seismic velocities in the uppermost oceanic crust age dependence and the fate of layer 2a it has been widely accepted that upper crustal seismic velocities gradually increase from low values in the youngest crust to typical crustal velocities 5 2 km s 1 over a period of 30 40 million years with a corresponding decrease in the observed heat flow deficiency caused by hydrothermal circulation some recent seismic studies however suggest that the seismic velocities actually rise rapidly nearly doubling in 5 km s 1 and velocities 3 km s 1 are not observed in crust older than 5 ma there is no detectable systematic change in the seismic properties of the uppermost oceanic crust after 10 million years median and average velocities increase from 2 3 to 4 4 km s 1 by 7 5 ma and remain unchanged thereafter if layer 2a is an interval capping the oceanic crust in which velocities are significantly lower than velocities that are typical of the underlying rocks layer 2b 5 2 km s 1 it follows that layer 2a does not disappear but is a persistent feature of the oceanic crust these results are consistent with geochemical evidence that most hydrothermal alteration occurs within 3 million years after the formation of the oceanic crust and suggest that there is little correlation between seismic velocities and ongoing hydrothermal circulation

the nicoya convergent margin a region of exceptionally low heat flow over 100 measurements of seafloor heat flow reveal that the accretionary complex adjacent to the nicoya peninsula is characterized by remarkably low heat flow values over the accretionary prism average 28 mw m and values in the trench and the ocean crust seaward of the trench average 14 mw m we attribute the low heat flow to effective hydrothermal cooling of the upper crust on the subducting plate and suggest that extensional faults created by flexure of the lithosphere enhance hydrothermal circulation thermal models show that subduction of low temperature crust combined with significant frictional heating at the decollement can explain the low and uniform heat flow disparity between heat flow values observed on the lower trench slope with model results suggests upward advection of heat by porewater flux through broadly distributed conduits

seismic velocities of the uppermost igneous crust versus age summary seismic refraction velocity data from the acoustic basement called layer 2a have been compiled for different mid ocean ridges the data from post 1970 studies show a strong correlation between velocity and basement age importantly velocities double in less than 10 myr for all ridges but for older crust up to 160 myr in age velocities do not increase significantly

geophysical evidence for hydration of the crust and mantle of the nazca plate during bending at the north chile trench water transported in subducting oceanic plates plays a key role in a number of phenomena including intraslab seismicity and arc magmatism however the locus of plate hydration and water distribution in crust and mantle of plates entering subduction zones is debated we present evidence for anomalously low seismic velocities and densities of the crust and upper mantle of the nazca plate at the north chile trench crustal seismic velocities at the trench are lower than velocities of mature fast spreading crust and even lower than velocities of highly extended slow spreading crust in addition the nazca plate at the north chile trench may contain an 20 km thick upper mantle layer with 17 serpentine which implies 2 5 wt water these results document pervasive rock alteration by water percolation linked to bending related extensional faulting

crustal structure across the pacific margin of nicaragua evidence for ophiolitic basement and a shallow mantle sliver seismic wide angle measurements across the pacific margin of nicaragua were carried out using ocean bottom hydrophones and land stations recording marine airgun shots the structure and the p wave velocity of the subducting cocos and overriding caribbean plates were determined by modelling wide angle data and further constrained by coincident seismic reflection borehole and gravity data the oceanic crust of the cocos plate is 5 5 km thick with a thin pelagic sediment cover the plate boundary can be traced to 40 km depth and is generally similar to configurations derived earlier from the local seismic network a major feature of the upper plate is an 80 km wide high velocity high density rock unit reaching from the front of the margin to about the middle of the shelf this wedge shaped unit is 15 km thick beneath the shelf edge and is composed of a 5 km thick upper part with velocities increasing from 3 5 km s 1 near the trench to 5 2 km s 1 at the shelf break overlying a 10 km thick lower part with velocities of 6 0 km s 1 analysis of the gravity field requires densities of 2 6 2 7 g cm 3 for the upper part and 2 9 g cm 3 for the lower part of this unit these velocities and densities suggest that the margin wedge is composed of ophiolitic rock similar to those sampled offshore guatemala and exposed on the nicoya peninsula costa rica the velocity structure of this ophiolitic unit ends about 50 km offshore landward the basement underneath the sandino basin is formed by older igneous rock drilled beneath upper cretaceous sedimentary rock onshore nicaragua beneath the ophiolitic basement we find a sliver with velocities typical of mantle material that begins at depths of 12 18 km and coincides with the down dip limit of the seismogenic zone mantle densities are required for the sliver to match the gravity data in a tectonic reconstruction the suture of an oceanic plateau on the farallon plate against the chortis block in upper cretaceous time is suggested suturing left the former trench and margin in deep water consistent with the late cretaceous to palaeocene deep water sediment of the rivas formation at the base of the sandino basin suture of the farallon plate and chortis block might have initiated the strike slip movement along the motagua polochic fault system the development of a new subduction zone in eocene oligocene times left the ophiolitic basement and a sliver of oceanic mantle attached to the chortis block and shifted the volcanic arc about 70 km southwestwards close to its present position

intraplate seismicity and related mantle hydration at the nicaraguan trench outer rise summary we examine micro earthquake records from a dense temporary array of ocean bottom seismometers obs and hydrophones that has been installed from september to november 2005 at the trench outer rise offshore nicaragua approximately 1 5 locatable earthquakes per day within the array of 110 120 km show the high seismic activity in this region seismicity is restricted to the upper 15 km of the mantle and hence where temperatures reach 350 400 c which is smaller than values observed for large mantle intraplate events 650 c determination of moment tensor solutions suggest a change of the stress region from tensional in the upper layers of the oceanic plate to compressional beneath the neutral plane between both regimes is located at 6 9 km beneath moho and thus very shallow fluids which are thought to travel through the tensional fault system into the upper mantle may not be able to penetrate any deeper the earthquake catalogue which seems to be complete for magnitudes above mw 1 6 1 8 suggests a strong change of the lithospheric rheology when approaching the trench and b factors that is the ratio between small and large earthquakes increase significantly in the closest 20 km to the trench axis implying that the crust and upper mantle is massively weakened and hence ruptures more frequently but under less release of stress we explain this with a partly serpentinized upper mantle

what are the origins of the antarctic intermediate waters transported by the north caledonian jet 1 the trajectories of several argo floats are studied to investigate the mid depth circulation in the southwest pacific that coincides with the spreading of the antarctic intermediate waters aaiw before entering the coral sea the floats converge and join the north caledonian jet ncj south of 15 s these observations suggest that the waters transported by this jet do not have a unique source hydrologic parameters and oxygen concentration confirm that the characteristics of the aaiw in the ncj result from a convergence and a mixing of waters from the northern limb of the subtropical gyre and from the south eastern new caledonian region the northern contribution is associated with the water masses transported by the north vanuatu jet that re circulate in the region between the d entrecasteaux reefs and the vanuatu archipelago it is shown that such complex dynamical features could be reproduced by numerical models when their horizontal resolution is sufficiently high

transition from amagmatic to volcanic margin mantle exhumation in the voring basin before the icelandic plume influence abstract the outer voring basin norwegian sea is an abnormally deep and distal part of the norwegian passive margin known as magma rich yet the lastest borehole drilled in its northernmost part led to a drastic change in the interpretation of seismic reflection data this update motivates a new model for the entire mesozoic eocene rifting period here i evaluate the link between tectonics serpentinization and magmatism in the norwegian margin from the ryazanian valanginian to the palaeocene eocene magmatic breakup maps of the regional nise sandstones and main regional unconformities specifically highlight the fundamental role of the nyk vema structure to understand the basin and to question the deformation timing related to deep structures the top of partially serpentinized mantle is supported by high amplitude reflectors clearly visible on certain seismic lines known as t reflector covered with gravity driven pre exhumation sediments and further deposits a new structural scheme based on the interpretation of 2d and 3d seismic data shows the complex organization of fault networks and constrains a major deformation period between the late campanian and the late palaeocene ocean scale complementary arguments are in good agreement with lower cretaceous mantle exhumation before becoming a volcanic margin in the palaeocene this segment of the norwegian margin appears as amagmatic until the late campanian at least i propose an integrated model since the onset of hyperextension in the end of the jurassic until the cessation of activity of the aegir ridge in the miocene this model has implications regarding the context dependent interpretation of lower crustal bodies the ubiquity of serpentinization and mantle exhumation worldwide and on the hydrocarbon system

electromagnetic detection of plate hydration due to bending faults at the middle america trench water plays an important role in the processes occurring at subduction zones since the release of water from the downgoing slab impacts seismicity and enhances arc volcanism geochemical indicators suggest that the nicaraguan slab is anomalously wet yet the mechanism of slab hydration remains poorly constrained extensional bending faults on the incoming oceanic plate of the middle america trench offshore nicaragua have been observed to penetrate to mantle depths suggesting a permeable pathway for hydration of the crust and serpentinization of the upper mantle low seismic velocities observed in the uppermost mantle of the incoming plate have been explained as serpentinization due to deep fluid penetration but could also be explained by intrinsic anisotropy and fractures in the absence of fluid circulation here we use controlled source electromagnetic imaging to map the electrical resistivity of the crust and uppermost mantle along a 220 km profile crossing the trench offshore nicaragua along the incoming plate our data reveal that crustal resistivity decreases by up to a factor of five directly with the onset of the bending faults furthermore a strong azimuthal anisotropy compatible with conductive vertical fault planes is observed only on the faulted trench seafloor the observed resistivity decrease and anisotropy can be explained by a porosity increase along vertical fault planes which we interpret as evidence that the lithospheric bending faults provide the necessary permeable fluid pathways required for serpentinization of the uppermost mantle this implies that most serpentinisation happens at the trench with the width of the faulting region and the density of fractures controlling the extent of upper mantle alteration this observation explains why the heavily faulted trench offshore nicaragua is associated with an anomalously wet slab whereas other sections of the middle america trench containing fewer bending faults have less fluid flux from the subducting slab

cascadia subducting plate fluids channelled to fore arc mantle corner ets and silica deposition in this study we first summarize the constraints that on the cascadia subduction thrust there is a 70 km gap downdip between the megathrust seismogenic zone and the episodic tremor and slip ets that lies further landward there is not a continuous transition from unstable to conditionally stable sliding seismic rupture occurs mainly offshore for this hot subduction zone ets lies onshore we then suggest what does control the downdip position of ets we conclude that fluids from dehydration of the downgoing plate focused to rise above the fore arc mantle corner are responsible for ets there is a remarkable correspondence between the position of ets and this corner along the whole margin hydrated mineral assemblages in the subducting oceanic crust and uppermost mantle are dehydrated with downdip increasing temperature and seismic tomography data indicate that these fluids have strongly serpentinized the overlying fore arc mantle laboratory data show that such fore arc mantle serpentinite has low permeability and likely blocks vertical expulsion and restricts flow updip within the underlying permeable oceanic crust and subduction shear zone at the fore arc mantle corner these fluids are released upward into the more permeable overlying fore arc crust an indication of this fluid flux comes from low poisson s ratios and vp vs found above the corner that may be explained by a concentration of quartz which has exceptionally low poisson s ratio the rising fluids should be silica saturated and precipitate quartz with decreasing temperature and pressure as they rise above the corner

outer slope faulting associated with the western kuril and japan trenches summary elongated fault escarpments on the outer slopes of the western kuril and japan trenches have been investigated through detailed swath bathymetric mapping numerous horsts and grabens formed by these escarpments were identified distinct n70 e linear alignment of the escarpments parallel to the magnetic anomaly lineations was revealed on the outer slope of the western kuril trench in the japan trench north of 39 00aen most of the escarpments are parallel to the trench axis and oblique to the magnetic lineations a zig zag pattern of faulting exists south of 39 00aen each topographic profile was decomposed by computer analysis into two curves representing 1 the smoothed long wavelength slope of the subducting ocean crust surface and 2 the short wavelength 10 km roughness of plateaus and valleys edged by outwardand inward facing fault escarpments throughout the surveyed areas escarpment heights increase from the crest of the trench outer swell down to a depth of about 6000 m on the slope of the outer trench wall but with no distinct increase below that depth no significant diverence is recognized in fault throws towards and away from the trench it can be concluded that these elongated escarpments originate from normal faults on the upper layer of the oceanic crust under extensional stress in a direction perpendicular to the trench axis which is caused by downward bending of the subducting lithosphere the relationship of escarpment height to escarpment length is similar to that obtained from normal fault escarpments in the east pacific rise crest the maximum length and height of escarpments are small in the kuril trench compared with those in the japan trench implying a diverence in mechanical strength depending on the fault orientation the crust is weakest along the inherited spreading fabric second weakest probably along the non transform ovset direction and strongest in directions very oblique to these orientations seamounts appear to be more rigid than normal ocean crust with no particular weak orientations resulting in fewer but larger faults along the axis of plate bending as most clearly represented in the subducting daiichi kashima seamount

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

extremely thin crust in the indian ocean possibly resulting from plume ridge interaction the thickness of the crust created at ocean spreading centres depends on the spreading rate and melt production in the mantle it is 5 8 km for a crust formed at slow and fast spreading centres and 2 4 km at ultra slow spreading centres away from hotspots and mantle anomalies the crust is generally thin at the fracture zones and thick beneath hotspots and large igneous provinces here we present results for the crust generated at the fast wharton spreading centre 55 58 ma ago using seismic reflection and refraction data we find that the crust over a 200 km segment of the wharton basin is only 3 5 4 5 km thick the thinnest crust ever observed in a fast spreading environment a thin crust could be produced by the presence of depleted and or cold mantle numerical simulations and recent laboratory experiments studying the impact of a hot plume under a lithosphere show that a curtain of weak cold downwellings of depleted mantle material is likely to develop around the edges of the hot plume pond because of a strongly temperature dependent viscosity of lithospheric material the hotter therefore less viscous bottom of the lithosphere can be mobilized by an impinging plume if sampled by a spreading centre the locally cold and depleted mantle should result in low production of melt we suggest that the observed thin crust in the wharton basin is likely to have been formed by the interaction between the kerguelen mantle plume and the wharton spreading centre 55 ma ago

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

the wet nicaraguan slab 1 nicaraguan volcanoes show globally high concentrations of geochemical tracers from dehydration of subducting crust which may reflect a slab with unusually high amounts of h2o to test this possibility we measure seismic velocities at the top of the subducted plate and compare them with predictions for hydrated mafic rocks regional seismic p waves for intraslab events at 100 150 km depth show a high frequency late arrival apparently trapped in a low velocity waveguide 2 5 6 km thick at the top of the downgoing plate 14 5 2 2 slower than surrounding mantle the velocities can be explained by 5 wt h2o in the subducted crust 2 3 times the hydration inferred for other slabs by similar methods this interpretation implies extensive hydration of the cocos plate off nicaragua perhaps enhanced by up dip fluid flow within the slab at 100 km depth

revised tectonic boundaries in the cocos plate off costa rica implications for the segmentation of the convergent margin and for plate tectonic models the oceanic cocos plate subducting beneath costa rica has a complex plate tectonic history resulting in segmentation new lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the east pacific rise from lithosphere formed at the cocos nazca spreading center they also define two early phase cocos nazca spreading regimes and a major propagator in addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of cocos plate over the galapagos hot spot the subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab newly identified seafloor spreading anomalies show oceanic lithosphere formed during initial breakup of the farallon plate at 22 7 ma and opening of the cocos nazca spreading center a revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the cocos nazca spreading center at 19 5 ma a major ridge jump reshaped its geometry and after 14 5 ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere a suspected cause of ridge jumps is an interaction of the cocos nazca spreading center with the galapagos hot spot

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

seismic evidence for fluids in fault zones on top of the subducting cocos plate beneath costa rica summary in the 2005 ticocava explosion seismology study in costa rica we observed crustal turning waves with a dominant frequency of 10 hz on a linear array of short period seismometers from the pacific ocean to the caribbean sea on one of the shot records from shot 21 in the backarc of the cordillera central we also observed two seismic phases with an unusually high dominant frequency 20 hz these two phases were recorded in the forearc region of central costa rica and arrived 7 s apart and 30 40 s after the detonation of shot 21 we considered the possibility that these secondary arrivals were produced by a local earthquake that may have happened during the active source seismic experiment such high frequency phases following shot 21 were not recorded after shots 22 23 and 24 all in the backarc of costa rica which might suggest that they were produced by some other source however earthquake dislocation models cannot produce seismic waves of such high frequency with significant amplitude in addition we would have expected to see more arrivals from such an earthquake on other seismic stations in central costa rica we therefore investigate whether the high frequency arrivals may be the result of a deep seismic reflection from the subducting cocos plate the timing of these phases is consistent with a shear wave from shot 21 that was reflected as a compressional s p and a shear s s wave at the top of the subducting cocos slab between 35 and 55 km depth the shift in dominant frequency from 10 hz in the downgoing seismic wave to 20 hz in the reflected waves requires a particular seismic structure at the interface between the subducting slab and the forearc mantle to produce a substantial increase in reflection coefficients with frequency the spectral amplitude characteristics of the s p and s s phases from shot 21 are consistent with a very high v p v s ratio of 6 in 5m thick slab parallel layers this result suggests that a system of thin shear zones near the plate interface beneath the forearc is occupied by hydrous fluids under near lithostatic conditions the overpressured shear zone probably takes up fluids from the downgoing slab and it may control the lower limit of the seismogenic zone

rate of serpentinization in seafloor environments the permeability of partially serpentinized peridotites has been measured near room temperature and found to be in the range of 10 11 to 10 12 darcys or 10 7 to 10 8 cm2 sec 1 comparable to shaley materials serpentinites act as semipermeable membranes to salt ions in ocean crust and deep groundwater environments serpentinization must be a continuous process controlled only by water access to ultramafic rocks through their cover given this it will generate high local strains and stresses with episodic cracking the serpentinization process may also contribute to heat flow and ocean floor topography

structure and ageing of oceanic crust at 14 s on the east pacific rise a wide angle reflection refraction survey was conducted along a flow line on the eastern flank of the east pacific rise at 14 s the data were acquired with a single 32 litre airgun and ocean bottom hydrophones obh and consist of six profiles on 0 5 8 3 myr old seafloor in total 17 obhs provided seismic records of excellent quality to generate 2 d velocity models traveltimes and amplitudes have been calculated using asymptotic ray theory a comparison of the traveltimes and amplitudes of p wave arrivals on all lines revealed important similarities that correlate well with the classical model of oceanic crust the uppermost crust is composed of a 0 4 0 7 km thick low velocity 2 9 4 6 km s 1 surficial layer layer 2a and is bounded at its base by a sharp velocity increase to 5 5 km s 1 at the top of layer 2b overall layer 2 is 1 4 1 8 km thick small seamounts frequently superposed on fast spreading crust are characterized by velocities lower than within the adjacent upper crust velocities 6 0 km s 1 exist throughout the edifices suggesting that the seamounts are dominantly extrusive decreasing amplitudes beyond 8 10 km offset constrain a moderate gradient lower crust layer 3 with velocities of 6 8 7 2 km s 1 crustal thickness is found to be remarkably uniform at 6 11 0 14 km nevertheless the seismic data provide evidence for age dependent variations in both the low velocity surficial layer and the upper mantle we find that velocities in layer 2a increase from 2 9 to 4 3 km s 1 velocities in the middle and lower crust show uniform values while velocities within the uppermost mantle increase gradually from 7 5 km s 1 to 8 0 km s 1 the most viable explanation for increasing velocities in the upper crust is that off axis hydrothermal circulation was accompanied by the precipitation of secondary minerals in open void spaces thereby decreasing the porosity and increasing the velocity low mantle velocities beneath young crust are suggested to be a temperature controlled phenomenon and may indicate the presence of partial melt in the upper mantle

the nature of deep focus earthquakes wadati 1928 first proved convincingly that some earthquakes occur at depths well beneath the earth s crust he essentially proposed the nomenclature used today by the international seismological centre isc i e earthquakes with focal depths exceeding 300 km are deep earthquakes and those with depths between 70 and 300 km are intermediate earth quakes however when there is little reason to distinguish between the two groups both types are called simply deep earthquakes to avoid confusion in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes and those with focal depths between 70 and 300 km as intermediate earthquakes deep earthquakes are of interest for at least four reasons first they are exceedingly common between 1964 and 1986 they constituted no less than 22 of all earthquakes having mb greater than 5 0 in the isc catalog second they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones one of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups or wadati benioff zones apparently delineates the cold down going cores of convection cells in the uppermost mantle third seismologists use body waves of deep earthquakes dispro portionately more often than those of shallow earthquakes to inves tigate core mantle and crustal structure this is because a deep earth quakes often possess relatively more impulsive sources b their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station and c surface waves do not contaminate these body phase arrivals on seismograms finally deep earthquakes are

the steeply subducting edge of the cocos ridge evidence from receiver functions beneath the northern talamanca range south central costa rica the deep structure of the south central costa rican subduction zone has not been studied in great detail so far because large parts of the area are virtually inaccessible we present a receiver function study along a transect of broadband seismometers through the northern flank of the cordillera de talamanca south costa rica below moho depths the receiver functions image a dipping positive conversion signal this is interpreted as the subducting cocos plate slab compatible with the conversions in the individual receiver functions in finite difference modeling a dipping signal such as the one imaged can only be reproduced by a steeply 80 dipping structure present at least until a depth of about 70 100 km below this depth the length of the slab cannot be determined because of possible scattering effects the proposed position of the slab agrees with previous results from local seismicity local earthquake tomography and active seismic studies while extending the slab location to greater depths and steeper dip angle along the trench no marked change is observed in the receiver functions suggesting that the steeply dipping slab continues until the northern flank of the cordillera de talamanca in the transition region between the incoming seamount segment and cocos ridge considering the time predicted for the establishment of shallow angle underthrusting after the onset of ridge collision the southern costa rican subduction zone may at present be undergoing a reconfiguration of subduction style where the transition to shallow underthrusting may be underway but still incomplete

contrasting subduction structures within the philippine sea plate hydrous oceanic crust and anhydrous volcanic arc crust we show contrasting subduction structures within the philippine sea plate inferred from active source wide angle reflection data previous studies showed that large amplitude reflections from the slab are observed in southwest japan and indicated that a thin low velocity layer with a high fluid content is formed along the top of the subducting oceanic crust on the contrary we found that the slab reflections have smaller amplitudes in the izu collision zone central japan where the izu bonin volcanic arc has been colliding subducting suggesting that such a low velocity layer does not exist beneath the collision zone this structural difference is also supported by p wave and s wave velocity anomalies by passive source tomography and electrical conductivity and correlates with the regional distribution of deep tremors and intraslab earthquakes both of which are induced by dehydration processes within the downgoing slab based on these comparisons we suggest that the original structure of the incoming plate controls the contrasting subducting systems typical oceanic plate absorbs water by hydrothermal circulation at spreading centers and or seawater infiltration at outer rises whereas volcanic arc crust consumes a large amount of hydrous minerals for melt production and metamorphoses to more stable anhydrous forms before subduction

abrupt thermal transition reveals hydrothermal boundary and role of seamounts within the cocos plate 1 new thermal data from 18 24 ma lithosphere on the cocos plate delineate contrasting subsurface thermal conditions in adjacent sections of crust heat flow through seafloor created at the east pacific rise is generally suppressed by 70 relative to conductive lithospheric cooling models whereas heat flow through adjacent similarly aged lithosphere generated at the cocos nazca spreading center is consistent with these models the transition between thermal regimes is remarkably abrupt only 2 5 km wide indicating a shallow hydrothermal origin the transition is more closely associated with differences in the distribution of basement outcrops than with tectonic boundaries demonstrating the importance of the former in extracting heat from the lithosphere on a regional basis

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

extinct spreading center in the labrador sea crustal structure from a two dimensional seismic refraction velocity model the labrador sea contains a rare example of an abandoned mid ocean ridge where active accretion of oceanic crust ceased due to a change in the spreading geometry of lithospheric plates seismic refraction data were collected along a line which transversely crosses the extinct spreading center two dimensional analyses of the refraction data using ray tracing and synthetic seismogram techniques reveal major variations in crustal thickness and velocity in relation to the axis of the extinct spreading center in the extinct spreading center a crustal thickness of approximately 4 km is determined compared with 5 5 km for the flanks substantial lateral variations in p wave velocities of the upper and lower crust are observed with a marked decrease within the extinct spreading center low velocities are also observed in the uppermost mantle underlying the extinct spreading center and are interpreted as being the result of hydrothermal alteration the anomalously low crustal velocities and crustal thinning are attributed to a decreasing supply of partial melt and increasing degree of tectonism at the slow spreading rates preceding extinction

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