lateral velocity resolution from three dimensional seismic refraction data summary a method for estimating the lateral velocity resolution from 3 d seismic refraction traveltimes is presented the method is non linear in that synthetic data calculated from checkerboard models are inverted using an iterative tomographic approach incorporating smoothness constraints and updated ray paths at each iteration two applications are presented 1 real data from the faeroe basin experiment using the ray coverage and noise level of approximately 50 000 picks and 2 ideal data from the same experiment corresponding to 100 per cent data recovery equivalent to about three times as many ray paths the refraction data constrain the sedimentary and basement structure to approximately 12 km depth a comparison of the results from the two data sets provides a qualitative check on the resolution method illustrates the pitfalls of using ray coverage as an indicator of resolution and demonstrates the potential resolution of currently feasible 3 d refraction experiments 32 checkerboard models consisting of alternating positive and negative velocity anomalies superimposed on the preferred final model of the basin were tested each with a diverent cell size position or orientation the problem of ray bending associated with the 5 per cent checkerboard anomalies is shown to be small using models with four cell sizes allowed for resolution estimates from 3 to 15 km this range being imposed by the frequency content of the data and the areal dimensions of the study area 44 26 km an operator centred on each model node measures the local semblance between the known and recovered checkerboard models a determination of the smallest well resolved cell size at each model node is then made using a 0 7 semblance threshold to provide a spatially dependent lateral resolution estimate resolution for the real data set varies from an average of just over 3 km at 2 km depth to 10 km at 10 km depth where there is a local maximum in ray coverage due to a concentration of ray bottoming points for the ideal data set the average lateral resolution drops ov roughly linearly from better than 3 km at 2 km depth to 6 3 km at 10 km depth the largest diverence between the resolution associated with the ideal and real data occurs at 7 2 km depth where it averages 4 6 km for the former and 10 8 km for the latter due to the near absence of ray bottoming points between 7 and 9 km the results show that the final model for the faeroe basin obtained by regularized inversion with smoothness constraints is consistent with the resolution estimates in the sense that the model does not contain structure with a wavelength smaller than the resolution estimate at any point the results for the ideal data show that good quality 3 d refraction experiments can provide a lateral resolution of no worse than the receiver spacing shot spacing for land data throughout most of the sampled volume

an alternative interpretation of the cayman trough evolution from a reidentification of magnetic anomalies magnetic data were collected during the wilkes 1973 and seacarib ii 1987 cruises to the cayman trough a new interpretation of magnetic data is carried out an isochron pattern is drawn up from our anomaly identifications an early eocene age 49 ma ypresian for cayman trough opening is proposed instead of the late oligocene or middle eocene ages suggested by previous studies our plate tectonic reconstruction is simpler and fits the on land geology jamaica and cuba and the tectonics our reconstruction shows a southward propagation of the spreading centre between magnetic anomalies 8 and 6 26 and 20 ma the trough width increases by 30 km in this period the southward propagation of the cayman spreading centre from the middle oligocene to the early miocene induced the development of the restraining bend of the swan islands the formation of a 1 km high scarp on the eastern trace of the cayman trough transform fault walton fault and the formation of a pull apart basin hendrix pull apart magnetic anomalies and magnetization maps give information about the deformation and the rocks the proposed evolutionary model of the cayman trough from the inception of seafloor spreading to the present configuration is presented in relation to the tectonic escape of the northern boundary of the caribbean plate from the maastrichtian to the present

surface based transport model for mixed size sediment we present a transport model for mixed sand gravel sediments fractional transport rates are referenced to the size distribution of the bed surface rather than subsurface making the model completely explicit and capable of predicting transient conditions the model is developed using a new data set of 48 coupled observations of flow transport and bed surface grain size using five different sediments the model incorporates a hiding function that resolves discrepancies observed among earlier hiding functions the model uses the full size distribution of the bed surface including sand and incorporates a nonlinear effect of sand content on gravel transport rate not included in previous models the model shares some common elements with two previous surface based transport models but differs in using the full surface size distribution and in that it is directly developed from a relatively comprehensive data set with unambiguous measurement of surface grain size over a range of flow transport rate and sediments

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

crustal anatomy of a transform continental margin abstract this paper describes new deep seismic reflection results across the major transform margin southwest of the grand banks off eastern canada an interpretation of these results is presented supplemented with previous seismic refraction gravity and magnetic measurements the results show a fairly sharp transition between oceanic and continental regions at the transform margin a narrow zone about 40 km wide separates the two regions and is interpreted to be a zone in which shearing has destroyed the original fabric of the continental crust faults in this region extend throughout the crust and there is some evidence that a major strike slip fault may extend some tens of kilometres into the upper mantle magmatism may have further modified the crust in this shear zone which is bounded on the oceanic side by a small seamount the continental crust is thinned by a factor of 3 or more in this zone which may be due to erosion or to flow in the lower crust during shearing the oceanic crust does not appear to thin significantly as the transform margin is approached crustal scale dipping reflectors over a confined region below the shelf are interpreted to represent the contact between two appalachian terranes the meguma and avalon terranes this supports earlier interpretations of a large magnetic lineament in the region as marking this boundary these crustal seismic data crossing a large transform margin are the first of their kind and will help us to constrain evolutionary models of transform margins the picture they provide is different in many respects from that across rifted margins and similar in others in some cases it might be difficult to distinguish between these two classes of atlantic type margins on the basis of crustal structure

seismic structure and segmentation of the axial valley of the mid cayman spreading center we report the results of a two dimensional tomographic inversion of marine seismic refraction data from an array of ocean bottom seismographs obss which produced an image of the crustal structure along the axial valley of the ultraslow spreading mid cayman spreading center mcsc the seismic velocity model shows variations in the thickness and properties of the young oceanic crust that are consistent with the existence of two magmatic tectonic segments along the 110 km long spreading center seismic wave speeds are consistent with exhumed mantle at the boundary between these two segments but changes in the vertical gradient of seismic velocity suggest that volcanic crust occupies most of the axial valley seafloor along the seismic transect the two spreading segments both have a low velocity zone lvz several kilometers beneath the seafloor which may indicate the presence of shallow melt however the northern segment also has low seismic velocities 3 km s in a thick upper crustal layer 1 5 2 0 km which we interpret as an extrusive volcanic section with high porosity and permeability this segment hosts the beebe vent field the deepest known high temperature black smoker hydrothermal vent system in contrast the southern spreading segment has seismic velocities as high as 4 0 km s near the seafloor we suggest that the porosity and permeability of the volcanic crust in the southern segment are much lower thus limiting deep seawater penetration and hydrothermal recharge this may explain why no hydrothermal vent system has been found in the southern half of the mcsc

evidence for transform margin evolution from the ivory coast ghana continental margin the relative motions between lithospheric plates can be reduced to three major types convergence divergence and transform motion convergence leads to an active margin and eventual collision divergence results in the construction of a rifted passive margin transform motion generates a transform or sheared margin although passive margins have been extensively studied1 2 and many models have been proposed for their origin and subsequent evolution3 5 little is known about transform margins with the exception of a few studies of their morphologies6 7 shallow structures or crustal sections8 9 here we present the main conclusions derived from a recent study of the northern gulf of guinea margins particularly off the eastern ivory coast and ghana where the continental margin is one of the best preserved examples of an extinct transform margin10 12 our observations support a four stage model for transform margin evolution technically active transform contacts first between normal continental crusts and then between thinned margins induce characteristic structures such as pull apart grabens and shear folds the next stage in which thermal exchange between oceanic and continental lithos pheres controls a complex subsidence is followed by the transition to a true intra oceanic fracture zone

supply and removal of sediment in a landslide dominated mountain belt central range taiwan abstract a strong coupling between hillslope and valley systems is often inferred for mountain landscapes dominated by bedrock landsliding we reveal the nature of this link using data sets on landsliding and sediment transport from two montane catchments draining the eastern central range of taiwan here the magnitude frequency distribution of landslides can be modeled by a robust power law but this scale invariance is not mirrored in the sediment discharge at the mountain front instead downstream sediment loads reflect a complex response to both sediment supply and ambient hydraulic conditions the rivers do not transport significant amounts of sediment unless it is provided by hillslope mass wasting in the catchment removal of landslide debris is a function of the transport capacity of the stream at the site of entry thus there is a dual supply and transport control on sediment loads in bedrock floored streams over a monitoring period of 25 yr the bulk of the sediment leaving the mountain bel

gravel bed river evolution in earthquake prone regions subject to cycled hydrographs and repeated sediment pulses sediment often enters rivers in the form of sediment pulses associated with landslides and debris flows this is particularly so in gravel bed rivers in earthquake prone mountain regions such as southwest china under such circumstances sediment pulses can rapidly change river topography and leave the river in repeated states of gradual recovery in this paper we implement a one dimensional morphodynamic model of river response to pulsed sediment supply the model is validated using data from flume experiments so demonstrating that it can successfully reproduce the overall morphodynamics of experimental pulses the model is then used to explore the evolution of a gravel bed river subject to cycled hydrographs and repeated sediment pulses these pulses are fed into the channel in a fixed region centered at a point halfway down the calculational domain the pulsed sediment supply is in addition to a constant sediment supply at the upstream end results indicate that the river can reach a mobile bed equilibrium in which two regions exist within which bed elevation and surface grain size distribution vary periodically in time one of these is at the upstream end where a periodic discharge hydrograph and constant sediment supply are imposed and the other is in a region about halfway down the channel where periodic sediment pulses are introduced outside these two regions bed elevation and surface grain size distribution reach a mobile bed equilibrium that is invariant in time the zone of fluctuation free mobile bed equilibrium upstream of the pulse region is not affected by repeated sediment pulses under the scenarios tested but downstream of the pulse region the channel reaches different fluctuation free mobile bed equilibriums under different sediment pulse scenarios the vertical bed structure predicted by the simulations indicates that the cyclic variation associated with the hydrograph and sediment pulses can affect the substrate stratigraphy to some depth copyright 2017 john wiley sons ltd

large floods alluvial overprint and bedrock erosion depending on their behaviour during extreme floods streams can be divided into two distinct classes which derive from a simple threshold model of transport dynamics flood cleaning streams erode during high flows and deposit during small and medium flows flood depositing streams deposit during high flows and erode during small and medium flows using published descriptions of the geomorphic effects of large floods rivers with a wide range of drainage areas and other characteristics are classified as either flood cleaning or flood depositing in bedrock channels this behaviour can lead to a feedback effect the overprint effect between sediment transport processes and bedrock erosion which can modulate long term bedrock erosion rates the overprint effect arises when alluvium covers the bedrock and typical alluvial channel forms e g meandering or braiding patterns armour layers or bedforms develop which influence sediment transport rates this effect may accelerate or decelerate sediment export from a reach causing increased or decreased long term bedrock erosion rates the overprint effect is illustrated using field data from the erlenbach switzerland and its implications for channel dynamics and bedrock erosion are discussed copyright 2012 john wiley sons ltd

effects of earthquake induced landslides on sediment budget and transport over a 50 yr period landsliding during a magnitude 7 7 earthquake in 1929 in northwestern south island new zealand delivered up to 400 000 m 3 km 2 of sediment to stream channels first to third order channels were buried to depths of as much as 10 m with sedimentary rock debris for distances ranging from 2 of the 92 km 2 study area because of the coarse grain size of the sedimentary rock debris and trapping of sediment in numerous landslide dammed lakes sand and pebble sized debris originating from landslides on a 6 6 km 2 area of granite has been transported downstream no more than 10 km at least 50 to 75 of this granitic debris is retained in the fourth order 92 km 2 catchment 50 yr after the earthquake the long retention times of sediment and the volume of debris supplied by landslides during a large earthquake suggest that earthquake induced landsliding is the principal sediment supply mechanism in the region

crustal structure of the french guiana margin west equatorial atlantic summary geophysical data from the amazon cone experiment are used to determine the structure and evolution of the french guiana and northeast brazil continental margin and to better understand the origin and development of along margin segmentation a 427 km long combined multichannel reflection and wide angle refraction seismic profile acquired across the southern french guiana margin is interpreted where plate reconstructions suggest a rift type setting the resulting model shows a crustal structure in which 35 37 km thick pre rift continental crust is thinned by a factor of 6 4 over a distance of 70 km associated with continental breakup and the initiation and establishment of seafloor spreading the ocean continent boundary is a transition zone up to 45 km in width in which the two layered oceanic type crustal structure develops although relatively thin at 3 5 5 0 km such thin oceanic crust appears characteristic of the margin as a whole there is no evidence of rift related magmatism either as seaward dipping sequences in the reflection data or as a high velocity region in the lower crust in the p wave velocity model and as a such the margin is identified as non volcanic in type however there is also no evidence of the rotated fault block and graben structures characteristic of rifted margins consequently the thin oceanic crust the rapidity of continental crustal thinning and the absence of characteristic riftrelated structures leads to the conclusion that the southern french guiana margin has instead developed in an oblique rift setting in which transform motion also played a significant role in the evolution of the resulting crustal structure and along margin segmentation in structural style

the structure evolution and symmetry of the magma poor rifted margins of the north and central atlantic a synthesis abstract magma poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting the serpentinization of the mantle beneath crust thinned to less than 8 2 km and the unroofing of a broad zone of mantle within the continent ocean transition accompanied by the development of detachment and other large offset faults these observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes although the paucity of magmatism may be explained by depth dependent extension of the lithosphere and by pre depleted sub crustal lithosphere rifting above initially cool sub lithospheric mantle also may explain the subsidence deficit observed at some margins synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere as it extends thins and cools ductile creeping layers in the mid and deep crust become progressively more brittle resulting in increased coupling between the upper and lower crust and eventually the embrittlement of the entire crust faults cutting from the surface across the moho bringing water into the mantle and causing its serpentinization increased coupling and the development of serpentine detachments predict the development of late stage asymmetry once the entire crust is brittle such detachments are imaged on some margins and inferred on others analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric similar analyses show that depth dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning instead this extension discrepancy may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting related to the changes in the rheological character and strength of the lithosphere as it is thinned complex polyphase faulting continues after complete crustal separation resulting in the exhumation of broad expanses of peridotitic basement the top of which is everywhere marked by an exhumed slip surface similar to the corrugated surface observed at mid ocean ridges the similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the cot and true oceanic crust difficult and possibly moot

crustal structure at the blake spur fracture zone from expanding spread profiles we present results from wkbj and reflectivity synthetic seismogram modeling of 10 reversed expanding spread profiles esps along flow lines parallel to the blake spur fracture zone across 140 ma atlantic crust these profiles provide detailed constraints on variations in crustal structure at the fracture zone seven profiles on either side of the fracture zone show a normal structure for old oceanic crust a 2 3 km thick upper layer with a steep velocity gradient is underlain by 4 5 km of crust with velocities of 6 5 7 2 km s and low gradient and a sharp transition to upper mantle velocities of 8 km s several velocity discontinuities were detected within the upper 2 3 km but these do not generally coincide with intracrustal reflectors detected by simultaneous normal incidence reflection profiles this structure shows little regional variation toward the fracture zone despite the small magnetic anomaly offset 12 km and the indistinct topographic signature of the fracture zone three esps within a 10 20 km wide ribbon centered on the fracture zone trough show clearly anomalous crustal structure relative to normal oceanic crust a 2 4 km thick high gradient upper layer is underlain by a thick prism of material with a velocity of 7 2 7 6 km s and high poisson s ratio probably at least 0 29 which is consistent with 15 30 serpentinization of upper mantle peridotites this interpretation requires the action of off axis hydrothermal circulation penetrating the cracked and relatively permeable fracture zone lithosphere to a depth of at least 7 km the original igneous crust in a narrow region close to the fracture zone is thus inferred to have been much thinner than adjacent normal crust which may imply a sharp reduction in the magma budget at the ends of the adjacent spreading segments

sedimentary impacts from landslides in the tachia river basin taiwan abstract a case study of coseismic landslides and post seismic sedimentary impacts of landslides due to rainfall events was conducted in the tachia river basin taichung county central taiwan about 3000 coseismic landslides occurred in the basin during the ml 7 3 chi chi earthquake in 1999 the deposits from these landslides provided material for numerous debris flows induced by subsequent rainfall events the estimated 4 1 107 m3 of landslide debris produced in the upland area caused sediment deposition in riverbeds and flash floods inundated downstream areas with sediment during torrential rains the landslide frequency size distributions for the coseismic landslides and the subsequent rainfall induced landslides were analyzed to determine the sediment budgets of the post seismic geomorphic response in the landslide dominated basin both the coseismic and the rainfall induced landslides show a power law frequency size distribution with a rollover it was found that the rainfall induced landslide magnitude was smaller than the coseismic one and that both have comparable negative scaling exponents in cumulative form of about 2 0 for larger landslides 10 2 km2 this may be attributed to ongoing movement or reactivation of old landslides and a natural stabilisation of small landslides between 10 4 and 10 2 km2 it is proposed that the characteristics of geological formations and rainfall as well as changes in landslide area are reflected in the power law distribution

crustal structure of the southwest indian ridge at the atlantis ii fracture zone the southwest indian ridge is a slow spreading end member of the mid ocean ridge system the deepest borehole penetrating the lower oceanic crust ocean drilling program hole 735b lies on the eastern transverse ridge of the atlantis ii fracture zone at 57 e a wide angle seismic survey in the vicinity of the borehole reveals a crustal structure that is highly heterogeneous to the east of atlantis bank on which hole 735b is located the crust consists of a 2 2 5 km thick high velocity gradient oceanic layer 2 and a 1 2 km thick low velocity gradient layer 3 the transform valley has a 2 5 3 km thick crust with anomalously low velocities interpreted to consist largely of highly serpentinized mantle rocks the seismically defined crust is thickest beneath the borehole where layer 2 is thinner and the lower crust is inferred to contain 2 3 km of partially serpentinized mantle the seismic velocity models are consistent with gravity data which show weak residual mantle bouguer anomalies because the regions of thinner crust have lower crustal densities stress variations deduced from mass balances between the transform valley floor and the adjacent transverse ridges are much larger than the likely threshold for lithospheric failure and therefore indicate that the relief is supported dynamically the variation of crustal thickness with spreading rate defined by data from the southwest indian ridge and elsewhere is consistent with models of melt generation in which the upwelling mantle is cooled by conductive heat loss at very slow spreading rates resulting in reduced melt generation under the spreading axis large segment scale variations in crustal thickness suggest subcrustal along axis migration of melt toward segment centers

experimental evidence for the effect of hydrographs on sediment pulse dynamics in gravel bedded rivers 1 gravel augmentation is a river restoration technique applied to channels downstream of dams where size selective transport and lack of gravel resupply have created armored relatively immobile channel beds augmentation sediment pulses rely on flow releases to move the material downstream and create conditions conducive to salmon spawning and rearing yet how sediment pulses respond to flow releases is often unknown here we explore how three types of dam releases constant flow small hydrograph and large hydrograph impact sediment transport and pulse behavior translation and dispersion in a channel with forced bar pool morphology we use the term sediment pulse generically to refer to the sediment introduced to the channel the zone of pronounced bed material transport that it causes and the sediment wave that may form in the channel from the additional sediment supply which can include input sediment and bed material in our experiments we held the volume of water released constant which is equivalent to holding the cost of purchasing a water volume constant in a stream restoration project the sediment pulses had the same grain size as the bed material in the channel we found that a constant flow 60 greater than the discharge required to initiate sediment motion caused a mixture of translation and dispersion of the sediment pulse a broad crested hydrograph with a peak flow 2 5 times the discharge required for entrainment caused pulse dispersion while a more peaked hydrograph 3 times the entrainment threshold discharge caused pulse dispersion with some translation the hydrographs produced a well defined clockwise hysteresis effecting sediment transport as is often observed for fine sediment transport and transportlimited gravel bed rivers the results imply a rational basis for design of water releases associated with gravel augmentation that is directly linked to the desired sediment behavior

the structure of oceanic core complexes controlled by the depth distribution of magma emplacement extension at mid ocean ridges can be accommodated by detachment faults forming oceanic core complexes that develop under low rates of magma intrusion modelling reveals that oceanic core complexes can also form under high rates of magma intrusion if the magma is injected into the lower ductile layer of the crust

anomalous seismic crustal structure of oceanic fracture zones summary the seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the north atlantic the crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3 anomalous crust is present in both large and small offset fracture zones since they are among the most common tectonic features in the ocean basins and are particularly closely spaced on slow spreading ridges fracture zones represent a major source of seismic crustal heterogeneity we interpret the anomalous crust as a thin intensely fractured faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that in places are extensively serpentinized the unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness culminating in the very thin crust of the fracture zones at either end of the ridge segment

crustal structure across the southwest newfoundland transform margin a seismic refraction survey providing deep crustal structure information of the continent ocean boundary across the south west newfoundland transform margin was carried out using large air gun sources and ocean bottom seismometer receivers continental crust 30 km thick beneath the southern grand banks p wave velocity 6 2 6 5 km s thins oceanward to a 25 km wide transition zone in the transition zone paleozoic basement of the grand banks 5 5 5 7 km s is replaced by a basement of oceanic volcanics and synrift sediments 4 5 5 5 km s seaward of the transition zone the crust is oceanic in character with a velocity gradient from 4 7 to 6 5 km s and a thickness of 7 8 km oceanic layer 3 is absent no significant thickness of intermediate velocity 7 km s material is present at the continent ocean transition indicating that no under plating of continental crust has taken place the continent ocean transition across the transform margin is much narrower than across rifted margins supporting the

the nature of the crust under cayman trough from gravity abstract considerable crustal thickness variations are inferred along cayman trough a slow spreading ocean basin in the caribbean sea from modeling of the gravity field the crust to a distance of 50 km from the spreading center is only 2 3 km thick in agreement with dredge and dive results crustal thickness increases to 5 5 km at distances between 100 and 430 km west of the spreading center and to 3 5 6 km at distances between 60 and 370 km east of the spreading center the increase in thickness is interpreted to represent serpentinization of the uppermost mantle lithosphere rather than a true increase in the volume of accreted ocean crust serpentinized peridotite rocks have indeed been dredged from the base of escarpments of oceanic crust rocks in cayman trough laboratory measured density and p wave speed of peridotite with 40 50 serpentine are similar to the observed speed in published refraction results and to the inferred density from the model crustal thickness gradually increases to 7 8 km at the far ends of the trough partially in areas where sea floor magnetic anomalies were identified basement depth becomes gradually shallower starting 250 km west of the rise and 340 km east of the rise in contrast to the predicted trend of increasing depth to basement from cooling models of the oceanic lithosphere the gradual increase in apparent crustal thickness and the shallowing trend of basement depth are interpreted to indicate that the deep distal parts of cayman trough are underlain by highly attenuated crust not by a continuously accreted oceanic crust

transient water and sediment storage of the decaying landslide dams induced by the 2008 wenchuan earthquake china article earthquake triggered landslide dams are potentially dangerous disrupters of water and sediment flux in mountain rivers and capable of releasing catastrophic outburst flows to downstream areas we analyze an inventory of 828 landslide dams in the longmen shan mountains china triggered by the mw 7 9 2008 wenchuan earthquake this database is unique in that it is the largest of its kind attributable to a single regional scale triggering event 501 of the spatially clustered landslides fully blocked rivers while the remainder only partially obstructed or diverted channels in steep watersheds of the hanging wall of the yingxiu beichuan fault zone the size distributions of the earthquake triggered landslides landslide dams and associated lakes a can be modeled by an inverse gamma distribution b show that moderate size slope failures caused the majority of blockages and c allow a detailed assessment of seismically induced river blockage effects on regional water and sediment storage monte carlo simulations based on volumetric scaling relationships for soil and bedrock failures respectively indicate that 14 18 of the estimated total coseismic landslide volume of 6 4 14 6 10 9 m 3 was contained in landslide dams representing only 1 4 of the 60 000 slope failures attributed to the earthquake these dams have created storage capacity of 0 6 10 9 m 3 for incoming water and sediment about 25 of the dams containing 2 of the total river blocking debris volume failed one week after the earthquake these figures had risen to 60 20 and 90 90 within one month and one year respectively thus also emptying 92 of the total potential water and sediment storage behind these dams within one year following the earthquake currently only 0 08 10 9 m 3 remain available as natural reservoirs for storing water and sediment while 0 19 10 9 m 3 i e about a third of the total river blocking debris volume has been eroded by rivers dam volume and upstream catchment area control to first order the longevity of the barriers and bivariate domain plots are consistent with the observation that most earthquake triggered landslide dams were ephemeral we conclude that the river blocking portion of coseismic slope failures disproportionately modulates the post seismic sediment flux in the longmen shan on annual to decadal timescales

volcanic tectonic structure of the mount dent oceanic core complex in the ultraslow mid cayman spreading center determined from detailed seafloor investigation the flanks of the ultraslow spreading mid cayman spreading center mcsc are characterized by domal massifs or oceanic core complexes occs the most prominent of these mt dent comprises lower crustal and upper mantle lithologies and hosts the von damm vent field vdvf 12 km west of the axial deep here presented auv derived swath sonar multibeam mapping and deep towed side scan sonar imagery lead to our interpretation that i slip along the occ bounding detachment fault is ceasing ii the termination zone where detachment fault meets the hanging wall is disintegrating iii the domed surface of the occ is cut by steep north south extensional faulting and iv the breakaway zone is cut by outward facing faults the vdvf and dispersed pockmarks on the occ s south flank further suggest that hydrothermal fluid flow is pervasive within the faulted occ on the axial floor of the mcsc bright acoustic backscatter and multibeam bathymetry reveal v a volcanic detachment hanging wall vi a major fault rifting the southern flank of mt dent and vii a young axial volcanic ridge intersecting its northern flank these observations are described by a conceptual model wherein detachment faulting and occ exhumation are ceasing during an increase in magmatic intrusion brittle deformation and hydrothermal circulation within the occ together this high resolution view of the mcsc provides an instructive example of how occs formed within an overall melt starved ultraslow spreading center can undergo magmatism hydrothermal activity and faulting in much the same way as expected in magmatically more robust slow spreading centers elsewhere

mantle wedge water contents estimated from seismic velocities in partially serpentinized peridotites 1 water carried into subduction zones with the down going plate and subsequently released by dehydration reactions at depth affects the composition of the mantle wedge triggers partial melting and affects subduction zone seismicity partially serpentinized peridotite may be a significant reservoir for water in the subducted plate the mantle wedge and the overriding plate here we develop a model that relates the degree of serpentinization and water content of partially serpentinized peridotites to their seismic p wave velocities in partially serpentinized ultramafic rocks a 1 decrease in p wave velocity corresponds to a 2 4 increase in serpentine content and a 0 3 0 18 moles m3 increase in h2o content up to a maximum of 13 where there is evidence of serpentinization mantle serpentine content is typically 15 corresponding to 4 5 wt h2o 6 10 moles m3

the 2010 haiti earthquake a complex fault pattern constrained by seismologic and tectonic observations after the january 12 2010 haiti earthquake we deployed a mainly offshore temporary network of seismologic stations around the damaged area the distribution of the recorded aftershocks together with morphotectonic observations and mainshock analysis allow us to constrain a complex fault pattern in the area almost all of the aftershocks have a n s compressive mechanism and not the expected left lateral strike slip mechanism a first order slip model of the mainshock shows a n264 e north dipping plane with a major left lateral component and a strong reverse component as the aftershock distribution is sub parallel and close to the enriquillo fault we assume that although the cause of the catastrophe was not a rupture along the enriquillo fault this fault had an important role as a mechanical boundary the azimuth of the focal planes of the aftershocks are parallel to the north dipping faults of the transhaitian belt which suggests a triggering of failure on these discontinuities in the western part the aftershock distribution reflects the triggering of slip on similar faults and or alternatively of the south dipping faults such the trois baies submarine fault these observations are in agreement with a model of an oblique collision of an indenter of the oceanic crust of the southern peninsula and the sedimentary wedge of the transhaitian belt the rupture occurred on a wrench fault at the rheologic boundary on top of the under thrusting rigid oceanic block whereas the aftershocks were the result of the relaxation on the hanging wall along pre existing discontinuities in the frontal part of the transhaitian belt

translation and dispersion of sediment pulses in flume experiments simulating gravel augmentation below dams 1 the effectiveness of gravel augmentation as a river restoration strategy depends on the extent and duration of the topographic and bed texture changes created by the pulse of added sediment previous work has emphasized the strong tendency for natural sediment waves to propagate primarily by dispersion however pulse translation may occur for gravel additions to armored channels downstream of dams where added sediments are finer than the preexisting bed material here we report results of a laboratory investigation in which we created an immobile armored bed and documented the spatial and temporal evolution of the bed topography and bed texture in response to gravel pulses of various volumes and grain sizes the introduced sediment waves evolved by a combination of translation and dispersion with a significant translational component pulse translation and dispersion can be readily discerned on a graph of the time evolution of the downstream cumulative distribution of elevation differences from the preexisting bed topography translation was most evident for smaller volumes of added sediment pulses of finer grained gravel moved through the flume more rapidly resulting in a larger magnitude but shorter duration of bed fining more work is needed to understand the influence of bar pool topography and flow magnitude and duration before the grain size and volume of gravel additions can be selected to achieve optimal patterns of pulse propagation

the crustal structure across the transform continental margin off ghana eastern equatorial atlantic forward modeling of a suite of wide angle seismic lines across the transform continental margin at the eastern end of the romanche fracture zone off ghana has shown the transition from continental to oceanic crust to be confined to a narrow 6 to 11 km wide zone located at the foot of the steep continental slope the structure of the adjacent oceanic and continental crusts has also been resolved these results are confirmed and enhanced by gravity and magnetic models the crust of the ocean continent transition zone is characterized by high velocities 5 8 7 3 km s 1 a high density 3 10 mg m 3 and high magnetizations 1 1 1 25 a m 1 this rules out a purely continental origin for the zone and suggests that it may be formed of basic igneous rocks intruded when the hot oceanic spreading center migrated along the margin the oceanic crust within 70 km of the margin is abnormally thin compared to normal atlantic oceanic crust and shows an average thickness of just 4 4 km we suggest that the region of abnormally thin oceanic crust is the result of a reduced magma supply due to a combination of closely spaced fracture zones and the conductive loss of heat from the upwelling oceanic mantle in small basins surrounded on at least three sides by cold continental lithosphere the preferred seismic model also shows a new fracture zone 30 km southeast of the ocean continent transition which is characterized by low velocities in the upper crust a lens shaped layer with velocities of 7 2 7 4 km s 1 at the base of the crust and a crustal thickness of just 3 4 km the continental crust appears largely unaffected by the proximity of the adjacent oceanic crust there is no evidence for underplating of the continental crust adjacent to the ocean continent transition zone

thermal uplift and erosion across the continent ocean transform boundary of the southern exmouth plateau thermal evolution of the continental lithosphere at a continent ocean transform margin is examined using a two dimensional heat conduction model all heating is assumed to result from the emplacement of new oceanic ridge against the continent the assumed initial continental temperature gradient is probably best applied to regions having experienced finite duration periods of rifting lasting tens of millions of years a deep seismic reflection profile across the southern paleo transform margin of the exmouth plateau northwest australia tests the predictions of the model using this reflection profile it is estimated that up to 3 5 km of sediments have been eroded from the continental rim diminishing to almost no erosion at 60 km from the continent ocean transform boundary this trend and these values for erosion can be matched approximately using the model under the condition of local isostasy a finite difference scheme is employed where the surface elevation is the result of the competing 1 thermal uplift up 2 surficial erosion down and 3 local isostatic rebound up in response to the erosion the model predicts that most of the erosion ceases by 40 ma after ridge emplacement and that of the order of 1000 km3 eroded sediments are shed for every 10 km of transform length

development of the continent ocean transform boundary of the southern exmouth plateau a two stage model is proposed to explain the principal tectonic and magmatic features observed in multichannel seismic reflection and refraction data across the southern transform margin of the exmouth plateau northwestern australia 1 the rifting stage in which detachment surfaces developed under conditions of extension at a high angle to the future transform and were later sheared by right lateral strike slip faulting final transform rupture was attended by large fault block rotation and mafic intrusions in conditions of pure shear 2 in the drifting stage as the oceanic ridge abutted the continent the continental rim was underplated at this location resulting in a 10 km thick 7 3 km s 3 g cm3 layer resulting in a permanent isostatic uplift of the crust and tilting of synrift sedimentary deposits this wedge extends laterally forming a thickened oceanic layer 3 transient heating of the continental lithosphere induced thermal uplift and erosion of up to 3 5 km of sedimentary units over a 50 km distance from the continent ocean contact

seamarc ii mapping of transform faults in the cayman trough caribbean sea seamarc ii maps of the southern wall of the cayman trough between honduras and jamaica show zones of continuous well defined fault lineaments adjacent and parallel to the wall both to the east and west of the cayman spreading axis these lineaments mark the present active traces of transform faults which intersect the southern end of the spreading axis at a triple junction the swan islands transform fault to the west is dominated by two major lineaments that overlap with right stepping sense across a large push up ridge beneath the swan islands the fault zone to the east of the axis named the walton fault is more complex containing multiple fault strands and a large pull apart structure the walton fault links the spreading axis to jamaican and hispaniolan strike slip faults and it defines the southern boundary of a microplate composed of the eastern cayman trough and western hispaniola the presence of this microplate raises questions about the veracity of caribbean plate velocities based primarily on cayman trough opening rates

the dominance of dispersion in the evolution of bed material waves in gravel bed rivers bed material waves are temporary zones of sediment accumulation created by large sediment inputs recent theoretical experimental and field studies examine factors influencing dispersion and translation of bed material waves in quasi uniform gravel bed channels exchanges of sediment between a channel and its floodplain are neglected within these constraints two factors influence relative rates of dispersion and translation 1 interactions between wave topography flow and bed load transport and 2 particle size differences between wave material and original bed material our results indicate that dispersion dominates the evolution of bed material waves in gravel bed channels significant translation requires a low froude number which is uncharacteristic of gravel bed channels and low wave amplitude which for a large wave can be achieved only after substantial dispersion wave material of small particle size can promote translation but it primarily increases bed load transport rate and thereby accelerates wave evolution copyright 2001 john wiley sons ltd

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data

assessment of crustal velocity models using seismic refraction and reflection tomography two tomographic methods for assessing velocity models obtained from wide angle seismic traveltime data are presented through four case studies the modelling inversion of wide angle traveltimes usually involves some aspects that are quite subjective for example 1 identifying and including later phases that are often difficult to pick within the seismic coda 2 assigning specific layers to arrivals 3 incorporating pre conceived structure not specifically required by the data and 4 selecting a model parametrization these steps are applied to maximize model constraint and minimize model non uniqueness however these steps may cause the overall approach to appear ad hoc and thereby diminish the credibility of the final model the effect of these subjective choices can largely be addressed by estimating the minimum model structure required by the least subjective portion of the wide angle data set the first arrival times for data sets with moho reflections the tomographic velocity model can be used to invert the pmp times for a minimum structure moho in this way crustal velocity and moho models can be obtained that require the least amount of subjective input and the model structure that is required by the wide angle data with a high degree of certainty can be differentiated from structure that is merely consistent with the data the tomographic models are not intended to supersede the preferred models since the latter model is typically better resolved and more interpretable this form of tomographic assessment is intended to lend credibility to model features common to the tomographic and preferred models four case studies are presented in which a preferred model was derived using one or more of the subjective steps described above this was followed by conventional first arrival and reflection traveltime tomography using a finely gridded model parametrization to derive smooth minimum structure models the case studies are from the se canadian cordillera across the rocky mountain trench central india across the narmada son lineament the iberia margin across the galicia bank and the central chilean margin across the valparaiso basin and a subducting seamount these case studies span the range of modern wide angle experiments and data sets in terms of shot receiver spacing marine and land acquisition lateral heterogeneity of the study area and availability of wide angle reflections and coincident near vertical reflection data the results are surprising given the amount of structure in the smooth tomographically derived models that is consistent with the more subjectively derived models the results show that exploiting the complementary nature of the subjective and tomographic approaches is an effective strategy for the analysis of wide angle traveltime data

constraints on crustal structure of adjacent occs and segment boundaries at 13 n on the mid atlantic ridge the 13 n segment of the mid atlantic ridge is an example of a morphologically well studied slow spreading ridge segment populated with oceanic core complexes occs in this paper we present the results of a 200 km long 2d seismic and gravity transect through this segment the bounding fracture zones to the south and the ridge discontinuity to the north we use this transect to consider the two end member models of occ evolution in which one referred to as the segment scale model implies they are interconnected with their detachments being part of a single segment long feature and the other the localised model that each occ is structurally isolated we show using the 7 5 km s 1 velocity contour as the base of crust marker that the crust is consistently relatively thin ridge parallel at 5 km thick on average and that beneath the occs the moho marks the top of a velocity gradient transition into the mantle rather than a distinct velocity discontinuity although each occ is not traversed in an identical structural location they show a different crustal velocity density structure with depth with along axis variations in this structure mirrored by the bathymetric deeps between them older occs have a contrasting velocity depth signature to the currently active 13 20 n occ the 13 20 n occ is distinct in that it does not show higher relative velocity at shallower crustal depth like its neighbours while the 13 30 n occ has an apparently thinner crust our combined p wave seismic travel time tomography and gravity forward modelling suggests that the occs of the 13 n segment are not interconnected at depth to the north of the 13 30 n occ our modelling also suggests that the crust is being magmatically refreshed or that the ridge axis is currently undergoing magmatic accretion with an associated ridge tip propagation occurring across the ridge discontinuity that marks its northern edge the profile also crosses the marathon and mercurius fracture zones that mark the southern limit of the 13 n segment and the southern ridge transform intersection outside corner along profile marathon fracture zone offsets younger 1 my vs 8 my oceanic crust than mercurius fracture zone 8 my vs 11 my when considered in combination both seismic and gravity modelling suggest crustal thinning in the direct vicinity of the bathymetric valley of marathon fracture zone coupled with a region of low density that most likely reflects serpentinisation of the uppermost mantle in addition the crust captured between fracture zones appears relatively rotated about an e w axis and uplifted to the north with the upwards motion accommodated on the northern lateral edge of the bathymetric depression rather than in its centre both the outside corner and the crust bounded by fracture zones have velocity depth characteristics similar to that of the 13 n segment occs rather than normally accreted oceanic crust particularly in the upper to middle crust overall our results support the localised model of occ evolution and suggest that fracture zones do not become locked immediately on transform to fracture transition as current models dictate

magmatic tectonic conditions for hydrothermal venting on an ultraslow spread oceanic core complex hydrothermal venting an important cooling mechanism of the earth supports a diverse array of seafloor and sub seafloor ecosystems that are sustained by large thermal and chemical fluxes vents have been found along even the slowest and coldest spreading centers calling into question the driving heat source for these vents the ultraslow spreading mid cayman spreading center in the caribbean sea which hosts the axial flank von damm vent field vdvf provides an opportunity to probe the mechanisms for venting at ultraslow spreading rates using active source seismic data from the 2015 cayseis cayman seismic experiment we determined the seismic velocities in the large massif beneath the vdvf we propose that this massif was produced by a pulse of on axis magmatism at ca 2 ma which was then followed by exhumation cooling and fracturing a low seismic velocity anomaly 5 km below the vdvf is evidence for either a cracking front mining lithospheric heat or intrusive magmatic sills both of which could drive ongoing deep hydrothermal fluid circulation we conclude that the transient magmatism and variable crustal thickness at ultraslow spreading centers create conditions for long lived hydrothermal venting that may be widespread and other vdvf like vents may be common in these areas

age and spreading history of the cayman trough as determined from depth heat flow and magnetic anomalies the opening of the cayman trough records aspects of relative motion between the north american and caribbean plates essential to accurate reconstruction of caribbean evolution assessment of opening rates based upon basement subsidence analysis as well as magnetic anomaly interpretation shows that the trough may have opened at one third to one half previously proposed rates depths to basement show that the crust has subsided away from the present axis in two stages a younger stage characterized by substantial and asymmetric greater to the east subsidence preceded by an older stage of slight increases of depth with distance depth to age relations calculated from subsidence curves which accommodate lateral as well as vertical cooling of the accreting slab indicate that the trough has opened at an overall rate of less than 15 mm yr since 25 30 ma and at 20 30 mm yr prior to 30 ma marine magnetic anomalies show lineations perpendicular to the trough including a distinct central anomaly but the pattern is neither well defined nor symmetrical about the present axis however partial anomaly sequences can be identified on both sides of the axis and the distribution of these sequences indicates that spreading may have involved frequent short ridge jumps mostly eastward the overall rate of opening as indicated by this interpretation of the magnetics is about 15 and 30 mm yr since and prior to 26 ma both subsidence and magnetics indicate that the trough opened by at least 45 50 ma

the ramesses experiment v crustal accretion at axial volcanic ridge segments a gravity study at 57 45 n on the slow spreading reykjanes ridge summary presented in this paper are the results of a two stage analysis of gravity data acquired during a multidisciplinary geophysical survey of a magmatically active axial volcanic ridge avr segment located at 57 45 n on the reykjanes ridge part of the slow spreading mid atlantic ridge south of iceland modelling of the free air anomaly in 2 d shows that across axis the observed anomaly results largely from density and layer thickness variation in the mid lower crust although seismic control on crustal thickness along axis is limited modelling also suggests that both crustal density and thickness also vary towards avr tips using the 2 d modelling results as crustal reference the residual mantle bouguer anomaly rmba is calculated to assess whether magma related density anomalies are present in the mantle and to investigate the structure of and relationship between adjacent avrs along axis rmba lows are associated with both an along ridge trend encompassing a number of adjacent avrs and with individual more topographically robust avrs modelling of the rmba low associated with the 57 45 n avr further suggests that along axis density variation is confined to the central region of this avr and that the anomaly can largely be accounted for by density variation within layer 3 and a degree of crustal thinning towards avr tips the nature of the along axis variation in crustal density further suggests that it may result from repeated phases of magma supply to the crustal system from the mantle within the resolution of the rmba modelling does not confirm or preclude the presence of a subcrustal density anomaly associated with retention of a small percentage of melt in the mantle however a melt free model for at least the top 40 km of the mantle is preferred as this is consistent with the results of modelling a coincident magnetotelluric data set the ridge trend characteristics of the rmba also suggest that magma delivery may take place along this trend and in an episodic fashion that controls initiation of an avr tectonomagmatic cycle avrs may therefore be created along a spreading orthogonal direction by tapping of a recent magma injection into the ridge trending crustal system with flow of this magma laterally along avr parallel faults and fissures accommodating spreading and crustal accretion

shallow structures and evolution of the ivory coast and ghana transform margin abstract seismic profiles across the transform continental margin off the ivory coast and ghana western africa illustrate the structural style resulting from the early cretaceous phase of shear stress which leads to the final separation between the african and brazilian continental margins in this area most of the characteristic tectonic features observed along this portion of margin asymmetric grabens on the ghanean platform folds of the deep ivory coast basin the ivory coast ghana marginal ridge are believed to result from progressive transform contacts between the african and brazilian continents as their margins were created during early cretaceous time a major tectonic unconformity inferred to be of upper albian lower cenomanian age may be a direct consequence of the final separation of the continental margins the later evolution of the transform margin is chiefly explained by thermal subsidence

transform continental margins part 2 a worldwide review we present a global inventory of transform continental margins based on both a worldwide comparison of continent ocean boundary identifications with oceanic fracture zones traces and a compilation of published regional studies this inventory increases the number of identified transform margins from 29 to 78 these margins represent 16 of continental margins in cumulative length and 31 of non convergent margins we include morphological data published upper crustal sections continent to ocean transition locations and published moho shape data in the new database this review confirms that continent to ocean transitions are sharper at transform margins than at divergent margins it also emphasizes the structural diversity of transform margins associated with one third of transform margins we define marginal plateaus as a new type of relief that corresponds to a flat but deep surface inside the continental slope and that may be inherited from crustal thinning prior to transform faulting transform margin initiation appears to be favoured along propagating oceans and within cold and thick lithospheres

geologic investigations in the cayman trough evidence for thin oceanic crust along the mid cayman rise the mid cayman rise is a 110 km long accreting plate boundary 2cm yr full rate characterized by a deep rift valley eighteen transponder navigated dives were made by the submersible alvin along the rift valley walls in two areas on opposite sides of the rift valley the relief of the walls is produced by a large number of inward facing small throw fault escarpments with vertical displacements ranging from a few tens of meters to perhaps as much as 400 m from these escarpments 142 samples were collected and gabbroic rocks 116 samples characterized by a wide range of compositions and a complex metamorphic history predominate in number over other rock types basalt serpentinized ultramafic we interpret the assemblage of rocks exposed on the escarpments to indicate that a shallow level magma reservoir exists beneath the axis of accretion at the mid cayman rise and creates anomalously thin oceanic crust with a carapace of only several hundreds of meters of diabase and basalt this carapace is und

crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin implications for thermal exchanges across the palaeotransform boundary summary crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin is determined by 2 d modelling of wide angle seismic data and gravity data the resulting models together with geological data recently collected by drilling on the margin are compared with available thermomechanical models of transform margins in particular we test the predicted effects on the transform margin crust of continental heating by the adjacent oceanic lithosphere these effects include i ductile flow of the lower continental crust either parallel to the transform direction or perpendicular to it from the continent towards the ocean and ii thermal uplift of the transform margin in the region of the cote d ivoire ghana transform margin the seismic and gravity data indicate that the continental crust remains uniform in thickness as the continent ocean boundary is approached the interpreted models lack a transitional domain between continental and oceanic crusts and abnormally thin oceanic crust is found along the margin these results exclude large scale ductile flow of lower continental crust during the margin formation the geological data are moreover not consistent with thermal uplift of the margin during the period of contact between the transform margin and the spreading centre our geophysical and geological data thus infer only moderate heating of the transform continental margin by the adjacent oceanic lithosphere moderate heating of the cote d ivoire ghana transform margin during margin formation implies that the thermal exchange between the transform margin and the adjacent spreading centre were lower than predicted by available thermal models two main factors may have contributed to reduce thermal exchanges across the continent ocean boundary the first factor is that the oceanic edge may have been colder than assumed by thermal models an abnormally cold spreading axis along the transform margin when the oceanic crust was emplaced is supported by the atypical structure of the oceanic crust along the cote d ivoire ghana transform margin the oceanic crustal structure is similar to that found along oceanic fracture zones where oceanic crust is also emplaced near a ridge transform intersection in both cases abnormally low thermal gradients at the spreading centre along the transform margin may be explained by the cumulative effect of i a cold edge effect of the adjacent continental lithosphere ii accretion at the end of a ridge segment away from the main magma supply point along the spreading axis and iii an efficient upward cooling of the oceanic lithosphere owing to deep hydrothermal circulation within the faulted lithosphere the second factor causing moderate thermal exchanges is the possibility of large amounts of water in oceanic lithospheric faults which reduces the lateral thermal conduction in the lithosphere since water is much less conductive than rock during the margin formation the oceanic edge thus seems to have acted as a thermal buffer between the cold continental margin and the hot spreading centre we propose that this thermal buffer hypothesis may be generalized to most transform margins implying a lower amount of continental heating than assumed by the available thermomechanical models

premonitory burst of seismicity and its significance for predicting large aegean earthquakes abstract the time variation of strain energy release prior to large m s 6 9 aegean shocks indicates that they are commonly preceded by a premonitory burst of seismicity in their focal regions several years prior to their occurrences a linear relationship independent of the type of stress field in the focal region is established implying that the higher the shock magnitude m s the longer the premonitory time the fact that there is no correlation between the size of the premonitory burst and the magnitude of the approaching earthquake is attributed to the great inhomogeneity of crustal material from place to place a second model showing a long term quasi exponential increase of the strain energy release towards the time of the main shock is applied in two cases the usefulness of the two models for predicting large earthquakes is discussed

post miocene cayman trough evolution a speculative model the cayman trough exhibits cross trough lineations which we interpret as crustal fabric formed parallel to the spreading center at the time of crustal generation alignment and variations of the spreading fabric are roughly symmetrically distributed about the mid cayman spreading center as are variations in the cross trough width of the cayman trough these suggest to us a simple eight stage model of stepwise varying spreading directions and rates for this model it is assumed that 1 tectonic spreading fabric is perpendicular to spreading direction 2 cross trough extension both widens the trough and lengthens the spreading center and 3 cross trough compression narrows the trough but does not shorten the spreading center combining the spreading fabric lineation data with these assumptions we predict a cross trough width variation that is essentially that of the cayman similar variations in spreading direction should be evidenced elsewhere on the plate boundary if the variations we observe do in fact record the history of changes in relative motion between the north american and caribbean plates

stochastic forcing of sediment supply to channel networks from landsliding and debris flow sediment influx to channel networks is stochastically driven by rainstorms and other perturbations which are discrete in time and space and which occur on a landscape with its own spatial variability in topography colluvium properties and state of recovery from previous disturbances the resulting stochastic field of sediment supply interacts with the topology of the channel network and with transport processes to generate spatial and temporal patterns of flux and storage that characterize the sedimentation regime of a drainage basin the regime varies systematically with basin area we describe how the stochastic sediment supply is generated by climatic topographic geotechnical and biotic controls that vary between regions the general principle is illustrated through application to a landscape where sediment is supplied by mass wasting and the forcing variables are deterministic thickening of colluvium random sequences of root destroying wildfires and random sequences of rainstorms that trigger failure in a population of landslide source areas with spatial variance in topography and colluvium strength landslides stop in channels or convert to scouring debris flows depending on the nature of the low order channel network sediment accumulates within these channels for centuries before being transferred downstream by debris flows time series of sediment supply transport and storage vary with basin scale for any combination of climatic topographic and geotechnical controls in a companion paper benda and dunne this issue we use simulations of timing volumes and locations of mass wasting to study the interaction between a stochastically forced sediment supply and systematic changes of storage and flux through channel networks

evolution of a sediment wave in an experimental channel the routing of bed material through channels is poorly understood we approach the problem by observing and modeling the fate of a low amplitude sediment wave of poorly sorted sand that we introduced into an experimental channel transporting sediment identical to that of the introduced wave the wave essentially dispersed upstream and downstream without translation although there was inconclusive evidence of translation late in the experiment when the wave was only 10 20 grain diameters high alternate bars migrated through zones of differing bed load transport rate without varying systematically in volume celerity or transport rate sediment that overpassed migrating bars was apparently responsible for dispersion of the wave the evolution of the wave was well predicted by a one dimensional model that contains no adjusted empirical constants numerical experiments demonstrate however that the theory does not predict sediment waves that migrate long distances downstream such waves can only be explained by the following processes not represented by the theory selective bed load transport spatial variations in bar and other form roughness the mechanics of mobile armor and perhaps other mechanisms

numerical model of sediment pulses and sediment supply disturbances in mountain rivers sediment pulses in rivers can result from many mechanisms including landslides entering from side slopes and debris flows entering from tributaries artificial sediment pulses can be caused by the removal of a dam this paper presents a numerical model for the simulation of gravel bedload transport and sediment pulse evolution in mountain rivers a combination of the backwater and quasi normal flow formulations is used to calculate flow parameters gravel bedload transport is calculated with the surface based bedload equation of parker in 1990 the exner equation of sediment continuity is used to express the mass balance at different grain size groups and lithologies as well as the abrasion of gravel the river is assumed to have no geological controls such as bedrock outcrops and immobile boulder pavements the results of nine numerical experiments designed to study various key parameters relevant to the evolution of sediment pulses are reported here results of the numerical runs indicate that the evol

episodic magmatism and serpentinized mantle exhumation at an ultraslow spreading centre mid ocean ridges spreading at ultraslow rates of less than 20 mm yr 1 can exhume serpentinized mantle to the seafloor or they can produce magmatic crust however seismic imaging of ultraslow spreading centres has not been able to resolve the abundance of serpentinized mantle exhumation and instead supports 2 to 5 km of crust most seismic crustal thickness estimates reflect the depth at which the 7 1 km s 1 p wave velocity is exceeded yet the true nature of the oceanic lithosphere is more reliably deduced using the p to s wave velocity vp vs ratio here we report on seismic data acquired along off axis profiles of older oceanic lithosphere at the ultraslow spreading mid cayman spreading centre we suggest that high vp vs ratios greater than 1 9 and continuously increasing p wave velocity changing from 4 km s 1 at the seafloor to greater than 7 4 km s 1 at 2 to 4 km depth indicate highly serpentinized peridotite exhumed to the seafloor elsewhere either magmatic crust or serpentinized mantle deformed and uplifted at oceanic core complexes underlies areas of high bathymetry the cayman trough therefore provides a window into mid ocean ridge dynamics that switch between magma rich and magma poor oceanic crustal accretion including exhumation of serpentinized mantle covering about 25 of the seafloor in this region

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

seismic waves velocities and anisotropy in serpentinized peridotites from xigaze ophiolite abundance of serpentine in slow spreading ridge the effect of serpentinization on seismic wave velocity and anisotropy has been analysed in 6 peridotite samples of the xigaze ophiolite having harzburgitic composition and a degree of serpentinization ranging from 3 to 70 we found i p and s wave velocities are linearly correlated with serpentine content ii anisotropy of p and s waves decreases with increasing serpentinization while in fresh peridotites anisotropy is of the order of 6 for 70 of serpentinization it drops to 1 for p waves and to 2 for s waves comparison of published laboratory velocity data in oceanic lithologies like gabbros and dolerites with serpentinized peridotite data shows the difficulty of distinguishing between gabbros and peridotites having a percentage of serpentine between 10 and 40 by seismic waves velocities only this comparison suggests that seismic velocity data are consistent with the existence of partialy serpentinized peridotites up to 40 of serpentinization in the oceanic crust formed at slow spreading ridges in accordance with geological observation

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

gps and seismic constraints on the m 7 3 2009 swan islands earthquake implications for stress changes along the motagua fault and other nearby faults summary we use measurements at 35 gps stations in northern central america and 25 seismometers at teleseismic distances to estimate the distribution of slip source time function and coulomb stress changes of the mw 7 3 2009 may 28 swan islands fault earthquake this event the largest in the region for several decades ruptured the offshore continuation of the seismically hazardous motagua fault of guatemala the site of the destructive ms 7 5 earthquake in 1976 measured gps offsets range from 308 millimetres at a campaign site in northern honduras to 6 millimetres at five continuous sites in el salvador separate inversions of geodetic and seismic data both indicate that up to 1 m of coseismic slip occurred along a 250 km long rupture zone between the island of roatan and the eastern limit of the 1976 m 7 5 motagua fault earthquake in guatemala evidence for slip 250 km west of the epicentre is corroborated independently by aftershocks recorded by a local seismic network and by the high concentration of damage to structures in areas of northern honduras adjacent to the western limit of the rupture zone coulomb stresses determined from the coseismic slip distribution resolve a maximum of 1 bar of stress transferred to the seismically hazardous motagua fault and further indicate unclamping of normal faults along the northern shore of honduras where two m 5 normal faulting earthquakes and numerous small earthquakes were triggered by the main shock

a model to predict the evolution of a gravel bed river under an imposed cyclic hydrograph and its application to the trinity river 1 major changes in the morphology of the trinity river in california such as narrowing of the cross section and sedimentation of fine sediment in pools occurred after the closure of a system of dams these changes caused a dramatic reduction in the salmonid population and a resulting decline of the fishery gravel augmentation regulated flood releases and mechanical channel rehabilitation are currently being implemented to help restore the aquatic habitat of the river the present paper describes a tool named the spawning gravel refresher for designing and predicting the effects of gravel augmentation in gravel bed rivers the tool assumes an imposed cycled hydrograph the model is calibrated and applied to the regulated reach of the trinity river in four steps 1 zeroing runs to reproduce conditions of mobile bed equilibrium as best can be estimated for the predam trinity river 2 runs to compare the predictions with the results of previous studies 3 runs at an engineering time scale to reproduce the effects of the dams and 4 runs to design gravel augmentation schemes in the fourth group of runs the combined effects of engineered flood flow releases and gravel augmentation are predicted at an engineering time scale the model indicates that the fraction of fine sediment in the surface layer and in the topmost part of the substrate should decrease when subjected to these two restoration measures with a consequent improvement of the quality of the spawning gravel

cote d ivoire ghana margin seismic imaging of passive rifted crust adjacent to a transform continental margin i i summary during may 1990 and january february 1991 an extensive geophysical data set was collected over the cote d ivoire ghana continental margin located along the equatorial coast of west africa the ghana margin is a transform continental margin running subparallel to the romanche fracture zone and its associated marginal ridge the cote d ivoire ghana ridge from this data set an explosive refraction line running 150 km ene wsw between 3 55 n 3 21 w and 4 23 n 2 4 w has been modelled together with wide angle airgun profiles and seismic reflection and gravity data this study is centred on the cote d ivoire basin located just to the north of the cote d ivoire ghana ridge where bathymetric data suggest that a component of normal rifting occurred rafher than the transform motion observed along the majority of the equatorial west african margin traveltime and amplitude modelling of the ocean bottom seismometer data shows that the continental moho beneath the margin rises in an oceanward direction from 24 km below sea level to 17 km in the centre of the line where the crust thins most rapidly there exists a region of anomalously high velocity at the base of the crust reaching some 8km in thickness this higher velocity region is thought to represent an area of localized underplating related to rifting modelling of marine gravity data collected coincident meth the seismic line has been used to test the bestfitting seismic model this modelling has shown that the observed free air anomaly is dominated by the effects of crustal thickness and that a region of higher density is required at the base of the crust to fit the observed data this higher density region is consistent in size and location with the high velocities required to fit the seismic data

the yakutat terrane dramatic change in crustal thickness across the transition fault alaska we present new constraints on the crustal structure of the yakutat terrane and evidence of the role of the transition fault in southern alaska the yakutat terrane south of yakutat bay includes crystalline crust that is 24 27 km thick overlain by sedimentary units that are 4 5 7 5 km thick the yakutat terrane crustal thickness and velocity structure are consistent with an oceanic plateau origin the southern edge of the yakutat terrane is bounded by the transition fault which is imaged as a near vertical fault zone 1 km wide the transition fault is coincident with a dramatic change in moho depth from 32 km for yakutat oceanic plateau crust to 11 5 km for pacific ocean crust occurring over a horizontal distance of 0 5 km there is no evidence for underthrusting of the pacific ocean crust beneath the yakutat terrane at the transition fault we argue that the yakutat terrane formed on the kula or farallon plate and was later juxtaposed next to younger pacific ocean crust by the transition fault

crustal structure accretionary processes and rift propagation a gravity study of the intermediate spreading valu fa ridge lau basin summary the valu fa ridge is an intermediate spreading full rate of 60 mm yr 1 ridge located in the lau basin in 1995 this ridge was surveyed using a multidisciplinary geophysical approach to image crust and upper mantle structure with the aim of investigating the processes of oceanic crustal accretion in a back arc tectonic environment as part of this experiment a network of gravity profiles was acquired together with seismic magnetic swath bathymetry and controlled source electromagnetic data presented in this paper are the results of forward modelling of a subset of the acquired gravity profiles two oriented ridge perpendicular and one ridge parallel using the preferred seismic models of turner et al 1999 as a basis of initial model construction in addition the gravity data set in its entirety has been used to calculate the mantle and residual mantle bouguer anomalies with the aim of investigating variability in crustal structure both density and layer thickness and the nature of the underlying upper mantle of particular interest are the overlapping spreading centre between the central and northern valu fa ridges where seismic modelling implies a generally thickened crust and a magma chamber located beneath the overlap basin rather than separate chambers supplying each ridge and the propagating rift tip and associated basin bounding pseudo fault modelling results suggest that the pre and post rift crusts have different compositional origins with lower densities required 12 km off axis to fit the observed free air gravity anomaly the locations of the transitions into regions of lower density correspond with those of turner et al 1999 derived from seismic modelling which in turn correspond in location to the rift related pseudo fault identified by wiedicke collier 1993 calculation and interpretation of the mantle and residual mantle bouguer anomalies also confirms the lower off axis densities and indicates a general increase in crustal thickness northwards towards the overlapping spreading centre between the central and northern valu fa ridge segments the presence of thicker crust corresponding to a region of negative mantle and residual mantle bouguer anomaly implies that the region surrounding the overlapping spreading centre has been in the recent geological past or is presently the site of an increased magma supply the residual mantle bouguer anomaly also reveals features related directly to off axis lateral variation in density and layer thickness associated with the southward propagation of the valu fa ridge into the havre trough modelling of the north south variation in crustal thickness along axis shows that the anomaly trend can be explained simply by thicker crust beneath each of the valu fa ridge segments and the overlapping spreading centre between the northern and central valu fa ridges the observation of segment centred crustal thickening is consistent with models of ridge segmentation that suggest that melt influx is located towards segment centres and that segment length is controlled by the maximum lateral extent to which this melt can flow macdonald et al 1988 tolstoy et al 1993 however the overlapping spreading centre remains anomalous from these models in that it also appears to be associated with thicker crust and an increased melt supply

sediment flux from a mountain belt derived by landslide mapping in humid uplands landsliding is the dominant mass wasting process in the western southern alps of new zealand landslides are scale invariant and have a power law magnitude frequency distribution independent studies from other regions suggest that this is a general property of landsliding this observation is of critical importance to the evaluation of the impact of events of different length scales over different time intervals on landscape evolution it is particularly useful when estimating regional geomorphic rates because it constrains the frequency and overall significance of extreme events which cannot otherwise be evaluated by integrating the complete response of the system we estimate the regional denudation rate due to landsliding to be 9 4 mm yr 1 sediment discharge from the western southern alps is dominated by landslide derived material

a mechanism to thin the continental lithosphere at magma poor margins crustal thinning where continental plates break apart can be accomplished by a system of conjugate concave downward faults instead of multiple normal faults through exhumation of mid crustal and mantle material where continental plates break apart slip along multiple normal faults provides the required space for the earth s crust to thin and subside1 after initial rifting however the displacement on normal faults observed at the sea floor seems not to match the inferred extension2 here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust 10 km by exhumation of mid crustal and mantle material this transitional crust is capped by sub horizontal detachment surfaces over distances exceeding 100 km with little visible deformation our rift model is based on numerical experiments constrained by geological and geophysical observations from the alpine tethys and iberia newfoundland margins3 4 5 6 7 8 9 furthermore we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust the presence of such lower crustal gabbros is well constrained for the alpine tethys system4 9 initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution this is achieved in our models by the formation of weak ductile shear zones

late cretaceous cenozoic tectonic transition from collision to transtension honduran borderlands and nicaraguan rise nw caribbean plate boundary

mode of slip and crust mantle interaction at oceanic transform faults oceanic transform faults connecting offset mid ocean spreading centres rupture quasi periodically in earthquakes up to about magnitude m 7 0 that are often preceded by foreshocks in addition to seismic slip a large portion of slip takes place as aseismic creep which likely influences initiation of large earthquakes although oceanic transform faults are one of the major types of plate boundaries the exact mode of slip and interaction between the seismic and aseismic motion remains unclear here we present a detailed model of the mode of slip at oceanic transform faults based on data acquired from a recent temporary deployment of ocean bottom seismometers at the blanco transform fault and existing regional and teleseismic observations in the model the crustal part of the fault is brittle and fully seismically coupled while the fault in the mantle shallower than the depth of the 600 c isotherm creeps partially and episodically the creep activates small asperities in the mantle that produce seismic swarms both mantle and the crustal zones release most of the plate motion strain during large magnitude earthquakes large earthquakes appear to be preceded by a brief episode of shallow mantle creep accompanied by seismic swarms which explains the observation of foreshocks and shows that mantle creep likely influences initiation of large seismic events earthquakes in the crust and mantle at transform faults are distinct yet coupled with seismic swarms in the mantle apparently preceding large earthquakes according to ocean bottom seismic monitoring of the blanco transform fault

sediment pulses in mountain rivers 1 experiments 1 sediment often enters rivers in discrete pulses associated with landslides and debris flows this is particularly so in the case of mountain streams the topographic disturbance created on the bed of a stream by a single pulse must be gradually eliminated if the river is to maintain its morphological integrity two mechanisms for elimination have been identified translation and dispersion according to the first of these the topographic high translates downstream according to the second of these it gradually diffuses away in any given river both mechanisms may operate this paper is devoted to a description of three controlled experiments on sediment pulses designed to model conditions in mountain streams each of the experiments began from the same mobile bed equilibrium with a set rate and grain size distribution of sediment feed in one experiment the median size of the pulse material was nearly identical to that of the feed sediment in the other two the pulse material differed in grain size distribution from the feed sediment being coarser in one case and finer in the other case in all cases the mode of pulse deformation was found to be predominantly dispersive a result that constitutes the main conclusion of this paper the pulses resulted in a notable but transient elevation of sediment transport rate immediately downstream when the pulse was coarser than the ambient sediment the bed downstream remained armored and a migrating delta formed in the backwater upstream when the pulse was finer than the ambient sediment translation was observed in addition to dispersion the presence of the finer material notably elevated the transport of ambient coarse material on the bed downstream in part 2 cui et al 2003 the experimental results are used to test a numerical model

oceanic core complex development at the ultraslow spreading mid cayman spreading center roughly a third of the global mid ocean ridge system spreads at 6 km spreading centers and thought to accrete some of the thinnest 3 km crust the mcsc generates end member mid ocean ridge basalt compositions and hosts recently discovered hydrothermal vents multibeam bathymetric data reveal that axial depth varies along the mcsc with intraridge rift walls defined by kilometer scale escarpments and massifs dredging and near bottom work has imaged and sampled predominantly basaltic lavas from the greatest axial depths and 15 peridotite surrounded by gabbroic rocks from the prominent massifs the gabbroic rocks exhibit wide compositional variation troctolites to ferrogabbros and in many places contain high temperature amphibolite to granulite facies shear zones gabbroic compositions primarily reflect the accumulation of near liquidus phases that crystallized from a range of basaltic melts as well as from interactions with interstitial melts in a subaxial mush zone magnetization variations inverted from aeromagnetic data are consistent with a discontinuous distribution of basaltic lavas and structurally asymmetric spreading these observations support an oceanic core complex model for mcsc seafloor spreading potentially making it a type example of ultraslow seafloor spreading through mush zone and detachment fault crustal processes

demerara plateau the structure and evolution of a transform passive margin summary the amazon cone experiment acquired two transects profile sd a across the demerara plateau part of the french guiana northeast brazil continental margin to better understand rift and transform style margin evolution profile a images an intermediate type margin formed as a result of trans tensional extension in this paper we describe the modelling of wide angle and multichannel seismic and gravity data from profile d to reveal whole crustal structure and features exhibiting transform characteristics combining these results with other studies in the region and comparing our results with young rift analogues we develop a model of along margin segmentation that explains the evolution of the west equatorial atlantic interpretation of the velocity depth model for profile d shows a 35 37 km thick continental crust which thins to 10 11 km over a distance of 320 km this thinning is accommodated in two regions the narrowest region associated with the ocean continent transition is interpreted to have formed by dextral shearing of the margin along major transform zones that accommodated the initial break up geometry of the central atlantic given the orientation of the margin relative to local fracture zone traces it is likely that the second region of thinning located 162 km landward of the ocean continent transition is a result of rifting suborthogonal to the profile orientation there is no evidence of rotated faulted blocks half graben structures or riftrelated magmatism manifest as either seaward dipping reflectors or as a high velocity region within the lower crust the demerara plateau is therefore interpreted as a margin segment comprising thinned continental crust bound to the north and south by transform type zones in which trans tensional extension is accommodated in contrast to profile a modelling suggests that the eastern margin exhibits a relatively broad region of crustal thinning associated with extension consistent with a rift type setting offshore unusually thin oceanic crust of 3 3 5 7 km thickness is identified which is consistent with similar observations further south in the absence of identifiable magnetic anomalies best estimates of the initial half spreading rate of 20 mm yr 1 suggest that the thin crust throughout the region is unlikely to be a result of ultra slow spreading but instead it is most likely due to a reduced magma supply at numerous long lived transform faults reflected by those presently offsetting the mid atlantic ridge in this equatorial setting

evolution of a landslide induced sediment wave in the navarro river california a streamside landslide delivered 60 000 m3 of mixed size sediment to the navarro river a sinuous gravel bed channel drainage area 535 km2 at the end of the annual high runoff period in spring 1995 the deposit formed a 9 m high dam that partially breached within several hours but recessional flows entrained little material until the following high runoff season the landslide afforded the opportunity to measure the evolution of a sediment wave from its inception to near obliteration and particularly to test relative tendencies for translation and dispersion of a sediment wave in a natural gravel bed channel this study represents a simple case the wave originated from a single input the preexisting channel was relatively uniform and resistant banks prevented adjustments in width we surveyed channel topography over a 1 5 4 5 km reach centered on the landslide dam each year from 1995 to 1999 and we sampled bed material downstream of the dam in 1995 and 1997 landslide material was coarser than ambient bed material but all sizes were mobilized by subsequent peak flows abrasion of weathered and fractured graywacke sandstone landslide material was roughly an order of magnitude greater than the ambient river gravel the sediment wave dispersed and mostly disappeared within a few years with no measurable translation sediment filled the reservoir created by the eroding landslide dam until throughput of bed load was restored in 1998 the stationary wave crest eroded until in 1999 it was 1 m higher e mail dsutherl fs fed us than the preslide elevation as the wave profile flattened its detectable leading edge extended downstream from 620 m in 1995 to 1600 m in 1997 downstream advance of the wave was associated with coarsening of bed material the sediment wave created a longitudinal disturbance in sediment transport by using the dam as a reference datum of zero bed load transport we computed longitudinal variations in annual bed load and suspended sediment transport rates in 100 m increments downstream of the dam these longitudinal variations were controlled by scour and fill of the bed and by abrasion of bed load particles bed load transport rates in the first and second years after the landslide increased in the landslide vicinity and then decreased downstream as sediment deposited behind the advancing leading edge of the wave the location of peak bed load transport rate advanced from the first year 400 m to the second 800 m we used a physically based one dimensional model cui et al 2002b to hindcast annual changes in transversely averaged bed elevation over the study reach agreement between measured and predicted bed elevations was very good this result supports our conclusion that once emplaced sediment waves in gravel bed rivers tend to disperse with little or no translation

seismic investigation of an active ocean continent transform margin the interaction between the swan islands fault zone and the ultraslow spreading mid cayman spreading centre

three dimensional temperature structure of a ridge transform ridge system abstract we have solved the problem of three dimensional heat transfer by conduction and material transport within a ridge transform ridge system the boundary conditions are those of a finite plate with temperatures fixed at the top and bottom we do not solve for the pattern of flow so must arbitrarily describe the material transport within triangular regions beneath the spreading centers flow is assumed to be vertical and at constant velocity with the volume of upwelling matching the volume of horizontal transport by the plates the steady state solution is approached by repeated finite steps in time each involving two stages first to represent material transport temperatures in the grid of points are shifted horizontally and or vertically to the point at which the associated particle would arrive given the time period and assumed velocity field second a three dimensional fourier transform technique is used to calculate the cooling which would occur in the static case during the same period we find that the geometry of flow in the upwelling region here parameterized by velocity of upwelling or width of the upwelling zone is more important than spreading rate in determining the temperature structure the results indicate that conductive cooling cannot alone account for deepening of the median valley towards the fracture zone for a 10 m y offset on the transform fault conductive cooling across the fracture zone to the older lithosphere affects the upwelling material only to a distance of about 10 km from the fault in contrast to the deepening of the median valley over several tens of kilometers observed on the mid atlantic ridge

magmatism versus serpentinization crustal structure along the 13 n segment at the mid atlantic ridge a region of oceanic core complexes occs exists at 13 n on the mid atlantic ridge that is regarded as a type site this site includes two occs at 13 20 n and 13 30 n thought to be in the active and dying stages of evolution and two together called the ashadze complex centred at 13 05 n that are considered to be relict here we describe the results of s wave seismic modelling along an 200 km long 2 d transect traversing south to north through both the mercurius and marathon fracture zones the southern outside corner of the 13 n segment the occs the ridge axis deviation in trend centred at 13 35 n and the youngest oceanic crust of the eastern ridge flank to the north our inversion model and the corresponding vp vs ratio show that the majority of the crust beneath the 13 30 n occ comprises metamorphosed lithologies that have been exhumed to the shallowest subseabed level while basaltic lithologies underlie the 13 20 n occ the transition between these contrasting crustal structures occurs over a distance of 1 9 and equivalent poisson s ratio of 0 3 indicates exhumed and or metamorphosed lithologies beneath the bathymetric depression between them and within the crust beneath the southern occ between the northern and southern flanks of the marathon fracture zone and northern flank of mercurius fracture zone the lower crust has a relatively low vp vs ratio suggesting that the deformation associated with marathon fracture zone which facilitates fluid ingress extends laterally within the lower crust marathon fracture zone itself is underlain by a broad zone of low s wave velocity 2 0 km s 1 up to 20 km wide from the seabed to at least the mid crust that is mirrored in a high vp vs ratio and lower density particularly deeper than 1 km below seabed within its bathymetric footprint volcanic domains are highlighted by a low vp vs ratio of 1 6 and equivalent poisson s ratio of 0 15 our combined seismic and density models favour the localized model of occ evolution they also show a considerable ridge parallel variability in the amount and distribution of magmatic versus metamorphosed crust our results suggest that the current focus of magmatism lies to the north of the 13 20 n occ where the magmatic accretion type seabed morphology observed is mirrored in the pattern of microseismicity suggesting that its inward facing median valley wall fault may link to the 13 20 n occ detachment surface magmatism and active faulting behind to the west the footwall breakaway of the 13 30 n occ and the microseismicity concentrated in a band along its southern flank suggest a readjustment of ridge geometry along axis is underway as part of this a transform offset is forming that will ultimately accommodate the 13 30 n occ in its inside corner on the eastern flank of the ridge axis to the north

effect of sediment pulse grain size on sediment transport rates and bed mobility in gravel bed rivers 1 sediment supply to gravel bed river channels often takes the form of episodic sediment pulses and there is considerable interest in introducing sediment pulses in stream restorations to alter bed surface grain size distributions and bed mobility a series of laboratory experiments was conducted in order to examine how sediment pulse grain size and volume affects the mobility of bed material in gravel bed channels pulses used in the experiments were composed of either the fine tail or the median of the subsurface bed material grain size distribution bed material refers to sediment in the channel prior to the pulse introduction exclusively both types of pulse were finer than the bed material surface median two pulse sizes were used which were either equivalent to the volume of sediment required to cover the entire bed one median subsurface bed material grain diameter deep full unit or 1 4 of this volume 1 4 unit the latter was designed to produce a transitory pulse with the exception of the 1 4 unit coarse pulse introduction of the sediment pulses to the channel caused dramatic increases in the bed load flux the coarse sediment pulses fine the bed surface and coarsen the bed load finer pulses also fine the surface but the bed load fines while the bed material load that excludes pulse material coarsens the greatest effects on the fractional transport occurred during the full unit fine pulse where the pulse covered the greatest bed surface and effectively smoothed the bed increasing near bed velocity and mobilizing the coarse particles overall the coarse pulses were not very effective at mobilizing bed material the large fine pulse mobilized 35 of the bed material surface 35 of its input weight and was most effective at mobilizing the surface however the small fine pulse mobilized 50 of its input weight as it passed through the channel making it the most efficient at mobilizing the bed material

how rivers react to large earthquakes evidence from central taiwan earthquakes and bedrock river incision are fundamental processes in the evolution of tectonically active landscapes yet little work has focused on understanding how a bedrock river responds to a single large earthquake data from the 1999 m w 7 6 chi chi earthquake in central taiwan show dramatic differences in river response that depend on the proximity to the rupture zone near the fault scarp vertical ground deformation intensifi es river incision on a time scale of years to decades while distal to the fault landslides induced by the earthquake mantle bedrock on the river bed with sediment impeding incision for decades to centuries as the material is evacuated this surprising spatial and temporal variability in bedrock incision caused by earthquakes has implications for the paleoseismic interpretation of bedrock terraces and for the evolution of tectonically active landscapes

kinematic reconstruction of the caribbean region since the early jurassic abstract the caribbean oceanic crust was formed west of the north and south american continents probably from late jurassic through early cretaceous time its subsequent evolution has resulted from a complex tectonic history governed by the interplay of the north american south american and paleo pacific plates during its entire tectonic evolution the caribbean plate was largely surrounded by subduction and transform boundaries and the oceanic crust has been overlain by the caribbean large igneous province clip since 90 ma the consequent absence of passive margins and measurable marine magnetic anomalies hampers a quantitative integration into the global circuit of plate motions here we present an updated quantitatively described kinematic reconstruction of the caribbean region back to 200 ma integrated into the global plate circuit and implemented with gplates free software our reconstruction includes description of the tectonic units in terms of euler poles and finite rotation angles our analysis of caribbean tectonic evolution incorporates an extensive literature review to constrain the caribbean plate motion between the american continents we use a novel approach that takes structural geological observations rather than marine magnetic anomalies as prime input and uses regionally extensive metamorphic and magmatic phenomena such as the great arc of the caribbean the clip and the caribbean high pressure belt as correlation markers the resulting model restores the caribbean plate back along the cayman trough and major strike slip faults in guatemala offshore nicaragua offshore belize and along the northern andes towards its position of origin west of the north and south american continents in early cretaceous time we provide the paleomagnetic reference frame for the caribbean region by rotating the global apparent polar wander path into coordinates of the caribbean plate interior cuba and the chortis block we conclude that formation of the caribbean plate west of the north and south americas as a result of panthalassa pacific spreading leads to a much simpler plate kinematic scenario than proto caribbean atlantic spreading placing our reconstruction in the most recent mantle reference frames shows that the clip originated 2000 3000 km east of the modern galapagos hotspot and may not have been derived from the corresponding mantle plume finally our reconstruction suggests that most if not all modern subduction zones surrounding the caribbean plate initiated at transform faults two of these along the southern mexican and nw south american margins evolved diachronously as a result of migrating trench trench transform triple junctions

nondilatant brittle deformation of serpentinites implications for mohr coulomb theory and the strength of faults we conducted deformation experiments to investigate the strength deformation processes and nature of the brittle ductile transition of lizardite and antigorite serpentinites a transition from localized to distributed deformation occurs as confining pressure increases from 200 to 400 mpa at room temperature deformation in both brittle localized and ductile distributed regimes is accommodated by shear microcracks which form preferentially parallel to the 001 cleavage axial microcracks mode i are infrequently observed volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant consistent with the shear dominated microcracking three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow 1 a lack of evidence for crystal plastic deformation 2 a positive pressure dependence of the maximum differential stress and 3 abundant evidence for brittle microcracking the weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the 001 cleavage combined with the low pressure dependence of strength the transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite rather than the more general byerlee s law if brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity serpentinites define an end member style of nondilatant brittle deformation this deformation style may result in extremely weak faults in nature and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present

deep seismic transects across the sheared western barents sea svalbard continental margin abstract we have used recently acquired deep seismic reflection and refraction profiles together with a large amount of conventional mcs data to study the 1000 km long sheared western barents sea svalbard margin with the objective of determining its crustal structure and tectonic history the margin developed from a megashear zone which linked the norwegian greenland sea and the eurasia basin during the eocene opening the margin is composed of two large shear segments and a central rifted margin segment associated with volcanism the continent ocean boundary is confined within a narrow zone 10 20 km at the sheared margin segments but is more obscure and partly masked by volcanics at the rifted margin the crustal structure and tectonic history differ significantly between the three margin segments in the north across the hornsund fault zone the crustal thickness changes abruptly from more than 30 km thick continental crusy on the svalbard platform to 6 8 km thick oceanic crust in the greenland sea this margin segment experienced oblique continent continent and partly continent ocean shear with both transtensional and transpressional component during eocene time while post eocene extension caused crustal stretching west of svalbard the volcanic province at the central rifted margin segment probably comprises an outer part of thick oceanic crust and an inner part of thinned continental crust covered by a mixture of volcanics and sediments this formed at a releasing bend in the margin in response to eocene oligocene extension and two phases of volcanism in the south the crustal thinning occurs over a wider area landward of the senja fracture zone here the margin bounds a basinal province in which as much as 18 20 km of sediments cover a highly attenuated crystalline crust the senja fracture zone evolved due to eocene transtensional continent continent shear followed by continent ocean shear and has been passive since earliest oligocone time we conclude that the first order crustal structure along the margin and its tectonic development is mainly the result of three controlling parameters 1 1 the pre breakup structure 2 2 the geometry of the plate boundary at opening and 3 3 the direction of relative plate motion the interplay between the direction of plate motion and the geometry of the plate boundary gave rise to striking differences in the structural development of the different margin segments

stochastic forcing of sediment routing and storage in channel networks the stochastic field of sediment supply to the channel network of a drainage basin depends on the large scale interactions among climatically driven processes such as forest fire and rainstorms topography channel network topology and basin scale during infrequent periods of intense erosion large volumes of colluvium are concentrated in parts of a channel network particularly near tributary junctions the rivers carry bed material and wash load downstream from these storage sites at different rates the bed material travels slowly creating transient patterns of sediment transport sediment storage and channel morphology along the channel network as the concentrations of bed material migrate along the network their waveforms can undergo changes by diffusion interference at tributary junctions and loss of mass through temporary sediment storage in fans and terraces and through particle abrasion which converts bed material to wash load we investigated how these processes might influence the sediment mass balance in channels of third and higher order in a 215 km 2 drainage basin within the oregon coast range over a simulated 3000 year period with a climate typical of the late holocene we used field measurements and a simulation model to illustrate interactions between the major controls on large scale processes functioning over long periods of time in complex drainage basins

seismic structure of the central tyrrhenian basin geophysical constraints on the nature of the main crustal domains in this work we investigate the crustal and tectonic structures of the central tyrrhenian back arc basin combining refraction and wide angle reflection seismic was gravity and multichannel seismic mcs reflection data acquired during the medoc mediterraneo occidental 2010 survey along a transect crossing the entire basin from sardinia to campania at 40 n the results presented include a 450 km long 2 d p wave velocity model obtained by the traveltime inversion of the was data a coincident density model and a mcs poststack time migrated profile we interpret three basement domains with different petrological affinity along the transect based on the comparison of velocity and velocity derived density models with existing compilations for continental crust oceanic crust and exhumed mantle the first domain includes the continental crust of sardinia and the conjugate campania margin in the sardinia margin extension has thinned the crust from 20 km under the coastline to 13 km 60 km seaward similarly the campania margin is also affected by strong extensional deformation the second domain under the cornaglia terrace and its conjugate campania terrace appears to be oceanic in nature however it shows differences with respect to the reference atlantic oceanic crust and agrees with that generated in back arc oceanic settings the velocities depth relationships and lack of moho reflections in seismic records of the third domain i e the magnaghi and vavilov basins support a basement fundamentally made of mantle rocks the large seamounts of the third domain e g vavilov are underlain by 10 20 km wide relatively low velocity anomalies interpreted as magmatic bodies locally intruding the mantle

crustal structure of north atlantic fracture zones seismic studies have established that large offset transforms along the slow spreading mid atlantic ridge exhibit anomalous crustal structures that fall well outside the range typically associated with oceanic crust seismically fracture zone crust in the north atlantic is extremely heterogeneous in both thickness and internal structure it is frequently quite thin 1 2 km thick and is characterized by low compressional wave velocities and the absence of a normal seismic layer 3 a more gradual crustal thinning can extend up to several tens of kilometers from these fracture zones anomalously thin crust has also been inferred from both seismic and gravity studies at smaller ridge axis discontinuities along the mid atlantic ridge the geological nature of the seismically anomalous crust found within atlantic fracture zones and how this crust forms are still controversial one interpretation consistent with available seismic observations is that the crust within north atlantic fracture zones consists of a thin intensely fractured and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places variations in apparent seismic crustal thickness along fracture zones may reflect different degrees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust the existence of a thinner crustal section in fracture zones can be explained by a reduced magma supply within a broad region near ridge offsets due to the three dimensional nature of upwelling beneath a segmented spreading center and by tectonic dismemberment of the crust by large scale detachment faults that form preferentially in the cold brittle lithosphere near the ends of segments