a self regulating model of bedrock river channel geometry 1 the evolution of many mountain landscapes is controlled by the incision of bedrock river channels while the rate of incision is set by channel shape through its mediation of flow the channel shape is itself set by the history of bedrock erosion this feedback between channel geometry and incision determines the speed of landscape response to tectonic or climatic forcing here a model for the dynamics of bedrock channel shape is derived from geometric arguments a normal flow approximation for channel flow and a threshold bed shear stress assumption for bedrock abrasion the model dynamics describe the competing effects of channel widening tilting bending and variable flow depth transient solutions suggest that channels may take 1 10 ky to adapt to changes in discharge implying that channel disequilibrium is commonplace if so landscape evolution models will need to include bedrock channel dynamics if they are to probe the effects of climate change

role of subsurface entrainment temperature te in the onset of el nino events as represented in an intermediate coupled model an improved intermediate coupled model icm is described for use in enso related modeling in the tropical pacific with ten baroclinic modes included in the vertical and horizonatally varying stratification taken into account one crucial component of the model is the way in which the subsurface entrainment temperature in the surface mixed layer te is explicitly used to determine the sea surface temperature sst variability an optimized procedure is developed to depict te using inverse modeling from an sst anomaly equation and its empirical relationship with the sea surface pressure variability the coupled system realistically produces interannual variability associated with enso cycles with a dominant 4 year oscillation the onset and development of el nino events from this icm are examined in view of the well known delayed oscillator paradigm an example is given for the evolution of la nina conditions in model year 2 to el nino conditions in year 4 right after a la nina event e g in year 2 there is a clear signature of reflections at the western boundary in early year 2 with related equatorial signals propagating eastward along the equator into the eastern basin in middle year 2 however these reflected signals on the equator do not directly lead to an onset of an el nino event at that time instead approximately 1 year delay a major el nino event is seen to develop in the following year late year 3 at a time when there is no reflected signal explicitly from the western boundary indicating that the origin of the el nino event cannot be directly ascribed to the reflection processes instead kelvin waves in the ocean that actually triggers the el nino event in early year 3 are generated by interior wind anomalies near the date line that are associated with the first appearance of warm sst anomalies off the equator persisted te anomalies off the equator in the western tropical pacific initiate the warm sst anomaly near the date line along the north equatorial countercurrent region which induces wind anomalies and an ocean atmosphere coupling leading to the el nino event in year 4 the relevance of these icm based results to other onset mechanisms of el nino and observations is also discussed

channeling of plume flow beneath mid ocean ridges we investigate the interaction of mantle plumes with mid ocean ridges in 3 d numerical convection models using a finite volume multigrid technique with local grid refinement we are able to incorporate a much stronger temperature dependence of viscosity than was possible in previous models this allows lateral viscosity variations of more than six orders of magnitude which is realistic in a ridge situation in models of ridge centered plumes with constant flux we determine how the shape and the along axis width of the plume head depends on various model parameters the plume material is significantly channeled along the axis of a slow spreading ridge if the plume viscosity is in the order of 1017 pa s the plume material spreads radially below the ridge if the spreading rate is fast or the viscosity contrast between plume and ambient mantle is small in time dependent calculations we investigate the temporal evolution of short term variations of the plume flux at slow spreading ridges these perturbations propagate in the form of pulses along the ridge with a speed much larger than the spreading rate our results support the hypothesis that pulsations of the iceland plume are the cause for v shaped topography and gravity anomalies at the reykjanes ridge

surface topography as key constraint on thermo rheological structure of stable cratons why some archean cratons survived for billions of years while the rest of the lithosphere has been reworked probably for several times is both enigmatic and fundamental for plate tectonics craton longetivity is mainly explained by their buoyancy and analyzed by testing gravitational stability of hardly detectable cratonic mantle keels as a function of a hypothesized thermo rheological structure destruction of some cratons suggests that buoyancy is not the only factor of their stability and many previous studies show that their mechanical strength is equally important the upper bounds on the integrated strength of cratons are provided by flexural studies demonstrating that te values equivalent elastic thickness in cratons are highest in the world 110 150 km yet the lower bounds on the integrated strength of stable cratons are still a matter of debate as well as the question on how this strength is partitioned between crust and mantle and which set of rheological parameters is most pertinent we show that primary observed cratonic features flat topography and frozen heterogeneous crustal structure represent the missing constraints for understanding of craton longevity the cratonic crust is characterized by isostatically misbalanced density heterogeneities suggesting that the lithosphere has to be strong enough to keep them frozen through time without developing major gravitational instabilities and topographic undulations hence to constrain thermo rheological properties of cratons one should investigate the stability of their topography and internal structure our free surface thermo mechanical numerical models demonstrate that craton stability cannot be warranted even by very high crustal strength so that dry olivine mantle and cold thick lithosphere are indispensable conditions we establish lower bound limits on the thermo rheological structure of cratons in particular we find that minimal te needed for long term stability of continents is approximately 90 110 km of which at least 70 km should be contributed by the lithosphere mantle

lithospheric thinning under the araripe basin ne brazil from a long period magnetotelluric survey constraints for tectonic inversion abstract the lithospheric architecture of the western borborema province and northern sao francisco craton of ne brazil has been investigated through analysis of long period magnetotelluric data acquired along a 700 km long survey using 12 instruments the survey samples several tectonic terrains in the province and penetrates into the adjacent sao francisco craton after crossing the araripe basin an aborted rift basin filled with mesozoic sediments that peak at 1000 m above mean sea level high conductivities are observed at shallow depths under the main precambrian shear zones that pervade the province consistent with tectonic reactivation and as a small patch embedded within the high resistivities that characterize the sao francisco craton high conductivities 25 m are also observed below 120 km depth between the patos and pernambuco lineaments right under the araripe basin flanked by resistive 120 m material immediately to the north and south this deep highly conductive body is found consistent with the presence of melt and aqueous fluids and is interpreted as shallow asthenospheric mantle bounded by thicker lithosphere we propose that extensional stresses in the mesozoic stretched and thinned the lithosphere under the araripe basin causing passive upwelling of asthenospheric material and lateral flow of the overlying lithosphere and resulting in thickening of the lithosphere under the flanks and uplift of the araripe basin we also hypothesize that thermal weakening of the lithospheric mantle perhaps sustained by channeling of asthenospheric flows under the basin would have caused regional stresses to concentrate in the brittle upper crust and contribute to basin inversion we thus propose that a combination of localized horizontal stresses and vertical buoyancy from underlying asthenospheric material are ultimately responsible for the actual topography of the araripe basin

mesozoic igneous activity in the maranha o province northern brazil 40ar 39ar evidence for separate episodes of basaltic magmatism low and high ti basaltic rocks of the maranha o province in northern brazil appear to be related to the opening of the atlantic ocean 40ar 39ar dating was carried out on powdered whole rock basalts both washed and unwashed in dilute nitric acid to document this relationship acid treatment does not alter the argon retention properties of retentive phases but is effective in removing alteration phases high ca k degassed at low laboratory temperatures our results identify that at least two periods of volcanic activity formed the maranha o province low ti basalts from the western sector are 200 190 ma and are related to the opening of the north atlantic high ti basalts from the eastern sector are 129 124 ma slightly younger than compositionally similar rocks in the serra geral flood basalt province of southern brazil and are related to the opening of the south atlantic other flood basalts such as the columbia river deccan rajmahal ferrar and siberian provinces were each predominantly formed in 1 2 ma the maranha o basalts remnants of separate igneous events spanning 70 ma should not be classed as a single flood basalt province the temporal complexities noted in the maranha o basaltic rocks appear to be related to the tracks of hotspots that led to the rifting of the atlantic ocean two sets of mesozoic basaltic rocks occur in southern africa india and south america resulting from events leading to the fragmentation and dispersal of sections of gondwanaland in each sub continent the two provinces were formed by events separated by 50 70 ma of these pairs only the maranha o and serra geral show geochemical similarities indicating compositionally distinct upper mantle source regions under sections of brazil separated by 3000 km

a mechanistic model for river incision into bedrock by saltating bed load 1 abrasion by bed load is a ubiquitous and sometimes dominant erosional mechanism for fluvial incision into bedrock here we develop a model for bedrock abrasion by saltating bed load wherein the wear rate depends linearly on the flux of impact kinetic energy normal to the bed and on the fraction of the bed that is not armored by transient deposits of alluvium we assume that the extent of alluvial bed cover depends on the ratio of coarse sediment supply to bed load transport capacity particle impact velocity and impact frequency depend on saltation trajectories which can be predicted using empirical functions of excess shear stress the model predicts a nonlinear dependence of bedrock abrasion rate on both sediment supply and transport capacity maximum wear rates occur at moderate relative supply rates due to the tradeoff between the availability of abrasive tools and the partial alluviation of the bedrock bed maximum wear rates also occur at intermediate levels of excess shear stress due to the reduction in impact frequency as grain motion approaches the threshold of suspension measurements of bedrock wear in a laboratory abrasion mill agree well with model predictions and allow calibration of the one free model parameter which relates rock strength to rock resistance to abrasive wear the model results suggest that grain size and sediment supply are fundamental controls on bedrock incision rates not only by bed load abrasion but also by all other mechanisms that require bedrock to be exposed in the channel bed

seismic evidence for a fossil mantle plume beneath south america and implications for plate driving forces a teleseismic travel time study reveals the presence of a fossil plume in the deep upper mantle beneath brazil which has apparently remained geographically fixed with respect to the overly ing continent despite thousands of kilometres of plate motion this result implies that the upper mantle and lithosphere beneath south america have remained coupled since the breakup of gondwanaland and may provide an answer to the long standing question of what forces drive continental plates

lamprophyric volcanism of cenozoic age on the sierra leone rise implications for regional tectonics and the stratigraphic time scale abstract igneous and sedimentary rocks have been recovered by dredging the upper slopes of krause seamount a feature lying on the sierra leone rise near 9 n 20 w in the eastern equatorial atlantic the igneous samples consist of lamprophyres and an ankaramite which are rarely encountered in an oceanic setting associated with these rocks are breccias made up of lamprophyric fragments set in a lithified calcareous matrix containing a rich planktonic foraminiferal assemblage of early eocene ypresian subzone p8a age whole rock k ar dating of the igneous samples proved unsuccessful owing to advanced alteration and consequent argon loss however within the breccias are numerous fresh crystal clasts of ti rich amphibole pyroxene and biotite which have compositions closely similar to phenocrysts observed in the lamprophyric rocks suggesting that they are derived directly from them k ar dating of separates of the three minerals yielded ages in the range 53 3 55 4 ma the concordant results provide a maximum age for the ypresian subzone p8a and are consistent with its position within recently established cenozoic time scales they indicate that krause seamount is appreciably younger than the adjacent gambia and sierra leone basins in common with other occurrences of lamprophyric rocks in the oceans those exposed on krause seamount are located in an area of old oceanic basement close to a large fracture zone which has been reactivated by late stage volcanic activity these relatively rare alkaline rocks may be generated by remelting of a volatile rich metasomatized layer which is present in the upper mantle beneath mesozoic ocean floor

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

an empirical parameterization of subsurface entrainment temperature for improved sst anomaly simulations in an intermediate ocean model abstract an empirical model for the temperature of subsurface water entrained into the ocean mixed layer te is presented and evaluated to improve sea surface temperature anomaly ssta simulations in an intermediate ocean model iom of the tropical pacific an inverse modeling approach is adopted to estimate te from an ssta equation using observed sst and simulated upper ocean currents a relationship between te and sea surface height ssh anomalies is then obtained by utilizing a singular value decomposition svd of their covariance this empirical scheme is able to better parameterize te anomalies than other local schemes and quite realistically depicts interannual variability of te including a nonlocal phase lag relation of te variations relative to ssh anomalies over the central equatorial pacific an improved te parameterization naturally leads to better depiction of the subsurface effect on sst variability by the mean upwelling of subsurface temperature anomalies as a result ssta simulations

plumes in a convecting mantle models and observations for individual hotspots the motion of hotspots and the deformation of their underlying plume conduits as calculated within models of global mantle flow are presented a new list of 44 possible hotspots with associated tracks has been compiled for all of them calculations have been performed under consideration of individual age and anomalous mass flux for three different models of plume buoyancy and mantle flow plume source depth has usually been assumed to be the top of d but an alternative source depth at the 670 km discontinuity has also been considered using models of relative plate motions and boundaries hotspot tracks on plates have been calculated and compared with age data ocean floor topography and distribution of volcanics on continents absolute plate motions have been redetermined under consideration of hotspot motion using a new least squares method for the hawaiian and yellowstone hotspots source locations and hotspot motion have been computed for a total of up to 23 different models the results show plume conduits being tilted with source regions at the d moving in the lowermost mantle flow generally toward large scale upwellings under southern africa and the south central pacific hotspot surface motion often represents the horizontal component of midmantle flow which is frequently opposite to plate motion toward ridges and away from subduction zones in particular almost all models tested predict southward motion of the hawaii and kerguelen hotspots and westward motion of the iceland hotspot for models including hotspot motion the agreement between calculated and observed hotspot tracks is frequently about as good as or better than for the fixed hotspot model but sometimes fixed hotspots give the best fit in some cases where the track ends at a subduction zone e g for the bowie hotspot results can give indications about the otherwise unknown age of the hotspot in other cases especially for the tahiti hotspot results suggest an origin shallower than d and in yet other cases particularly east africa the failure of the hotspot models used supports other evidence indicating the presence of comparatively broad upwellings rather than localized plumes

alkaline volcano of paleocene age on the southern guinean margin mapping petrology 40ar 39ar laser probe dating and implications for the evolution of the eastern equatorial atlantic abstract a segment of the southern guinea plateau margin s g p m was surveyed during the 1988 equamarge ii cruise of the r v jean charcot detailed seabeam mapping reveals a huge 3000 m high volcano which was called the nadir seamount culminating at 840 m below sea level and 6 adventive cones volcanics dredged between 2100 and 1300 m of water depth consist of hyaloclastite breccias and subaerial alkali basalts recovered as both massive blocks and eolian pebbles with a desert varnish the data are consistent with the growth of a volcano up to emergence and thereafter its dismembering and subsidence volcanological petrographic and geochemical features ree of the volcanics present similarities with those observed 1 in early basalts from fuerteventura canary islands and maio cape verde islands 2 in lamprophyric volcanism from the sierra leone rise and 3 in volcanics from the gorringe bank 40ar 39ar laser probe dating was performed on two biotite phenocrysts from a massive basalt using step heating and spot fusion procedures both experiments concordantly display a plateau age of 58 6 0 3 ma and an integrated age of 58 6 0 7 ma respectively the nadir seamount provides the third evidence of paleocene alkaline volcanism in the eastern atlantic after the gorringe bank and the northern sierra leone rise krause seamount these volcanoes are presumed to have been controlled by fractures zones the ages and compositions of the nadir and krause seamounts respectively suggest a connection between the volcanic chains of both the sierra leone rise and guinea f z which may have recorded the same hotspot track

a latest cretaceous hotspot and the southeasterly tilt of britain the southeasterly regional dip of england and wales is usually ascribed to miocene movements loss of up to 2 km of cover from northern england shown by apatite fission track analysis has been related to uplift and erosion induced by a mantle hotspot but its effects were far more widespread than considered hitherto outcrop patterns throughout much of england wales and eastern ireland can be related to a hotspot beneath the irish sea uplift occurred within the late maastrichtian and erosion continued into the late palaeogene after post oligocene down faulting of the irish sea the modern drainage was initiated on the eroded surface unconformities provide the best means of dating orogenic movements in some localities the time interval between the youngest rocks preserved beneath the unconformity and the oldest preserved above may be small enough to enable the dating of fold movements to be determined with some precision thus in southeastern britain the major e w structures of the hampshire basin the weald and the london basin can be dated as miocene as rocks of oligocene age are involved in the folding in the hampshire basin and pliocene sands rest in erosional pockets of the chalk of se england because fold movements tend to affect large areas of the crust at discrete intervals of time the southeasterly tilt of britain evident from the fact that a nw se transect from anglesey to london crosses outcrops of rocks getting progressively younger from the precambrian of anglesey to the tertiary of the london basin has

enso simulation and prediction with a hybrid coupled model a hybrid coupled model hcm consisting of a tropical pacific ocean and global atmosphere is presented the ocean component is a linear reduced gravity model of the upper ocean in the tropical pacific the atmospheric component is a triangular 30 horizontal resolution global spectral general circulation model with 18 unevenly spaced levels in the vertical in coupling these component models an anomaly coupling strategy is employed a 40 yr simulation was made with hcm and the variability in the tropical pacific was compared to the observed variability the hcm produces irregular enso events with a broad spectrum of periods between 12 and 48 months on longer timescales approximately 48 months the simulated variability was weaker than the observed and on shorter timescales approximately 24 months the simulated variability was too strong the simulated variability is asymmetric in the sense that the amplitude of the warm events is realistic but there are no significant cold events an ensemble of 60 hindcast predictions was made with the hcm and the skill was compared to other prediction systems in forecasting sea surface temperature anomalies in the eastern pacific the hcm is comparable to the other prediction systems for lead times up to 10 months the anomaly correlation coefficient for the eastern pacific ssta remains above 0 6 for lead times of up to 11 months consistent with the 40 yr simulation hindcasts of cold events have little skill particularly when compared to hindcasts of warm events specific hindcasts also demonstrate that the hcm also has difficulty predicting the transition from warm conditions to normal or cold conditions

understanding cratonic flood basalts the origin of continental flood basalts has been debated for decades these eruptions often produce millions of cubic kilometers of basalt on timescales of only a million years although flood basalts are found in a variety of settings no locale is more puzzling than cratonic areas such as southern africa or the siberian craton where strong thick lithosphere is breached by these large basaltic outpourings conventionally flood basalts have been interpreted as melting events produced by one of two processes 1 elevated temperatures associated with mantle plumes and or 2 adiabatic decompression melting associated with lithospheric thinning in southern africa however there are severe problems with both of these mechanisms first the rifting circumstances of several wellknown basaltic outpourings clearly reflect lithospheric control rather than the influence of a deep seated plume specifically rift timing and magmatism are correlated with stress perturbations to the lithosphere associated with the formation of collisional rifts second the substantial lithospheric thinning required for adiabatic decompression melting is inconsistent with xenolith evidence for the continued survival of thick lithosphere beneath flood basalt domains as an alternative to these models we propose a new two stage model that interprets cratonic flood basalts not as melting events but as short duration drainage events that tap previously created sublithospheric reservoirs of molten basalt formed over a longer time scale reservoir creation existence stage i requires long term e g 1 ma supersolidus conditions in the sublithospheric mantle that could be maintained by an elevated equilibrium geotherm appropriate for the archean a slow thermal perturbation e g thermal blanketing or large scale mantle upwelling or a subduction related increase in volatile content the drainage event stage ii occurs in response to an abrupt stress change in the lithosphere which leads to the initiation and propagation of lithospheric dikes such a model accounts for the short eruption time of flood basalts the evidence for lithospheric control of the eruptions and the continued survival of cratonic lithosphere following these magmatic events the most notable consequence of this model is that it implies the existence of large reservoirs of magma comparable to the eruption volume of flood basalts that have been and are still likely present beneath stable continental lithopshere

the denudation history of the onshore continental margin of se brazil inferred from apatite fission track data a suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast brazil are presented some of the results reflect resetting related to the eruption of the parana continental flood basalt 130 ma and four others are directly related to late cretaceous magmatism 80 ma the majority of the results were obtained from precambrian basement rocks and sediments from the parana basin the fission track ages broadly increase inland from 60 90 ma on the coastal plain to 300 ma in the continental hinterland the track length distributions show a characteristic trend being negatively skewed at the coast and showing increasing bimodality inland except for the oldest samples which are more unimodal these trends do not reflect rift related heating and resetting but are the result of protracted denudation since the opening of the south atlantic the results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland the topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology for the region further south model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long wavelength topography unless the flexural rigidity is more than 1025 n m in this case the initial elevation of the rift margin would need to be 2 5 km in order to accommodate the amount of denudation inferred from the fission track data however it is possible that the effective flexural rigidity varies across the margin becoming greater in the continental interior alternatively the broad scale wavelength topography may be the result of magmatic underplating related to the parana continental flood basalt event if so estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5

three dimensional simulations of plume lithosphere interaction at the hawaiian swell rapid lithospheric thinning by mantle plumes has not been achieved in numerical experiments performed to date efficient thinning depends on small scale instabilities that convectively remove lithospheric material these instabilities are favored by hotter plumes or stronger temperature dependence of viscosity and a simple scaling independent of rheology controls their onset this scaling allows extrapolation of the results of numerical experiments to the earth s mantle mantle plumes between 100 and 150 kelvins hotter than the background mantle should exhibit small scale convective rolls aligned with the plate motion the unusual variation in heat flow across the hawaiian swell may be due to such instabilities it was found that the spreading of the plume creates a downwelling curtain of material that isolates it from the rest of the mantle for distances of at least 1000 kilometers from the plume origin this isolation has important consequences for the geochemical heterogeneity of the lithosphere and upper mantle

a new plumbing system framework for mantle plume related continental large igneous provinces and their mafic ultramafic intrusions abstract the magmatic components of continental large igneous provinces lips include flood basalts and their plumbing system of giant mafic dyke swarms radiating linear and the recently discovered circumferential type mafic sill provinces a lower crustal magmatic underplate mafic ultramafic m um intrusions associated silicic magmatism and associated carbonatites and kimberlites this paper proposes a new plumbing system framework for mantle plume related continental lips that incorporates all of these components and provides a context for addressing key thematic aspects such as tracking magma batches upstream and downstream and their geochemical evolution assessing the setting of m um intrusions and their economic potential interpreting deep magmatic component identified by geophysical signatures and estimating magnitudes of extrusive and intrusive components with climate change implications this plumbing system model and its associated implications needs to be tested against the rapidly improving lip record

geochemical evidence for the trindade hotspot trace columbia seamount ankaramite abstract the columbia seamount 825 km offshore from brazil at 20 s lies on the east west trace of the trindade hotspot continental and oceanic magmatism believed to have originated with this hotspot is alkalic and sio2 undersaturated and dates from 85 ma in southern brazil to

controls on the channel width of rivers implications for modeling fluvial incision of bedrock on the basis of the manning equation and basic mass conser vation principles we derive an expression for scaling the steady state width w of river channels as a function of discharge q channel slope s roughness n and width to depth ratio a w 5 a a1 2 2 3 3 8 q 3 8 s 23 16 n 3 8 we propose that channel width to depth ratio in addition to roughness is a function of the material in which the channel is developed and that where a river is con fined to a given material width to depth ratio and roughness can be assumed constant given these simplifications the expression emulates traditional width discharge relationships for rivers incis ing bedrock with uniformly concave fluvial long profiles more sig nificantly this relationship describes river width trends in terrain with spatially nonuniform rock uplift rates where conventional discharge based width scaling laws are inadequate we suggest that much of observed channel width variability in river channels confined by bedrock is a simple consequence of the tendency for water to flow faster in steeper reaches and therefore occupy smaller channel cross sections we demonstrate that using conventional scaling relationships for channel width can result in underestima tion of stream power variability in channels incising bedrock and that our model improves estimates of spatial patterns of bedrock incision rates

fate of mantle plume material trapped within a lithospheric catchment with reference to brazil 1 local regions of thin lithosphere act as catchments of hot buoyant plume material unless replenished the trapped plume material cools by convection to the mantle adiabat by several tens of million years in particular currently hot material from the 130 ma parana starting plume head is unlikely to supply 85 ma to recent volcanism on the mainland of brazil and the martin vaz and fernando hot spots rather a plume tail may now underlie southern brazil where tomographic studies detect conduit shaped velocity anomaly through the upper mantle if the tomographic study in fact found a plume tail the track crossed the amazon rift at 85 ma and since then lateral flow along thin regions of the lithosphere from the tail fed widespread feeble volcanism including the flow line hot spots of martin vaz and fernando

an embedding method for improving interannual variability simulations in a hybrid coupled model of the tropical pacific ocean atmosphere system an embedding approach is developed and tested to improve el nino southern oscillation enso simulations in a hybrid coupled model hcm focusing on the ocean thermocline effects on sea surface temperature sst in the eastern equatorial pacific the noaa gfdl modular ocean model mom 3 is coupled to a statistical atmospheric model that estimates wind stress anomalies based on a singular value decomposition svd of the covariance between observed wind stress and sst anomalies analogous to the cane zebiak cz coupled model a separate sst anomaly model is explicitly embedded into the z coordinate ocean general circulation model ogcm the three components exchange predicted anomalies within the coupled system the ogcm provides anomalies of ocean currents in the surface mixed layer and the thermocline depth which are used to calculate sst anomalies from the embedded sst model wind anomalies are then determined according to the statistical atmospheric model which in turn force the ogcm results from uncoupled and coupled runs with and without the embedding are compared with the standard coupling the system exhibits similar behavior to previous hcms including interannual variability with a dominant quasi biennial oscillation and a westward propagation of sst anomalies on the equator these characteristics suggest that the horizontal advection is playing a more important role than the vertical advection in determining sst changes over the eastern equatorial pacific incorporating the embedded sst anomaly model with which the thermocline effects on sst can be enhanced in the eastern equatorial pacific has a significant impact on performance of the hcm the embedded hcm exhibits more realistic sst variability and coupled behavior characterized by 3 4 yr oscillations and a more standing sst pattern along the equator the results support the hypothesis that current physical parameterizations in the ogcm provide insufficient thermal linkage between the thermocline and the sea surface in the eastern equatorial pacific it is demonstrated that the long known deficiency of some ogcms in their depiction of the thermocline and its interactions with sst may contribute to unrealistic coupled variability in hcms of enso the embedding approach not only provides a diagnosis for parameterization deficiencies in current ogcms but pending progress on this difficult problem provides a straightforward means to bypass it and improve coupled model performance

lateral flow and ponding of starting plume material many mantle plumes started with voluminous heads of hot material that ascended as diapirs through the mantle flood basalts radial dike swarms and frequently continental breakup are associated with the impingement of starting plume heads on the base of the lithosphere the buoyant plume head material ponds at the base of the lithosphere and then flows laterally relief on the base of the lithosphere acts as an upside down drainage pattern with enclosed catchments for this flow that is plume material preferentially collects beneath regions where the lithosphere is locally thin such as thermally subsiding sedimentary basins an example is uplift associated with the iceland starting plume which inverted the mesozoic irish sea basin additional relief at the base of the lithosphere is associated with rifting and seafloor spreading in the case of the northern north sea material from the iceland starting plume ponded beneath a basin with locally thin lithosphere rifting breached the enclosed catchment allowing the ponded plume material to drain across the new passive margin toward very thin lithosphere produced by seafloor spreading rifting also creates a conduit for the flow of plume material away from the region underlain by the starting plume head an example is the volcanic passive margin off nova scotia which is 2000 km from the center of the bahama starting plume head numerical and analytical calculations indicate that lateral flow of plume material consistent with this observation is expected if its viscosity is around 1018 pa s

channel width response to differential uplift 1 the role of channel width and slope adjustments to differential uplift in rivers within actively deforming terrains remains contentious here high resolution topographic surveying of formerly antecedent outwash channels demonstrates marked changes in channel width as a primary response to differential uplift for five late quaternary alluvial paleochannels crossing small folds along the active ostler fault zone of southern new zealand nearly continuous measurements of paleochannel width and concomitant incision reveal abrupt narrowing of widths toward minimum values at channel positions coincident with the initial uplift when the magnitude of differential uplift is sufficiently small narrowing alone permits these channels to remain antecedent in the context of a unit stream power model for fluvial erosion observed limits on the magnitude of channel narrowing suggest that above some threshold amount of differential uplift continued incision requires concomitant changes in channel gradient thus when crossing small growing folds alluvial rivers simply narrow their channels whereas larger folds that demand greater incision prompt an initial narrowing followed by channel steepening

improving sst anomaly simulations in a layer ocean model with an embedded entrainment temperature submodel abstract in this study an improved sea surface temperature sst anomaly ssta solution for the tropical pacific is presented by explicitly embedding into a layer ocean general circulation model ogcm a separate ssta submodel with an empirical parameterization for the temperature of subsurface water entrained into the ocean mixed layer te instead of using subsurface temperature directly from the ogcm te anomalies for the embedded ssta submodel are calculated from a historical data based empirical procedure in terms of sea level sl anomalies simulated from the ogcm an inverse modeling approach is first adopted to estimate te anomalies from the ssta equation using observed sst and simulated upper ocean currents from the ogcm a relationship between te and sl anomalies is then obtained by utilizing an empirical orthogonal function eof analysis technique the empirical te parameterization optimally leads to a better balanced depiction of the subsurface effect on sst variability by the mean upwelli

channeling at the base of the lithosphere during the lateral flow of plume material beneath flow line hot spots chains of volcanic edifices lie along flow lines between plume fed hot spots and the thin lithosphere at ridge axes discovery and euterpe musicians seamounts are two examples an attractive hypothesis is that buoyant plume material flows along the base of the lithosphere perpendicular to isochrons the plume material may conceivably flow in a broad front or flow within channels convectively eroded into the base to the lithosphere a necessary but not sufficient condition for convective channeling is that the expected stagnant lid heat flow for the maximum temperature of the plume material is comparable to the half space surface heat flow of the oceanic lithosphere two dimensional and three dimensional numerical calculations confirm this inference a second criterion for significant convective erosion is that it needs to occur before the plume material thins by lateral spreading scaling relationships indicate spreading and convection are closely related mathematically the nusselt number ratio of convective to conductive heat flow in the plume material scales with the flux volume per time per length of flow front of the plume material a blob of unconfined plume material thus spreads before the lithosphere thins much and evolves to a slowly spreading and slowly convecting warm region in equilibrium with conduction into the base of the overlying lithosphere three dimensional calculations illustrate this long lasting and hence observable state of plume material away from its plume source a different flow domain occurs around a stationary hot plume that continuously supplies hot material the plume convectively erodes the overlying lithosphere trapping the plume material near its orifice the region of lithosphere underlain by plume material grows toward the ridge axis and laterally by convective thinning of the lithosphere at its edges the hottest plume material channels along flow lines geologically the regions of lithosphere underlain by either warm or hot plume material are likely to extend laterally away from the volcanic edifices whether or not channeling occurs

embedding a sst anomaly model in to a z coordinate oceanic gcm for producing el nino oscillation in the tropical pacific climate system n n the sensitivity of simulated interannual variability to thermocline impacts on sea surface temperature sst is examined in a hybrid coupled model hcm of the tropical pacific climate system consisting of the latest version of the gfdl noaa modular ocean model mom 3 and the atmospheric component of the cane zebiak cz coupled model standard coupling of the two components has difficulty producing el nino like oscillations apparently due in part to the ocean general circulation model 39 s ogcm problems in the far eastern equatorial pacific where the modeled subsurface influence on the sst is very weak analogous to the cz model a separate diagnostic sst anomaly model is explicitly embedded into the z coordinate ogcm serving as an interface between the ocean and atmosphere in the hcm the three components exchange anomaly information only this embedding strategy has a significant effect on the coupled variability in the model system incorporating the embedded sst anomaly equation in which the thermocline effect on sst is enhanced in the eastern equatorial pacific the hcm simulations exhibit oscillations on interannual time scales for a range of parameters it is shown that the most realistic coupled behavior e g 3 4 year oscillation results from the appropriate depiction of entrainment temperature te in association with the thermocline depth anomalies the results support the hypothesis that current mixing parameterization in the ogcm is problematic in depicting vertical turbulent processes

intraplate seismicity in se brazil stress concentration in lithospheric thin spots summary intraplate seismicity has generally poor correlation with surface geological patterns except for major extensional features such as aborted continental rifts which may act as weak zones it is usually difficult to find simple geology based models to explain differences in seismic activity in stable continental regions seismicity in brazil is clearly not uniform and a few areas of higher activity have been identified however the seismic areas show almost no correlation with the main geological provinces which is typical of other intraplate settings a recent uppermantle tomography study in se and central brazil using approximately 8500 p wave and 2000 pkp wave arrivals recorded in 59 sites since 1992 has mapped p wave velocity anomalies from lithospheric depths down to 1300 km in this region higher seismic activity occurs preferentially in areas with low p wave velocities at 150 250 km depth the low p wave velocities are interpreted as shallower asthenosphere in such areas a hotter geotherm will reduce the strength of the lithospheric upper mantle causing most of the intraplate forces to be concentrated in the brittle upper crust the low velocity anomalies coincide with late cretaceous provinces of alkaline intrusions the proposed ponding of the trindade plume head beneath lithospheric thin spots is consistent with our tomography results suggesting that plume effects may have helped to preserve lithosphere asthenosphere topography although other factors are also important the present data show that stress concentrations resulting from lithosphere asthenosphere topography should play an important role in explaining the intraplate seismicity in the brazilian platform

the bahia seamounts test of a hotspot model and a preliminary south american late cretaceous to tertiary apparent polar wander path abstract detailed bathymetric and magnetic data collected over the bahia seamounts in the western south atlantic brazil basin are analyzed six paleomagnetic poles are determined one from three seamounts sharing a ridge and five from isolated seamounts two poles fall near a continental late cretaceous south american pole and are assumed to be of late cretaceous age another seamount has a preliminary 40 ar 39 ar age of 62 4 ma we check this age and estimate the ages of other seamounts by rotating their poles into north american coordinates and comparing them with north american paleomagnetic reference poles these ages and the characteristic magnetic polarities of all bahia seamounts are used to test hotspot models of formation of the seamount group the northern and central bahia seamount chains could have formed from a single hotspot the southern bahia chain may require a second hotspot the two postulated hotspots were probably concurrent and separated by 150 to 200 km the paleomagnetic poles trace out a smooth curve from the late cretaceous poles to a pole that is indistinguishable from the spin axis which we estimate as having an early eocene age the pole ages remain preliminary until better age dates are found and rocks are recovered from more seamounts

on the impact of local feedbacks in the central pacific on the enso cycle abstract while sea surface temperature sst anomalies in the eastern equatorial pacific are dominated by the thermocline feedback in the central equatorial pacific local wind effects such as zonal advection are important as well el nino southern oscillation enso simulations with a linear model improve markedly if these effects are included as a local wind stress feedback on sst an atmosphere model that reacts both to eastern and central pacific sst anomalies is needed for producing a realistic enso cycle first simulations are studied of a linear 1 5 layer reduced gravity ocean model and a linear sst anomaly equation forced by observed monthly wind stress if only the thermocline feedback is present in the sst equation sst can be simulated well in the eastern pacific but contrary to observations central pacific sst is out of phase with the eastern pacific if a wind stress feedback is added in the sst equation as a term proportional to the zonal wind stress correlations between observed an

upper mantle structure of the borborema province ne brazil from p wave tomography implications for rheology and volcanism data were collected with support of the instituto nacional de ciencia e tecnologia em estudos tectonicos inct et of the centro nacional de desenvolvimento cientifico e tecnologico cnpq grant number 57 3713 2008 01 and the borborema deep electromagnetic and seismic experiment bodes cnpq grant number 400743 2014 0 flsn acknowledges support from the coordenacao de aperfeicoamento de pessoal de nivel superior capes through a 4 yr scholarship to complete his phd at the universidade federal do rio grande do norte ufrn jj thanks cnpq for his research fellowship cnpq grant number 304421 2015 4 ms acknowledges support by brazilian science without border program grant number 40 2174 2012 7

the response of a coupled model of enso to observed estimates of stochastic forcing in this work the role that observed intraseasonal atmospheric variability may play in controlling and maintaining enso variability is examined to this end an asymptotically stable intermediate coupled model of el nino southern oscillation enso is forced with observed estimates of stochastic forcing which are defined to be the part of the atmospheric variability that is apparently independent of the ocean circulation the stochastic forcing sf was estimated from 51 yr 1950 2000 of ncep ncar reanalyses of surface winds and net surface heat flux 32 yr 1950 81 of reconstructed sea surface temperatures sst and 19 yr 1982 2000 of reynolds sst in the tropical pacific the deterministic component of the surface wind and heat flux anomalies that can be linearly related to sst anomalies was estimated using the singular value decomposition of the covariance between the anomaly fields and was then removed from the atmospheric anomaly fields to recover the stochastic component of the ocean surface forcing principal component analysis reveals that the stochastic component has no preferred mode of variability exhibits decorrelation times of a few days and has a spectrum that is indis tinguishable from red noise a 19 yr stochastically forced coupled model integration qualitatively shows some similarities with the observed equatorial sst the robustness of this result is checked by performing different sensitivity experiments the model mostly exhibits a linear and nonnormal response to the low frequency tail of sf using the ideas of generalized linear stability theory the dynamically important contributions of the sf are isolated and it is shown that most of the variability in the stochastically forced model solution is produced by stochastically induced kelvin waves forced in the western and central pacific moreover the two most dynamically important patterns of stochastic forcing which account for 71 of the expected variance in the model response describe eastward propagation of the forcing similar to the mjo the results of this study support the hypothesis that a significant fraction of enso variability may be due to sf and suggest that a better understanding of the influence of sf on the ocean surface in the western central pacific may be required in order to understand the predictability of enso

numerical modeling of flexural stresses in the upper crust of ne brazil implications for stress regimes in intraplate settings abstract we model the contribution of the stresses generated by the topography and density heterogeneities in the stress field of ne brazil and the interplay between these and regional stresses our model consists of a horizontally layered elastic slab the lithosphere that floats above an inviscid fluid the asthenosphere where the surface load is generated by the topography and bathymetry and the internal loads are calculated from the bouguer anomalies the deviatoric flexural stresses are calculated in the fourier domain our results showed local flexural stresses with magnitudes comparable to those of the plate wide stresses and mainly controlled by the long wavelength bouguer anomalies scenarios for the total stress field were calculated by superposing different values for an e w oriented regional compression we predict the stress axes orientations and stress regimes for the equatorial margin and the pernambuco shear zone pesz region however we highlight the importance of spreading stresses not considered here in rotating the maximum horizontal compression shmax predictions the prediction of stress regime and stress axes orientation in pesz using low to zero regional compression suggest that a decoupling of the shallow stresses from the plate wide stresses might occur in this region another possible explanation found was that the spreading stresses might counterbalance the regional stresses we conclude that the superposition of principally flexural and plate wide stresses explains reasonably the observed stress regimes and shmax orientations in a great part of ne brazil furthermore flexural stresses are playing an important role in the reactivation of the pesz possibly controlling the stress field

lithosphere structure beneath the phanerozoic intracratonic basins of north america four intracratonic basins of north america the hudson bay michigan illinois and williston basins have similar ages and are close to one another yet they exhibit different subsidence histories characterised by different time scales and sediment thicknesses they can be explained by local lithosphere thinning and by the cooling of the induced thermal anomaly within the framework of 1d thermal models for vertical heat transport each basin requires a different lithosphere thickness or a different boundary condition at the base of the lithosphere heat flow and seismic studies show that beneath the north american craton the lithosphere is too thick for the assumption of purely vertical heat transfer to be valid thermal models are developed to account for finite thermal anomaly width and for two types of basal boundary conditions fixed temperature or fixed heat flux different subsidence histories are explained by deep lithospheric anomalies of different sizes the stability of thick continental roots requires the mantle part of the lithosphere to be compositionally buoyant with respect to normal convecting mantle localised lithospheric thinning due for example to plume penetration results in the emplacement of compositionally denser mantle into the lithosphere this represents a load which drives permanent flexure the cooling time and the characteristics of flexure allow constraints on the dimensions of these deep lithospheric anomalies there are no solutions for lithosphere thicknesses less than 170 km the williston and illinois basins are associated with wide v200 km and thin anomalies v100 km whereas the michigan and hudson bay are located on top of narrow v100 km and tall v200 km anomalies fl 2000 elsevier science b v all rights reserved

controls of mantle plumes and lithospheric folding on modes of intraplate continental tectonics differences and similarities summary mantle plume activity and lithospheric folding by far field stresses exerted from plate boundaries are two important end members as mechanisms for continental intraplate deformation the topographic expression of mantle plume impingement on continental lithosphere and lithospheric folding has some striking similarities observations from a number of areas in europe s intraplate lithosphere demonstrate that these mechanisms commonly interact in space and time we present the results of thermomechanical modelling addressing the role of factors such as the presence of a hot upper mantle the spatial dimensions of the plume and the time constants involved in the temporal succession of plume activity and lithospheric folding by stress accumulation in intraplate continental lithosphere the results demonstrate that both the processes plume lithosphere interactions and folding may interact resulting either in strong amplification attenuation or modification of their surface expression these inferences are compatible with a number of key observations on the nature and the temporal succession of topography evolution in the alpine foreland the pannonian basin the scandinavian continental margin and the iberian peninsula

on the dependence of hindcast skill on ocean thermodynamics in a coupled ocean atmosphere model abstract three different mechanisms for the generation of enso sst anomalies within a simplified tropical pacific ocean model are examined thermocline depth changes ekman induced upwelling anomalies and zonal advection changes the effect of varying the relative influence of these terms on the realism of tropical pacific coupled models is analyzed the principal tool used to assess such realism is hindcast skill with forced ocean and oscillatory behavior also being examined of the mechanisms considered thermocline perturbations are shown to be crucially important for high coupled model hindcast skill furthermore it is concluded that the realism of the model as measured by hindcast skill deteriorates markedly when the influence on sst of ekman upwelling becomes greater than a small fraction of the thermocline influence this provides strong evidence for the hypothesis that ekman upwelling anomalies which are essentially a local response to wind stress anomalies have only a small influence on th

migrating cretaceous eocene magmatismin the serra do mar alkaline province se brazil melts from the deflected trindade mantle plume department of geological sciences university of durham south road durham dh1 3le uk department of earth sciences university of cambridge downing street cambridge cb2 3eq uk department of physics the university newcastle upon tyne ne1 7ru uk department of geology mcmaster university 1280 main street west hamilton ont canada l8s 4m1 departmento de geoquimica e recursos minerais instituto de geociencias universidade de brasilia

enso simulated by intermediate coupled models and evaluated with observations over 1970 98 part i role of the off equatorial variability enso simulations are investigated in 30 yr integrations of various intermediate coupled models and compared with observed sst wind and thermocline depth anomalies over the tropical pacific the cane and zebiak model simulates warm events with a period close to the observations but with westerlies that are located 30 8 east of them and thermocline anomalies in the western pacific that are much shallower between two warm events the model simulates a series of three weak and short cold sst peaks and hardly ever simulates easterlies the sst in the eastern equatorial pacific is not sensitive to thermocline depth anomalies but to the anomalous downwelling of surface currents induced by ekman shear the model simulates a pair of very strong cyclonic wind stress curl anomalies on both sides of the equator in the eastern off equatorial domain between 78 and 158 lat these are necessary to maintain the oscillatory regime so are the ocean meridional rossby modes higher than 5 the thermocline zonal slopes required to balance the off equatorial curl anomalies are about three times steeper than the ones required to balance the zonal stress along the equator thus the off equator exerts an excess of zonal pressure which by continuity affects the equatorial ocean and plays a crucial role in reversing and triggering the growing events six months after the warm peaks the whole ocean between 158s and 158n is significantly upwelled the equatorial oceanic heat content is recharged from the south prior to a warm event contrary to simulations when the model is driven by observed wind anomalies increasing the friction in the baroclinic ocean does not decrease the off equatorial variability but significantly alters the low frequency oscillations that are no longer enso like introducing the parameterization of subsurface temperature derived from hydrographic profiles in the ocean component does not allow the coupled model to recover cold events as in a forced context introducing the parameterization of convection derived from high cloud temperature measurements is the most effective improvement but results still poorly agree with observations and are in contrast with the simulations driven by observed sst biased toward westerlies in the central pacific upwelled thermocline in the west and warm sst in the east thus modifying the ocean component only or the atmosphere only does not have the same impact on simulations as in a forced context the coupling allows new mechanisms to grow and govern the model behavior one of them is the slow meridional oceanic mass adjusment in quasi sverdrup balance with the winds

the guinea continental margin an example of a structurally complex transform margin abstract the guinean continental margin corresponds to a wide hinge zone which developed between the jurassic central atlantic and the cretaceous equatorial atlantic the guinea marginal plateau that characterizes the southern border of this margin is a wide triangular morphological feature bounded to the west by a typical rifted margin slope segment while its southern edge corresponds to a narrow and morphologically complex continental slope probably resulting from intracontinental transcurrent motion during the early cretaceous on the basis of seismic profiles recorded in two selected areas located along the southern guinea margin a seismic stratigraphy is established at depth the plateau is underlain by a thick mesozoic and cenozoic sedimentary wedge made of two main ensembles a lower ensemble or sequence 1 consists of early cretaceous clastics an upper ensemble includes a series of four distinct sequences attributed to albian to tertiary deposits eastward the sedimentary cover of the southern plateau edge is locally pierced by volcanic bodies of paleocene age beneath the southern margin of the guinea plateau the sedimentary sequences appear deformed by faults and folds resulting from at least two main tectonic episodes a first event only affected sequence 1 which is cut by a set of extensional faults a second tectonic phase including folding reverse faulting and transcurrent faulting is responsible for the deformation of sequences 1 and 2 during this tectonic episode former normal faults have been reactivated as reverse faults resulting in a structural inversion a third tectonic phase was responsible for structures transverse to the general e w trend of the southern guinea margin and consisting of n s trending normal faults and associated volcanoes the spatial distribution of the tectonic trends clearly substantiates a polyphase tectonic activity that can be related to plate motion changes during early stages of the continental separation a first stage is characterized by a divergent rifting leading to n s trending synsedimentary normal faults during the cretaceous the tectonic activity increased as a general transform motion occurred between the guinea plateau and its american conjugate margin the demerara plateau local tensional stresses were responsible for a splay fault system active near the tip of the guinea plateau in late albian times a slight adjustment of the africa plate with respect to the south american one resulted in local squeezing of the whole sedimentary wedge after this short compressive event final continental separation occurred in an extensional regime characterized by a general collapse along two major scarps and volcanic activity

fluvial incision and tectonic uplift across the himalayas of central nepal the pattern of fluvial incision across the himalayas of central nepal is estimated from the distribution of holocene and pleistocene terraces and from the geometry of modern channels along major rivers draining across the range the terraces provide good constraints on incision rates across the himalayan frontal folds sub himalaya or siwaliks hills where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces farther north and upstream in the lesser himalaya prominent fill terraces were deposited probably during the late pleistocene and were subsequently incised the amount of bedrock incision beneath the fill deposits is generally small suggesting a slow rate of fluvial incision in the lesser himalaya the terrace record is lost in the high range where the rivers are cutting steep gorges to complement the terrace study fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate this approach allows quantification of the effect of variations in channel slope width and discharge on the incision rate of a river the determination of incision rates requires an additional lithological calibration the two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared in the sub himalaya river incision is rapid with values up to 10 15 mm yr it does not exceed a few millimeters per year in the lesser himalaya and rises abruptly at the front of the high range to reach values of 4 8 mm yr within a 50 km wide zone that coincides with the position of the highest himalayan peaks sediment yield derived from the measurement of suspended load in himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range the observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat ramp flat geometry of the main himalayan thrust fault the morphology of the range reflects a dynamic equilibrium between present day tectonics and surface processes the sharp relief together with the high uplift rates in the higher himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the tibetan plateau driven by late cenozoic climate change or late miocene reactivation of the main central thrust

the response of a stochastically forced enso model to observed off equatorial wind stress forcing abstract this study investigates the response of a stochastically forced coupled atmosphere ocean model of the equatorial pacific to off equatorial wind stress anomaly forcing the intermediate complexity coupled enso model comprises a linear first baroclinic mode ocean shallow water model with a steady state two pressure level 250 and 750 mb atmospheric component that has been linearized about a state of rest on the plane estimates of observed equatorial region stochastic forcing are calculated from ncep ncar reanalysis surface winds for the period 1950 2006 using singular value decomposition the stochastic forcing is applied to the model both with and without off equatorial region wind stress anomalies i e poleward of 12 5 latitude it is found that the multiyear changes in the equatorial pacific thermocline depth background state induced by off equatorial forcing can affect the amplitude of modeled sea surface temperature anomalies by up to 1 c moreover off equatorial wind stress anom

a three region conceptual model for central pacific el nino including zonal advective feedback abstractthe simple zonal two region framework of the recharge paradigm can accurately manifest the traditional eastern pacific ep type of el nino southern oscillation enso as its major warming

probability based methods for quantifying nonlinearity in the enso robust statistical tools have been used to investigate non normality and nonlinearity of the el nino southern oscillation enso in observations and coupled model simulations the analysis confirms previous suggestions that the observed nino 3 sea surface temperature sst anomalies are positively skewed the non linearity is estimated using a simple nonlinear stochastic model which relates the sea surface temperature anomalies to the observed thermocline depth anomalies in the nino 3 region there is evidence that saturation of sst only occurs when the thermocline is deep the nonlinearity has also been estimated for the nino 3 sst indices from twenty four different coupled models participating in the el nino simulation intercomparison project ensip large differences are found between models and observations in particular the majority of the coupled models underestimate the nonlinearity seen in the observed nino 3 sea surface temperature index more than half of the models have nino 3 sst indices that are normally distributed at 99 confidence level only a few models exhibit significant nonlinearity yet this tends to be rather different in character from the nonlinearity seen in the observations furthermore no significant association is found between the means and the spread nor between the spread and the skewness for the different coupled model nino 3 sst indices

geologic constraints on bedrock river incision using the stream power law denudation rate in unextended terranes is limited by the rate of bedrock channel incision often modeled as work rate on the channel bed by water and sediment or stream power the latter can be generalized as ka m s n where k represents the channel bed s resistance to lowenng whose variation with lithology is unknown a is drainage area a surrogate for discharge s is local slope and m and n are exponents whose values are debated we address these uncertainties by simulating the lowering of ancient river profiles using the finite difference method we vary m n and k to match the evolved profile as closely as possible to the corresponding modern river profile over a time period constrained by the age of the mapped paleoprofiles we find at least two end member incision laws ka 0 3 0 5 s for australian rivers with stable base levels and k f a 0 1 0 2 s n for rivers in kauai subject to abrupt base level change the long term lowering rate on the latter expression is a function of the frequency and magnitude of knickpoint erosion characterized by k f incision patterns from japan and california could follow either expression if they follow the first expression with m 0 4 k varies from 10 7 10 6 m 0 2 yr for granite and metamorphic rocks to 10 5 10 4 m 0 2 yr for volcaniclastic rocks and 10 4 10 2 m 0 2 yr for mudstones this potentially large variation in k with lithology could drive strong variability in the rate of long term landscape change including denudation rate and sediment yield

the role of the southward wind shift in both the seasonal synchronization and duration of enso events near the end of the calendar year when el nino events typically reach their peak amplitude there is a southward shift of the zonal wind anomalies which were centred around the equator prior to the event peak previous studies have shown that enso s anomalous wind stresses including this southward shift can be reconstructed with the two leading eofs of wind stresses over the tropical pacific here a hybrid coupled model is developed featuring a statistical atmosphere that utilises these first two eofs along with a linear shallow water model ocean and a stochastic westerly wind burst model this hybrid coupled model is then used to assess the role of this meridional wind movement on both the seasonal synchronization as well as the duration of the events it is found that the addition of the southward wind shift in the model leads to a christmas peak in variance similar to the observed timing although with weaker amplitude we also find that the added meridional wind movement enhances the termination of el nino events making the events shorter while this movement does not appear to play an important role on the duration of la nina events thus our results strongly suggest that the meridional movement of enso zonal wind anomalies is at least partly responsible for seasonal synchronization of enso events and the duration asymmetry between the warm el nino and cool la nina phases

downstream variations in the width of bedrock channels field surveys of channel width w and drainage area a in bedrock channel reaches reveal relationships where w 5 ca b similar to the classic hydraulic geometry of alluvial channels data from five mountain channel networks support the assumption used in many landscape evolution models that an alluvial hydraulic geometry relationship where b 5 0 3 0 5 holds for bedrock channel systems although there is substantial local variability in channel width in bedrock channel systems there is no systematic difference in width versus drainage area relations for the surveyed bedrock and alluvial reaches in sedimentary lithologies in coastal oregon and washington in contrast bedrock channels were narrower and therefore had deeper flow than alluvial channels with equal drainage areas in the granite and limestone terrain of the yuba river california in addition data from the mokelumne river show that bedrock channel width decreases substantially downstream at the contact between relatively weak limestone and more erosion resistant granite but that channel slope does not change appreciably across contacts between these two lithologies data from coastal oregon drainage basins further show systematic channel widening after flood flows and debris flow impacts we conclude that downstream variations in the width of bedrock channels generally follow traditional hydraulic geometry relations but also reflect the local influence of longitudinal patterns of bedrock erosivity and disturbance history

the linear response of enso to the madden julian oscillation the possibility that the tropical pacific coupled system linearly amplifies perturbations produced by the madden julian oscillation mjo is explored this requires an estimate of the low frequency tail of the mjo using 23 yr of ncep ncar reanalyses of surface wind and reynolds sst we show that the spatial structure that dominates the intraseasonal band i e the mjo also dominates the low frequency band once the anomalies directly related to enso have been removed this low frequency contribution of the intraseasonal variability is not included in most enso coupled models used to date its effect in a coupled model of intermediate complexity has therefore been studied it is found that this mjo forcing mjo can explain a large fraction of the interannual variability in an asymptotically stable version of the model this interaction is achieved via linear dynamics that is it is the cumulative effect of individual events that maintains ensos in this model the largest coupled wind anomalies are initiated after a sequence of several downwelling kelvin waves of the same sign have been forced by mjo the cumulative effect of the forced kelvin waves is to persist the small sst anomalies in the eastern pacific just enough for the coupled ocean atmosphere dynamics to amplify the anomalies into a mature enso event even though mjo explains just a small fraction of the energy contained in the stress not associated with enso a large fraction of the modeled enso variability is excited by this forcing the characteristics that make mjo an optimal stochastic forcing for the model are discussed the large zonal extent is an important factor that differentiates the mjo from other sources of stochastic forcing

global upper mantle tomography with the automated multimode inversion of surface and s wave forms summary we apply the automated multimode inversion of surface and s wave forms to a large global data set verify the accuracy of the method and assumptions behind it and compute an s vvelocity model of the upper mantle crust 660 km the model is constrained with 51 000 seismograms recorded at 368 permanent and temporary broadband seismic stations structure of the mantle and crust is constrained by waveform information both from the fundamentalmode rayleigh waves periods from 20 to 400 s and from s and multiple s waves higher modes in order to enhance the validity of the path average approximation we implement the automated inversion of surface and s wave forms with a three dimensional 3 d reference model linear equations obtained from the processing of all the seismograms of the data set are inverted for seismic velocity variations also relative to a 3 d reference in this study composed of a 3 d model of the crust and a one dimensional 1 d global average depth profile in the mantle below waveform information is related to shear and compressional velocity structure within approximate waveform sensitivity areas we use two global triangular grids of knots with approximately equal interknot spacing within each a finely spaced grid for integration over sensitivity areas and a rougher spaced one for the model parametrization for the tomographic inversion we use lsqr with horizontal and vertical smoothing and norm damping we invert for isotropic variations in s and p wave velocities but also allow for s wave azimuthal anisotropy in order to minimize errors due to possible mapping of anisotropy into isotropic heterogeneity the lateral resolution of the resulting isotropic upper mantle images is a few hundred kilometres varying with data sampling we validate the imaging technique with a spectral element resolution test inverting a published global synthetic data set computed with the spectral element method using a laterally heterogeneous mantle model we are able to reconstruct the synthetic model accurately this test confirms both the accuracy of the implementation of the method and the validity of the jwkb and path average approximations as applied in it reviewing the tomographic model we observe that low s v velocity anomalies beneath mid ocean ridges and backarc basins extend down to 100 km depth only shallower than according to some previous tomographic models this presents a close match to published estimates of primary melt production depth ranges there in the seismic lithosphere beneath cratons unambiguous high velocity anomalies extend to 200 km pronounced low velocity zones beneath cratonic lithosphere are rare where present south america tanzania they are neighboured by volcanic areas near cratonic boundaries the images of these low velocity zones may indicate hot material possibly of mantle plume origin trapped or spreading beneath the thick cratonic lithosphere

bulk crustal properties of the borborema province ne brazil from p wave receiver functions implications for models of intraplate cenozoic uplift abstract we investigate variations in crustal thickness and bulk vp vs ratio across the borborema province of ne brazil by analyzing teleseismic p to s conversions recorded at 52 seismic stations in the province the borborema province represents the western portion of a larger neoproterozoic mobile belt that resulted from the assembly of gondwanaland and that split from the african continent during mesozoic times the evolution of the province after continental breakup was marked by episodes of diffuse intraplate magmatism perhaps leading to uplift of the borborema plateau in the cenozoic a number of geodynamic models have been proposed to explain coeval cenozoic magmatism and uplift in the province which invoke either thermal anomalies under the plateau and related mantle upwellings channeling along lithospheric thin spots from a distant mantle plume and small scale convection at the continental edge alternatively plateau uplift might have resulted from thickening of the crust after depth dependent stretching of the continental lithosphere in the mesozoic most of the models imply mafic underplating of the plateau s crust in order to fully explain its elevated topography but the volume of such mafic underplate varies among them our results show that i the crust is 32 40 km thick under the borborema plateau ii the crust is generally thinner about 30 33 km under the lower topographies surrounding the plateau and iii vp vs ratios are in the 1 68 1 80 range for both regions of higher and lower topography no apparent correlation is observed between vp vs ratio and crustal thickness our results suggest that compositional differences between thick and thin crust across the borborema province are minimal and that models of plateau uplift involving a small volume of mafic underplate provide a more plausible explanation for the observed topography of the borborema plateau

dynamic adjustments in channel width in response to a forced diversion gower gulch death valley national park california we studied the 1941 diversion of furnace creek wash drainage area 439 km 2 into gower gulch 5 8 km 2 as an experiment in the transient response of a channel to a large change in water and sediment discharge we measured sequential changes in valley width using a time series of aerial photographs 1948 1995 airborne laser elevation data from 2005 and a field survey we found that the response of the system varied depending on the prediversion channel morphology and bedrock geology in two steep knickzone segments narrowing knickpoint retreat and bedrock incision dominated indicating a detachment limited response in the relatively low gradient main part of gower gulch widening dominated as the coarse postdiversion sediment load covered the channel bed this transport limited part of the system has undergone only modest incision and adjustments in gradient over long periods the lowering rate of gower gulch probably depends on knickpoint retreat but the present day response of this non steady state system is a hybrid of incision and narrowing in detachment limited reaches and widening in transport limited reaches this system demonstrates the importance of evolving channel geometry in setting the transient response of rivers to changes in forcing parameters

thermo mechanical controls on intra plate deformation and the role of plume folding interactions in continental topography thermo tectonic age and inherited structure exert the main controls on the bulk strength of the lithosphere in intraplate settings mechanical decoupling within the lithosphere strongly affects the interaction between deep earth and surface processes thermo mechanical models demonstrate the particular importance of the rheological stratification of the lithosphere in the preservation of ancient cratonic blocks in the surface expression of plume and mantle lithosphere interactions and their impact on the dynamic topography in general the same is true for the effect of large scale lithospheric folding on intraplate basin formation and associated differential vertical motions initiation of continental lithosphere subduction crucial for linking orogenic deformation to intraplate deformation appears to be facilitated by plume lithosphere interactions we present a discussion of these aspects focusing on better process understanding of continental deformation in the context of a number of well documented cases of intraplate deformation

western pacific sst prediction with an intermediate el nino prediction model abstract to improve forecasting skills in the western pacific sea surface temperature sst the authors utilized and modified an intermediate el nino prediction model the original model does not have the major sst thermodynamics for western pacific sst variability so it cannot simulate interannual variation in the western pacific correctly therefore the authors have introduced some modifications such as heat flux and vertical mixing into the dynamical model in order to capture sst thermodynamics more realistically the modified model has better forecast skill than the original one not only for the western pacific but also for the eastern central pacific the model has predictive skill up to 6 months lead time as judged by a correlation exceeding 0 5

local lithospheric relief associated with fracture zones and ponded plume material 1 buoyant plume material ponds at the base of oceanic lithosphere it is a natural probe for the relief on the base of the lithosphere owing to plate age because it flows laterally along an upside down drainage pattern geomorphic and geochemical studies of near ridge and on ridge hot spots indicate as expected that the oceanic lithosphere thickens as it ages corridors of thin lithosphere channel plume material along the axis while the thick lithosphere at the end of ridge segments dams this along axis flow channel hot spots extend from plumes toward ridge axes however a good correlation between local lithospheric thickness and local age is not evident at fracture zones within the old ocean basins such a correlation would be expected if the lithosphere cooled only by conduction then pressure release melting and volcanoes would occur preferentially near fracture zones where plume material cascades from thick old to thin young lithosphere this concentration of volcanic edifices does not occur this lack of correlation between lithosphere thickness and plate age is an expected consequence of small scale convection at the base of the lithosphere scaling relationships and numerical models indicate that modestly vigorous convection of several milliwatt per meter squared is sufficient to perturb local lithospheric relief beneath fracture zones without changing the gross correlation of lithospheric thickness with plate age typically the step in lithospheric thickness moves toward the old side of the fracture zone

implications of sediment flux dependent river incision models for landscape evolution 1 developing a quantitative understanding of the factors that control the rate of river incision into bedrock is critical to studies of landscape evolution and the linkages between climate erosion and tectonics current models of long term river network incision differ significantly in their treatment of the role of sediment flux we analyze the implications of various sediment flux dependent incision models for large scale topography in an attempt 1 to identify quantifiable and diagnostic differences between models that could be detected from topographic data or from the transient responses of perturbed systems and 2 to explain the apparent ubiquity of mixed bedrock alluvial channels in active orogens although certain forms of the various models can be discarded as inconsistent with morphological data we find that the relative intrinsic concavity indices of detachment and transport limited systems defined herein largely dictate whether the various models can be tied to distinctive steady state morphologies preliminary data suggest that no such diagnostic differences may exist and other methods must be developed to test models accordingly we develop and explore differences in the scaling behavior of topographic relief and the extent of detachment versus transport limited channels as a function of rock uplift rate that may allow discrimination among various models further we explore potentially diagnostic differences in the rates and patterns of transient channel response to changes in rock uplift rate in addition to general differences between detachment and transport limited systems our analysis identifies an interesting hysteresis in landscape evolution hybrid channels at the threshold between detachment and transport limited conditions are expected to act as detachment limited systems in response to an increase in rock uplift rate or base level fall and as transport limited systems in response to a decrease in rock uplift rate especially during postorogenic topographic decline the analyses presented set the stage for field studies designed to test quantitatively the various river incision models that have been proposed

seismic velocity anomalies beneath se brazil from p and s wave travel time inversions 1 we present the result from teleseismic travel time inversions for p and s wave data mainly recorded at portable broadband stations in se brazil the stations were deployed at 45 sites within an area 1000 1700 km during the years 1992 1999 more than 10 000 relative p and s wave arrival times including core phases were obtained from the waveforms using a new coherence functional these p and s relative phase times are independently inverted for slowness perturbations earthquake relocations and station corrections the final models represent the least amount of structure required to explain the residuals within a defined standard error the robust and consistent features in the velocity anomaly models are interpreted and their resolution is tested with synthetic case inversions we confirm the existence of a cylindrical low velocity anomaly beneath the parana basin which has been interpreted by vandecar et al 1995 as the fossil conduit through which the initial tristan da cunha plume head traveled to generate the parana etendeka flood basalts about 130 ma we now show that this low velocity cylindrical structure seems to be confined to the upper mantle beneath the upper mantle the velocity anomalies show a n s oriented pattern which we interpret as due to the nazca plate subducted slab at lithospheric depths the archean southern part of the sao francisco craton shows high velocities down to 200 300 km all areas with late cretaceous postrift alkaline intrusions are characterized by low velocities at lithospheric depths

geomorphic response of coastal streams to low intermediate and high rates of uplift medocino triple junction region northern california analysis of three dimensional morphological properties of 24 coastal drainage basins that have evolved in areas of low 3 m 1 000 yr rates of uplift near the mendocino triple junction mtj coastal northern california identified channel gradients as the best indicator of tectonism in the landscape lower order tributaries best reflect tectonically controlled differences the largest streams examined of third order are able to adjust to most base level change and maintain their profile form whereas lower order streams farther upstream tend to accumulate the effects of net base level fall and have steepest profiles in the areas of highest uplift rates variations in steepness of first order channel gradients indicate that 1 high uplift rates in the wake of the mtj have existed for at least 73 000 yr and 2 differential tilt of the region to the north and south of the current locus of highest uplift rate is occurring in association with regional uplift although first order streams are excellent indicators of areas of both high uplift rates and regional differential tilting they are less useful in distinguishing between areas of low and intermediate uplift rate analysis of the longitudinal profile of the main trunk stream of ten of the 24 drainage basins using the stream gradient index was more useful to categorize broadly the uplift rates and to distinguish between streams in low and intermediate uplift rate areas although the hypotheses tested herein do not identify the exact mechanism off uplift in the mtj region they do indicate that the nature of deformation is most likely regional tilt rather than crustal shortening with localized compressional folding and thrust faulting they also indicate that geomorphic responses trail in the wake of the uplift rate response caused by development of a slab window beneath the north american plate 1 maximum uplift rates occur 9 km south of the northern boundary of the slab window 2 maximum mean first order channel gradient occurs 16 km south of the northern edge of the slab window and 3 maximum drainage basin relief occurs 20 km south of the northern edge of the slab window associated lag times between passage of the southern edge of the subducted gorda plate slab and geomorphic responses are 160 000 yr for maximum uplift rates 290 000 yr for maximum channel gradients and 370 000 yr for maximum relief