the initiation of subduction by crustal extension at a continental margin summary we investigate how subduction may be triggered by continental crust extension at a continental margin the large topography contrast between continental and oceanic domains drives the spreading of continental crust over oceanic basement subduction requires the oceanic plate to get submerged in mantle so that negative buoyancy forces may take over and drive further descent this is promoted by two mechanisms loading by continental crust bends the oceanic plate downwards extension in the continental domain induces crustal thinning which acts to raise mantle above the oceanic plate in this model the width of the continental region undergoing extension is an important control parameter the main physical controls are illustrated by laboratory experiments and simple theory for elastic flexure coupled to viscous crustal spreading three governing dimensionless parameters are identified one involves the poorly constrained oceanic plate buoyancy we find that the oceanic plate can be thrust to depths larger than 40 km even if it is buoyant enabling metamorphic reactions and density increase in the oceanic crust another parameter is the ratio between the width of the continental extension region and the flexural parameter for the oceanic plate initiating subduction is easier if the continent thins over a short lateral distance or if the oceanic plate is strong the third important parameter is the ratio of oceanic plate thickness to initial continental crust thickness such that a weak plate and a thick crust do not favour subduction thus the change from a passive to an active margin depends on the local characteristics of the continental crust and is not determined solely by the age and properties of the oceanic lithosphere it is shown that the spreading of continental crust induces uplift of the margin as the adjacent seafloor subsides evidence for the emplacement of continental crust over oceanic basement at passive margins is reviewed

global near ir maps from gemini n and keck in 2010 with a special focus on janus patera and kanehekili fluctus abstract we have imaged io in the near infrared 1 5 m with adaptive optics on the 10 m w m keck ii and the gemini north telescopes we have constructed global maps from the data taken in 2010 although numerous hot spots are visible the maps are dominated by two volcanic centers loki patera and kanehekili fluctus we have examined in detail the thermal emission from janus patera and kanehekili fluctus using images obtained between 2003 and 2010 and have created a timeline of volcanic activity at these locations utilizing data from a wide variety of sources based on our data and this timeline we find that the thermal emission from janus patera is relatively steady indicative of an active lava lake the thermal emission from kanehekili fluctus however is quite variable and is indicative of the emplacement of extensive lava flows in 2010 the thermal emission from this location was 10 times higher than ever seen before indicative of a particularly voluminous eruption of lava

io eruptions at pillan and the time evolution of pele and pillan from 1996 to 2015 abstract observations obtained with the near infrared camera nirc2 coupled to the adaptive optics system on the 10 m w m keck ii telescope on mauna kea hawaii on 14 august 2007 revealed an active and highly energetic eruption at pillan at 245 2 0 7 w and 8 5 0 5 s a one temperature blackbody fit to the data revealed a blackbody temperature of 840 40 k over an area of 17 km2 with a total power output of 500 gw using davies davies a g 1996 icarus 124 1 45 61 io flow model we find that the oldest lava present is less than 1 2 h old having cooled down from the eruption temperature of 1400 k to 710 k this young hot lava suggests that an episode of lava fountaining was underway in addition to an examination of this eruption we present data of the pele and pillan volcanoes obtained with the same instrument and telescope from 2002 through 2015 these data reveal another eruption at pillan on ut 28 june 2010 model fits to this eruption yield a blackbody temperature of 600 700 k over an area of 60 km2 radiating over 600 gw on ut 18 february 2015 an energetic eruption was captured by the infrared telescope facility irtf via mutual event occultations the eruption took place at 242 7 1 w and 12 4 1 s i e in the eastern part of pillan patera subsequent observations showed a gradual decrease in the intensity of the eruption images obtained with the keck telescope on 31 march and 5 may 2015 revealed that the locations of the eruption had shifted by 120 160 km to the nw in contrast to the episodicity of pillan pele has been persistent observed in every appropriate 4 7 m observation pele was remarkably consistent in its thermal emission from the galileo era through february 2002 when a blackbody temperature of 940 40 k and an area of 6 5 km2 was measured since that time however the radiant flux from what is likely a apparently large overturning lava lake has gradually subsided over the next decade by a factor of 4 while the location of the thermal source was moving back and forth between areas roughly 100 km to the w of the 2002 location and an area roughly 100 km to the se of the 2002 location

back arc strain in subduction zones statistical observations versus numerical modeling 1 recent statistical analysis by lallemand et al 2008 of subduction zone parameters revealed that the back arc deformation mode depends on the combination between the subducting vsub and upper vup plate velocities no significant strain is recorded in the arc area if plate kinematics verifies vup 0 5 vsub 2 3 cm a in the hs3 reference frame arc spreading shortening occurs if vup is greater lower than the preceding relationship we test this statistical law with numerical models of subduction by applying constant plate velocities far away from the subduction zone the subducting lithosphere is free to deform at all depths we quantify the force applied on the two converging plates to sustain constant surface velocities the simulated rheology combined viscous non newtonian and brittle behaviors and depends on water content the influence of subduction rate vs is first studied for a fixed upper plate after 950 km of convergence steady state slab pull the transition from extensional to compressive stresses in the upper plate occurs for vs 1 4 cm a the effect of upper plate velocity is then tested at constant subduction rate upper plate retreat advance with respect to the trench increases extension compression in the arc lithosphere and increases decreases the subducting plate dip our modeling confirms the statistical kinematic relationship between vsub and vup that describes the transition from extensional to compressive stresses in the arc lithosphere even if the modeled law is shifted toward higher rates of upper plate retreat using our set of physical parameters e g 100 km thick subducting oceanic plate and short term simulations our results make valid the choice of the hs3 reference frame for assessing plate velocity influence on arc tectonic regime the subduction model suggests that friction along the interplate contact and the mantle stokes reaction could be the two main forces competing against slab pull for upper mantle subductions besides our simulations show that the arc deformation mode is strongly time dependent

fault trends on the seaward slope of the aleutian trench implications for a laterally changing stress field tied to a westward increase in oblique convergence 1 normal faults along the seaward trench slope sts commonly strike parallel to the trench in response to bending of the oceanic plate into the subduction zone this is not the circumstance for the aleutian trench where the direction of convergence gradually changes westward from normal to transform motion gloria side scan sonar images document that the aleutian sts is dominated by faults striking oblique to the trench west of 179 e and east of 172 w these images also show a pattern of east west trending seafloor faults that are aligned parallel to the spreading fabric defined by magnetic anomalies the stress strain field along the sts is divided into two domains west and east respectively of 179 e over the western domain sts faults and nodal planes of earthquakes are oriented oblique 9 46 to the trench axis and 69 90 to the magnetic fabric west of 179 e sts fault strikes change by 36 from the e w trend of sts where the trench parallel slip gets larger than its orthogonal component of convergence this rotation indicates that horizontal stresses along the western domain of the sts are deflected by the increasing obliquity in convergence an analytical model supports the idea that strikes of sts faults result from a superposition of stresses associated with the dextral shear couple of the oblique convergence and stresses caused by plate bending for the eastern domain most nodal planes of earthquakes strike parallel to the outer rise indicating bending as the prevailing mechanism causing normal faulting east of 172 w sts faults strike parallel to the magnetic fabric but oblique 10 26 to the axis of the trench on the basis of a coulomb failure criterion the trench oblique strikes probably result from reactivation of crustal faults generated by spreading

influence of the yangtze river and grain size on the spatial variations of heavy metals and organic carbon in the east china sea continental shelf sediments abstract bulk heavy metal fe mn zn cu pb cd al organic carbon and carbonate concentrations grain sizes and c13 of the organic carbon distributions were studied in sediments collected throughout the east china sea continental shelf and the yangtze river delta the results demonstrated that terrigenous sediments from the yangtze river is a dominating factor controlling the spatial variations of heavy metals and organic carbon concentrations on the east china sea continental shelf in addition grain size and recent anthropogenic influences are also major factors modifying the spatial and vertical variations of heavy metals large spatial variations with a band type distribution of heavy metals grain size organic carbon and carbonate were observed higher concentrations of heavy metal and light c13 of the organic carbon were found primarily in the deltaic and inner shelf sediments the band type distribution generally followed the coastline with little variations in the north south direction away from the delta and inner shelf west east direction most heavy metal concentrations decreased rapidly with the exception of cd where high concentrations of cd were also found in the carbonate rich shelf break sediments coarse grained relict sediments and biogenic carbonate are two primary diluting agents for the fine grained aluminosilicate sediments from the yangtze river with high concentrations of heavy metals unusually high concentrations of cu pb and cd showed both spatially and vertically that more pollution prevention measures are needed in the yangtze river drainage basin in order to prevent further heavy metal pollution of the east china sea inner continental shelf

the role of elasticity in slab bending previous studies showed that plate rheology exerts a dominant control on the shape and velocity of subducting plates here we perform a systematic investigation of the role of elasticity in slab bending using fully dynamic 2 d models where an elastic viscoelastic or viscoelastoplastic plate subducts freely into a purely viscous mantle we derive a scaling relationship between the bending radius of viscoelastic slabs and the deborah number de which is the ratio of maxwell time over deformation time we show that de controls the ratio of elastically stored energy over viscously dissipated energy and find that at de 10 2 substantially less energy is required to bend a viscoelastic slab to the same shape as a purely viscous slab with the same intrinsic viscosity elastically stored energy at higher de favors retreating modes of subduction via unbending while trench advance only occurs for some cases with de 1 where most zones have low de 0 1 slabs with de 10 2 either have very low viscosities or they may be yielding in which case our de estimates may be underestimated by up to an order of magnitude potentially pointing towards a significant role of elasticity in 60 of the subduction zones in support of such a role of elasticity in subduction we find that increasing de correlates with increasing proportion of larger seismic events in both instrumental and historic catalogues

outer rise stress changes related to the subduction of the juan fernandez ridge central chile 1 although outer rise seismicity is less common than interplate seismicity in subduction zones a significant level of seismicity has occurred between the nazca trench and juan fernandez ridge in central chile during the past 20 years we first study the 9 april 2001 mw 7 0 event and determine its focal mechanism depth and source time function by body waveform inversion from teleseismic broadband data the results indicate tensional faulting in the upper part of the mechanical lithosphere its strike 41 is similar to those observed in events down dip of the slab at about 100 km depth which could indicate that these earthquakes occur in preexisting structures formed at the trench compressive outer rise events have also occurred during the 1980s in front of the rupture zone of the 1985 mw 7 8 valparaiso earthquake to understand their relation with the state of stress of the lithosphere we construct yield stress envelopes of the oceanic lithosphere including static and dynamic stresses dynamic stresses are due either to slab pull ridge push resistive and drag forces we explain the sequence of compressive and tensional events by the accumulation of stress prior to 1985 when the subduction is assumed to be locked and after by the unlockage of the subduction by the valparaiso interplate event the yield stress envelope analysis enables us to quantify the accumulation of compressive forces before 1985 and the tensional force after

the relationship between depth age and gravity in the oceans we reassess the applicability of the thermal plate cooling model to the subsidence of the north pacific atlantic and north indian ocean basins we use a new numerical plate model in which the thermophysical parameters of the lithosphere vary with temperature according to the results of laboratory experiments and the ridge temperature structure is consistent with the thickness of the oceanic crust we first attempt to exclude thickened crust from our data set and then to exclude swells and downwellings by masking regions of the data that remains that have significant gravity anomalies when there exists a clear regional correlation between intermediate wavelength gravity and topography we find that the average variation of depth with age is consistent with conventional half space models until about 90 myr thereafter the departure from the half space cooling curve is more rapid than predicted using simple conductive plate cooling models the depth age curves in the pacific and atlantic show 250 m of temporary shallowing between the ages of 90 130 myr a result consistent with the outcome of experiments on the initiation of small scale boundary layer convection the results do not change significantly if the estimated component of the gravity arising from plate cooling is subtracted prior to calculation of the correlation between gravity and topography a 90 km thick conductive plate is nevertheless a reasonable model for the average temperature structure of the oldest part of the pacific ocean lithosphere in the pacific the broad topographic undulations associated with the line island swell the hawaiian swell and surrounding basins have correlated gravity anomalies and an admittance of approximately 30 mgal km 1 and are likely to result from convective circulation in the upper mantle in the northeast atlantic the intermediate wavelength admittance over the cape verde swell is similar in the northwest atlantic over the bermuda swell it is slightly larger but not as well constrained

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

strength of the lithosphere of the galilean satellites abstract several approaches have been used to estimate the ice shell thickness on callisto ganymede and europa here we develop a method for placing a strict lower bound on the thickness of the strong part of the shell lithosphere using measurements of topography the minimal assumptions are that the strength of faults in the brittle lithosphere is controlled by lithostatic pressure according to byerlee s law and the shell has relatively uniform density and thickness under these conditions the topography of the ice provides a direct measure of the bending moment in the lithosphere this topographic bending moment must be less than the saturation bending moment of the yield strength envelope derived from byerlee s law the model predicts that the topographic amplitude spectrum decreases as the square of the topographic wavelength this explains why europa is rugged at shorter wavelengths 10 km but extremely smooth and perhaps conforming to an equipotential surface at longer wavelengths 100 km previously compiled data on impact crater depth and diameter schenk p m 2002 nature 417 419 421 on europa show good agreement with the spectral decrease predicted by the model and require a lithosphere thicker than 2 5 km a more realistic model including a ductile lower lithosphere requires a thickness greater than 3 5 km future measurements of topography in the 10 100 km wavelength band will provide tight constraints on lithospheric strength

on the accuracy of digital bathymetric data the global 5 arcmin gridded topography data etopo 5 are based on contour maps rather than original soundings and have large errors the artificial statistical distribution generated by digitizing contours makes these data unsuitable for use in regression models for depth age variations their amplitude spectrum is bounded by a frequency 4 power law so that they should not be used when the gravity topography transfer function is important such as in studies of flexural isostasy and mantle convection i assess the accuracy of 14 491 069 digital ship soundings in 2253 cruise surveys collected between 1955 and 1992 in the lamont doherty earth observatory on line data base by analyzing 329 058 crossover errors coes at intersecting ship tracks five percent of cruises with internal coes yield root mean square coe amplitudes exceeding 500 m all of these have errors in digitizing two way travel time from analog precision depth recorder traces twenty eight cruises were found which had errors caused by misinterpretation of the nominal sound velocity used when travel times were reported as nominal depths two nominal sound velocities in common use differ by 2 5 an amount which is often undetectable producing uncertainties in depth of this magnitude ship data have been acquired at different rates over time with the peak of activity in the early 1970s although present technologies can yield very accurate data these are acquired at a rate which is small with respect to the total available data the cumulative median global coe has remained constant at 26 m since the late 1970s most recent data acquisition has been in the northern hemisphere oceans and the oldest and least accurate data are in the southern oceans where median coes are 100 250 m the majority of the data in the south pacific were acquired before the advent of satellite navigation

thermal stresses and the bimodal distribution of elastic thickness estimates of the oceanic lithosphere observations of plate flexure beneath seamounts and at deep sea trenches suggest that the mechanical strength of the oceanic lithosphere increases with age the mechanical thickness h seems to follow the depth to a particular isotherm in the cooling plate model a detailed review of available estimates of elastic thickness te shows that the strength deduced from studies at seamounts is significantly different from the strength inferred at trenches and fracture zones whereas te beneath seamounts approximately follows the 400 c isotherm h at trenches follows the depth to the 700 800 c isotherms the cause of this bimodal distribution is not fully understood thermal bending stresses due to cooling of the lithosphere have been suggested to play an important role in the flexural deformation at transform faults and fracture zones and their distribution with depth is consistent with intraplate seismicity this paper evaluates the effects of thermal stresses on deformation beneath seamounts and at trenches and how their interaction with flexural stresses may modify plate strength the results indicate that thermal bending stresses will bias the moment curvature relationship such that a plate of a certain strength will appear relatively weaker beneath a seamount positive curvature but relatively stronger at a trench negative curvature this systematic bias appears to be in qualitative agreement with published te estimates for old lithosphere the theory predicts a diminishing amount of bias the general lack of sufficient te determinations from old ocean basins and the uncertainties associated with modeling of trenches prohibit a detailed analysis of this part of the te versus age relationship i conclude that the scatter in the elastic thickness estimates appears to be caused by a combination of lithospheric reheating during seamount emplacement bias introduced by thermal stresses and variable amount of yielding of the plate beneath seamounts in proportion to seamount size and lithospheric age at the time of loading

lithospheric flexure modelling seaward of the chile trench implications for oceanic plate weakening in the trench outer rise region summary the chilean subduction zone presents a unique opportunity to study trench outer rise deformation of the subducting oceanic lithosphere at different thermal ages the shape of the outer rise for plate ages ranging from 0 to 50 ma is predicted by using an elastic plate model with variable elastic thickness te x as a function of the distance measured from the trench axis in addition the uncertainties of our results are estimated by performing a monte carlo type analysis and we considered explicitly the sediment loading effect on the lithospheric flexure in regions where the trench is heavily sedimented the results show a systematic reduction in te of up to 50 per cent or reduction in the flexural rigidity d of up to 90 per cent from the peak of the outer rise to the trench axis the reduction in te and d observed in most of the bathymetric profiles is coincident with i high plate curvatures 5 10 7 m 1 ii strong bending moments 10 16 n m iii pervasive fracturing and faulting of the oceanic basement as imaged by high resolution bathymetry data and iv reduction in crustal and mantle seismic velocities the reduction in flexural rigidity towards the trench suggests a weakening of the oceanic lithosphere and is interpreted to be caused partially by fracturing and a likely increase in fluid pore pressure in general our estimates do not show consistent increases in elastic thickness as a function of plate age this result suggests that either te is independent of plate age or te depends strongly on other factors these factors could include lithospheric weakening due to hydro fracturing and the loading history of the plate prior to subduction

regional variations in subsidence rate of oceanic plates a global analysis a global analysis of the dependence of seafloor depth on crustal age has been performed for each large oceanic plate independently each plate has been divided into regions bounded by major tectonic features a total of 32 regions have been considered bathymetry corrected for sediment loading has been plotted as a function of crustal age for each region except for three regions western pacific between 10 s and 40 n north american plate between 24 n and 38 n and african plate between 10 n and 24 n where ocean floor older than 80 100 ma flattens with age in other regions depth increases linearly with age1 2 subsidence rate over each region has been computed by linear regression large variations up to a factor 2 are reported from one region to another asymmetrical subsidence is also observed in a number of areas however the largest asymmetries are less than the regional variations in subsidence rate occurring on individual plates regional variations in subsidence rate appear linearly related to regional variations in ridge crest topography shallow ridge subsiding quickly and deep ridge subsiding slowly hence seafloor depth subsides at a rate modulated by the value of the initial axial depth this observation suggests that regional variations in subsidence rate reflect the thermal regime of the mantle beneath mid ocean ridges rather than lateral temperature variations inside the asthenosphere involving small scale convection

impact of the 90 e ridge at the indo burmese subduction zone imaged from deep seismic reflection data abstract as a result of the indo burmese active hyper oblique subduction part of the bay of bengal is presently subducting eastward below the burmese microplate the nature and structure of this basin is still poorly understood due to the lack of seismic data we have conducted two deep penetration seismic reflection surveys in the north eastern bay of bengal providing the first high resolution seismic image of the bengal basin fill and basement on basis of these data we are able to trace the 90 e ridge much more northward than previously thought i e up to 20 n along the indo burmese plate boundary we found out that the surface deformation the deep structure of the subduction zone and the geometry of the plate boundary could all be strongly influenced by the impact of a prominent asperity the 90 e ridge these effects are variable along the margin between 15 n and 18 n the ridge asperity brushes the active burmese plate boundary that strikes n10 e at this latitude all the structures framing the indo burmese wedge have a similar n10 e trend deformation at the plate boundary is mainly strike slip this is confirmed by the absence of subducted slab at depth as indicated by both seismicity and tomography the small component of shortening along this plate boundary is probably accommodated partly by the flexure of the ridge and partly within the deformed upper plate north of 19 n the ridge vanishes progressively the absence of basement topography together with the large amount of sediments provided by the brahmaputra delta facilitates the fast westward growth of the indo burmese wedge the seismicity fits a well developed subducted slab at depth in the narrow transition zone between 18 n and 19 n the 90 e ridge northern tips collides with the burmese microplate this collision could explain the rise of a subsuface flat and ramp system offshore ramree and cheduba islands and the strong uplift of the indo burmese wedge in mount victoria area

seasat observations of flexure evidence for a strong lithosphere abstract outer rises seaward of subduction zones arise due to flexure of the oceanic lithosphere a mechanical model of this flexure is used to predict the shape of the associated geoid signal these predicted geoid signals are compared with seasat altimeter profiles over outer rises adjacent to nine major subduction zones the nine zones include the aleutian mariana izu bonin puerto rico lesser antilles peru chile middle america south sandwich philippine and kuril the comparisons with seasat data provide estimates of effective elastic thickness he and maximum bending moment mm for each zone it is shown that 1 1 these thicknesses hc are approximately proportional to the square root of lithospheric age 2 2 the maximum bending moments mm increase roughly as the cube of thickness hc these relations 1 and 2 are both very similar to those predicted by a layered rheology model brittle elastic ductile of the lithosphere derived from mechanical experiments on rocks according to this model thermally activated creep is the mechanism of ductile flow in the lower lithosphere hence an isotherm 740 50 defines the bottom of the lithosphere thus old oceanic lithosphere may have significant strength to depths in excess of 60 km or about half that of the thermal lithosphere

the thermal signature of volcanic eruptions on io and earth abstract we investigate a spectrum based technique to identify the style of active volcanic eruptions on jupiter s moon io thermal remote sensing of io has had to rely primarily on low spatial resolution data similar to low spatial resolution satellite data applied to detecting and charting the temporal evolution of terrestrial hot spots these terrestrial analyses use data from sensors designed to monitor the weather and sea surface temperature on io such low spatial resolution data are used to classify eruption styles modes of emplacement by means of several criteria related to the temporal evolution of the infrared spectrum associated with the eruptive activity at each hot spot which we term thermal signature we find that the ratio of the emission at 2 and 5 m and how this ratio changes with time is often diagnostic of eruption style even in low spatial resolution data tests using thermal data for terrestrial ground truth cases show that our classification system is valid on earth the results of our analysis can be used to aid in the design of future space based instruments that can be used for volcano monitoring on io as well as earth

antarctic marine gravity field from high density satellite altimetry summary closely spaced satellite altimeter profiles 5 km collected during the geosat geodetic mission geosat gm and those planned for the extended ers 1 mission are easily converted to grids of vertical gravity gradient and gravity anomaly as profile spacing decreases it becomes increasingly difficult to perform a crossover adjustment on the original geoid height profiles without introducing large cross track gradients if one is only interested in the horizontal and vertical derivatives of the gravitational potential however adjustment of the profile is unnecessary the long wavelength radial orbit error is suppressed well below the noise level of the altimeter by simply taking the along track derivative of each profile ascending and descending slope profiles are then interpolated onto separate uniform grids these two grids are summed and differenced to form comparable grids of east and north vertical deflection using laplace s equation the vertical gravity gradient is calculated directly from the vertical deflection grids fourier analysis is required to construct gravity anomalies from the two vertical deflection grids these techniques are applied to high density 2 km profile spacing geosat gm profiles in antarctic waters 60 s to 72 s gridding and interpolation are performed using the method of projection onto convex sets where the smoothness criteria corresponds to upward continuation through 4 km of ocean the resultant gravity grids have resolution and accuracy comparable to shipboard gravity profiles after adjustment of a dc shift in the shipboard gravity profiles 5 mgal the rms difference between the ship and satellite gravity is 5 5 mgal many interesting and previously uncharted features are apparent in these new gravity maps including a propagating rift wake and a large leaky transform along the pacific antarctic rise

intraplate earthquakes and the state of stress in oceanic lithosphere abstract the sources of stress responsible for oceanic intraplate earthquakes and their usefulness for inferring the tectonic stress field are investigated through a synthesis of the source mechanisms of 58 of the largest such events determined by inversion of long period p and sh waveforms further constraints are provided by the global distribution of oceanic intraplate seismicity and a calculation of the seismic moment release per unit volume of seismogenic lithosphere as a function of lithosphere age although frequently associated with relict tectonic features large oceanic intraplate earthquakes are generally reliable indicators of the state of stress in the source region most oceanic intraplate seismicity occurs in young lithosphere

seasat observations of lithospheric flexure seaward of trenches lithospheric flexure seaward of deep ocean trenches is evident in seasat altimeter observations of the marine geoid in fact mechanical models of lithospheric flexure can be tested directly on the seasat altimeter data we have used a simple elastic model for the oceanic lithosphere and after least squares adjustments have recovered estimates of model parameters including outer rise or amplitude or wavelength and effective lithospheric thickness effective lithospheric thicknesses have been recovered for six regions the mariana the kuril the philippine the aleutian the izu bonin and the middle america or s these results support the proposition that effective thickness te increases with age of lithosphere in approximate accord with the relation te c age1 2 where c 4 km m y 1 2 in fact our altimetric results agree more closely with this relation than do published results based on bathymetric data the close agreement with the thickness age relation suggests that there is no longer any need to assume that significant horizontal compression acts across the kuril marianas and izu bonin trenches this thickness age relation implies that flexural strength of the oceanic lithosphere is temperature controlled

behaviour of the oceanic lithosphere at subduction zones plastic yield strength from a finite element method abstract using the finite element method to study the deflection of the oceanic lithosphere at subduction zones shows that deflection of an elastic plastic model agrees with observed bathymetry data except for 2 examples kermadec and mariana where the lithospheric behaviour remains elastic seaward of the trench seven of the cases we studied could be easily fitted and enabled us to compute an effective yield strength the average value of which is about 3 8 to 4 8 kbar and seems to be the same in every case

very high resolution seismo acoustic imaging of seagrass meadows mediterranean sea implications for carbon sink estimates 1 posidonia oceanica is a widespread coastal mediterranean seagrass which accumulates in its subsurface large quantities of organic material derived from its roots rhizomes and leaf sheaths embedded in sandy sediments these organic deposits may be up to several meters thick as they accumulate over thousands of years forming the matte whose high content in organic carbon plays a major role in the global ocean carbon cycle in this study very high resolution seismo acoustic methods were applied to image the subsurface features of a p oceanica seagrass meadow at portlligat cadaques girona spain in the nw mediterranean sea our findings yield fresh insights into the settling of the p oceanica meadow in the study area and define with unprecedented detail the potential volume occupied by the matte a strong reflector located from 4 3 to 11 7 m depth was recognized in several seismo acoustic profiles as the substratum on which p oceanica first settled in the study area a 3d bathymetric model of this substratum allowed us to reconstruct the portlligat palaeo environment prior to the settling of p oceanica which corresponded to a shallow coastal setting protected from the open sea a core drilled in the meadow at portlligat revealed the presence of a 6 m thick dense matte composed of medium to coarse sandy sediments mixed with plant debris and bioclasts radiocarbon datings revealed a constant accretion rate of the matte of about 1 1 m kyr gravelly bioclastic deposits observed at the base of the core correspond to the base of the matte and gave a date of 5616 46 cal yr bp for the first time very high resolution marine geophysical techniques allowed us to accurately define the volume occupied by p oceanicamatte which in the study area reaches up to almost 220 000 17 400 m3 this result is an important step forward in our efforts to estimate the size of the carbon sink represented by p oceanica meadows along the mediterranean coasts significantly contributing to the biosphere carbon cycle

on the deformation cycle of a strongly coupled plate interface the triple earthquakes of 16 march 1963 15 november 2006 and 13 january 2007 along the kurile subduction zone 1 the great plate interface kurile earthquake of 15 november 2006 mw 8 3 was immediately followed by normal aftershocks in the outer rise of the central kurile subduction zone two months later at a distance 50 km from this event the 13 january 2007 mw 8 1 earthquake with a pure normal mechanism occurred in the outer rise of the central kurile the 15 november 2006 earthquake ruptured an area of 280 150 km2 with maximum slip of almost 7 m while the 13 january 2007 earthquake ruptured an area of 240 40 km2 with a maximum slip of almost 14 m these two significant events were preceded by a major reverse earthquake on 16 march 1963 mw 7 7 that took place close to the location of the 13 january 2007 earthquake these very different earthquakes demonstrate the different stages of the deformation cycle along strongly coupled plates at the central kurile arc and present a rare case of stress regime reversal over a short time span in order to understand the details of these three earthquakes we inverted for the source rupture processes of the earthquakes and delineated the location and dimension of asperities the role of gravity and topography anomalies and their correlation to the asperity distribution as well as the long term seismicity is studied in detail using trench parallel bouguer anomaly we have identified the differences in the asperities and the location of larger normal outer rise aftershocks as well as the location of 13 january 2007 earthquake we also show that the long term seismicity of the central kurile arc is confined to the intraslab and not the plate interface the static stress transfer for these earthquakes showed that the stress on optimally oriented thrust faults is increased in the northeastern part of the rupture area of the 15 november 2006 earthquake absence of major and great earthquakes to the northeastern immediate vicinity of the rupture area of the 15 november 2006 earthquake supports the hypothesis that this area is mature for the next strong earthquake along the kurile arc

temporal and three dimensional spatial analyses of the frequency magnitude distribution near long valley caldera california summary the 3 d distribution of the b value of the frequency magnitude distribution is analysed in the seismically active parts of the crust near long valley caldera california the seismicity is sampled in spherical volumes containing n 150 earthquakes and centred at nodes of a grid separated by 0 3 km significant variations in the b value are detected with b ranging from b 0 6 to b 2 0 high b value volumes are located near the resurgent dome and at depths below 5 km at mammoth mountain b values are found to be much lower south of the long valley caldera we interpret this to indicate that an active magma body has advanced from depths below 8 km to depths of 4 to 5 km beneath mammoth mountain in 1989 and that anomalous crust either highly fractured or containing unusually high pore pressure such as is the case in the vicinity of active magma bodies exists north of the seismically active area beneath the resurgent dome at all depths we also investigate the spatial distribution of temporal variations of the frequency magnitude distribution by introducing diverential b value maps b values increased from b 0 8 to b 1 5 underneath mammoth mountain at the onset of the 1989 earthquake swarm and remained high thereafter this suggests that an intrusion permanently altered the average distribution of cracks at 5 10 km depth or that the pore pressure permanently increased we propose that high b values are a necessary but not suycient condition near a magmatic body and therefore spatial b value mapping can be used to aid in the identification of active magma bodies

alternating thin versus thick skinned decollements example in a fast tectonic setting the misool onin kumawa ridge west papua abstract the misool onin kumawa ridge eastern indonesia is a broad anticline in the lower plate of the seram subduction system in the south it lies between the seram accretionary wedge and the young lengguru fold and thrust belt the evolution results from what appears to be a continuum of deformation through three major stages 1 formation of a messinian thin skinned fold and thrust belt over a shaly silty permian paleocene unit 2 a pliocene thick skinned event responsible for the uplift of the ridge possibly induced by the onset of continental subduction and 3 recent pleistocene deformation when thin skinned tectonics resumed in the seram trough currently the seram wedge abuts the ridge transferring compression northward into the salawati basin the jumps of active detachment levels may be a response to changes in subduction parameters velocity rugosity etc during the transition between oceanic and continental subduction or at least from thinned crust to thicker continental crust

strain rate dependency of oceanic intraplate earthquake b values at extremely low strain rates we discovered a clear positive dependence of oceanic intraplate earthquake oceq b values on the age of the oceanic lithosphere oceq b values in the youngest 20 ma oceanic lithosphere exceed 1 5 which is significantly higher than the average worldwide earthquake b value around 1 0 on the other hand the b value of intraplate earthquakes in the ninety east sumatra orogen where oceanic lithosphere has an anomalously higher strain rate compared with normal oceanic lithosphere is 0 93 which is significantly lower than the oceq b value about 1 9 with the same age 50 110 ma thus the variation in b values relates to the strain rate of the oceanic lithosphere and is not caused by a difference in thermal structure we revealed a negative strain rate dependency of the b value at extremely low strain rates 1 5 in oceanic lithosphere 20 ma old imply that future improvement in seismic observation will capture many smaller magnitude oceqs which will provide valuable information on the evolution of the oceanic lithosphere and the driving mechanism of plate tectonics

plate force due to bending at subduction zones 1 bending of the lithosphere at subduction zones generates stresses that contribute to the force balance on plates we determine the net horizontal force on subducting plates by imposing a simple kinematic description of strain in the equations that govern the mechanical equilibrium of a thin viscous sheet the magnitude of the force depends on the velocity thickness and curvature of the subducting plate using representative values for old oceanic lithosphere we obtain an effective horizontal force that is nearly 40 of the buoyancy force due to slabs in the upper mantle

the mechanism of postseismic deformation triggered by the 2006 2007 great kuril earthquakes received 21 january 2011 accepted 15 february 2011 published 24 march 2011 1 in 2006 2007 a doublet of great earthquakes mw 8 struck in the center of the kuril subduction zone a thrust event followed by an extensional event our observations of the kuril gps array in 2006 2009 outline a broad zone of postseismic deformation with initial horizontal velocities to 90 mm a and postseismic uplift we show that most of the postseismic signal after the great kuril doublet is caused by the viscoelastic relaxation of shear stresses in the weak asthenosphere with the best fitting maxwell viscosity in the range of 5 10 10 17 pa s an order of magnitude smaller than was estimated for several subduction zones we predict that the postseismic deformation will die out in about a decade after the earthquake doublet our results suggest large variations among subduction zones in the asthenospheric viscosity one of the most important rheological parameters citation kogan m g n f vasilenko d i frolov j t freymueller g m steblov b w levin and a s prytkov 2011 the mechanism of postseismic deformation triggered by the 2006 2007 great kuril earthquakes geophys res lett 38 l06304 doi 10 1029 2011gl046855

state of stress within a bending spherical shell and its implications for subducting lithosphere summary the state of stress within a bending spherical shell has some special features that are caused by sphericity while most lithospheres are more like spherical shells than flat plates our ideas of the state of stress have been dominated by flat plate models as a consequence we might be missing some important aspects of the state of stress within subducting lithospheres in order to examine this problem we analyse spherical shell bending problems from basic equations we present two approaches to solve sphericalshell bending problems one by the variational approach which is suitable for globalscale problems and the other by the asymptotic equation which is valid to first order in h r where h is the thickness of the lithosphere and r is its curvature radius i e under the assumption of small curvature the form of the equation for displacement shows that wavelengths of deformation are determined by the spherical elastic evect and the gravitational buoyancy evect for which only the latter evect is included in the usual flat plate formulations in the case of the earth the buoyancy force is dominant and consequently spherical evects are suppressed to a large extent this explains why flat plate models have been successful for earth s lithospheric problems on the other hand the state of stress shows interesting spherical evects while bending fibre stress along the subduction zone is always important bending stress along the trench strike direction can also be important in particular when the subduction zone arc is small numerical results also indicate that compressive normal stress along the trench strike direction is important when a subduction zone arc is large these two stresses the bending stress and the compressive normal stress both along the trench strike direction may have important implications for intraplate earthquakes at subduction zones

s and p velocity heterogeneities within the upper mantle below the baltic shield abstract upper mantle structure beneath the baltic fennoscandian shield is investigated using non linear tomographic inversion of relative arrival time residuals 52 selected teleseismic earthquakes recorded by 45 broadband stations of the swedish national seismological network snsn provide 1532 good quality s wave relative arrival times sv and sh arrival time residuals were initially analyzed independently providing two separate models these reveal several consistent major features many of which are also consistent with p wave results lateral velocity variations of 3 4 are observed to depths of at least 470 km the correlation between the sh and sv models is investigated and shows a pattern of minor but significant differences down to around 150 200 km depth below which the models are essentially similar direct cell by cell comparison of the model velocities reveals a similar pattern with velocity differences between the models of up to 4 numerical tests show that differences in the two s wave models can only be partially attributed to noise and limited resolution and some features are attributed to the effect of large scale anisotropy one of the significant and sharp discrepancies between the s models coincides with a presumed boundary between archean and proterozic domains suggesting different anisotropic characteristics in the two regions

comparison of marine gravity from shipboard and high density satellite altimetry along the mid atlantic ridge 30 5 35 5 s we compare new marine gravity fields derived from satellite altimetry with shipboard measurements over a region of more than 120 000 square kilometers in the central south atlantic newly declassified satellite data were employed to construct free air anomaly maps on 0 05 degree grids sandwell and smith 1992 marks et al 1993 an extensive gravity and bathymetry dataset from four cruises along the mid atlantic ridge from 30 5 35 5os provides a benchmark for testing the two dimensional resolution and accuracy of the satellite measurements where their crosstrack spacing is near their widest the satellite gravity signal is coherent with bathymetry in this region down to wavelengths of 26 km t2 0 5 compared to 12 5 km for shipboard gravity residuals between the shipboard and satellite datasets have a roughly normal distribution the standard deviation of satellite gravity with respect to shipboard measurements is nearly 7 mgal in a region of 140 mgal total variation whereas the internal standard deviation at crossovers for gps navigated shipboard data is 1 8 mgal the differences between shipboard and satellite data are too large to use satellite gravity to determine crustal thickness variations within a typical ridge segment

variations in subsidence rates along intermediate and fast spreading mid ocean ridges summary the nature of subsidence near the ridge crest of the intermediate and fast spreading mid ocean ridges of the indian and pacific oceans is investigated using surface ship bathymetry and magnetics profiles the ridge can be divided into discrete sections apparently bounded by distinct structural features such as major fracture zones in which bathymetry plotted against crustal age forms a well defined envelope with a width roughly the amplitude of the local bathymetry the averaged bathymetry in all of the regions studied follows closely a square root of age subsidence curve which in most regions has a subsidence coefficient c1 in the range of 340 390 m myr 1 2 the best fitting subsidence curve however never reproduces the amplitude of the axial topographic high the most notable region displaying unusual behaviour is the east pacific rise between 9 s and 22 s in this region the western flank of the ridge is subsiding at 200 225 m myr 1 2 while the eastern flank is subsiding at normal rates of 350 400 m myr 1 2 other anomalous areas include the region between the easter island hot spot and the chile rise triple junction in which the ridge crest is shallow and which is subsiding at rates of about 290 m myr 1 2 and the region east of the australia antarctic discordance in which the northern flank is subsiding at 440 m myr 1 2 this area may also be subsiding asymmetrically although there is not much data from the southern flank the asymmetric subsidence in the 9 s 22 s region of the east pacific rise begins immediately at the ridge crest and the low subsidence rates on the west flank continue to at least 12 myr old crust oligocene aged crust on the western flank is subsiding at more normal rates but is 500 m shallow with respect both to the world wide average and to the conjugate crust on the eastern flank the simplest model to explain these observations is that the western flank is underlain by a hotter mantle perhaps as the result of upwelling resulting from the large scale return circulation from the trenches depending on the depth of compensation the observed asymmetry could result from a lateral temperature gradient of 0 05 0 10 c km 1 and a total lateral temperature variation of under 100 c

anisotropic horizontal thermal contraction of young oceanic lithosphere inferred from stress release due to oceanic intraplate earthquakes how freely the oceanic lithosphere contracts horizontally due to thermal contraction is important information because it reflects the boundary condition of the oceanic lithosphere which includes information regarding the magnitude of driving resisting forces of plate tectonics we investigated the horizontal thermal contraction of young oceanic lithosphere using an analysis of the intraplate stress release due to oceanic intraplate earthquakes oceqs and numerical simulations the stress release due to oceqs in young oceanic lithosphere 5 15 ma shows significant differences between the spreading directional component and the ridge parallel component the extensional stress release of the ridge parallel component is six times as large as that of the spreading directional component while the compressional stress release of the ridge parallel component is one seventh that of the spreading directional component we conducted a numerical simulation of the thermal stress evolution of the oceanic lithosphere to investigate how the difference in the horizontal contraction rates between the spreading direction and the ridge parallel direction can explain the observed anisotropic stress release the result indicates that young oceanic lithosphere 5 15 ma barely contracts in the ridge parallel direction only 0 30 of the spreading directional contraction rate while it contracts freely in the spreading direction due to the weakness of the oceanic ridge strength and the low viscosity asthenosphere from the results we constrained the magnitude of the basal traction working on the bottom of the oceanic lithosphere to be smaller than 0 44 mpa

lithospheric 3 d flexure modelling of the oceanic plate seaward of the trench using variable elastic thickness summary whendescribingthemechanicalbehaviourofthelithospheremodelledasathinplate themost important parameter corresponds to itsflexural rigidity which is commonly expressed through the effective elastic thickness te this parameter is a measure of the stiffness of the plate and defines the maximum magnitude and wavelength of those surface loads that can be supported without suffering unelastic deformation realistic 3 d models of the flexural response of the lithosphere near the trench are scarce because of the mathematical and computational complexity we present a method for determining the flexure of the lithosphere caused by the combined effect of 3 d seamount loading and bending of the lithosphere near the trench our method consists on solving numerically the flexure equations of the reissner mindlin thin plate theory including variable thickness using the finite element method with mesh adaptation the method was applied to study the flexure of the oceanic nazca lithosphere beneath the o higgins seamount group which lies 70km seaward of the chile trench the results show that an elastic thickness te of 5km under the seamounts a te of 15km far from the trench and a te of 13km near the trench can explain both the down deflection of the oceanic moho and bending of the oceanic lithosphere observed in seismic and gravity profiles in order to study the impact of high trench curvature on the morphology of the outer rise we apply the same methodology to study and model the flexure of the lithosphere in the arica bend region 14 s 23 s results indicate that the te values are overestimated if the 3 d trench curvature is not included in the modelling

relating rapid plate motion variations to plate boundary forces in global coupled models of the mantle lithosphere system effects of topography and friction abstract recent high resolution models of past plate motions and their comparison with plate motion models inferred from space geodetic techniques reveal a number of short term variations in global plate velocities over the past 10 myrs such variations serve as powerful probe into the nature and magnitude of plate boundary forces because they are unlikely to originate from changes in mantle buoyancy forces which evolve on longer time scales here we explore the constraints of the velocity record using a novel coupled modeling approach of global neo tectonic simulations combined with realistic plate driving forces obtained from mantle circulation models mcms to arrive at simple global budgets of mantle lithosphere and plate boundary forces we focus on three plate boundary systems along the nazca south america plate margin the aleutian trench and the india australia plate boundary to show that gravitational spreading from high topography in the andes and tibet contributes substantially to the global plate tectonic force balance and that this contribution is sufficient to explain some 35 of recent velocity changes over the earth s surface including among others the observed 30 convergence reduction between the nazca south america plates our models make a number of specific predictions such as significant lateral variations in plate coupling forces along a given margin revealed by trench parallel gravity and bathymetry anomalies and the occurrence of large earthquakes as well as differences by as much as a factor of five from margin to margin they also support the notion of a relatively young plate boundary separating the india and australia plates which has been previously suggested based on independent observations importantly we find that the modeled nazca south america convergence reduction explains recent spreading rate variations in the south atlantic and south pacific which points to the importance of far field effects on the adjacent continents in explaining the spreading record of oceanic basins our numerical results demonstrate a that detailed budgets of forces acting upon plates can be obtained and b support the notion of strong forcing along weak plate boundaries

accurate measurements of residual topography from the oceanic realm in the oceans our understanding of plate subsidence as a function of age permits residual depth anomalies to be identified and mapped these anomalies may reflect dynamic topography and could be an important means for constraining convective circulation of the sublithospheric mantle here we analyze a global database of seismic reflection and wide angle profiles from heavily sedimented oceanic crust which abuts continental lithosphere at 449 locations we calculated water loaded subsidence compared it with a reference age depth relationship and determined residual depth we then combined these spot measurements of residual depth with observations from mid oceanic ridges and from selected ship track bathymetry to construct a global map of residual depth our results suggest that the amplitude of residual depth varies by up to 1 km with wavelengths of order 103 km we compare our residual depths with free air gravity and seismic tomographic anomalies our results show that residual depths correlate with long wavelength gravity anomalies in contrast correlations between residual depths and vertically averaged shear velocity anomalies within the upper and or the lower mantle are weaker the largest discrepancies occur at short 1000 km wavelengths these combined observations suggest that residual depth anomalies could be generate by density variations within a thin 102 km low viscosity layer beneath the lithosphere our global compilation should play a significant role in helping to refine predictive geodynamical models

characterization of soils beneath a posidonia oceanica meadow the study of a 475 cm core spanning 4316 y bp sampled in a posidonia oceanica meadow portlligat bay nw mediterranean allowed us to make the first detailed description of the soil below this endemic seagrass the sediments under p oceanica often referred to as mat or matte are low density average bulk density of 0 69 g dw cm 3 marine soils mainly composed of siliciclastic 46 and biogenic carbonated 46 fine grained sediments particles 1 mm and finer organic matter 3 of som 1 mm the mat is heterogeneous and has a high total organic matter tom content in the upper layers average of 32 in the top 50 cm after ca 500 years of burial after which the layers become increasingly dominated by inorganic fractions ranging from 20 to 1 of tom in 50 475 cm the tom content in the mat decreases exponentially at an overall rate of 0 0005 y 1 p oceanica sediments have been found to contain the highest areal stocks of tom and organic c 194 kg dw tom m 2 and 79 kg corg m 2 respectively out of all seagrasses the average corg refractory burial rates were estimated to be 21 g corg m 2 y 1 carbon as carbonates accreted at a rate of 54 g ccarb m 2 y 1 for the mat thickness studied the two c fractions yielded a total stock that is among the highest ever recorded in terrestrial and marine soils 282 kg ctot m 2 the mat was also found to trap large amounts of sediment rate 899 g dw m 2 y 1 stock 3342 kg dw m 2 and in particular muddy silt and clay fractions rate 120 g dw m 2 y 1 stock 417 kg dw m 2 the results obtained in this study provide sound additional proof of the valuable role p oceanica plays in stabilizing coastal sediments and sequestering c and also of its potential as a holocene palaeorecord the p oceanica mats located at shallow depths 2 m can be tentatively classified as a limnic subaquatic histosols calcaric eutric world reference base for soil resources 2007

two new rare high effusion outburst eruptions at rarog and heno paterae on io observations obtained with the near infrared camera nirc2 coupled to the adaptive optics system on the 10 m w m keck ii telescope on mauna kea hawaii on 15 august 2013 at 15 30 ut revealed two large outburst class volcanic eruptions on io follow up observations five days later showed that both eruptions had substantially faded the most energetic eruption was at rarog patera at a location near 305 w 42 s a smaller one occurred further south at 310 w and 57 s close to heno patera total radiant fluxes at rarog patera on august 15 were of order 500 gw sr m at wavelengths between 2 and 4 m and close to 250 gw sr m at 1 6 m indicative of an effective temperature of 1040 k over an area of 120 km 2 and a total thermal emission of nearly 8 tw at heno patera the 4 m flux measured 250 gw sr m and 90 gw sr m at 2 2 m suggestive of an effective temperature of 720 k over an area of more than 300 km 2 and a total thermal emission of 5 6 tw fits of the davies davies a g 1996 icarus 124 1 45 61 io flow model indicate that these two eruptions are vigorous and the exposed surfaces are mostly very young no older than 4 5 min at rarog patera and a few hours at heno patera the model fits suggest that in both locations lava fountaining is taking place a highly energetic style of volcanism using follow up observations taken between august 20 and september 7 we estimate peak effusion rates between 5 10 4 and 10 5 m 3 s

sediment retention by a mediterranean posidonia oceanica meadow the balance between deposition and resuspension the role of posidonia oceanica in promoting sediment stability and accretion was studied in a 15 m deep meadow at fanals point nw mediterranean spain by comparing particle deposition within the meadow and adjacent bare sediment small sediment traps were used to measure deposition within and above the meadow and over bare sand a model based on measurements of particle deposition at increasing distances from the bottom was used to partition the total depositional flux between primary sediment particles deposited for the first time at the measuring site and resuspended deposition sediment particles that have been previously deposited at the measuring site measurements were conducted monthly over a year to establish the magnitude and seasonality of deposition and to form a balance of particle transport at the annual time scale significant differences in total deposition were found over time ranging from 1 5 to 500 g dw m 2 d 1 including those between bare and vegetated sediments the effect of p oceanica in increasing primary deposition at an annual scale was modest however p oceanica significantly buffered sediment resuspension which was reduced more than three fold compared to the unvegetated bottom the annual flux of deposition was dominated by settling of resuspended materials which represented 85 of the total flux within the meadow but 95 of the total deposition on bare sand thus seagrass meadows reduce resuspension in the nw mediterranean littoral thereby contributing to increased sediment retention and therefore reducing erosion in the coastal zone

constraints on yield strength in the oceanic lithosphere derived from observations of flexure summary the wavelength and amplitude of outer rises seaward of sub duction zones and arches surrounding islands and seamounts are used to parameterize flexure profiles in terms of the moment and curvature at the first zero crossing the data show the clear age dependence in the mechanical thickness of the lithosphere up to 60 100myr saturation of moment at large curvature is interpreted in terms of a depth dependent yield strength for the lithosphere using relations adopted from laboratory experiments of rock deformation a comparison of theoretical curves with observed moments indicates that old oceanic lithosphere has no long term strength below about 40 km depth with no difference between 100 and 165 myr old crust moderate axial loading forces 200 mpa can explain most variations in the moment curvature observations except in the case of the kuril trench which appears anomalous given the age of the crust regional tension causes greater variability in moment as compared to regional compression because of the greater slope in the brittle failure envelope under tension the observations point to a lithosphere weaker than the prediction from experimental deformation of rocks of the possible weakening mechanisms elevated pore fluid pressure on faults does not predict the correct age dependence and is incompatible with earthquake focal mechanisms our favoured explanation is that the activation energy q appropriate for ductile flow at geological strain rates is lower than the values derived from laboratory extrapolations of dry olivine data taken at high temperatures if recent oceanic geotherms are reliable q in the lower lithosphere must be lower than 100kcal mol 1 the method used here is most appropriate for trench profiles with curvatures greater than 10 7m 1 for lower curvatures such as along seamount profiles small errors in the curvature estimate cause large changes in rheological parameters

analytical solutions for 3 d flexural deformation of semi infinite elastic plates summary the theory of elastic plate flexure has played a prominent role in isostatic studies and investigations of lithospheric deformation caused by vertical tectonism simple analytical asymmetrical solutions exist when the boundaries of the plate are far removed from the area where the load is applied when deformation is driven by end loads i e bending moments and shear forces the problem is usually simplified to yield a 2 d solution that approximates a cross section of the 3 d solution we present analytical green s functions for the point force response of semi infinite elastic plates resting on an inviscid substratum and being acted upon by constant in plane forces and arbitrary end loads this solution may be used to study deformation and stresses caused by loads close to a free geological boundary e g a trench or weak fracture zone we also present green s functions for the case when two semi infinite plates of different rigidities are mechanically coupled and subject to a point load this solution may be useful when studying deformation near mechanically strong fracture zones or at ocean continent transitions the analytical solutions provide simple alternatives to and calibration of complex numerical finite element or finite difference solutions and furthermore give additional physical insight into the geological process to be examined the analytical methods are used to demonstrate the different stress patterns that may arise near weak and strong geological boundaries we also illustrate how the observed scatter in 2 d determinations of elastic thickness at subduction zones may partly have its origin in 3 d effects

ecosystem engineering by annual intertidal seagrass beds sediment accretion and modification seagrasses are generally known as ecosystem engineers as they reduce flow velocities in their canopies in perennial subtidal meadows this usually leads to increased net sedimentation rates and reduction of the grain size the present study aims to describe the contribution of annual seagrass populations to these processes and elucidate the temporal dynamics sediment accretion and grain size modification were experimentally tested by transplanting seedlings of an annual intertidal eelgrass population to an unvegetated tidal flat within the planting units 79 shoots m 2 4 7 mm of sediment accreted whereas in the most dense parts of these units 199 shoots m 2 accretion amounted to 7 1 mm the silt fraction 63 m increased and the sand fraction 63 500 m decreased in the eelgrass beds which provides evidence that higher silt content in seagrass beds is the result and not the cause of seagrass presence annual intertidal eelgrass beds significantly contribute to the immobilisation of sediment during the growing season with its magnitude depending on canopy density during winter the accumulated sediments were released again and could even induce additional erosion possible consequences of these sediment dynamics for the larger scale functioning of estuarine ecosystems are discussed

lithospheric bending at subduction zones based on depth soundings and satellite gravity a global study of trench flexure was performed by simultaneously modeling 117 bathymetric profiles original depth soundings and satellite derived gravity profiles a thin elastic plate flexure model was fit to each bathymetry gravity profile by minimization of the l norm the six model parameters were regional depth regional gravity trench axis location flexural wavelength flexural amplitude and lithospheric density a regional tilt parameter was not required after correcting for age related trend using a new high resolution age map estimates of the density parameter confirm that most outer rises are uncompensated we find that flexural wavelength is not an accurate estimate of plate thickness because of the high curvatures observed at a majority of trenches as in previous studies we find that the gravity data favor a longer wavelength flexure than the bathymetry data a joint topography gravity modeling scheme and fit criteria are used to limit acceptable parameter values to models for which topography and gravity yield consistent results even after the elastic thicknesses are converted to mechanical thicknesses using the yield strength envelope model residual scatter obscures the systematic increase of mechanical thickness with age perhaps this reflects the combination of uncertainties inherent in estimating flexural wavelength such as extreme inelastic bending and accumulated thermoelastic stress the bending moment needed to support the trench and outer rise topography increases by a factor of 10 as lithospheric age increases from 20 to 150 ma this reflects the increase in saturation bending moment that the lithosphere can maintain using a stiff dry olivine rheology we find that the lithosphere of the gdh1 thermal model stein and stein 1992 is too hot and thin to maintain the observed bending moments moreover the regional depth seaward of the oldest trenches 150 ma exceeds the gdh1 model depths by about 400 m

the accuracy of some lithospheric bending parameters summary a procedure is described for iterative least squares inference of bending parameters from lithospheric deflection profiles the method is applied to several bathymetric profiles seaward of subduction zones and used to study the accuracy with which these parameters and related ones such as plate thickness and moment curvature relations can be determined the presence of typical bathymetric undulations particularly at wavelengths of the order of the flexural wavelength and greater prevents the inference of effective elastic thickness from single profile data to better than 12 km 2 mixing of elastic and inelastic model estimates of effective thickness further increases scatter it is found that the sensitivity of inferred bending moments on the outer trench wall to typical uncertainties of regional bathymetric slope is substantial and inferred estimates of inelastic behaviour need to be interpreted with caution

sequence stratigraphy structure and tectonic history of the southwestern ontong java plateau adjacent to the north solomon trench and solomon islands arc the ontong java plateau ojp is the largest and thickest oceanic plateau on earth and one of the few oceanic plateaus actively converging on an island arc we present velocity determinations and geologic interpretation of 2000 km of two dimensional multi channel seismic data from the southwestern ontong java plateau north solomon trench and northern solomon islands we recognize three megasequences ranging in age from early cretaceous to quaternary on the basis of distinct interval velocities and seismic stratigraphic facies megasequence oj1 is early cretaceous upper igneous crust of the ojp and correlates with basalt outcrops dated at 122 125 ma on the island of malaita the top of the overlying megasequence oj2 a late cretaceous mudstone unit had been identified by previous workers as the top of igneous basement seismic facies and correlation to distant deep sea drilling project ocean drilling program sites indicate that oj2 was deposited in a moderately low energy marine environment near a fluctuating carbonate compensation depth that resulted in multiple periods of dissolution oj2 thins south of the stewart arch onto the solomon islands where it is correlated with the kwaraae mudstone formation megasequence oj3 is late cretaceous through quaternary pelagic cover which caps the ontong java plateau it thickens into the north solomon trench and seismic facies suggest that oj3 was deposited in a low energy marine environment we use seismic facies analysis sediment thickness structural observations and quantitative plate reconstructions of the position of the ojp and solomon islands to propose a tectonic magmatic and sedimentary history of the southwestern ontong java plateau prior to 125 ma late jurassic and early cretaceous oceanic crust formed from 125 to 122 ma the first mantle plume formed igneous crust oj1 between 122 and 92 ma marine mudstone oj2 and kwaraae mudstone of malaita solomon islands was deposited on ontong java plateau at 92 ma a second mantle plume caused widespread volcanism on the plateau from 92 to 15 ma pelagic carbonate sediment oj3 was deposited at 15 ma the southern ontong java plateau was deformed by normal faults during its approach toward the north solomon trench finally from 4 to 0 ma the malaita accretionary prism formed during collision between a substantially thicker portion of the ontong java plateau and the solomon islands arc flexure of the ontong java plateau near the trench caused coeval normal faulting

an iterative spectral solution method for thin elastic plate flexure with variable rigidity s u m m a r y thin plate flexure theory provides an accurate model for the response of the lithosphere to vertical loads on horizontal length scales ranging from tens to hundreds of kilometres examples include flexure at seamounts fracture zones sedimentary basins and subduction zones when applying this theory to real world situations most studies assume a locally uniform plate thickness to enable simple fourier transform solutions however in cases where the amplitude of the flexure is prominent such as subduction zones or there are rapid variations in seafloor age such as fracture zones these models are inadequate here we present a computationally efficient algorithm for solving the thin plate flexure equation for non uniform plate thickness and arbitrary vertical load the iterative scheme takes advantage of the 2 d fast fourier transform to perform calculations in both the spatial and spectral domains resulting in an accurate and computationally efficient solution we illustrate the accuracy of the method through comparisons with known analytic solutions finally we present results from three simple models demonstrating the differences in trench outer rise flexure when 2 d variations in plate rigidity and applied bending moment are taken into account although we focus our analysis on ocean trench flexure the method is applicable to other 2 d flexure problems having spatial rigidity variations such as seamount loading of a thermally eroded lithosphere or flexure across the continental oceanic crust boundary

sediment deposition and production in se asia seagrass meadows abstract seagrass meadows play an important role in the trapping and binding of particles in coastal sediments yet seagrass may also contribute to sediment production directly through the deposition of detritus and also the deposition of the associated mineral particles this study aims at estimating the contribution of different seagrass species growing across an extensive range of deposition to inorganic carbonate and non carbonate and organic sediment production total daily deposition measured with sediment traps varied from 18 8 2 0 g dw m 2 d 1 in silaqui philippines to 681 1 102 g dw m 2 d 1 in bay tien vietnam these measurements correspond to a single sampling event and represent sedimentation conditions during the dry season in se asia coastal areas enhalus acoroides was the most common species in the seagrass meadows visited and together with thalassia hemprichii was present at sites from low to very high deposition halodule uninervis and cymodocea species were present in sites from low to medium deposition the mineral load in seagrass leaves increased with age and was high in e acoroides because it had the largest and long lived leaves up to 417 mg calcium carbonate per leaf and 507 mg non carbonate minerals per leaf and low in h uninervis with short lived leaves 4 mg calcium carbonate per leaf and 2 mg non carbonate minerals per leaf in se asia seagrass meadows non carbonate minerals accumulate at slower rates than the production of calcium carbonate by the epiphytic community consequently the final loads supported by fully grown leaves were as average lower than calcium carbonate loads our results show that organic and inorganic production of the seagrasses in se asia represents a small contribution maximum of 15 of the materials sedimented on a daily base by the water column during the sampling period the contribution of the carbonate fraction can be locally significant i e 34 in silaqui in areas where the depositional flux is low but is minor

marine geodynamics depth age revisited more than 20 years have passed since the subsidence of the seafloor away from midocean ridges was explained by the cooling of the lithosphere figure 1 a thermal boundary layer at the earth s surface turcotte and oxburgh 1967 although the general concept has proven quite sound and found much practical application the details of the model have never been completely agreed upon for example if no process limits the growth of the thermal boundary layer the seafloor should continue to deepen as the square root of lithospheric age in contrast the observations indicate that little deepening occurs for lithosphere aged greater than about 100 ma ma million years before present early on this fact was explained by supposing that there was a limit to the depth of cooling of the boundary layer such that the lithosphere cools as a plate of finite thickness rather than as a semi infinite half space mckenzie 1967 according to the plate model the depth of old lithosphere approaches a maximum as the geothermal gradient in the plate reaches thermal equilibrium however this theoretical model merely provided a better fit to the observations without addressing the fundamental question of what process might limit the growth of the earth s uppermost thermal boundary layer

broadscale postseismic gravity change following the 2011 tohoku oki earthquake and implication for deformation by viscoelastic relaxation and afterslip the analysis of grace gravity data revealed postseismic gravity increase by 6 gal over a 500 km scale within a couple of years after the 2011 tohoku oki earthquake which is nearly 40 50 of the coseismic gravity change it originates mostly from changes in the isotropic component corresponding to the mrr moment tensor element the exponential decay with rapid change in a year and gradual change afterward is a characteristic temporal pattern both viscoelastic relaxation and afterslip models produce reasonable agreement with the grace free air gravity observation while their bouguer gravity patterns and seafloor vertical deformations are distinctly different the postseismic gravity variation is best modeled by the biviscous relaxation with a transient and steady state viscosity of 1018 and 1019 pa s respectively for the asthenosphere our calculated higher resolution viscoelastic relaxation model underlying the partially ruptured elastic lithosphere yields the localized postseismic subsidence above the hypocenter reported from the gps acoustic seafloor surveying

seismicity in the outer rise forearc region and configuration of the subducting lithosphere with special reference to the japan trench along nine subduction zones where the largest mw or ms 7 6 normal faulting earthquakes have occurred beneath the outer rise there is an apparent correlation between a large curvature of the subducting slab and regions of low background seismicity in the forearc in six cases a gentle inner trench is also observed taking advantage of the excellent coverage of teleseismic stations and extensive previous studies using local data we investigate the nature of this spatial correlation between seismicity and bathymetry along the honshu forearc in some detail an aseismic region in the forearc lies directly west of the rupture zone of the 1933 sanriku earthquake mw 8 4 a great normal faulting earthquake that most likely has ruptured the entire brittle portion of the lithosphere in the outer rise along this particular cross section of the japan trench large to moderate sized interplate earthquakes span a short interval of depths between approximately 14 and 31 km about half the interval observed along an adjacent profile to the south 12 45 km the aseismic portion of the plate interface between depths of approximately 30 and 45 km has persisted for over 100 years the occurrence of lithospheric normal faulting earthquakes in the outer rise region indicates that bending moment within the subducting slab limited by yield strength of the lithosphere has nearly reached an asymptotic value moment saturation at this state the curvature of the plate can increase with little increase in bending moment as such any stresses transmitted across the plate interface will be relaxed by a permanent increase in the plate s curvature and little elastic strain can accumulate across the interplate thrust zone to produce large earthquakes thus the saturation of bending moment in the subducting lithosphere offers a straightforward interpretation for the observed correlation between seismicity in the outer rise forearc region and configuration of the subducting lithosphere

observations of flexure and the rheology of oceanic lithosphere the principal evidence for the long term i e 1 myr mechanical behaviour of the oceanic lithosphere has come from studies of how it deforms in response to large loads such as volcanoes and sediments a model widely used to explain the deformation is an elastic plate in which flexural rigidity depends on the thermal age of the lithosphere at the time of loading an elastic model however is time invariant and does not take into account temporal changes that may occur in the flexural rigidity as a consequence of loading there is evidence for example that the flexural rigidity of the oceanic lithosphere also depends on load age being large at short times and small at long times thus competing effects may exist between thermal cooling which strengthens the lithosphere and some form of load induced stress relaxation which weakens it in order to investigate the relative roles of these processes we have developed a multilayered viscoelastic model which is based on the results of experimental rock mechanics that creep in the lithosphere is a thermally activated process and on a thermal structure that is given by the plate cooling model by comparing the predictions of the model with a new compilation of flexural rigidity estimates we have found that if the upper mantle viscosity is 1020 pa s the activation energy that best describes the long term mechanical behaviour of the oceanic lithosphere is 120 kj mol 1 this parameter pair explains the dependence of flexural rigidity on both plate and load age it also helps account for the subsidence and uplift history of oceanic islands and the stratigraphic patterns that develop in the flexural moats that flank them at atolls a multilayered viscoelastic model explains the rapid subsidence that follows shield building and does not require that the age of the lithosphere that supports a volcano is thermally re set to a younger value our studies suggest that while the oceanic lithosphere has viscoelastic properties the viscosity of its upper layers is so much higher than that of its lower layers that in effect it behaves as a thin elastic plate on long timescales there will therefore be a certain permanence to the observations of oceanic flexure such that they may be used with some confidence to evaluate the tectonic setting of individual features of the seafloor and in some cases their age

constraints on subducting plate strength within the kermadec trench four specially designed surveys parallel to the kermadec trench allow localized estimates of plate strength within the subducting pacific plate to be made the transfer function between topography and gravity is estimated for five trench parallel ship tracks at distances of 25 110 km from the trench axis we find a clear reduction in the magnitude and peak wavelength of the transfer function from the outer rise to the trench axis the change in the transfer function indicates a decrease in plate strength and is consistent with a reduction in flexural rigidity by 3 5 orders of magnitude or a decrease in effective elastic thickness by more than 15 km such a large magnitude decrease in the effective elastic strength suggests that the plate has little or no elastic strength within the trench and that viscous stresses play an important role in transferring slab pull forces to the subducting plate and regulating plate speeds in subduction zones

observations and temperatures of io s pele patera from cassini and galileo spacecraft images abstract pele has been the most intense high temperature hotspot on io to be continuously active during the galileo monitoring from 1996 2001 a suite of characteristics suggests that pele is an active lava lake inside a volcanic depression in 2000 2001 pele was observed by two spacecraft cassini and galileo the cassini observations revealed that pele is variable in activity over timescales of minutes typical of active lava lakes in hawaii and ethiopia these observations also revealed that the short wavelength thermal emission from pele decreases with rotation of io by a factor significantly greater than the cosine of the emission angle and that the color temperature becomes more variable and hotter at high emission angles this behavior suggests that a significant portion of the visible thermal emission from pele comes from lava fountains within a topographically confined lava body high spatial resolution nightside images from a galileo flyby in october 2001 revealed a large relatively cool

paleoshoreline record of relative holocene sea levels on pacific islands abstract understanding the history of relative holocene sea levels on pacific islands is important for constraining fundamental geodynamic theories interpreting the environments of early human occupation sites and forecasting future environmental conditions on the islands an observational paleoshoreline record is provided by emergent paleoshoreline indicators formed at higher relative sea levels hence standing at higher elevations than modern counterparts emergent paleoshoreline notches in limestone seacliffs record paleo high tide levels and emergent paleoreef flats record paleo low tide levels whereas emergent paleobeachrock locally records paleo intertidal levels both paleonotches and paleoreefs occur along the coasts of high standing islands exposing volcanic bedrock and uplifted reef complexes but low lying coralline atolls lack sufficient relief to preserve paleonotches controls on relative holocene sea level include global eustatic and regional hydro isostatic changes in ambient sea level relative to island landmasses and shifts in the elevations of islands relative to sea level caused by thermal subsidence of the oceanic lithosphere or thermally rejuvenated loci of hotspot volcanism by flexure of the lithosphere under the load of growing volcanic edifices hawaii samoa society islands by arching of the lithosphere over trench forebulges loyalty islands niue bellona rennell and by tectonism within forearc belts between active volcanic chains and trenches mariana islands tonga vanuatu the dominant pattern of relative sea level change where not overprinted by local tectonism or lithospheric flexure was a uniform early holocene rise in eustatic sea level followed by a regionally variable late holocene hydro isostatic drawdown in sea level the resultant was a mid holocene highstand in relative sea level that affected the development of shoreline morphology throughout the tropical pacific ocean the earliest human migrations into intra oceanic island groups of both the northwestern and southwestern pacific ocean followed close on the heels of the mid holocene sea level highstand and took advantage of newly attractive coastal environments engendered by sea level drawdown the effects of the mid holocene highstand were modified to varying degrees in different island groups by geodynamic uplift or subsidence

40ar 39ar geochronology and the paleoposition of christmas island australia northeast indian ocean abstract the christmas island seamount province is an extensive zone of volcanism in the northeast indian ocean consisting of numerous submerged seamounts and flat topped guyots within this region lies two subaerial island groups christmas island and the cocos keeling archipelago christmas island has experienced multiple episodes of volcanism that are exposed sporadically along its coastline here we dated these volcanics using 40ar 39ar geochronology and analysed them for paleomagnetism the oldest exposed volcanism occurred in the eocene between 43 and 37 ma this is followed by a time gap of 33 million years before the eruption of a younger episode of pliocene age 4 32 0 17 ma it has however been suggested by previous workers that there is a much older late cretaceous event beneath the limestone which is unexposed in addition this study conducted the first paleomagnetic analysis of samples from christmas island to determine its paleoposition and the paleomagnetic polarity of the sampled sites two normal and two reversal magnetic events have been recorded that agree with the geomagnetic reversal timescales late eocene 38 39 ma palaeomagnetic data suggest a palaeolatitude of 43 5 11 2 9 0 which is further south than palaeolatitudes around 30 s derived from existing plate reconstruction models for the australian plate however the late eocene palaeomagnetic data are limited only two sites and secular variation may not have been averaged out during the pliocene ca 4 ma we estimate a palaeolatitude of approximately 13 s the presence of the late eocene ages at christmas island correlates well with the cessation of spreading of the wharton ridge 43 ma the initiation of spreading along the south east indian ridge and the transit of christmas island over a broad low velocity zone in the upper mantle this suggests that changes in stress regimes following the tectonic reorganisation of the region prior to 43 ma may have allowed deeper origin mantle melts to rise similarly changes in the plate s stress regime at the flexural bulge of the sunda java subduction zone may be implicated in renewed melting at 4 ma suggesting that tectonic stresses have exerted a first order effect on the timing and emplacement of volcanism at christmas island

sedimentology and budget of a recent carbonate mound florida keys the sedimentology of a recent carbonate mound is investigated to further our understanding of mound building communities surface and subsurface mound sediments and the overall sediment budget of mounds nine sedimentary facies of the surface of tavernier mound florida keys are described these sediments are composed of neogoniolithon halimeda porites mollusc and foraminiferal grains and lime mud muds rich in aragonite and high magnesian calcite show little mineralogical variation over the mound surface geochemical evidence suggests that the mud is mainly formed from breakdown of codiacean algae and thalassia blade epibionts production rates of the facies are established from in situ growth rate experiments and standing crop surveys annual calcium carbonate production is c 500gm 2 intermediate between reef and other bay and lagoonal environment production rates in the caribbean the internal structure of the mound studied from piston cores and sediment probes indicates that seven facies can be identified five of these can be related to the present day facies and occur in the upper part of the mound gravel mound stage the remaining two facies characterized by molluscs and aragonite rich muds occur in the lower part of the mound mud mound stage and are most similar to facies from typical florida bay mud mounds mangrove peats within the mound indicate former intertidal areas and c14 dates from these peats provide a time framework for mound sedimentation the mound appears to have formed because of an initial valley in the pleistocene surface which accumulated mud in a shallow embayment during the holocene transgression a sediment budget for the mound is presented which compares production rates from present day facies with subsurface sediment masses during the mud mound stage production rates were similar to accumulation rates and the mound was similar to the present day mounds of florida bay during the gravel mound stage 3400 yr bp present day conditions were more normal marine and the establishment of porites and neogoniolithon on the mound increased production rates 10 over accumulation rates this excess sediment is thought to be transported off the mound to the surrounding seabed models are proposed which divide carbonate mounds on the basis of internal versus external sediment supply comparisons are made with other recent and ancient mounds similarities exist between the roles of the biotic components of late palaeozoic mounds but major differences are found when structures and early diagenesis are compared

variations in oceanic plate bending along the mariana trench we quantify along trench variations in plate flexural bending along the mariana trench in the western pacific ocean a 3 d interpreted flexural deformation surface of the subducting pacific plate was obtained by removing from the observed bathymetry the effects of sediment loading isostatically compensated topography based on gravity modeling age related lithospheric thermal subsidence and residual short wavelength features we analyzed flexural bending of 75 across trench profile sections and calculated five best fitting tectonic and plate parameters that control the flexural bending results of analysis revealed significant along trench variations the trench relief varies from 0 9 to 5 7 km trench axis vertical loading v 0 from 0 73 x 10 12 to 3 17 x 10 12 n m and axial bending moment m 0 from 0 1 x 10 17 to 2 7 x 10 17 n the effective elastic plate thickness seaward of the outer rise region t e m ranges from 45 to 52 km while that trench ward of the outer rise t e m ranges from 19 to 40 km this corresponds to a reduction in t e of 21 61 the transition from t e m to t e m occurs at a breaking distance of 60 125 km from the trench axis which is near the outer rise and corresponds to the onset of observed pervasive normal faults the challenger deep area is associated with the greatest trench relief and axial vertical loading while areas with seamounts at the trench axis are often associated with more subtle trench relief smaller axial vertical loading and greater topographic bulge at the outer rise c 2014 elsevier b v all rights reserved

ontong java plateau and late neogene changes in pacific plate motion aided by a new geometrical technique called hot spotting we recently proposed a new model for the absolute motion of the pacific plate this model called wk97 assumed the hawaiian hotspot is under loihi and that there was a change in plate motion at 3 6 ma the model located the louisville hotspot close to the hollister ridge a shallow volcanic ridge associated with a geoid high seismicity and recent volcanism most of the tectonic implications of wk97 however were not addressed furthermore although several recent studies suggest that the geochemistry of hollister lavas may indicate a mixture between louisville melt material and pacific antarctic ridge mid ocean ridge basalt these studies strongly disagree with wk97 and consider it seriously flawed here we discuss numerous tectonic predictions of wk97 and present a large amount of observational evidence in support of it we find that a late neogene collision between the ontong java plateau and the northern margin of the australia plate appears to have altered the motion of the pacific plate as inferred from hotspot volcanism by forcing it to rotate counter clockwise this rotation about a low latitude pole appears to have induced right lateral shear stress along the entire pacific plate divergent boundary which resulted in the formation of extensional transform faults microplates and propagating ridges this change in motion also appears to have triggered concomitant circum pacific tectonism including trench migration and back arc rifting the wk97 suggests that long lived plume fed volcanism on the pacific plate appears very limited perhaps only to five to six hotspots hawaii louisville cobb caroline and perhaps marquesas and bowie the geometry of these chains are consistent with our model despite some evidence for volcanic age progressions most other pacific neogene hotspots are most likely crack spots i e sites of extensional volcanism at preexisting zones of weakness reactivated by plate stresses indeed several volcanic ridges such as hollister ridge near the louisville hotspot appear to have formed in response to stresses following the change in absolute plate motion wk97 also obviates the need for simultaneous different directions of mantle flow in the south central pacific transpression at the san andreas and alpine strike slip faults and aleutian arc explosivity since 6 ma also appear to be consequences of the changes in pacific plate motion furthermore both the wk97 model and observed relative plate motions suggest that adjoining plates have changed their absolute motions as well and that large changes in absolute plate motion do not necessarily imply large changes in relative plate motion our findings support slab pull as the dominant plate tectonic driving force highlight the rheological difference between strong oceanic lithosphere and weak continental crust and suggest that wk97 presently provides a unifying albeit preliminary framework for relating both intraplate and circum pacific tectonism and volcanism in the late neogene

flexure of subducted slabs the subducted lithosphere is regarded as a thin elastic plate that bends as a consequence of slab pull the pressure of the asthenospheric flow induced by the subduction motion and the pressure exerted by the asthenospheric motion relative to the lithosphere in westward subductions the latter factor enhances the slab pull but in eastward subductions it opposes it as a result the subduction angle changes continuously with depth following an elastic profile it is smaller in eastward subductions and larger in those having a westward direction the application of the model to 13 subducted slabs shows a good fit between the observed and the calculated shapes of the slabs

reappraising elastic thickness variation at oceanic trenches we reassess the variation of elastic thickness as a function of lithospheric plate age using a global database of bathymetric and free air gravity profiles which are perpendicular to oceanic trenches as in many previous studies our starting point is the well known floating elastic plate model in order to remove the influence of short wavelength features not associated with lithospheric bending adjacent profiles from 10 myr bins have been stacked together to construct average profiles with standard deviations each average profile was then inverted in a two stage procedure first singular value decomposition was used to determine two unknown flexural parameters together with a regional slope and offset for any given elastic thickness this procedure was repeated for a range of elastic thicknesses second residual misfit was plotted as a function of elastic thickness and the global minimum was identified this two stage procedure makes no prior assumptions about magnitude of the load size of the bending moment or whether the elastic plate is broken continuous we obtained excellent fits between theory and observation for both bathymetric and gravity profiles from lithosphere with an age range of 0 150 ma the shape of the residual misfit function indicates the degree of confidence we have in our elastic thickness estimates the lower limit of elastic thickness is usually well determined but upper limits are often poorly constrained inverse modeling was carried out using a range of profile lengths 250 300 500 and 700 km in general our estimates show no consistent increase of elastic thickness as a function of plate age this surprising result is consistent with recent reassessments of elastic thickness beneath seamounts and implies either that elastic thickness is independent of plate age or that elastic thickness cannot be measured with sufficient accuracy to reveal such a relationship modeling of short free air gravity profiles 250 300 km does tentatively suggest that elastic thickness increases linearly from 5 to 10 km between 0 and 20 ma and from 10 to 15 km between 20 and 150 ma this variation roughly matches the depth to the 200 c isotherm which corresponds to an homologous temperature of 0 4 for wet peridotite unfortunately for longer profile lengths there is no temporal dependence and elastic thicknesses vary considerably for all plate ages bathymetric profile modeling yields similar results although uncertainties are larger

lithospheric rheology and flexure at artemis chasma venus artemis chasma is an arcuate 2000 km diameter zone of convergence and lithospheric underthrusting on venus inelastic flexure modeling of the topography at artemis combined with observations of trench tectonics allow us to document local temperature gradients below 4 k km 1 and an immense compressive in plane force at the trench lithospheric rheology on venus may be stronger than would be predicted from conventional extrapolations of rock mechanics experiments in particular the brittle surface strength must reach a few tens of megapascals to satisfy the observed lack of flexurally induced surface faulting elastic plate bending models provide an adequate estimate of the bending moment at artemis but they fail to constrain or even recognize the in plane force the inelastic analysis implies an in plane force of the order of 1 1014 n m 1 a potential driving force is thermal thinning of regionally thick lithosphre in the highlands to the north the low heat flux at artemis which is a comparatively young structure is compatible with the notion that venus has experienced a prolonged period of cooling in the last several hundred million years the inference of such exceptionally low thermal gradients embraces three end member possibilities 1 the surface age is 600 ma 2 the lithospheric thermal age is greater than the surface age and or 3 the upper mantle temperature is anomalously low 1550 k

an explanation for the age independence of oceanic elastic thickness estimates from flexural profiles at subduction zones and implications for continental rheology most properties of oceanic lithosphere are widely observed to be dependent on the age of the plate such as water depth heat flow and seismogenic thickness however estimates of the effective elastic thickness of oceanic lithosphere based on the deflection of the plate as it enters a subduction zone show little correlation with the age of the incoming lithosphere this paradox requires reconciliation if we are to gain a full understanding of the structure rheology and behaviour of oceanic lithosphere here we show that the permanent deformation of the plate due to outer rise faulting combined with uncertainties in the yield stress of the lithosphere the in plane forces transmitted through subduction zones and the levels of noise in bathymetric and gravity data prevents simple elastic plate modelling from accurately capturing the underlying rheological structure of the incoming plate the age independent estimates of effective elastic thickness obtained by purely elastic plate modelling are therefore not likely to represent the true rheology of the plate and hence are not expected to correspond to the plate age similar effects may apply to estimates of elastic thickness from continental forelands with implications for our understanding of continental rheology

lithospheric flexure and thermal anomalies plots of effective elastic thickness te of the oceanic lithosphere as a function of age when elastically deformed show a general increase in te with the square root of age although there is much scatter some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks which makes te for more sharply bent plates underestimate the true depth tm to the rheological boundary at the base of the high strength mechanical lithosphere after converting effective values te to true thicknesses tm it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones from several other lines of evidence such as depth anomalies seamount subsidence and heat flow data it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere the reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature this dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere since the mechanical plate thickness depends principally upon the temperature at one depth specifically the depth at which the temperature reaches 550 600 c depending on strain rate flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating compared to quantities dependent on depth integrals of temperature such as swell height and subsidence the thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first 10 ma after loading if initially reheating is confined to depths close to and below the 550 c isotherm the flexural data combined with depth anomaly information for hawaii point to extremely rapid reheating localized in the lower lithosphere rejuvenation in the society islands on the other hand is compatible with slower methods of heat injection involving the entire lithospheric column

source mechanisms of earthquakes near mid ocean ridges from body waveform inversion implications for the early evolution of oceanic lithosphere to investigate the early tectonic evolution of oceanic lithosphere we present a synthesis of the characteristics of near ridge earthquakes i e earthquakes which occur off the axis of major mid ocean spreading centers but in lithosphere less than 35 m y old near ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems with the exception of the central indian ocean and the eastern part of the cocos plate which have experienced a high rate of earthquake activity during that time for 32 near ridge earthquakes we determined the double couple orientation seismic moment centroid depth and source time function using a formal inversion technique based on matching synthetic and observed p and sh waveforms all types of faulting styles are observed seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m y and occurs almost entirely in the uppermost mantle from just below the moho to depths at which the temperature estimated from standard thermal models exceeds 800 c thrust faulting is not observed at depths greater than 10 km below the seafloor the deepest near ridge earthquakes are generally characterized by normal faulting focal mechanisms with the t axis oblique or perpendicular to the local spreading direction most of these earthquakes occur in the central indian ocean the transition from mechanisms characteristic of ridge axes to those characteristic of the near ridge environment occurs within the first few million years for lithosphere less than 35 m y old horizontal compressive stress associated with the cooling and subsidence of the lithosphere ridge push does not appear to be the dominant source of stress released in earthquakes this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere this bound together with the observed depth of faulting implies that the strain rate associated with seismic deformation is locally at least 10 15 s 1 we propose that thermoelastic stresses associated with plate cooling play an important role in near ridge earthquakes this source of stress can account for the mechanisms and depths of many near ridge earthquakes and for the concentration of activity in very young lithosphere the comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis the higher level of near ridge seismicity in the indian ocean may be the result of modification or enhancement of the more typical near ridge stress state by such local processes as the continental collision between india and asia or active secondary convection beneath the young lithosphere of the indian plate

bending of spherical lithosphere axisymmetric case summary effects of sphericity are commonly ignored in the lithospheric bending problem in order to examine its effects i solve a simple axisymmetric spherical shell model the full solution and the asymptotic solution are derived from the basic equations and their relationship to the flat plate solution is examined for displacement effects of sphericity are small and use of the flat plate solution produces results that are numerically indistinguishable from those of the spherical solution the most significant effect of sphericity appears in the stress in particular the normal stress along the strike direction of the trench this stress is approximately given by eu r where e is young s modulus u is the vertical deformation of the shell and r is its radius of curvature if the shell lithosphere is bent downwards and reaches 30 km this stress can become about 5 kbar in the earth while plastic behaviour may set in under such high pressure conditions and analysis beyond elasticity theory may be required sphericity may be a cause of large compressive stress in the trench strike direction this stress may play an important role in forming the overall shape of the earth s subduction zones

the relationship between plate curvature and elastic plate thickness a study of the peru chile trench the age of the nazca plate where it enters the peru and northern chile trenches varies from 30 ma in the north to 45 ma in the south as its dip beneath the south american continent steepens from 13 to 30 if the elastic thickness te of oceanic lithosphere depends only on its age and therefore thermal state we would expect that te determined from fitting the flexure of the lithosphere over the outer rise as revealed in the depth geoid and gravity anomalies would increase from the peru trench in the north to the northern chile trench further south we find that the opposite is true the lithosphere appears suffer outboard of the peru trench than it does offshore chile to explain deflections of the lithosphere seaward of peru the isotherm controlling the elastic ductile transition must be near 800 c providing the thermal structure of the plate is that predicted by the standard thermal plate model because the decrease in plate stiffness to the south is correlated with an increase of plate curvature over the outer rise and outer trench wall we interpret our result in terms of inelastic yielding of the oceanic lithosphere when bent to high strains the magnitude of the reduction in strength however suggests that the standard oceanic yield strength envelope underestimates the amount of failure for lithosphere bent to high strains unless the stress field off northern chile is characterized by 100 mpa level tension the more highly bent segment of subducting lithosphere at this trench also dips at a steeper angle at greater depth beneath the continent but detailed analysis of plate geometry does not support a relationship between slab dip as determined by earthquake hypocenters and elastic behavior over the outer rise

geographic variability in organic carbon stock and accumulation rate in sediments of east and southeast asian seagrass meadows organic carbon oc stored in the sediments of seagrass meadows has been considered a globally significant oc reservoir however the sparsity and regional bias of studies on long term oc accumulation in coastal sediments have limited reliable estimation of the capacity of seagrass meadows as a global oc sink we evaluated the amount and accumulation rate of oc in sediment of seagrass meadows and adjacent areas in east and southeast asia in temperate sites the average oc concentration in the top 30 cm of sediment was higher in seagrass meadows 780 1080 mol g 1 than in sediments without seagrass cover 52 430 mol g 1 the average oc in the top 30 cm of subtropical and tropical seagrass meadow sediments ranged from 140 to 440 mol g 1 carbon isotope mass balancing suggested that the contribution of seagrass derived carbon to oc stored in sediments was often relatively minor temperate 10 40 subtropical 35 82 tropical 4 34 and correlated to the habitat type being particularly low in estuarine habitats stock of oc in the top meter of sediment of all the studied meadows ranged from 38 to 120 mg ha 1 the sediment accumulation rates were estimated by radiocarbon dating of six selected cores 0 32 1 34 mm yr 1 the long term oc accumulation rates calculated from the sediment accumulation rate and the top 30 cm average oc concentration for the seagrass meadows 24 101 kg ha 1 yr 1 were considerably lower than the oc accumulation rates previously reported for mediterranean posidonia oceanica meadows 580 kg ha 1 yr 1 on average current estimates for the global carbon sink capacity of seagrass meadows which rely largely on mediterranean studies may be considerable overestimations

passive and active seismological study of bending related faulting and mantle serpentinization at the middle america trench water transported within the subducting oceanic lithosphere into the earth s interior affects a wealth of subduction zone processes including intraslab earthquakes and arc magmatism in recent years growing evidence suggests that much of the hydration of oceanic plates occurs at the trench ocean slope right before subduction here normal faults are created while the rigid lithosphere bends into the trench offshore of middle america multi channel seismic reflection imaging suggests that bending related faults cut into the uppermost mantle providing a mechanism for hydration and transformation of mantle peridotites into serpentinites seismic wide angle reflection and refraction data were collected coincident with one of the seismic profiles where the faults have been imaged travel time inversion provides evidence that both crustal and uppermost mantle velocities are reduced with respect to the velocity structure found in mature oceanic crust away from deep sea trenches if mantle velocity reduction is solely produced by hydration velocities indicate 10 15 of serpentinization in the uppermost 3 km of the mantle where seismic data provide enough resolution a small network of ocean bottom hydrophones deployed for about a month detected 3 local micro earthquakes per day earthquake epicentres align with fault scarps at the seafloor and continuous earthquake activity might be an important process to facilitate the percolation of seawater into the upper mantle

rapid postseismic relaxation after the great 2006 2007 kuril earthquakes from gps observations in 2007 2011 1 the 2006 2007 doublet of mw 8 earthquakes in the kuril subduction zone caused postseismic transient motion in the asthenosphere which we observed on the kuril gps array in 2007 2011 here we show that the maxwell asthenospheric viscosity that best fits the geodetic data increased by nearly an order of magnitude over the interval of 4 years from 2 1017 to 1 1018 pa s these effective values of viscosity can be explained by a power law rheology for which strain rate is proportional to stress raised to a power n 1 the apparent change in viscosity can also be caused by other factors such as coupling between afterslip and viscoelastic flow the open and intriguing question in connection with postseismic data after the kuril earthquake doublet is the magnitude of the long term asthenospheric viscosity which shall be revealed by continued observations an asthenosphere with viscosity of about 1 1019 pa s is favored by the postseismic deformation still observed several decades after the 1960 chile and 1964 alaska mw 9 earthquakes however postseismic deformation associated with the 1952 southern kamchatka mw 9 earthquake currently is not observed in the northern kurils an indication that the long term asthenospheric viscosity in the kurils is lower than that in chile and alaska

effect of oxygen on degradation rate of refractory and labile organic matter in continental margin sediments in order to study the effect of oxygen on degradation rate of bulk sedimentary organic matter we developed a new incubation method in contrast to most previous experiments 1 all of the sediment undergoing decomposition was maintained under oxygenated or oxygen free conditions and 2 organic matter of varying lability was studied the production of co2 during incubations of sediment in glass test tubes corrected for dissolution precipitation of calcium carbonates was used as a measure of degradation rates the laboratory experiments using surficial and buried continental shelf and slope sediment from the open skagerrak northeastern north sea demonstrated that the effect of oxygen on the degradation rate of sedimentary organic matter is a function of the lability of the decomposing material fresh material was degraded with little difference in rates in the presence or absence of oxygen whereas old material was decomposed significantly up to 3 6 times faster with oxygen than without oxygen an implication of these findings is that bioturbation by exposing old buried material to oxygen may enhance integrated organic carbon oxidation in marine sediments this constitutes a previously unexplored mechanism by which faunal reworking may stimulate carbon degradation the anoxic decomposition rates of organic material buried at 20 cm depth in sediment were the lowest measured we found however that the extent of oxidation of this buried old sediment was considerably larger than that of surficial sediment under oxygenated conditions which indicated that the oxic anoxic oxic redox transitions deposition under oxic conditions burial under anoxia and reexposure to oxygen promoted degradation our results therefore also suggest that the extent of long term decomposition of sedimentary organic material is smaller under oxygenated or anoxic conditions alone than when the material is exposed to the repeated activities of both oxic and anoxic microorganisms