compilation of several shields may contain detrital au and u and oxide facies iron formations associated felsic volcanics contain no recognized volcanogenic cu zn deposits in the superior province the following metallogenic associations are noted 2 mafic plain sequences contain komatiite flow related ni deposits cu ni and platinum group deposits are associated with flows and coeval intrusions 3 arc volcanic sequences contain volcanogenic cu zn sulphide deposits and scarce deposits of porphyry type cu mo and 4 timiskaming sequences and related plutonism are related to lode au

the role of subsidence in shelf widening around ocean island volcanoes insights from observed morphology and modeling abstract on reefless volcanic islands insular shelves are thought to have formed essentially by the combined effects of wave erosion and glacio eustatic sea level oscillations subsidence however has also been recognized as having an important role in the development of these morphologies yet few studies have quantified the relative contribution of subsidence to shelf generation and development particularly to shelf width a better understanding of this contribution however is key to understand the long term evolution of coasts at volcanic islands particularly given that subsidence may exacerbate the effects of marine erosion in this study we assess quantitatively the role of subsidence in shelf development by comparing real cross shore shelf profiles with modeled profiles using varied rates of subsidence to achieve this we used shelf bathymetric profiles from faial and sao jorge islands in the azores which we compared with predictions of a numerical model of coastal erosion that has been calibrated previously against other field data the first set of model runs was made to calibrate the model by determining the values that produced shelves with break depths widths and profile shapes that were similar to those observed the second set of runs which served to evaluate the contribution of subsidence to shelf widening revealed that subsidence may have been responsible for increasing shelf widths by almost 2 5 times relative to shelves formed only by the combined effects of wave erosion and glacio eustatic oscillations modeling shelf formation on the same islands but with increased subsidence rates up to 2 2 mm yr resulted in shelf profiles up to 19 km wide a value 3 to 6 times greater than observed on these islands our study therefore reinforces the idea that subsidence is a key contributor to the generation of broad insular shelves given its role in enhancing coastal retreat our shelf evolution modeling also suggests that notwithstanding the crucial role subsidence plays in increasing the width of shelves on islands subjected to energetic wave regimes as it happens in the azores and hawaii modification by erosion is important enough to result in shelf morphologies that are not constructional in essence but rather dominantly erosive surfaces this study therefore contributes to a better understanding of how insular shelves and submarine terraces form and develop by proving important new quantitative insights of the role of subsidence on the generation of these morphologies

model for the sigmoidal curvature of submarine slopes sigmoidal slope profiles are common on the margins of continents and intracratonic basins we propose that these profiles consist of two genetically different segments the lower concave part reflects the exponential decrease of the sedimentation rate with increasing distance from the source at the shelf edge the upper convex part represents the gradational boundary between the shelf domain where sediment is moved by waves and wave induced currents and the slope domain where sediment is moved by gravity the boundary is narrow and the shelf edge is sharp if sedimentary base level remains stationary during progradation the boundary becomes gradational and the shelf edge becomes rounded if storms and sea level cycles induce significant fluctuations of base level this model is supported by observations on holocene sedimentary systems as well as numerical simulations with the program strata based on the diffusion equation for sediment dispersal

the record of oligocene middle miocene paleoenvironmental changes in a carbonate platform iodp exp 359 maldives indian ocean abstract the carbonate edifice of the maldives has an exceptional record of the global climate and ocean current reconfigurations during the neogene that set the path for the establishment of the present day global climate system the study of seismic profiles sedimentary facies microfacies and ichnofabrics of the oligocene miocene platform slopes allows reconstructing the different stages of basin evolution the focus is on the characterization of paleoenvironmental conditions along the slope at iodp expedition 359 drill sites as well as subtle changes of the slope depositional regime controlled by variations in the frequency and intensity of the platform shedding and the basin oxygenation natural gamma radiation and ichnofabric logs are used as a proxy for sea floor ventilation in the inner sea area which is today surrounded by atolls from the oligocene 25 5 ma to the end of the middle miocene climate optimum 15 ma the maldives carbonate platform slopes were poorly oxygenated the opening of the eastern passages through the maldives carbonate platform at 17 5 ma led to an improvement of the sea floor oxygenation sea level fluctuations controlled the sediment input from the platform onto the slope that resulted in changes of the fossil and trace fossil assemblages after the evolution from a ramp to a flat topped platform with well defined bank edges the platform developed hanging shoulder and more sigmoidal clinoforms this correlates to the global decline of coral reefs during the middle miocene climate transition 15 ma and a general sea level lowering the associated slope deposits show abundant channels and cyclic steps are rich in reworked bioclasts and have ichnofabrics pointing towards a good oxygenation of the sea floor the example of the maldives carbonate platform therefore shows how integrating seismic geometries and compositional changes i e microfacies and ichnofabrics allow identifying changes in sea level and in the current pattern as the main controlling factors on carbonate platform evolution

archean greenstone belt magmatism and the continental growth mantle evolution connection constraints from th u nb lree systematics of the 2 7 ga wawa subprovince superior province canada abstract an extensive database including th u nb ree systematics for diverse magmatic and sedimentary lithologies of 2 7 ga wawa greenstone belts provide new constraints on the mechanism of crustal growth in the southern superior province and controls on its composition the greenstone belts are characterized by collages of oceanic plateaus oceanic island arcs and trench turbidites these lithotectonic fragments were tectonically assembled in a large subduction accretion complex following juxtaposition these diverse lithologies were collectively intruded by syn kinematic ttg tonalite trondhjemite granodiorite plutons and ultramafic to felsic dykes and sills with subduction zone geochemical signatures intra oceanic basalts are characterized by near flat ree patterns and nb u and nb th ratios generally greater than primitive mantle values consistent with positive nd values they are associated with komatiites the association being interpreted as an ocean plateau sequence erupted from a mantle plume bimodal arc volcanic sequences trench turbidites and contemporaneous ttg suites are characterized by fractionated ree with nb u and nb th ratios less than primitive mantle values mixing hyperbolae between oceanic plateau and magmatic arc sequences pass through the estimated composition of bulk continental crust suggesting that crustal growth in the late archean was by tectonic sedimentary and chemical mixing of oceanic plateau and arc sequences at convergent plate boundaries mixing calculations suggest that oceanic plateau and subduction zone components in the wawa continental crust are represented by 6 12 and 88 94 respectively high nb u and nb th ratios of plateau tholeiitic basalts are interpreted as a complementary reservoir to arc magmatism low nb u and nb th hundreds of millions of years prior to recycling of oceanic lithosphere through a subduction zone high nb u nb th and its incorporation into a mantle plume from which 2 7 ga plateau tholeiites erupted the variably high nb u ratios of the plateau basalts are consistent with early extraction of large quantities of the protoliths magmatic precursor of continental crust from the southern superior province asthenospheric mantle