sea surface and subsurface circulation dynamics off equatorial peru during the last 17 kyr the complex deglacial to holocene oceanographic development in the gulf of guayaquil eastern equatorial pacific is reconstructed for sea surface and subsurface ocean levels from isotope geochemical proxies based on marine sediment cores at sea surface southern sourced cold coastal water and tropical equatorial surface water tropical surface water are intimately related in particular since 10 ka independent sea surface temperature proxies capturing different seasons emphasize the growing seasonal contrast in the gulf of guayaquil which is in contrast to ocean areas further offshore cold coastal water became rapidly present in the gulf of guayaquil during the austral winter season in line with the strengthening of the southeast trades while coastal upwelling off peru gradually intensified and expanded northward in response to a seasonally changing atmospheric circulation pattern affecting the core locations intensively since 4 ka bp equatorial surface water instead was displaced and tropical surface water moved northward together with the equatorial front at subsurface the presence of equatorial under current sourced equatorial subsurface water was continuously growing prominently since 10 8 ka b p during heinrich stadial 1 and large parts of the bolling allerod and similarly during short holocene time intervals at 5 1 4 ka b p and 1 5 0 5 ka b p the admixture of equatorial subsurface water was reduced in response to both short term weakening of equatorial under current strength from the northwest and emplacement by tropical equatorial surface water considerably warming the uppermost ocean layers

a seismic reflection and gloria study of compressional deformation in the gorringe bank region eastern north atlantic seismic reflection and gloria side scan sonar data obtained on rrs charles darwin cruise cd64 reveal new information on the styles of deformation in the gorringe bank region at the eastern end of the azores gibraltar plate boundary previous studies suggest that gorringe bank was formed by the overthrusting of a portion of the african plate upon the eurasian plate the new seismic data show however that the most intensely deformed region is located south of gorringe bank on the northern flanks of a nw se trending submarine ridge which includes the ampere and coral patch seamounts the deformation is expressed as long wavelength up to 60 km large amplitude up to 800 m folds in the sediments and underlying acoustic basement which in places are associated with one or more reverse faults and as a fabric of short wavelength folds up to 3 km with a ne trend in contrast the same sedimentary units when traced beneath the flanking plains are undeformed except for some faults with a small throw 30 m some of which offset the seafloor gloria data show that recent deformation is broadly distributed over the region structural trends rotate from 45 in the west to 70 in the east of the region nearly perpendicular to the nw verging plate motion vectors as determined from plate kinematic models flexure modelling suggests that a portion of gorringe bank has loaded 152 ma oceanic lithosphere and that a maximum of 50 km of shortening has occurred at gorringe bank since the mid miocene our observations support a model in which there is no single plate boundary in the region rather that the deformation is distributed over a 200 330 km wide zone

hydraulic control of flow over the sill in observatory inlet observations of density velocity and acoustic backscatter obtained over the sill in observatory inlet british columbia during the ebb tide illustrate several features of internal hydraulic control in a tidally forced stratified flow following high water slack the flow field was initially determined by critical control of the second internal mode at the sill crest as the flow speed increased a transition occurred and control changed to an approach control of the second internal mode located upstream of the sill crest whereas the critical control represents a minimum energy condition at the crest the approach control corresponds to a maximum energy condition upstream of the sill crest later when the ebb currents were just past their maximum values the control condition changed to critical control of the first internal mode the concept of approach control of flow over topography has not previously been discussed in the literature the data described here are believed to represent the first field observations of this phenomenon the changing hydraulic conditions are analyzed in terms of two and three layer hydraulic models model results are consistent with the evolution of the flow patterns although the model cannot reproduce the apparent hysteresis in the response after maximum ebb the concept of an approach control is demonstrated with a simple two layer laboratory model

kinematics of the western africa eurasia plate boundary from focal mechanisms and gps data summary the western mediterranean displays a complex pattern of crustal deformation distributed along tectonically active belts developed in the framework of slow oblique plate convergence we used earthquake and global positioning system gps data to study the present day kinematics and tectonics of the africa eurasia plate boundary in this region crustal seismicity and focal mechanisms analysed in terms of seismic moment release and seismic deformation outline the geometry of major seismic belts and characterize their tectonics and kinematics continuous gps data have been analysed to determine euler vectors for the nubian and eurasian plates and to provide the global frame for a new mediterranean gps velocity field obtained by merging continuous and campaign observations collected in the 1991 2005 time span gps velocities and displacements predicted by the nubia eurasia rotation pole provide estimates of the deformation accommodated across the tectonically active belts the rather simple deformation occurring in the atlantic region characterized by extension about perpendicular to the middle atlantic and terceira ridges and right lateral motion along the gloria transform fault turns into a complex pattern of deformation occurring along broader seismic belts where continental lithosphere is involved our analysis reveals a more complex fragmentation of the plate boundary than previously proposed the roughly e w trending mainly compressive segments i e southwestern iberia northern algeria and southern tyrrhenian where plate convergence is largely accomodated across rather localized deformation zones and partially transferred northward to the adjacent domains i e the algero balearic and tyrrhenian basins are interrupted by regions of more distributed deformation i e the rif alboran betics tunisia libya and eastern sicily or limited seismicity i e the strait of sicily which are characterized by less homogeneous tectonics regimes mainly transcurrent to extensional in correspondence of the observed breaks tectonic structures with different orientation interfere and we find belts with only limited deformation i e the high and middle atlas the tunisian atlas and the offshore tunisia libya belt that extends from the plate boundary into the nubian plate along pre existing tectonic lineaments our analysis suggest that the sicilian pelagian domain is moving independently from nubia according to the presence of a right lateral and extensional decoupling zone corresponding to the tunisia libya and strait of sicily deformation zone despite the space variability of active tectonic regimes plate convergence still governs most of the seismotectonic and kinematic setting up to the central aeolian region in general local complexities derive from pre existing structural features inherited from the tectonic evolution of the mediterranean region on the contrary along calabria and the apennines the contribution of the subducted ionian oceanic lithosphere and the occurrence of microplates i e adria appear to substantially modify both tectonics and kinematics finally gps data across the gibraltar arc and the tyrrhenian calabria domain support the hypothesis that slab rollback in these regions is mostly slowed down or stopped

on the initiation of subduction estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation three environments proposed as preferential sites of incipient subduction are investigated passive continental margins transform faults fracture zones and extinct ridges none of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins the attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates this is the only tectonic force sufficient to trigger the nucleation of a new subduction zone the ubiquitous distribution of transform faults and fracture zones combined with the common proximity of these features to mature subduction complexes suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere

finite dispersal of a separative nepheloid plume by an internal hydraulic jump in a tropical mountainous river estuary this paper investigates the dynamics of an internal hydraulic jump in a river plume and associated suspended sediment dispersal field investigations were undertaken into the river plume generated by the herbert river australia following a moderate flood event induced by cyclone fritz in 2004 the forced plume experiences an abrupt transition from supercritical to subcritical via an internal hydraulic jump as defined by a mode 1 internal froude number computed using the phase speeds from the taylor goldstein equation the hydraulic theory of a two layer stratified flow was used to identify the plume shape and the mechanical energy loss within the jump the hydraulic jump energy loss is primarily transferred to the buoyancy driven potential energy uplifting the river plume intense stratification decreases the bottom stress damping the resuspension therefore a separative nepheloid dispersal system occurs at the jump section both the upper and lower nepheloid flows are confined to the inner shelf but have different dispersal behaviors and mechanisms the upper nepheloid flow which is primarily controlled by advection and settling satisfies an exponential decay law of the total suspended sediment concentrations versus the offshore distance the lower nepheloid flow dominated by deposition is detached seaward near the lift off point of the river plume a turbidity front associated with the jump may accumulate a large quantity of suspended sediments enhancing sediment release from the river plume these findings will promote in depth understanding of both the cross shelf sediment dispersal and muddy deposit on the shelf

the quest for the africa eurasia plate boundary west of the strait of gibraltar the missing link in the plate boundary between eurasia and africa in the central atlantic is presented and discussed a set of almost linear and sub parallel dextral strike slip faults the swim1 faults that form a narrow band of deformation over a length of 600 km coincident with a small circle centred on the pole of rotation of africa with respect to eurasia was mapped using a new swath bathymetry compilation available in the area offshore sw portugal these faults connect the gloria fault to the rif tell fault zone two segments of the plate boundary between africa and eurasia the swim faults cut across the gulf of cadiz in the atlantic ocean where the 1755 great lisbon earthquake m 8 5 8 7 and tsunami were generated providing a new insight on its source location

seismic and gravity constraints on the nature of the basement in the africa eurasia plate boundary new insights for the geodynamic evolution of the sw iberian margin we present a new classification of geological domains at the africa eurasia plate boundary off sw iberia together with a regional geodynamic reconstruction spanning from the mesozoic extension to the neogene to present day convergence it is based on seismic velocity and density models along a new transect running from the horseshoe to the seine abyssal plains which is combined with previously available geophysical models from the region the basement velocity structure at the seine abyssal plain indicates the presence of a highly heterogeneous thin oceanic crust with local high velocity anomalies possibly representing zones related to the presence of ultramafic rocks the integration of this model with previous ones reveals the presence of three oceanic domains offshore sw iberia 1 the seine abyssal plain domain generated during the first stages of slow seafloor spreading in the ne central atlantic early jurassic 2 the gulf of cadiz domain made of oceanic crust generated in the alpine tethys spreading system between iberia and africa which was coeval with the formation of the seine abyssal plain domain and lasted up to the north atlantic continental breakup late jurassic and 3 the gorringe bank domain made of exhumed mantle rocks which formed during the first stages of north atlantic opening our models suggest that the seine abyssal plain and gulf of cadiz domains are separated by the lineament south strike slip fault whereas the gulf of cadiz and gorringe bank domains appear to be limited by a deep thrust fault located at the center of the horseshoe abyssal plain

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

moment tensor inversion for iberia maghreb earthquakes 2005 2008 abstract we present and discuss a set of 77 moment tensor solutions for earthquakes in the iberia maghreb region from mid 2005 to the end of 2008 with moment magnitudes ranging from 3 2 to 6 0 inversion is based upon full three component intermediate period waveforms recorded at regional seismic broadband stations following the recent densification of permanent broadband networks and large scale temporary deployments at present more than 150 stations are recording in spain portugal and morocco this unprecedented station density allows analysis of small events from available short distance recordings and in general leads to more reliable source estimates due to data redundancies that permit elimination of waveforms affected by noise or complicated propagation effects the solutions for 2005 2008 constitute an important upgrade of the iberia maghreb moment tensor inventory to 225 solutions to date enhancing the image of seismotectonics at the compressive n algerian margin in the betic alboran shear zone and at the transpressive sw iberian margin as well as providing valuable constraints on seismic deformation in the western betics and the iberian foreland where little information has been previously available new solutions for the foreland and three recent seismic series in the western betics show strike slip and reverse faulting style contrasting with the dominance of normal faulting in the adjacent areas towards east in these areas as well as at the sw iberian margin faulting orientations are heterogeneous including solutions with opposite kinematics this indicates control by local stress conditions and points to fault interaction along the n algerian margin a counterclockwise rotation of slip vectors of thrust events from east to west becomes apparent several solutions for the area offshore cape st vincent are located at sub moho depths between 40 and 60 km supporting a large brittle layer thickness in the oceanic lithosphere

tidal and atmospheric forcing of the upper ocean in the gulf of california 1 sea surface temperature variability two years of satellite infrared imagery 1984 1986 are used to examine the sea surface temperature sst variability in the northern gulf of california empirical orthogonal functions eofs of the temporal and spatial sst variance for 20 monthly mean images show that the dominant sst patterns are generated by spatially varying tidal mixing in the presence of seasonal heating and cooling these patterns are modified in the fall and winter when shelf temperatures south of tiburon island cool in response to upwelling favorable winds these same winds bring cold dry air from off the continental united states causing local cooling of the shallow northern shelf during the rest of the year the broad shallow shelves are warmer than offshore the seasonally reversing temperature patterns are consistent with recent hydrographic observations which show a cyclonic surface circulation in the summer and a weaker anticyclonic circulation during the rest of the year atmospheric forcing of the northern gulf appears to occur over large spatial scales area averaged ssts for the guaymas basin island region and northern basin show significant fluctuations which are highly correlated these fluctuations in sst correspond to similar fluctuations in the air temperature which are related to synoptic weather events over the gulf during periods of particularly low wind speeds the air temperature over the gulf increases dramatically by afternoon intense heating of the sea surface results in the appearance of warm sst anomalies in the satellite data these ssts are approximately 2 c warmer than surrounding ssts and most likely occur as a result of a spatially varying wind field a regression analysis of the sst relative to the fortnightly tidal range shows that tidal mixing occurs over the sills in the island region as well as on the shallow norther shelf mixing over the sills however occurs as a result of large breaking internal waves or internal hydraulic jumps which mix water over the upper 300 500 m this mixing pumps heat away from the surface deep into the water column there by maintaining the cool ssts since mixing occurs over greater depths in the island region the temperatures there are much colder than those generated by tidal mixing on the shallow shelves resulting in the persistent pool of cool water evident in the satellite data this cooler water is mixed horizontally by the basin scale circulation lowering the ssts over much of the northern gulf these reduced ssts have a large impact on the surface heat flux by lowering the saturation vapor pressure of the air as a result the amount of heat lost to the atmosphere through the latent or evaporative heat flux is reduced this may explain why the gulf of california gains heat on an annual average rather than losing heat as occurs in the mediterranean and red seas where tidal mixing is not significant

rapid and widespread dispersal of flood sediment on the northern california margin the dispersal of flood sediment from small river systems is a poorly studied yet potentially important aspect of active continental margin sedimentation in january 1995 during a flood with a 30 yr return period the eel river northern california delivered an estimated 25 3 106 t metric tons of fine grained 62 m sediment to the ocean the flood formed a distinct layer on the sea bed that was centered on the 70 m isobath extended for 30 km along shelf and 8 km across shelf and was as thick as 8 5 cm but contained only 6 106 t of sediment thus 75 of the flood derived sediment did not form a recognizable deposit but was instead rapidly and widely dispersed over the continental margin stratigraphic models of and compilations of sediment flux to active continental margins need to take the dispersive nature of small river systems into account

hyperpycnal plume formation from riverine outflows with small sediment concentrations a series of laboratory experiments has been conducted in order to elucidate the sediment induced mixing processes accompanying riverine outflows specifically the discharge of a warm fresh particle laden fluid over a relatively dense cool brine in a parameter regime analogous to recently acquired field measurements hypopycnal surface plumes were subject to a convective instability driven by some combination of heat diffusing out of the warm fresh sediment laden plume and particle settling within it convection was robust in the presence or absence of intense turbulence at sediment concentrations as low as 1 kg m 3 and took the form of millimetre scale sediment laden fingers descending from the base of the surface plume a consequence of the convective instability of the original hypopycnal plume is the generation of a hyperpycnal bottom riding flow the experiments presented here indicate that natural river outflows may thus generate hyperpycnal plumes when sediment concentrations are 40 times less than those required to render the outflow heavy relative to the oceanic ambient the resulting hyperpycnal plumes may play an important role in transporting substantial quantities of sediment to the continental slope and beyond

morphotectonics and strain partitioning at the iberia africa plate boundary from multibeam and seismic reflection data abstract the gulf of cadiz off sw iberia and the nw moroccan margin straddles the cryptic plate boundary between africa and eurasia a region where the orogenic alpine compressive deformation in the continental collision zone passes laterally to the west to strike slip deformation a set of new multibeam bathymetry multi channel and single channel seismic data presented here image the main morphological features of tectonic origin of a significant part of the gulf of cadiz from the continental shelf to the abyssal plain these morphotectonic features are shown to result from the reactivation of deeply rooted faults that changed their kinematics from the early mesozoic rifting through the late cretaceous paleogene collision to the pliocene quaternary thrusting and wrenching the old faults control deep incised more than 100 km long canyons and valleys several effects of neotectonics on deep water seabed are shown these include i the complex morphology caused by wrenching on the 230 km long wnw ese faults that produced en echelon folds on the sediments ii the formation of up to 5 km wide crescent shaped scours at roughly 4 km water depth by reactivation of thrusts iii 10 km long creep folds on the continental slope and iv the formation of landslides on active fault escarpments the present day deformation is partitioned on ne sw thrusts and wnw ese to w e strike slip faults and is propagating northwards on n s trending thrusts along the west iberia margin from 35 5 n to 38 n which should be considered for seismic hazard

settling driven convection a mechanism of sedimentation from stratified fluids convection driven by sediment particles may play an important role in sedimentation from the base of buoyant hypopycnal plumes for example fluvial plumes in stratified estuaries and lakes black smokers on the ocean floor volcanic clouds and coastal currents in addition to the well known double diffusive convection mechanism another mode of convective instability development is by settling across the density interface we performed laboratory experiments to investigate this fingering convective instability mechanism and its effect on particle distribution in the water column and deposition at the bed a simple theoretical model of finger formation at a fluid density interface is developed based on an analogy with thermal plume formation at a flat heated plate this model which involves a thickening interface layer that becomes gravitationally unstable relative to the ambient fluid is in good agreement with measurements of finger size and instability wavelength from visualization experiments since fingering at the density interface drives larger scale convection in the fluid below a mass balance model of the lower layer assuming strong mixing i e uniform sediment concentration is successfully applied to predict sediment concentration in the water column and deposition at the bed strong mixing can be assumed since convective velocities are usually much greater than the particle fall velocities as convection proceeds the sediment concentrations in the two layers approach each other and convection will die out using the model equations we develop analytical expressions for the time when convection ceases and the portion of sediment remaining in the water column

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

crustal structure of the madeira tore rise eastern north atlantic results of a dobs wide angle and normal incidence seismic experiment in the josephine seamount region summary during november december 1988 an extensive geophysical data set was collected over the josephine seamount which is located at the northeasterly end of the madeira tore rise in the eastern north atlantic the josephine seamount lies at the intersection of the madeira tore rise and azores gibraltar fracture zone the latter representing the africa eurasia plate boundary in this part of the eastern north atlantic from this data set a 275 km long explosive refraction line has been modelled together with wide angle airgun profiles seismic reflection and gravity data the velocity depth model shows that the crust either side of the seamount is typically oceanic in character however beneath the seamount there exists a region of anomalously high velocity and crustal thickening to a depth of about 17 18 km gravity modelling also suggests that the josephine seamount is compensated by a crustal root and that the josephine seamount madeira tore rise system is in local isostatic equilibrium calculations of the flexural rigidity and effective elastic thickness of the lithosphere in this region suggest that the madeira tore rise formed contemporaneously with the lithosphere on which it lies this age of crustal loading is consistent with the proposal that the madeira tore rise is an aseismic ridge which formed at or near the mid atlantic ridge

river plume sedimentation modeling for sequence stratigraphy application to the eel margin northern california abstract sediment deposition in the coastal zone from riverine input is of key importance in understanding the construction of sediment sequences through time a turbulent jet model is used to simulate sediment input to the continental margin from a riverine source turbulent jet models track the momentum which is the first order force carrying most of the sediment flowing into the ocean the froude number at the river mouth remains supercritical during floods of the eel river justifying the use of a jet model for the initial dispersion of the sediment laden plume sediment is deposited from the plume prior to the centerline froude number decreasing to its critical value empirically derived first order removal rate constants are used for each grain size in an advection diffusion equation to predict the areal extent of sediment deposition one strength of using a jet model is its ability to simulate deposits on a daily time step over intermediate time scales thousands of years using a limited number of input parameters a number of simulations of the eel river plume are shown for conditions of the present day and 18 000 yr bp during the last glacial maximum the model results for modern conditions show that a majority of the deposition occurs within 20 km north of the river mouth and is typically confined to within 10 km of the coastline corresponding well with the shelf sands on the eel margin however the initial mud deposit from a 1995 flood was farther offshore and northward additional advection of the mud fraction must occur after it leaves the surficial plume and prior to its initial deposition the model runs of 18 000 yr bp with 100 m lower sea level wetter and colder climate and the river mouth assumed to be at the canyon show that the plume is able to transport sand silt and clay out of the canyon and onto the continental slope

paleoclimate history of galapagos surface waters over the last 135 000 yr abstract records of planktonic and benthic foraminiferal 18 o and planktonic mg ca from core tr163 22 just northwest of the galapagos islands reveal a detailed 250 450 year resolution climate history of the region over the last 135 thousand years kyr sea surface temperatures ssts reconstructed from g ruber mg ca averaged 24 3 0 4 c during the holocene 22 6 0 6 c during marine isotope stage mis 2 3 and 4 and 26 0 0 9 c during mis 5e changes in sst lead changes in both planktonic and benthic 18 o by an average 3 kyr suggesting that sst changes in this region predated continental ice volume changes changes in sst display clear millennial scale variability especially in marine isotope stage 3 with behavior somewhat similar to antarctic proxy air temperature and south pacific sst records removal of the temperature component from the planktonic 18 o record demonstrates that glacial interglacial 18 o water changes at this site were 1 0 0 2 similar to estimates for mean ocean shifts implying that salinity changes due to regional hydrological variation between the last glacial maximum and holocene in the galapagos region were minimal comparison between the tr163 22 sst record and an sst record from core tr163 19 north of the equatorial front reveals a largely similar broad scale climate history suggesting that changes in the galapagos region were caused by large scale forcing rather than by local or regional dynamical changes changes in atmospheric greenhouse forcing are the most plausible explanation for the observed large scale climate changes in the eastern equatorial pacific

effects of ambient currents and waves on gravity driven sediment transport on continental shelves abstract observations from several shelf environments show that down slope gravity driven transport may constitute an important mode of suspended sediment dispersal across shelves and highlight the influence of ambient waves and currents on gravity induced sediment flux the phenomena discussed here involve high concentrations of suspended sediment mixed with seawater and thus differ in genesis from hyperpycnal plumes released directly from sediment laden rivers the field sites examined are the gulf of bohai off the mouth of the yellow river huanghe the northern california shelf off the mouth of the eel river and the louisiana shelf west of the mouths of the mississippi river off the yellow river rapid down slope transport over distances of a few km occurred when frictional resistance induced by strong along shelf currents was temporarily relaxed more prolonged down slope motion over longer distances occurred following floods of the eel river when wave and current agitation provided turbulence to sustain gravity driven transport of fluid mud on the louisiana inner shelf the down slope gravity force was much weaker but observations suggest that thin gravity flows may still have occurred in the presence of waves a simple analytical theory is developed that incorporates the influence of ambient shelf currents on gravity driven transport of suspended sediment this theory is quantitatively consistent with the observations from the three sites if the supply of easily suspended sediment is less than the capacity of ambient currents including waves to carry sediment then intense turbulence limits gravity induced sediment transport by increasing the drag at the base of the flow when ambient currents abruptly cease rapid down slope transport can then occur over short distances until the sediment settles such flows do not remain intensely turbulent because the slope of the continental shelf is too gentle to induce shear instability within the gravity flow the maximum sustained rate of gravity induced sediment transport occurs when ambient currents are strong but the supply of easily suspended sediment exceeds the resuspension capacity of the ambient currents feedback then leads to values of the gradient richardson number ri within the flow that are near the critical value of 1 4 this partially damps bottom drag but still allows the generation of sufficient turbulence to maintain sediment in suspension observations also indicate systematic relationships among ri the supply of easily suspended sediment and the bottom drag coefficient acting on the gravity flow

cyclone pumping sediment partitioning and the development of the great barrier reef shelf system a review abstract the modern great barrier reef gbr is part of the world s largest and best known mixed terrigenous carbonate continental margin the gbr shelf contains three shore parallel sedimentary belts an inner shelf zone of terrigenous sedimentation at depths of 0 22 m a middle shelf zone of sediment starvation at depths of 22 40 m and an outer shelf reef tract with its inner edge at ca 40 m depth these zones are controlled by the dynamics of northward fair weather along shelf drift driven by southeasterly trade winds and by the regular passage of tropical cyclones cyclones cause wind driven north directed middle shelf flows in excess of 130 cm s which erode the seabed concentrate the sparse mobile sediment into sand ribbons and advect suspended load onto the outer part of the nearshore terrigenous sediment prism and into inter reef depocentres within the outer shelf reef tract cyclones largely control the input of new sediment into the gbr system via river flooding seabed erosion or reef breakage they also help to control the partitioning and dispersion of the three main shore parallel belts of sediment and hence stratigraphic accumulation acting as a sediment pump especially during interglacial highstands cyclones have exerted great control on the development of the modern gbr province and its sediments by maintaining a broad shelf parallel zone of episodically mobilised sediment and scoured seabed upon which coral reefs have been unable to form cyclones may also have partly controlled the timing of initiation of the first gbr at 0 6 mybp contrary to current models gbr storm beds are most likely to be preserved intact close to the shoreline and become coarser grained away from the shoreline for the central gbr highstand shedding only applies to carbonate sediment at the scale of local reefs system wide oceanographic controls cause high rates of carbonate sedimentation on the slope during both sea level rise and highstand concomitantly terrigenous sediment accumulates fastest on the slope during sea level rise and slowest during sea level lowstand and highstand

the termination of the equatorial undercurrent in the eastern pacific abstract the termination of the equatorial undercurrent euc in the eastern tropical pacific has been studied through analysis of the historical hydrographic station data in the region bounded by 5 n 10 s 80 w and 100 w three distinctive hydrographic features associated with the euc are used along with dynamic topography to trace the mean path of the euc and to investigate aspects of its seasonal variation these features are 1 the 13 c thermostad 2 the high salinity core and 3 the high dissolved oxygen concentration tongue the thermostad is thickest just south of the equator to the west of the galapagos islands with a decreasing maximum extending southeast to the coast of south america near 7 s the thermostad is poorly developed in the mean near the equator east of the galapagos islands this pattern appears to be produced by convergence of eastward flow in the euc and in the southern subsurface countercurrent as the euc flows southeastward from the galapagos this interpretation is supported by the geostrophic flow calculated from the mean dynamic topography the isolated high salinity core is found to the south of the equator and is traced all the way to the coast of south america the high oxygen tongue associated with the euc splits at the galapagos with the southward projecting branch disappearing rapidly as an identifiable feature the other branch of the tongue continues around the islands to the north and along the equator to the coast of ecuador some evidence of recirculation of euc water in the south equatorial current is seen to the northwest of the galapagos a strong seasonal pulsation of the euc affects these features in a consistent way with the pulse occurring first west of the galapagos and later along the coast the thermostad first reaches maximum development and then a pronounced downwelling of the thermocline occurs rapidly leading to minimum thermostad extent associated wtih this downwelling is an eastward protruding high salinity tongue near the equator which is later seen as an isolated core along the coast at 4 s and 6 s pressure gradients along the equator and the coast inferred from dynamic topography suggest that this pulse is associated with an increase in the flow of the euc and ultimately the peru chile undercurrent the pressure gradient reversal along the equator occurs prior to the annual wind reversal suggesting that remote forcing plays a role in the seasonal pulse of the euc in the eastern pacific

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

spatio temporal variability of the equatorial front in the eastern tropical pacific from remote sensing salinity data 2010 2015 abstract in the eastern pacific the equatorial front ef separates the relatively cold and salty equatorial surface water to the south from the relatively warm and fresh surface tropical water to the north although previous studies have succinctly described the main ef characteristics little is known about its spatial pattern and temporal variability beyond seasonal scale in this study we used 5 years of remote sensing sea surface salinity sss data acquired by smos in a synoptic and continuous manner to describe the spatio temporal variability of the haline ef hef after validating the sss satellite dataset with concomitant in situ data we developed an algorithm based on maximum sss gradients to automatically identify the hef position on every smos sss map this algorithm applied to the 4 day resolution smos sss product allows us to investigate the spatio temporal variability of the hef between 2010 and 2015 seasonally west of the galapagos islands the hef is centered at 2 n and shows relatively weak meridional displacements in contrast east of the galapagos archipelago the hef varies on average between 4 s in spring and 2 n in autumn at intraseasonal scales the hef shows meridional fluctuations of 1 25 around its average position these intraseasonal fluctuations are partly related to kelvin waves which was more noticeable since 2014 over the 5 year studied period the hef position showed a northward trend of 0 3 0 4 latitudinal degrees per year between 2010 and 2015 however the relatively short time series does not permit to investigate whether this low frequency trend is due to long term variability or interdecadal fluctuations

lagrangian study of the panama bight and surrounding regions near surface circulation of the panama bight and surrounding regions 0 9 n 73 w 90 w was studied using satellite tracked drifter trajectories from 1979 2004 this region encompasses three major currents showing typical velocities of 30 cm s 1 1 the eastward north equatorial counter current necc 2 the near circular panama bight cyclonic gyre pbcg and 3 the westward south equatorial current sec we do not observe significant modification of the mean surface circulation during el nino southern oscillation events even if the sec is slightly reinforced during relatively warm el nino periods at seasonal scales the circulation is strongly controlled by the activity of the panama wind jet in boreal winter the currents are stronger and an anticyclonic cell is present west of the pbcg this dipole leads to a strong 200 km wide southward current which then disappears during the rest of the year in summer the three major currents have reduced intensity by 30 40 large scale current vorticity shows that the upwelling associated with the pbcg is also 3 4 times stronger in winter than during summer months the kinetic energy is largely dominated by eddy activity and its intensity is double in winter than during summer ageostrophic motions and eddy activity appear to have a substantial impact on the energy spatial distribution in the necc and sec regions lagrangian scales are anisotropic and zonally enhanced in the direction of the mean currents the typical integral time and length scales of these regions are 2 5 days and 50 60 km in the zonal direction and 1 5 days and 25 30 km in the meridional direction lateral eddy diffusivity coefficients are on the order of 11 14 107 cm2 s 1 zonally and 5 6 107 cm2 s 1 meridionally in contrast in the pbcg region the lagrangian characteristics are isotropic with typical timescales of 1 7 days space scales of 30 km and eddy diffusivity coefficients of 6 107 cm2 s 1 in both directions

unprecedented recent warming of surface temperatures in the eastern tropical pacific ocean climate variability in the tropical pacific ocean influences climate across much of the planet a diatom based record of sea surface temperatures from el junco lake galapagos reveals that the most recent half century is the warmest 50 year period in the past 1 200 years through its intimate connection with the el nino southern oscillation system climate variability in the tropical pacific ocean influences climate across much of the planet but the history of temperature change in the tropical pacific ocean during recent millennia is poorly known the available annually resolved records1 2 are discontinuous and rarely span more than a few centuries longer records at coarser temporal resolution suggest that significant oceanographic changes observed at multi year to multi century resolution have had important effects on global climate3 4 5 here we use a diatom record from el junco lake galapagos to produce a calibrated continuous record of sea surface temperature in the eastern tropical pacific ocean at subdecadal resolution spanning the past 1 200 years our reconstruction reveals that the most recent 50 years are the warmest 50 year period within the record because our diatom based sea surface temperature index resembles northern hemisphere temperature reconstructions we suggest that with continued anthropogenic warming the eastern tropical pacific ocean may continue to warm

factors influencing the island mass effect of the galapagos archipelago 1 enhanced phytoplankton biomass in the wake of the galapagos islands is thought to result from an island mass effect ime fueled by upwelling of the equatorial undercurrent euc and by natural iron enrichment from the island platform annual means of five variables describing the thermocline the pycnocline and the availability of nitrate at the surface were derived from the world ocean atlas 1998 woa98 the first principal component of these variables explained 55 8 of the variance corroborating that the galapagos ime is associated with features of the euc mainly a shallow thermocline pycnocline and its vertical spreading in the vicinity of the galapagos regression analysis of seawifs derived chlorophyll a chl on the woa98 variables indicated that the depth of the thermocline and nitrate availability explain 91 9 of the chl variance a secondary ime of enhanced chl levels associated with the wind sheltered area north of isabela island was evident in the residual variability

high resolution measurements made across a tidal intrusion front high spatial resolution current and hydrographic measurements have been made across a tidal intrusion front in the james river estuary virginia during the first few hours of flood the current measurements were made with a surface towed acoustic doppler current profiler which was used in both profile and side scan configurations the profile data show a 30 50 cm s 1 inflow of water from the chesapeake bay a time evolving across front density gradient and downward velocity beneath the front of about 15 cm s 1 the side scan observations directly measure the across front change in current and provide a view of the three dimensional character of the frontal interface the near surface horizontal strain rate a critical parameter for understanding the modulation of surface waves across the front is estimated to be about 0 04 s 1 using phillips 1984 theory as a guide fronts with such large strain rates as these should be discernible in imagery from the ers 1 c band synthetic aperture radar for wind speeds of 4 m s 1

3 d velocity structure from simultaneous traveltime inversion of in line seismic data along intersecting profiles summary a method for simultaneously inverting in line seismic traveltime data recorded along a network of intersecting linear profiles is presented consistency of the models at the intersection points is assured and permits 3 d structural and velocity variations to be inferred by interpolation between the profiles common model parameters are used at the intersection points thereby minimizing the number of independent model parameters and maximizing the degree of constraint of each 2 d model the method has two main applications 1 to obtain the most constrained set of 2 d models for each profile if subsequent 3 d modelling is not possible and 2 to provide a 3 d starting model for subsequent 3 d modelling if there is sufficient azimuthal data coverage some ideal line geometries are suggested but the method is applicable to any set of two or more lines with one or more intersection point a new interpretation of crustal seismic data from the peace river arch region of the western canada sedimentary basin consisting of four profiles is presented using the simultaneous inversion method for both refracted and reflected arrivals the results are generally consistent with a previous interpretation of the data in which each profile was modelled independently of the others using 2 d forward modelling however the models obtained by inversion contain less lateral heterogeneity that more accurately reflects the resolution limits of the data this illustrates the need to reconsider data that have only been analysed by forward modelling

thrust wrench interference tectonics in the gulf of cadiz africa iberia plate boundary in the north east atlantic insights from analog models abstract in the gulf of cadiz key segment of the africa iberia plate boundary north east atlantic ocean three main different modes of tectonic interference between a recently identified wrench system swim and the gulf of cadiz accretionary wedge gcaw were tested through analog sand box modeling a an active accretionary wedge on top of a pre existent inactive basement fault b an active strike slip fault cutting a previously formed inactive accretionary wedge and c simultaneous activity of both the accretionary wedge and the strike slip fault the results we obtained and the comparison with the natural deformation pattern favor a tectonic evolution comprising two main steps i the formation of the gulf of cadiz accretionary wedge on top of inactive tethyan related basement faults middle miocene to 1 8 ma ii subsequent reactivation of these basement faults with dextral strike slip motion 1 8 ma to present simultaneously with continued tectonic accretion in the gcaw these results exclude the possibility of ongoing active swim wrench system cross cutting an inactive gcaw structure our results also support a new interpretation of the swim wrench system as fundamentally resulting from strike slip reactivation of an old tethyan related plate boundary

seismic moment assessment of earthquakes in stable continental regions iii new madrid 1811 1812 charleston 1886 and lisbon 1755 summary the sizes of three major or great historical earthquakes are reassessed using the isoseismal area regression tools developed in parts i and ii of this study of stable continental region scr seismicity the earthquakes are 1811 new madrid central united states and its following sequence 1886 charleston coastal south carolina and 1755 lisbon oceanic intraplate off the continental shelf of portugal the analysis confirms the large size of these events and for the first time places constraints on the uncertainty of their seismic moment release because of the exceptionally low seismic wave attenuation of eastern north america ena a separate north american regression of seismic moment on isoseismal area was developed additionally the unknown western extents of the new madrid isoseismal areas were calibrated with the patterns of the m 6 3 6 6 1843 and 1895 earthquakes application of part ii analysis procedures with these corrections yields new madrid size estimates expressed as moment magnitude of m 8 1 0 31 for the 1811 december 16 m 7 8 0 33 for the 1812 january 23 and m 8 0 0 33 for the 1812 february 7 principal events the charleston earthquake s magnitude decreases from m 7 4 to m 7 3 0 26 after compensation for the effect of coastal plain sediments on its inner isoseismals intensity regressions for lisbon are calibrated against the isoseismal pattern of the nearly co located m 7 8 1969 st vincent earthquake which in this case increases the predicted size of lisbon from m 8 4 to m 8 7 0 39 these size estimates are supported by data from independent phenomena extent and severity of liquefaction the maximum distance of induced landslides and for lisbon tsunami wave amplitudes estimated source parameters are controlled by crustal or lithospheric temperature which governs the depth extent of brittle faulting using estimated continental and oceanic geotherms viable fault lengths are 30 80 km for charleston 120 180 km for 1811 new madrid and 180 280 km for lisbon for average displacements of 2 4 m 8 11 m and 10 14 m respectively and for average static stress and strain drops at the estimated seismic moments of this study the 1811 new madrid and the 1755 lisbon events are respectively the largest known scr and oceanic lithosphere earthquakes

seafloor hydrothermal systems driven by the serpentinization of peridotite 1 heat balance models predict that heat released upon serpentinization of peridotites can result in a wide range of hydrothermal venting temperatures temperatures ranging from a few to a few tens of c are likely temperatures 300 c may be possible in conjunction with significant heat input from below low fluid flow rate 10 kg s a high rate of serpentinization 100 kg s or some combination of these factors the volume of fresh lithosphere accessible to serpentinization may limit the duration of an episode of hydrothermal venting when the serpentinization rate exceeds 10 kg s hydrothermal venting driven by high rates of serpentinization may occur episodically as new reaction surfaces are made available by tectonic activity or crack propagation given the uncertainty of several parameters the current episode of venting at the lost city vent field on the mid atlantic ridge may range from 102 to 104 years

internal hydraulic flows on the continental shelf high drag states over a small bank observations of currents hydrography and turbulence provide unambiguous evidence for hydraulic control of flow over an isolated three dimensional topographic feature on oregon s continental shelf the flow becomes critical at the crest of the bank forming a strong supercritical downslope flow in the lower layer farther downstream internal hydraulic jumps form as the bottom flow becomes subcritical as a consequence turbulence is greatly enhanced in the bottom boundary layer in the sheared interface above the swiftly flowing bottom current and in the internal hydraulic jump the dissipation rate of turbulent energy is consistent with the mean energy removal rate for a hydraulic jump in an idealized two layer flow this enhanced turbulence constitutes a high drag state of the flow in which the form drag introduced by the topography exerts significant influence on the flow around it and mixing is increased 102 103 times the background values

serpentinization faults and their role in the tectonics of slow spreading ridges two major families of faults dominate the tectonics of slow spreading ridges the transform faults paralleling the spreading direction and the gloria faults observed by large scale side scan sonar normal to it the observation of microearthquakes to 8 km depth beneath the mid atlantic ridge axis implies that not only the transform faults but the gloria faults as well penetrate into the mantle since large horizontal temperature gradients are often associated with them both types of fault provide the means of circulating large volumes of sea water through the oceanic mantle under these conditions serpentinization of upper mantle rocks will occur in a predictable way its influence on the tectonic development of slow spreading ridges is discussed

seismic evidence of exhumed mantle rock basement at the gorringe bank and the adjacent horseshoe and tagus abyssal plains sw iberia the gorringe bank is a gigantic seamount that separates the horseshoe and tagus abyssal plains offshore sw iberia in a zone that hosts the convergent boundary between the africa and eurasia plates although the region has been the focus of numerous investigations since the early 1970s the lack of appropriate geophysical data makes the nature of the basement and thus the origin of the structures still debated in this work we present combined p wave seismic velocity and gravity models along a transect that crosses the gorringe bank from the tagus to the horseshoe abyssal plains the p wave velocity structure of the basement is similar in the tagus and horseshoe plains it shows a 2 5 3 0 km thick top layer with a velocity gradient twice stronger than oceanic layer 2 and an abrupt change to an underlying layer with a five fold weaker gradient velocity and density is lower beneath the gorringe bank probably due to enhanced fracturing that have led to rock disaggregation in the sediment starved northern flank in contrast to previous velocity models of this region there is no evidence of a sharp crust mantle boundary in any of the record sections the modelling results indicate that the sediment overlays directly serpentinite rock exhumed from the mantle with a degree of serpentinization decreasing from a maximum of 70 80 under the top of gorringe bank to less than 5 at a depth of 20 km we propose that the three domains were originally part of a single serpentine rock band of nature and possibly origin similar to the iberia abyssal plain ocean continent transition which was probably generated during the earliest phase of the north atlantic opening that followed continental crust breakup early cretaceous during the miocene the nw se trending eurasia africa convergence resulted in thrusting of the southeastern segment of the exhumed serpentinite band over the northwestern one forming the gorringe bank the local deformation associated to plate convergence and uplift could have promoted pervasive rock fracturing of the overriding plate leading eventually to rock disaggregation in the northern flank of the gb which could be now a potential source of rock avalanches and tsunamis

seasonal patterns of sea surface temperature and ocean color around the galapagos regional and local influences abstract monthly climatologies of satellite derived sea surface temperature sst from avhrr and ocean color from ocean color and temperature scanner and sea viewing wide field of view sensor around the galapagos archipelago are used to estimate the mean seasonal cycle of these properties and their relationship to the equatorial circulation in this oceanographically complex region harmonic analysis of the climatological time series indicates a best fit with annual and semi annual constituents the annual amplitude is the dominant signal in sst corresponding to the basin wide seasonal cycle of warming and cooling associated with the north south migration of the intertropical convergence zone influx of upwelled water from the panama bight into the northeastern part of the study area is also consistent with the annual signal an empirical orthogonal function decomposition identifies two main spatial patterns with amplitude time series representing out of phase annual cycles the dominant mode corresponds to the strengthening of the equatorial front and the south equatorial current during the second part of the year this mode explains 92 2 of the sst variance and 82 9 of the ocean color variance the second mode is consistent with the topographically induced upwelling of the equatorial undercurrent on the western side of the archipelago and with influx of upwelled panama bight water on the eastern side both reaching their peak during the first part of the year this mode accounts for 6 of the sst variance and 7 7 of the ocean color variance the seasonal evolution of water column temperature and nitrate from world ocean atlas 1998 climatologies is consistent with the satellite derived patterns a slight tilt aligned with the east west axis of the galapagos platform outlined by the 2000 m isobath is evident in all property fields suggesting that the presence of the archipelago introduces a small but noticeable perturbation to the large scale currents and property gradients of the eastern equatorial pacific

a decadal record of underflows from a coastal river into the deep sea four periods of extreme turbidity were measured in monterey submarine canyon over the past 12 yr these turbid events occurred simultaneously with the four largest flood events of the nearby salinas river they filled the canyon with fresher warmer and apparently buoyant water that extended to depths below 1 km the low salinity signature must reflect underflow of the salinas river plume that was driven to depth by negative buoyancy contributed from the suspended sediment a flux estimate for the 1995 event suggests that one half or more of the river s suspended sediment load was carried down the canyon in the underflow underflows may contribute significantly to the carbon budget of the continental slope

direct measurements of upper ocean currents and water properties across the tropical pacific during the 1990s abstract meridional sections of upper ocean zonal currents potential temperature and salinity are estimated at ten longitudes from 143 e to 95 w using conductivity temperature depth and acoustic doppler current profiler data from 172 synoptic sections taken in the tropical pacific between 138 e and 86 w mostly in the 1990s data reduction is carried out in a potential isopycnal and mixed layer framework to preserve a sharp pycnocline a mixed layer water property extrema and velocity extrema mean zonal currents potential temperatures and salinities are produced at each longitude the seasonal cycles of these fields are also estimated as well as a simple el nino southern oscillation enso cycle zonal sections along the equator are also presented properties of the near equatorial zonal currents including transports temperatures and salinities are estimated separately from the synoptic sections the seasonal cycles of these quantities and their correlations with the southern oscillation index are investigated the work is distinguished from most existing literature in that direct estimates of zonal velocity are combined with contemporaneous temperature and salinity data allowing trans pacific estimates of near equatorial current transports and properties including those of the northern branch of the south equatorial current the new guinea coastal undercurrent and the equatorial undercurrent

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

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

directions of lithosphere interactions in the pamir hindu kush junction inferred from anisotropic tomography the pamir and hindu kush are examples of a puzzling collision system where a complex junction of colliding lithospheric plates coexists with intermediate depth seismicity at 300 km in this study

the seismic structure of 140 myr old crust in the western central atlantic ocean summary a detailed seismic refraction experiment using explosive sources and ocean bottom hydrophone obh receivers was carried out over mesozoic magnetic anomaly m17 about 300 km south west of bermuda amplitude and travel time interpretations show this 140myr old crust to be uniform on the seismic scale over a lateral distance of almost 1ookm the best estimate of the velocity structure consists of an average 700 m of sediment overlying a total thickness of igneous crust of 7 2 km the principle components beginning with a 5 km s l velocity at the top of layer 2 are a 0 4 km thickness with gradient 1 1 s a 1 9km thickness with gradient 0 64s 1 7 km thickness with gradient 0 1 s a 3 2 km thickness with zero gradient and constant velocity of 7 km s below which lies a 0 5 km thick moho transition zone the uppermost few kilometres of the upper mantle apparently has little or no velocity gradient the normal incidence two way reflection time through this structure agrees to better than 0 1 s with the location of the moho reflection seen on the ipod usgs multichannel reflection profile that passes within 1 km of this experiment the travel times of the converted shear wave arrivals constrain poisson s ratio in the igneous crust to 0 28 k 0 01 a delay time study of a 20km radius circle of shots fired around a small 6 km five element array of ocean bottom hydrophone receivers confirms the uniformity of structure of this old crust on the scale of a few kilometres the observed differences in shallow crustal delay time between the five obh instruments can be wholly attributed to differences in sediment thickness beneath the receivers as determined by a deep towed hydrophone seismic reflection profile these experiments were located clear of fracture zones or other structural anomalies in a region of well defined linear constant amplitude magnetic anomalies we propose that this was the primary cause of our result of lateral uniformity of structure on the scale of a few kilometres we suggest that the reason that such a simple uniform structural model for oceanic crust is not supported by the historical seismic refraction dataset is that a large proportion of the old experiments were poorly located relative to fracture zones and other structural anomalies unknown at the time and the

thrust wrench interference between major active faults in the gulf of cadiz africa eurasia plate boundary offshore sw iberia tectonic implications from coupled analog and numerical modeling abstract analog and numerical modeling experiments were carried out to investigate the tectonic interference between intersecting major active strike slip and thrust faults in the gulf of cadiz africa eurasia plate boundary offshore sw iberia the obtained results show that newly mapped tectonic features located in the fault intersection area corner zone consist mostly in oblique dextral reverse faults that accommodate significant strain partitioning modeling of this corner zone faults show that they have endured some degree of rotation displaying successive evolving geometries and kinematics numerical modeling results further show that an interbedded shallow soft layer accounting for a regional late miocene gravitational chaotic unit could explain the mild bathymetric expression of the fault pattern in the corner zone moreover a recognized depth discrepancy between the upper crust interference fault pattern and the lithospheric mantle seismicity is interpreted as a manifestation of similar thrust wrench tectonic interference at different lithospheric depths accordingly an intermediate lower crust upper mantle aseismic i e softened depth domain could be explained by pervasive alteration serpentinization prompted by fluid percolation through fault related fractures associated with the newly revealed corner zone fault network overall obtained results reinforce the relevance of a thrust wrench multi rupture seismic scenario as the main cause for the moderate seismicity mw

closure of the africa eurasia north america plate motion circuit and tectonics of the gloria fault we examine the closure of the current plate motion circuit between the african north american and eurasian plates to test whether these plates are rigid and whether the gloria fault is an active transform fault we also investigate the possible existence of microplates that have been previously proposed to lie along these plate boundaries and compare the predicted direction of motion along the african eurasian plate boundary in the mediterranean with the direction of slip observed in earthquakes from marine geophysical data we obtain 13 transform fault azimuths and 40 3 m y average spreading rates 34 of which are determined from comparison of synthetic magnetic anomaly profiles to 140 observed profiles slip vectors from 32 earthquake focal mechanisms further describe plate motion detailed magnetic surveys north of iceland provide 11 rates in a region where prior plate motion models had few data magnetic profiles north of the azores triple junction record a rate of 24 mm yr 4 mm yr slower than used by prior models gloria and sea beam surveys accurately measure the azimuths of seven transform faults our plate motion model fits six of the seven within 2 two transform faults surveyed by gloria side scan sonar lie near famous area transform faults a and b and give azimuths 13 clockwise of them because recent studies show that short offset transforms such as transforms a and b are in many places oblique to the direction of plate motion we exclude azimuths from transforms with less than 35 km offset the best fitting and closure enforced vectors fit the data well except for a small systematic misfit to the slip vectors on right lateral slipping transforms slip vectors tend to be a few degrees clockwise of plate motion and mapped fault azimuths whereas on left lateral slipping transforms slip vectors tend to be a few degrees counterclockwise of plate motion and mapped fault azimuths we search the long eurasia north america boundary for evidence of an additional plate but find no systematic misfits to the data in particular if a spitsbergen plate exists and moves relative to eurasia its motion is less than 3 mm yr an africa eurasia euler vector determined by adding the eurasia north america and africa north america euler vectors is consistent with the gloria fault trend and with slip vectors from eastern azores gibraltar ridge focal mechanisms a small circle centered at the africa eurasia closure enforced pole fits the trace of the gloria fault the model in which closure was enforced predicts 4 mm yr slip across the azores gibraltar ridge and west northwest convergence near gibraltar 45 more oblique than suggested by a recent model based on compressive axes of focal mechanisms moreover our model predicts directions of plate motion that agree well with northwest trending slip vectors from thrust earthquakes between gibraltar and sicily because closure enforced vectors fit the data nearly as well as the best fitting vectors we conclude that the data are consistent with a rigid plate model and with the gloria fault being a transform fault

seasonal variability in the equatorial pacific abstract five to nine years of observations from the tropical atmosphere ocean array in the tropical pacific are used to document the seasonal cycle of surface winds zonal currents sst thermocline depth and dynamic height along the equator the normally westward surface zonal current reverses direction in late boreal spring and early summer this seasonal variation in the zonal surface current propagates westward as do seasonal variations in the equatorial undercurrent and zonal surface winds at the equator at 5 n and 5 s the seasonal variations in the 20 c isotherm and dynamic height also propagate westward conversely at the equator in the eastern and central pacific the variations in 20 c isotherm depth and dynamic height propagate eastward these seasonal variations are interpreted by means of a simple dynamical model based on linear equatorial wave theory model results indicate that seasonal variability between 5 n and 5 s is dominated by wind forced equatorial kelvin waves and first meridi

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles the iam 9 transect at 40 20 n we present a crust and mantle velocity structure for the west iberia passive continental margin derived from a 320 km long wide angle seismic profile acquired in the southern iberia abyssal plain we observe a 170 km wide ocean continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault bounded blocks of continental crust the profile lies 40 km south of the transect sampled by ocean drilling program odp legs 149 and 173 the transition zone structure can be divided into an upper layer 2 4 km thick with velocities of between 4 5 and 7 0 km s 1 and generally a high velocity gradient 1 s 1 and a lower layer up to 4 km thick with a velocity of 7 6 km s 1 and a low velocity gradient a weak moho reflection in this zone was seen only on wide angle profiles at an offset of 30 km the upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope conversely the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust on the coincident seismic reflection profile fault bounded crustal blocks identified in unequivocal extended continental crust are not observed in the transition zone the upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges but no oceanic layer 3 velocity structure is present while magnetic anomalies have been identified within the transition zone they have not been modeled successfully as seafloor spreading magnetic anomalies nor do they generally form long linear margin parallel features finally odp boreholes 40 km north of our profile and within the interpreted transition zone have recovered up to 140 m thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material we conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust although current melting models predict a considerably thicker crust of decompression melt products we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite

seismic evidence for the presence of jurassic oceanic crust in the central gulf of cadiz sw iberian margin abstract we investigate the crustal structure of the sw iberian margin along a 340 km long refraction and wide angle reflection seismic profile crossing from the central gulf of cadiz to the variscan continental margin in the algarve southern portugal the seismic velocity and crustal geometry model obtained by joint refraction and reflection travel time inversion reveal three distinct crustal domains the 28 30 km thick variscan crust in the north a 60 km wide transition zone offshore where the crust abruptly thins 20 km and finally a 7 km thick and 150 km wide crustal section that appears to be oceanic in nature the oceanic crust is overlain by a 1 3 km thick section of mesozoic to eocene sediments with an additional 3 4 km of low velocity unconsolidated sediments on top belonging to the miocene age gulf of cadiz imbricated wedge the sharp transition between continental and oceanic crust is best explained by an initial rifting setting as a transform margin during the early jurassic that followed the continental break up in the central atlantic the narrow oceanic basin would have formed during an oblique rifting and seafloor spreading episode between iberia and africa that started shortly thereafter bajocian and lasted up to the initiation of oceanic spreading in the north atlantic at the tithonian late jurassic earliest cretaceous the velocity model displays four wide prominent south dipping low velocity anomalies which seem to be related with the presence of crustal scale faults previously identified in the area some of which could well be extensional faults generated during this rifting episode we propose that this oceanic plate segment is the last remnant of an oceanic corridor that once connected the alpine tethys with the atlantic ocean so it is in turn one of the oldest oceanic crustal fragments currently preserved on earth the presence of oceanic crust in the central gulf of cadiz is consistent with geodynamic models suggesting the existence of a narrow westward retreating oceanic slab beneath the gibraltar arc alboran basin system

the diffuse plate boundary of nubia and iberia in the western mediterranean crustal deformation evidence for viscous coupling and fragmented lithosphere a spatially dense gnss based crustal velocity field for the iberian peninsula and northern africa allows us to provide new insights into two main tectonic processes currently occurring in this area in particular we provide for the first time clear evidence for a large scale clockwise rotation of the iberian peninsula with respect to stable eurasia euler pole component n42 612 w1 833 clockwise rotation rate of 0 07 deg myr we favor the interpretation that this pattern reflects the quasi continuous straining of the ductile lithosphere in some sectors of south and western iberia in response to viscous coupling of the nw nubia and iberian plate boundary in the gulf of cadiz we furnish evidence for a fragmentation of the western mediterranean basin into independent crustal tectonic blocks which are delimited by inherited lithospheric shear structures among these blocks an oceanic like western algerian one is currently transferring a significant fraction of the nubia eurasia convergence rate into the eastern betics se iberia and likely causing the eastward motion of the baleares promontory these processes can be mainly explained by spatially variable lithospheric plate forces imposed along the nubia eurasia convergence boundary

from unthinned continent to ocean the deep structure of the west iberia passive continental margin at 38 n abstract the crustal and lithospheric mantle structure at the south segment of the west iberian margin was investigated along a 370 km long seismic transect the transect goes from unthinned continental crust onshore to oceanic crust crossing the ocean continent transition oct zone the wide angle data set includes recordings from 6 obss and 2 inland seismic stations kinematic and dynamic modeling provided a 2d velocity model that proved to be consistent with the modeled free air anomaly data the interpretation of coincident multi channel near vertical and wide angle reflection data sets allowed the identification of four main crustal domains i continental east of 9 4 w ii continental thinning 9 4 w 9 7 w iii transitional 9 7 w 10 5 w and iv oceanic west of 10 5 w in the continental domain the complete crustal section of slightly thinned continental crust is present the upper ucc 5 1 6 0 km s and the lower continental crust lcc 6 9 7 2 km s are seismically reflective and have intermediate to low p wave velocity gradients the middle continental crust mcc 6 35 6 45 km s is generally unreflective with low velocity gradient the main thinning of the continental crust occurs in the thinning domain by attenuation of the ucc and the lcc major thinning of the mcc starts to the west of the lcc pinchout point where it rests directly upon the mantle in the thinning domain the moho slope is at least 13 and the continental crust thickness decreases seaward from 22 to 11 km over a 35 km distance stretched by a factor of 1 5 to 3 in the oceanic domain a two layer high gradient igneous crust 5 3 6 0 km s 6 5 7 4 km s was modeled the intra crustal interface correlates with prominent mid basement 10 15 km long reflections in the multi channel seismic profile strong secondary reflected pmp phases require a first order discontinuity at the moho the sedimentary cover can be as thick as 5 km and the igneous crustal thickness varies from 4 to 11 km in the west where the profile reaches the madeira tore rise in the transitional domain the crust has a complex structure that varies both horizontally and vertically beneath the continental slope it includes exhumed continental crust 6 15 6 45 km s strong diffractions were modeled to originate at the lower interface of this layer the western segment of this transitional domain is highly reflective at all levels probably due to dykes and sills according to the high apparent susceptibility and density modeled at this location sub moho mantle velocity is found to be 8 0 km s but velocities smaller than 8 0 km s confined to short segments are not excluded by the data strong p wave wide angle reflections are modeled to originate at depth of 20 km within the lithospheric mantle under the eastern segment of the oceanic domain or even deeper at the transitional domain suggesting a layered structure for the lithospheric mantle both interface depths and velocities of the continental section are in good agreement to the conjugate newfoundland margin a 40 km wide oct having a geophysical signature distinct from the oct to the north favors a two pulse continental breakup

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

seismicity source mechanisms and tectonics of the azores gibraltar plate boundary abstract a study and partial revision of the seismicity and source mechanisms of earthquakes are presented for the oceanic plate boundary of eurasia and africa from the azores islands to the strait of gibraltar focal mechanisms of 36 earthquakes wave form modelling of 4 and seismic moments and dimensions of 28 have been calculated these results indicate that the boundary is divided into three sections with different characteristics the azores the central section and the east section the azores section has high seismicity of moderate magnitude and a low slip rate 0 76 cm yr the predominant mechanism is normal or transform faulting with horizontal tensions on average in a n25 e direction in the central section large earthquakes occur with strike slip right lateral motion along faults in an e w direction there is a high slip rate 3 39 cm yr the east section is of more complex nature large earthquakes also occur but the predominant mechanism is reverse faulting with a horizontal pressure axis in the n30 w direction resulting in underthrusting of africa from the slip vectors the pole of rotation for the african plate is located at 28 2 n 21 1 w nearer to the boundary than previous locations close to the continent there is a continuation in the ocean crust of some structural features of the iberian peninsula

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

holocene changes in eastern tropical pacific climate inferred from a galapagos lake sediment record abstract paleoclimate records from the tropical pacific suggest the early to mid holocene was a period of reduced el nino southern oscillation enso variability with a transition to modern increased enso frequency occurring some time in the last few thousand years however the nature and timing of this shift remains uncertain due to the discontinuous nature and or coarse resolution of many enso proxies as well as a lack of agreement between previously published records a new continuous climate record from el junco crater lake in the galapagos islands reveals several abrupt changes in lake level and precipitation through the holocene hydroclimatic model simulations suggest that el junco lake level responds sensitively to increases in precipitation associated with el nino events rising during wet el nino events and falling during the intervening dry periods grain size data from el junco sediment cores indicate past lake level variability likely associated with changing seasonal precipitation and enso frequency the grain size data suggest increased precipitation intensity prior to 9000 120 cal years bp and after 4200 130 cal years bp as well as a two step increase in precipitation at 3200 160 and 2000 100 cal years bp maximum holocene precipitation and inferred enso variability occurred between 2000 100 and 1500 70 cal years bp during the same period that six other independent proxy records suggest higher enso frequency and longer stronger el nino events decreasing sediment carbon nitrogen c n ratios in el junco sediments indicate rising lake levels from the early holocene to present corroborating the grain size data the inferred increase in precipitation at 4200 130 cal years bp and at 2000 100 cal years bp coincides with decreasing southwest asian and east asian monsoon intensity suggesting tropical pacific climate and the asian monsoon were interconnected systems at centennial to millennial timescales during the holocene a weakening trend in the asian monsoon and the trend toward wetter conditions at el junco also coincide with a trend toward cooler and drier conditions inferred from cariaco basin sediment proxies from the mid holocene to present suggesting the migration of the intertropical convergence zone itcz likely influenced hydrological changes in both the eastern tropical pacific and the asian monsoon region during the holocene

modes of continental rifting as a function of ductile strain localization in the lithospheric mantle analogue and numerical models have shown that the strength of the lithospheric mantle controls the mode of lithosphere deformation in extension the presence or absence of a high strength brittle mantle respectively leads to localized or distributed rifting however first order geophysical data question the existence of such a brittle mantle here we use 2 d finite element large strain modelling to quantify the impact of a ductile localizing mantle instead of brittle in triggering continental rifting as a novelty the mantle rheology considers the effect of grain boundary sliding during strain induced grain size reduction which may promote a significant strength drop and subsequent strain localization at low mantle temperature 700 800 c our results reveal that such ductile localizing mantle implies varying modes of continental rifting that mainly depend on both the amount of weakening in the ductile mantle and the strength of the lower ductile crust a medium to strong lower crust implies coupling between the upper crust and ductile localizing mantle yielding to narrow continental rifting in contrast a weak lower crust implies decoupling between the upper crust and ductile localizing mantle giving rise to a switch from distributed faulting at incipient strain to localized faulting at large strain ductile strain localization in the lithospheric mantle is therefore sufficient to trigger continental rifting although a critical amount of weakening is required such ductile localizing mantle provides a relevant geological and mechanical alternative to the brittle mantle it moreover provides a wider variety of modes of upper crustal faulting that are commonly observed in nature

evidence for active subduction beneath gibraltar we report on a marine seismic survey that images an active accretionary wedge west of gibraltar ramp thrusts offset the seafloor and sole out to an east dipping decollement indicating ongoing westward vergent tectonic shortening new traveltime tomographic re sults image a slab of oceanic lithosphere descending from the atlantic domain of the gulf of cadiz passing through intermediate depth 60 120 km seismicity beneath the west ernmost alboran sea and merging with a region of deep focus earthquakes 600 660 km below granada spain together these new data provide compelling evidence for an active east dipping subduction zone

the crustal structure of the cocos ridge off costa rica 1 the submarine cocos ridge in the northwestern panama basin a bathymetric feature more than 1000 km long and 250 500 km broad is about 2 km shallower than the adjacent basin it is generally interpreted as the trace of the galapagos hot spot two 127 and 260 km long seismic wide angle sections were recorded along and across this ridge offshore the osa peninsula costa rica crustal thickening is seen everywhere along the sections on the northwestern outer ridge flank increased thickness is exclusively attributed to the upper crust and expressed by 2 km thick flow basalts the quepos plateau caps the upper crust in this area toward the center of the cocos ridge the moho deepens from 11 12 to 21 km depth and crustal thickening is almost entirely attributed to the lower crust which makes up 80 of the crust and is three times the thickness of normal oceanic lower crust it is homogeneously structured and the velocities which range from 6 5 km s at the top to 7 35 km s at the base are comparable to normal lower crust under these depth conditions and suggest no differences to a gabbroic rock composition similarities to the crustal velocity structure of iceland central kerguelen plateau and broken ridge are consistent with a formation of this 13 15 ma old cocos ridge segment by excessive magmatism in a near plate boundary setting

a suspended sediment front in the severn estuary a zone of marked local gradient in the regional suspended solids field has been located along the axis of the severn estuary this front occupies virtually the same position on ebb and flood and on spring and neap tides although the amplitude of the gradient fluctuates thermohaline fronts have been described in coastal waters1 2 suspended sediment fronts associated with the discharge of major rivers into the sea are common3 5 whilst fronts caused by convergence of secondary circulations within estuaries have also been reported6 7 the previous work describes features with significant differences in the salinity or temperature fields in which any suspended sediment contrasts present are incidental and do not of themselves produce large horizontal density variations in contrast in the severn strong thermo haline gradients in the vicinity of the suspended solids front have not been detected the mechanisms for the formation and maintenance of the front are not yet known

estuary plumes and fronts in shelf waters a layer model abstract a layer model that treats fronts as discontinuities is developed to study the steady state behavior of shallow estuary plumes on the continental shelf the complete range of earth rotation effect is evaluated from small scale or nonrotating plumes kelvin number equal zero to large scale rotating plumes kelvin number equal order one supercritical flow is assumed in the outlet channel and the method of characteristics is used to compute the flow downstream nonrotating plumes have strong boundary fronts and concentrate their greatest layer depth and mass transport offshore near the front but form no coastal current rotating plumes have boundary fronts that weaken soon after discharge form a turning region where coriolis action deflects the flow toward shore and subsequently set up a coastal current soon after its formation this coastal current is bounded offshore by a strong front called the coastal front across which the momentum balance changes from nearly inertial in the turning regi

temperature dependence of frictional healing of westerly granite experimental observations and numerical simulations 1 temperature is believed to have an important control on frictional properties of rocks yet the amount of experimental observations of time dependent rock friction at high temperatures is rather limited in this study we investigated frictional healing of westerly granite in a series of slide hold slide experiments using a direct shear apparatus at ambient temperatures between 20 c and 550 c we observed that at room temperature coefficient of friction increases in proportion to the logarithm of hold time at a rate consistent with findings of previous studies for a given hold time the coefficient of friction linearly increases with temperature but temperature has little effect on the rate of change in static friction with hold time we used a numerical model to investigate whether time dependent increases in real contact area between rough surfaces could account for the observed frictional healing the model incorporates fractal geometry and temperature dependent viscoelasoplastic rheology we explored several candidate rheologies that have been proposed for steady state creep of rocks at high stresses and temperatures none of the tested laws could provide an agreement between the observed and modeled healing behavior given material properties reported in the bulk creep experiments an acceptable fit to the experimental data could be achieved with modified parameters in particular for the power law rheology to provide a reasonable fit to the data the stress exponent needs to be greater than 40 alternative mechanisms include time dependent gouge compaction and increases in bond strength between contacting asperities

age variations of oceanic crust poisson s ratio inversion and a porosity evolution model porosity in the oceanic crust is one of the most important factors influencing measured seismic velocities porosity is particularly important in the uppermost young crust where rapid variations in velocities with depth and crustal age are observed knowledge of the concentration and aspect ratios of inferred crack populations can be improved considerably if estimates of poisson s ratio are available from observations of compressional and shear seismic velocities p and s in this paper i present a joint seismic waveform inversion for p and s velocities are found while maximizing or minimizing poisson s ratio using a hypothesis testing mechanism i apply this method to ocean bottom hydrophone data in 140 ma atlantic crust the resulting solution corridor agrees with laboratory measurements without the low poisson s ratio anomalies at depths of 0 8 1 5 km found by spudich and orcutt 1980 and au and clowes 1984 on younger 15 ma pacific crust compiling other published p and s solutions an age dependent pattern emerges none of the solutions for crust older than 60 ma display the poisson s ratio anomaly i propose a simple crustal evolution model using thin and thick cracks to explain these observations thin cracks preferentially close at shallow depths in the crust producing the localized poisson s ratio anomaly sealing of all cracks by hydrothermal deposits as the crust ages restores the seismic velocities to consistency with laboratory measurements this model is consistent with similar models of crack populations and their evolution from shallow measurements

reevaluation of magnetic chrons in the north atlantic between 35 n and 47 n implications for the formation of the azores triple junction and associated plateau 1 in this paper we present a new magnetic compilation for an area of the north atlantic located between 35 n and 47 n and up to anomaly 33r we also present a strategy to pick magnetic isochrones and compute finite rotation poles this technique is based on a continuous reduction to the pole technique and some basic assumptions regarding the direction of the remanent magnetization vector a cost function that measures the misfit between interpreted and rotated isochrones and the systematic exploitation of the parameter space is used to compute the best set of finite eulerian rotations for the chrons 5 6 6c 11 12 13 18 20 21 22 23 24 25 26 28 29 30 32 33 and 33r this set of chrons and poles is used to discuss the evolution of the north atlantic close to iberia and in particular the onset and early development of the azores triple junction area we show that the relative motion between the eurasian and the african plates can be coherently described in terms of rigid plate kinematics respecting both the anomalies shapes and the precise location of the main structural elements of the area the pico fracture zone the east azores fracture zone and the gloria fault we distinguish from the magnetic point of view two different areas of the azores plateau the south azores domain where almost undisturbed nnw magnetic lineations can be found and the azores domain close to the topographic highs and with no systematic magnetic stripping with the exception of a few recent lineations probably matuyama and brunhes we present an approximate reconstruction of the plate configuration after chron 18 to conclude that the attachment of iberia to eurasia was younger than previously thought lower miocene triggering the formation of the azores domain in which stretching took place essentially in the last 20 ma at an average rate of 3 8 mm a and progressively attaching the south azores domain to the african plate by a northward progression of the triple junction