chirp 2 7 khz echo characters of the south korea plateau east sea styles of mass movement and sediment gravity flow abstract the south korea plateau is a complex of ridges seamount chains troughs and a basin detailed analysis of 12 019 line km chirp 2 7 khz subbottom profiles from the south korea plateau and the eastern continental slope of korea reveals a distinctive zonal distribution of echo types depending systematically on seafloor morphology pelagites hemipelagites type i 2 and basement highs type iii 1 prevail on the ridge summits seamount chains and the upper to middle part of the eastern continental slope of korea and are commonly bounded downslope by creeps type iii 3 and slides slumps type iv 1 that occur extensively over the entire slope areas of the ridges seamount chains and eastern continental slope the mass movement deposits change downslope to debrites and turbidites types ii iii 2 iv 2 and iv 3 in the troughs and onnuri basin suggesting successive downslope evolution from slide and slump to debris flow and turbidity current the voluminous creeps slides and slumps over the entire slope areas of the plateau and eastern continental slope deeper than 300 m in water depth were most likely generated by frequent seismic shakings

kaikoura canyon new zealand active conduit from near shore sediment zones to trench axis channel kaikoura canyon is one of a few major active conduits between a near shore sediment transport system and a deep ocean channel it is the sink for mobile zones of gravel sand and mud that migrate northwards off northeastern south island new zealand it is presently the primary source for the 1500 km long hikurangi channel which supplies overbank turbidites to a filled trench an ocean plateau basin and a distal fan drift swath data seismic profiles side scan sonographs aerial photographs cores grab samples and current meter data are used to define the shape and texture of the canyon and adjacent shelf in order to better understand how coastal processes and canyon interact to supply sediment to the deep ocean kaikoura canyon is 60 km long up to 1200 m deep and generally u shaped in profile its head is within 500 m of the shore and within 200 m of rocky projections from the shore platform in a mountain backed bay without large rivers the canyon head incises the 18 m depth contour and boulders pebble gravel and megarippled coarse sand reach the canyon rim fine sand migrating northward along the shelf under the influence of waves and currents is trapped in a southward projecting canyon head gully which incises the thickest part of the holocene sediment prism it is estimated that about 1 5 106 m3 of sediment falls into the canyon head each year tensional fractures around the canyon rim suggest that sediments in the canyon head gully are unstable gravel turbidites with post last glacial age shells are at or near the seabed in the lower canyon but are blanketed by many thin silt and sand possibly storm generated turbidites in the upper canyon the top gravel contains a twig that is about 170 years old suggesting that the last major collapse in the canyon head coincides with many onshore rockfalls triggered by rupture of a major strike slip plate boundary fault in about 1833 an underlying gravel is about 300 years old and may again coincide with fault rupture most of the large earthquake triggered failures may ignite to form self perpetuating autosuspension flows that feed a 1500 km long deep sea turbidite channel

quaternary history of the antarctic circumpolar current evidence from the scotia sea within the scotia sea the axis of the antarctic circumpolar current acc is geographically confined and sediments therefore contain a record of palaeo flow speed uncomplicated by acc axis migration we outline holocene and last glacial maximum lgm current controlled sedimentation using data from 3 5 khz profiles cores and current meter moorings geophysical surveys show areas of erosion and deposition controlled by neogene basement topography deposition occurs in mounded sediment drifts or flatter areas where 500 1000 m of sediment overlies acoustic basement 3 5 khz profiles show parallel continuous sub bottom reflectors with highest sedimentation rates in the centre of the drifts and reflectors converging towards marginal zones of non deposition locally on the flanks of continental blocks e g south georgia downslope processes are dominant the absence of mudwaves on the sediment drifts may result from the unsteadiness of acc flow a core transect from the acc axis south to the boundary with the weddell gyre shows a southward decrease in biogenic content controlled by the polar front and the spring sea ice edge both these features lay farther north at lgm the cores have been dated by relative abundance of the radiolarian cycladophora davisiana and by changes in the biogenic ba content a palaeoproductivity indicator sedimentation rates range from 3 to 17 cm ka the grain size of holocene sediments shows a coarsening trend from south to north consistent with strongest bottom current flow near the acc axis though interpretation is complicated by the presence of biogenic grains year long current meter records indicate mean speeds from 7 cm s in the south to 12 cm s in the north with benthic storm frequency increasing northwards lgm sediments are predominantly terrigenous and show a clearer northward coarsening trend with well sorted silts in the northern scotia sea assuming a constant terrigenous source this implies stronger acc flow at the lgm contrasting with weaker weddell gyre flow deduced from earlier work

deep water facies processes and models a review and classification scheme for modern and ancient sediments abstract a review of previous work on modern and ancient deep water facies processes and models is presented with a new classification scheme involving 40 distinct facies related to 15 conceptually distinct facies groups these facies are fixed points in a spectrum of facies generated in a process continuum from resedimentation processes through semi permanent bottom currents to pelagic settling in essence the scheme is descriptive of the sedimentary attributes of sediments although it is designed to aid interpretation of possible sediment transport deposition processes the classification scheme is three tier with facies classes groups and constituent facies and is hierarchical to allow flexibility in its use there are seven facies classes with classes a e defined largely on the basis of grain size differences class f on the basis of internal organization and class g on composition the facies classes are class a gravels muddy gravels gravelly muds and pebbly sands with 5 gravel grade class b sands with 80 sand grade and 80 mud 40 silt and 0 20 sand class e muds and clays with 95 mud grade for the purpose of large scale mapping or reconnaissance fieldwork either the level of facies classes or groups may be appropriate whereas for more detailed sedimentology the more detailed facies level will be necessary

eustatic sea level controls on the flushing of a shelf incising submarine canyon turbidity currents are the principal processes responsible for carving submarine canyons and maintaining them over geological time scales the turbidity currents that maintain or flush submarine canyons are some of the most voluminous sediment transport events on earth long term controls on the frequency and triggers of canyon flushing events are poorly understood in most canyon systems due to a paucity of long sedimentary records here we analyzed a 160 m long ocean drilling program odp core to determine the recurrence intervals of canyon flushing events in the nazare canyon over the last 1 8 m y we then investigated the role of global eustatic sea level in controlling the frequency and magnitude of these canyon flushing events canyon flushing turbidity currents that reach the iberian abyssal plain had an average recurrence interval of 2770 yr over the last 1 8 m y previous research has documented no effect of global eustatic sea level on the recurrence rate of canyon flushing however we find that sharp changes in global eustatic sea level during the mid pleistocene transition 1 2 0 9 ma were associated with more frequent canyon flushing events the change into high amplitude long periodicity sea level variability during the mid pleistocene transition may have remobilized large volumes of shelf sediment via subaerial weathering and temporarily increased the frequency and magnitude of canyon flushing turbidity currents turbidite recurrence intervals in the iberian abyssal plain have a lognormal distribution which is fundamentally different from the exponential distribution of recurrence intervals observed in other basin turbidite records the lognormal distribution of turbidite recurrence intervals seen in the iberian abyssal plain is demonstrated to result from the variable runout distance of turbidity currents such that distal records are less complete with possible influence from diverse sources or triggering mechanisms the changing form of turbidite recurrence intervals at different locations down the depositional system is important because it ultimately determines the probability of turbidity current related geohazards

basaltic breccias of the clipperton fracture zone east pacific sedimentation and tectonics in a fast slipping oceanic transform investigations of the clipperton transform fault a fast slipping 11 cm yr transform plate boundary offsetting the east pacific rise indicate that the most common types of rocks occurring in this setting comprise a distinctive suite of sedimentary breccias and mudstones these rocks occur within a narrow 5 km wide domain of rugged terrain inferred to be the surface expression of strike slip faulting the sedimentary breccias are composed of angular basaltic clasts set in a fine grained matrix of very angular basaltic mineral and glass fragments and a subordinate amount of their altered equivalents the mudstones consist of material similar to the breccia matrix but also include a small amount of pelagic carbonate these sediments represent the deposits of small active debris fan systems located at the base of slopes flanking the elongate transform parallel basins of the region the active mass wasting and breccia formation occur in young crust as it passes through the transform fault domain the sediments are the products of physical and chemical disintegration and tectonic abrasion of basaltic crust initially formed at the ridge transform intersections most of the fine grained fraction consists of rock flour formed by frictional movement along fault surfaces and shear fractures its abundance reflects the high strain rates of this fast slipping transform fault as a result of differential vertical movements of crustal blocks associated with wrench tectonics these sediments undergo multiple cycles of erosion and redeposition as sections of the floor of the transform valley escape into the aseismic limbs of the fracture zone the breccias and mudstones an preserved in the basal sedimentary stratigraphy of the oceanic crust these distinctive assemblages may serve as indicators of oceanic crust from transform fault zones expose in some ophiolites

new high resolution acoustic data from the braided system of the orinoco deep sea fan abstract the interpretation of approximately 100 km of em12 multibeam bathymetry and backscatter data together with high resolution topas profiles obtained during a transit over the orinoco deep sea fan near the barbados accretionary prism shows that the braided drainage pattern seen on gloria images by belderson et al 1984 corresponds to wide 10 km low relief

acoustic facies and depositional processes in the upper submarine canyon swatch of no ground bay of bengal abstract the swatch of no ground song is a canyon that incises the bengal shelf deeply over more than 100 km length landward of the shelf edge and 100 km west of the ganges brahmaputra meghna river mouth it acts as conduit for the fluvially discharged sediment that bypasses the shelf and reaches the deep sea fan what transport deposition and mass wasting processes contribute to the sediment export and what sediment structures and main pathways are associated with these processes is studied by closely spaced bathymetric hydrosweep swath sounder and parasound sediment echosounder profiles over the upper song an analogue side scan sonar profile completes the acoustic data set from the parasound sediment echosounder data three main acoustic facies types related to undisturbed normal deposition mass wasting and erosive transport are defined and mapped with respect to the hydrosweep bathymetry of the song generally nearly all facies types show an unusually high sediment echosounder signal penetration of 80 100 m indicating that unconsolidated high porosity sediments occur on the canyon floor flanks and banks additionally the acoustic facies distribution map yields that about half of the canyon floor is covered by undisturbed well stratified fine grained muddy and coarse grained graded beds they are deposited from a suspension plume transported to the canyon either during fine weather conditions by westward oriented tidal currents or by storm events that mobilize sediments on the inner shelf curved or inclined faults or growth faults are found at all steep flanks indicating that active mass wasting processes occur along all channels and gullies on both canyon flanks such mass wasting processes are probably initiated by severe storms earthquakes high sediment load or excess pore pressure and lead to slides slumps or mudflows they stop in regions with low inclination angle and form acoustically transparent deposits of various extents on the channel and gully floors and on the southwestern canyon thalweg current marks identified on the canyon floor by high backscatter amplitudes in the side scan sonar images indicate that low density turbidity currents episodically flow down the canyon they are generated in an area of very high sedimentation rates close to the canyon head by high energy conditions like severe storms or earthquakes additionally a small gravity driven flow along the main channel s obviously continuously erode the steeper northeastern branches and inhibit sediment deposition in the more gentle southwestern channel below 450 500 m water depth together with larger scale mudflows and low density turbidity currents this gravity driven flow contributes to an active growth of the channel levee system in the deep sea fan

shelfbreak gullies products of sea level lowstand and sediment failure examples from bowser basin northern british columbia abstract the transition from cyclothemic shelf to shale dominated slope deposits callovian to oxfordian that accumulated along the northern convergent margin of bowser basin is delineated by spectacular conglomerate filled channels or gullies the gullies representing the shelf slope break formed in two stages 1 slumping of upper slope outer shelf sediment created topography which determined 2 the course of fluvial distributary type channels and focused gravel dominated sediment transport during relative sea level lowstands gully fill consists predominantly of debris flow deposits locally incorporated into small lowstand fan deltas that prograded from gully margins lowstand fluvial channels overlie shelf cycles incise the underlying highstand deposits and are overlain by fossiliferous transgressive sandstone however the shelfbreak gullies deposits are up to 10 times thicker than those of the associated lowstand fluvial channels although the lowstand fluvial channels likely acted as bypass conduits for gravel and sand during times of low sea level the initiation and overdeepening of the shelfbreak gullies by slumping probably took place during both high and low relative sea levels deposition within gullies may also have continued during relative highstands from sediment in storage on the outer shelf the implications for sequence stratigraphic models are shelf break gullies can form at any stage of sea level fluctuation gully thickness may not be related to the magnitude of lowstand incision on the adjacent shelf and gullies formed by sediment gravity failure do not provide quantitative information about the magnitude of sea level fall

morphostructure of the galicia continental margin and adjacent deep ocean floor from hyperextended rifted to convergent margin styles abstract a joint study and mapping of the morphological features and architecture of the galicia margin based on high resolution multibeam bathymetry data and multichannel seismic profiles provides new insights of the margin morphostructure tectonic processes are the major control of the margin s morphology where the imprint of the north atlantic rifting and opening and later subduction of the bay of biscay and compressional events are still well preserved the west galicia margin which is up to 350 km wide is composed from east to the west of inner transitional and outer sectors the inner sector shows a graded upper slope strongly dissected by a system of submarine canyons and valleys that flow into the 170 km long and 40 60 km wide nw se valle inclan valley the transitional sector which is up to 75 km wide displays a smooth relief dominated by landslides and giant craters finally the outer sector is characterised by several banks n s sub parallel ridges and elongated valleys that project seaward over 200 km i e galicia bank half graben deep galicia margin and peridotite ridge domains the north galicia margin is characterised by a system of terraces up to 30 km wide that form a stepped slope followed by an abrupt lower slope affected by large scale rotational failures the galicia margin constitutes an exceptional area within the atlantic margins for identifying different morphostructure styles that characterised the main stages of the margin s evolution along the wilson cycle the galicia margin hosts the transition between two end members of continental margin styles a western hyperextended rifted margin being the conjugate with the newfoundland margin during the late jurassic and a northern convergent margin now fossilised associated with the partial subduction of the southern sector of the bay of biscay underneath the north iberian margin which occurred in paleocene eocene times thus the morphostructure of the west galicia margin still reflects the detached fault systems and tilted blocks that resulted from the mesozoic north atlantic rifting furthermore the coruna and jean charcot seamounts which are adjacent to the galicia margin correspond to spreading ridges formed by the simultaneous opening of north atlantic ocean and the bay of biscay in the upper cretaceous the rest of the terraced morphology of the north galicia margin is derived from extensional faults caused by an overriding of the iberian plate in relation to the subduction of the bay of biscay oceanic crust during cenozoic times since the late neogene compressional tectonics resulting from the reorganisation of the eurasia iberia and africa plates is also reflected on the present day morphology as the following i an elevation of the biscay abyssal plain up to 150 m over the iberia abyssal plain ii deep seated landslides associated with an uplift of the galicia bank and iii an orientation of submarine canyons along nw se strike slip faults

deep water sediment wave fields bottom current sand channels and gravity flow channel lobe systems gulf of cadiz ne atlantic a study of the seafloor of the gulf of cadiz west of the strait of gibraltar using an integrated geophysical and sedimentological data set gives new insights into sediment deposition from downslope thermohaline bottom currents in this area the mediterranean outflow mo begins to mix with north atlantic waters and separates into alongslope geostrophic and downslope ageostrophic components changes in bedform morphology across the study area indicate a decrease in the peak velocity of the mo from 1 m s 1to 0 5 m s 1 the associated sediment waves form a continuum from sand waves to muddy sand waves to mud waves a series of downslope oriented channels formed by the mo are found where the mo starts to descend the continental slope at a water depth of a700 m these channels are up to 40 km long have gradients of 0 5 a fairly constant width of a2 km and a depth of a75 m sand waves move down the channels that have mud wave covered levees similar to those seen in turbidite channel levee systems although the channel size and levee thickness do not decrease downslope as in typical turbidite channel systems the channels terminate abruptly where the mo lifts off the seafloor gravity flow channels with lobes on the basin floor exist downslope from several of the bottom current channels each gravity flow system has a narrow slightly sinuous channel up to 20 m deep feeding a depositional lobe up to 7 km long cores from the lobes recovered up to 8 5 m of massive well sorted fine sand with occasional mud clasts this work provides an insight into the complex facies patterns associated with strong bottom currents and highlights key differences between bottom current and gravity flow channel levee systems the distribution of sand within these systems is of particular interest with applications in understanding the architecture of hydrocarbon reservoirs formed in continental slope settings

morphological feature analyses of the prestige half graben on the sw galicia bank abstract morphological analysis was carried out on the sw flank of the galicia bank as part of detailed geological studies of the half graben where the prestige oil tanker wreckage is located broad multibeam echosounder simrad em 12s 120 and seismic reflection airgun data were collected on board rv hesperides three groups of morphological features were identified as tectonic erosive and depositional the main tectonic morphological feature is a n s oriented fault scarp in the eastern part of the study area main erosive features are gullies slide scars e w oriented inter lobe channels and a nne ssw oriented main channel dominant depositional morphologies comprise thirteen e w oriented sedimentary wedges seven large e w to ne sw oriented sedimentary lobes and the relatively smooth sedimentary layer covering the eastern and western ridges that comprise the galicia bank s sw flank characterization classification and distribution of these features indicate that the study area constitutes a half graben structure comprising a n s oriented normal fault scarp abruptly connecting steep gradients in the eastern area with the gentle slope of the central area a thick but narrow wedge of sediment has accumulated at the foot of the scarp as a result of mass movement processes that govern the formation of a series of short lived wedges overlapping lobes and also channels this particular kind of slope apron system corresponds to a tectonic controlled drainage pattern from a sharply tilted hanging wall block supplied by sediment from the ridge finally we can conclude that the prestige stern and bow are located at the steep fault scarp close to a channel head and at the axis of inter lobe channels developed by mass movement processes thus morphologic tectonic and sedimentary factors can be considered as potential hazards

morphological characteristics of the basque continental shelf bay of biscay northern spain their implications for integrated coastal zone management this contribution integrates and analyzes data from high resolution multibeam seismic profiles bathymetric lidar and surficial sediment data for the geomorphological seascapes characterisation and process description of the basque inner and middle continental shelf northern spain from the data obtained the basque shelf is characterised by a heterogeneous seafloor where on a small spatial scale different morphologies and sedimentary processes can be observed tectonic activity and sea level changes together with present processes of sediment supply and climatic conditions have a critical influence on the present configuration of the continental shelf and the distribution of seafloor types on the basis of all of the datasets seafloor classification bedform analysis long term sea level change induced seafloor features and anthropogenic features over the seafloor are described as a result three distinct zones have been identified for the basque shelf related to the main geomorphological features and seascapes as summarised below i a western part with a northwestern orientation which coincides with the prevailing wave direction this zone is characterised by a predominantly rocky substrate except within the mouth of the nervion estuary where sandy sediments appear to infill a paleo channel ii a central part which is oriented towards the northeast and receives less wave energy within this zone a sedimentary seabed is predominant sorted bedforms are well developed and can be detected in water depths in excess of 90 m iii an eastern zone which is characterised by sedimentary seafloor and shore terraces here a flat rocky seafloor is covered by a thin layer of sediments throughout the study area the bathymetry and sedimentary features of the shoreface and inner shelf are controlled mainly by climatic conditions coastal dynamics and the underlying geological framework the integration of these data sets together with their interpretation is a valuable source of information for integrated coastal zone management iczm and constitutes a useful tool for implementing various european directives

tsunamigenic seismogenic structures neotectonics sedimentary processes and slope instability on the southwest portuguese margin tectonically active structures prone to cause devastating earthquakes and tsunamis e g the lisbon 1755 earthquake were investigated during the unesco ioc training through research 10 ttr 10 cruise on the southwest portuguese continental margin using single channel seismic profiles a 3 5 khz hull mounted sea bottom profiler 10 khz okean long range side scan sonar 30 khz oretech deep towed side scan sonar and a high resolution deep towed sea bottom profiler these data allowed the definition of new active faults and the establishment of morphological criteria for the classification of active faults in the study area landslides associated with the activity of a major tectonic structure the marques de pombal fault and other areas with clear signs of mass wasting phenomena were mapped a slope to basin sedimentary system comprising 21 sedimentary ridges up to 20 km long was mapped and described it was found that the sediments are mainly transported into the deep basins by mass transport processes across the steepest fault scarps forming a channel levee system while gravitational slides slumps dominate the shallower slopes the sedimentary ridges with an elevation of 40 50 m 50 60 ms twt above the seafloor are imaged on the high resolution seismic profiles as an alternation of high and low amplitude reflectors it is shown that the pereira de sousa fault its plateau and the principes de avis plateau are experiencing uplift according to sedimentary and morphological criteria

instabilities along ocean margins a geomorphological and geotechnical perspective due to the growing development of offshore activities along margins in the search for more natural resources there has been a greater need for understanding the mechanics of slope formation in these areas one aspect of this relates to the problems associated with slope stability issues this article presents an perspective view of some geomorphological and geotechnical aspects related to instabilities along ocean margins in order to properly address the issues of slope stability and other mass movements we need to improve our technology in the field of sampling high resolution seismic and in situ measurements ultimately the goal is to be able to carry out proper risk assessment analysis pertaining to submarine mass movement this could be achieved by integrating the geotechnical characterisation of mass movements into a risk assessment methodology which can then be applied on a regional basis with the initiation of a project on continental slope stability costa during the year 2000 major advances are expected on our understanding of the mechanics of submarine mass movements

stress pattern in portugal mainland and the adjacent atlantic region west iberia the portuguese mainland territory is located close to the azores gibraltar plate boundary in a tectonic setting responsible for significant neotectonic and seismic activities however few data concerning the present regional lithospheric stress field were available as testified by recently published maps of stress indicators for the europe and mediterranean regions one of the authors already presented a synthesis on this subject cabral 1993 where geological and geophysical stress indicators were considered in this paper we introduce new information mainly a considerable amount of borehole breakout data the updated data set comprises 32 reliable stress indicators showing a mean azimuth of 145 standard deviation 21 for the maximum horizontal stress direction shmax on the average the geological data are rotated clockwise and the focal mechanism data deviated anticlockwise to that azimuth while the borehole elongation results are consistent with the mean shmax trend these differences in stress trend suggest a regional progressive rotation of the shmax direction from nnw sse to wnw ese since the upper pliocene to estimate stress trajectories new and published stress indicators in the adjacent atlantic area and northern africa were also investigated showing a very uniform nw se shmax trend in west iberia a high level of horizontal compressive stress acting oblique to the western portuguese continental margin is inferred and interpreted in view of a proposed regional geodynamical model of activation of this passive margin with the nucleation of a subduction zone in the atlantic sw of iberia at the gorringe submarine bank which is propagating northward along the base of the continental slope at the transition between thinned and normal continental crust

poroelastic modeling of seismic boundary conditions across a fracture permeability of a fracture can affect how the fracture interacts with seismic waves to examine this effect a simple mathematical model that describes the poroelastic nature of wave fracture interaction is useful in this paper a set of boundary conditions is presented which relate wave induced particle velocity or displacement and stress including fluid pressure across a compliant fluid bearing fracture these conditions are derived by modeling a fracture as a thin porous layer with increased compliance and finite permeability assuming a small layer thickness the boundary conditions can be derived by integrating the governing equations of poroelastic wave propagation a finite jump in the stress and velocity across a fracture is expressed as a function of the stress and velocity at the boundaries further simplification for a thin fracture yields a set of characteristic parameters that control the seismic response of single fractures with a wide range of mechanical and hydraulic properties these boundary conditions have potential applications in simplifying numerical models such as finite difference and finite element methods to compute seismic wave scattering off nonplanar e g curved and intersecting fractures

sedimentary processes on the northwestern iberian continental margin viewed by long range side scan sonar and seismic data abstract the effects of an eastern boundary current in the north atlantic have been mapped from about 39 north latitude along the iberian margin to as far north as 43 30 north latitude at the western margin of galicia bank the geostrophic current has produced sediment drifts that are covered with bedforms the sediment drifts are difficult to detect on gloria long range side scan sonar data but are easily resolved on seismic reflection records as anomalously thick accumulations of sediment banked against either buried or outcropping basement highs the bedforms ornamenting the drift surfaces were subdivided into 1 000 m water depth intervals and their dimensions were tabulated there are few bedforms in water depths less han 2 000 m but from depths between 2 000 and 4 000 m they are numerous and have a mean wavelength of 695 m bedforms from depths greater than 4 000 m have a mean wavelength of 999 m the different wavelengths from different water depths suggest two distinct and separated boundary flows the wave heights of all bedforms found in water depths greater than 2 000 m are less than 10 m in order to investigate the continuity of sediment drifting through geological time the stratigraphic section drilled at dsdp site 398 was reinterpreted and using seismic reflection profiles was traced throughout the northern iberian margin together the lithostratigraphic and seismic data indicate that sediment drifting developed along this margin in the eocene the lithofacies of the eocene section is t e oldest to have numerous layers of sand and silt an unconformity separates the eocene section from the latest miocene pliocene section the unconformity is interpreted to be the result of the initial pulses of mediterranean outflow that followed the messinian desiccation events a second period of sediment drifting commenced during the pliocene once the mediterranean basin filled and the flow out of the strait of gibraltar resumed

high resolution seismic stratigraphy of the galicia bank region and neighbouring abyssal plains nw iberian continental margin abstract the high resolution seismic stratigraphy of the galicia bank region and adjacent deposits of the neighbouring iberian and biscay abyssal plains was included as part of the geological studies conducted in the area where the oil tanker prestige wreck is located this seismic stratigraphy is characterized by five seismic units 5 to 1 from oldest to youngest lying above an irregular acoustic basement defined by a highly fractured system of horsts and grabens these faulted systems have controlled the local depositional architecture deforming fracturing relocating and distributing sediments since the valanginian three depositional models of facies can be recognized on the galicia bank region and surrounding abyssal plains bank escarpment and abyssal plain facies the bank facies reflects the depositional evolution from initial filling to obliterating stages of the horsts and grabens the escarpment facies has a local distribution and represents deposition associated with active slopes of the faulted escarpments the abyssal plain facies represents gravity flow deposits coming mostly from the bounded fault flanks of the galicia bank region the valanginian to quaternary sedimentary evolution of the galicia bank region reflects depositional responses to tectonic condition the occurrence of mass movements confirms that the syn rift period valanginian to aptian age along the galicia margin was diachronous beginning in the interior basin and shifting later toward the west of the galicia bank region deep galicia margin the albian to quaternary post rift evolution is characterized by a decrease in tectonic activity and sedimentary variations in the style of deposition valley and related features mass movement deposits contourites and drape deposits conditioned by changes in the sedimentary processes paleoenvironments and tectonic reactivation of the regional slopes the effects of tectonic movements persist as evidenced in areas where faulted escarpments and highs outcrop and in areas of near surface faulting the syn rift and post rift stages have also conditioned the depositional architecture of the prestige sinking area the evolution of the surrounding abyssal plains does not reflect the same pattern to tectonic condition

reexposure and advection of 14c depleted organic carbon from old deposits at the upper continental slope represents an additional source of 14 c depleted organic carbon supplied to the ocean in parallel with the weathering of fossil organic carbon delivered by rivers from land to understand the dynamics and implications of this reexposure at the shelf edge a biogeochemical study was carried out in the gulf of lions mediterranean sea where erosive processes driven by shelf dense water cascading are currently shaping the seafloor at the canyon heads mooring lines equipped with sediment traps and current meters were deployed during the cascading season in the southwestern canyon heads whereas sediment cores were collected along the sediment dispersal system from the prodelta regions down to the canyon heads evidence from grain size x radiographs and 210 pb activity indicate the presence in the upper slope of a shelly coarse surface stratum overlying a consolidated deposit this erosive discontinuity was interpreted as being a result of dense water cascading that is able to generate sufficient shear stress at the canyon heads to mobilize the coarse surface layer eroding the basal strata as a result a pool of aged organic carbon d 14 c 944 5 24 7 mean age 23 650 3 321 ybp outcrops at the modern seafloor and is reexposed to the contemporary carbon cycle this basal deposit was found to have relatively high terrigenous organic carbon lignin 1 48 0 14 mg 100 mg oc suggesting that this material was deposited during the last low sea level stand a few sediment trap samples showed anomalously depleted radiocarbon concentrations d 14 c 704 4 62 5 relative to inner shelf d 14 c 293 4 134 0 mid shelf d 14 c 366 6 51 1 and outer shelf d 14 c 384 47 8 surface sediments therefore although the major source of particulate material during the cascading season is resuspended shelf deposits there is evidence that this aged pool of organic carbon can be eroded and laterally advected downslope

detrital and early diagenetic processes in late pleistocene and holocene sediments from the sw galicia bank inferred from high resolution enviromagnetic and geochemical records abstract u channel based high resolution magnetic and geochemical measurements supported by scanning electron microscopy x ray diffraction and fluorescence and grain size analyses of discrete samples were carried out on six sediment cores recovered from the sw flank of the galicia bank at the prestige s wreck area five magnetochemical facies reflecting the different sedimentary environments i e hemipelagic contouritic turbiditic sediment provenances and early diagenetic redoxomorphic processes i e mn peaks are defined on the basis of a cluster analysis the records showed climatically forced detrital signatures of regional importance associated with continental slope sedimentary processes the chronostratigraphy and correlation of the cores is based on 20 14 c ams dating recognition of heinrich like events h1 to h3 and well documented holocene climatic features from a geochemical and enviromagnetic perspective the late pleistocene to holocene evolution of the area shows two different sedimentation contexts mis 2 is dominated by down slope depositional processes the highest sedimentation rates 9 1 cm ky were achieved during this interval the major turbidite activity is recorded between 31 000 yr bp and 13 500 yr bp successive arrivals of ird h1 to h3 are also identified during this period in contrast holocene sedimentation was dominated by a mixture of pelagic hemipelagic and along slope depositional processes and presented considerable lower sedimentation rates 3 cm ky bottom currents intensification at 7000 yr bp and 4500 yr bp coinciding with late holocene warm events are also recognised during this stage

western extension of the iberian european plate boundary during the early cenozoic pyrenean convergence a new model abstract a dense seismic net work allows of drawing the western prolongation of the northern boundary of the iberian plate this early cenozoic deformed boundary is disrupted by two prominent nw se transform faults a kinematic reconstruction with a nw late cretaceous lower eocene motion of the iberian plate is proposed the azores biscay rise is interpreted as a short lived subduction zone whereas king s trough could be a transform fault

galicia bank sediment transport activity in response to continuous sedimentary instability dynamics a geotechnical perspective the sedimentary instability dynamics occurring over time throughout the isolated galicia bank atlantic ocean nw iberian peninsula have been studied using a sedimentological and geotechnical approach featuring a quantitative assessment of slope stability under different scenarios including earthquake activity the erosion of the scarps in the galicia bank includes the continuous development of small instabilities evacuated along gullies and channels or deposited on slopes these deposits may be subsequently mobilized evolving into new sedimentary gravity flows e g turbidity flow the studied sediments consist mostly of poorly to very poorly sorted sands and silts transported by turbidity currents and occasionally by debris flow processes the sediments in the study area identified as normally consolidated and located on gently sloping areas gradients less than 5 may become unstable if low magnitude seismic events occur pga 10 without any trigger other than oversteepening in contrast overconsolidated sediments may remain stable under static conditions and may become unstable on slope gradients 10 when earthquakes occur with the maximum peak ground acceleration pga 0 34 the sedimentological and geotechnical models presented herein are complementary approaches that can be utilized to understand the long term sedimentary instability dynamics observed within the study area such results are critical for better understanding sedimentary models of the dismantling processes of deep seamounts located far away from continental sedimentary inputs

seismicity analysis at the prestige oil tanker wreck area galicia margin nw of iberia an evaluation of the geological hazard related to the sinking of the prestige oil tanker in november 2002 in the galicia margin nw of iberia was supported by the spanish research agency it included a specific seismic study of the local activity and of the regional seismicity in this margin results of which are presented here published global catalogues show that diffuse seismicity of moderate magnitude events up to 5 6 mb can be associated with the northern flank of the galicia margin with a roughly e w distribution however event locations in this area are poorly constrained due to the inappropriate geometry of permanent seismic networks we collected and analyzed new data from a dense deployment of 10 obs around the wreck site and from a network of 12 land stations at the nw corner of iberia during a 7 months operation period the land network permitted location of 120 events with magnitudes ranging from 1 6 to 4 8 more than 30 of which were not reported by the spanish permanent array confirming the existence of a moderate seismicity in this area five new focal solutions have been obtained for events on land providing additional information about the active faults in the region the obs network ran for 1 month and allowed the location of three earthquakes of magnitude 2 5 near the sinking site four more events have been identified with just a rough determination of their epicenters based on polarization of the p wave arrivals besides these earthquakes the obs data recorded numerous seismic events of different types they include i groups of very short and regularly spaced pulses that can be explained as acoustic calls generated by finback whales ii a large number of short duration seismic events 2 3 s long identified only at individual obs iii local intense monochromatic signals lasting for up to 4 h that can be described as harmonic tremors these last two types of seismic events might be related to hydrothermal activity within the uppermost sediments and deserve additional studies to appropriately establish the seismic hazard of this area 2007 elsevier b v all rights reserved

the relative importance of gravity induced versus current controlled sedimentation during the quaternary along the mideast u s outer continental margin revealed by 3 5 khz echo character abstract a detailed echo character map based on more than 50 000 km of 3 5 khz echograms reveals the relative importance of contour currents turbidity currents and mass wasting processes for the upper 50 100 m of sea floor sediments along the u s continental margin between southern georges bank and cape hatteras nineteen individual echo types are mapped and interpreted as deposits produced either by current controlled or gravity induced processes based on echo type distribution analysis of selected sediment core samples and the published results of other workers turbidity currents and mass wasting have apparently been the primary depositional agents throughout 60 of the region in contrast contour currents alone appear to have influenced only 10 of the region however combined processes i e non separable interaction between downslope and parallel to contour processes have influenced an estimated 25 of the margin the remaining 5 of the study area constitutes the outermost continental shelf echo types here reflect the influence of fluvial deltaic and shallow marine sedimentary processes changes in the relative distributions of various echo types with sea floor depth indicate that the dominant sedimentary processes change downslope and are significantly different within each physiographic province deposits formed by mass wasting dominate the continental slope where 50 of the sea floor is eroded and gullied by slides slumps and the heads of debris flows deposits developed under the influence of combined processes are most wide spread on the upper continental rise and cover 40 of the sea floor primarily in the form of deep sea channel levees modified by contour currents on the lower continental rise turbidites form more than 50 of the sea floor deposits the downslope deposits turbidity currents slumps debris flows etc were emplaced primarily during the pleistocene glacioeustatic sea level lowering moved beaches and rivers across the exposed continental shelf to the shelf edge where massive fluxes of terrigenous sediment could be delivered to the continental slope and rise by turbidity currents significant amounts of this sediment were transported through submarine canyons and leveed deep sea channels to the lower continental rise and formed the lower continental rise terrace a flat turbidite basin ponded behind the hatteras outer ridge southwest of the wilmington channel this glacially derived influx was diminished however depositional patterns indicate that downslope processes continued to be dominant at least as far south as cape hatteras where rapid deposition on the continental slope and uppermost continental rise created instabilities slope failure produced slumps slides and debris flows and provided an additional volumetrically important redistribution of sediment downslope the influence of contour currents is clearly observed only in those areas topographically shielded from the masking effects of downslope processes in particular along the seaward side of the buried chesapeake drift and the hatteras outer ridge

seismic signatures of permeability in heterogeneous porous media in homogeneous poroelastic systems the permeability tensor practically does not influence propagating seismic waves in the low frequency range 0 1000 hz see e g schmitt 1989 gelinsky and shapiro 1996 in this paper we show that this situation changes in heterogeneous systems such as layered or fractured sediments due to the heterogeneities of poroelastic structures the attenuation of p waves is influenced by the permeability in an enhanced way we show however that such a seismic permeability can differ very strongly from the hydraulic permeability

multibeam backscatter as a tool for sea floor characterization and identification of oil spills in the galicia bank this study is focused on the potential of using the multibeam acoustic backscatter both for sea floor characterization of the galicia bank and for identification of sea bottom oil spills an extended multibeam data set was acquired during the rg 1 and rg 2 surveys aboard rv hesperides in 2003 for the geological and geophysical characterization of the galicia bank where the oil tanker prestige wreck is located 3 d images have been made by integrating bathymetry and backscatter data multibeam data have been additionally supported by gravity core information in order to correlate sea floor acoustic backscatter with the surficial sediments and their physical properties the major factors that seem to control the zonation of the backscatter intensity images are i the type of the sedimentary processes gravitational turbiditic hemipelagic which control at the same time the grain size distribution and ii the sea bottom morphologies such as basement outcrops however there are some backscatter halos that cannot be explained by these processes in the backscatter maps the bow and stern of the prestige wreck are clearly marked by a high backscatter intensity anomaly whose existence is discussed

holocene sediment deposition on the poverty slope margin by the muddy waipaoa river east coast new zealand the margin off the raukumara peninsula east coast north island of new zealand is characterised by high terrigenous sediment flux the dramatic effects of historic land use change the rapid uplift of mesozoic and soft tertiary sediments and by complex sediment tectonic interactions on a steep and unstable continental slope the poverty bay shelf and slope indentation are adjacent to the waipaoa river which delivers 15 mt year 1 of mud this study presents new data from the poverty slope the seaward extension of the waipaoa dispersal system a 3 km3 lobe of postglacial sediment emerges from the poverty gap and feathers down onto the upper slope downslope the poverty indentation has received a significant holocene flux of terrigenous sediment accumulating in two mid slope basins the paritu trough and a smaller lower slope basin below the south paritu ridge hemipelagic sediment has accumulated on the mid slope at around 0 05 g cm 2 year 1 0 06 cm year 1 since the mid late holocene seismic reflection echo type and backscatter imagery indicate mud deposits on the mid slope debris at the base of regularly spaced upper slope gullies and a large avalanche deposit in the upper paritu trough the consistency of the tephrostratigraphy indicates that despite slope instability hemipelagic sedimentation is the dominant background process operating at millennial time scales at least since the mid holocene the poverty slope should be included in any budget calculation of post colonisation increased sediment yield for the margin the poverty canyon has probably been largely inactive during the holocene highstand

abyssal current influence on the southwest bermuda rise and surrounding region abstract echograms 3 5 khz and bottom photographs reveal that the northward flowing antarctic bottom water aabw has strongly influenced the modern depositional regime on the southwest bermuda rise the spatial distribution of echo character types the orientation and nature of current controlled structures and limited current meter data show that aabw flows with varying intensities along three primary pathways around and over the southwest bermuda rise the main core of aabw flows clockwise around the eastern and western flanks of the southern bermuda rise roughly parallel to the 5400 m isobath this current bifurcates at 28 30 n 69 w where a portion flows northeast over the southwest bermuda rise and the remainder continues north along the physiographic boundary between the southwest bermuda rise and the hatteras abyssal plain secondary ribbons of aabw branch off the main core of aabw during its southerly journey along the southeastern bermuda rise and flow west through fracture zones finally a diffuse northward flowing aabw sweeps the entire southwest bermuda rise a progression of current controlled bedforms occurs beneath the main path of the aabw reflecting the spatially varying current velocities and sediment supply the main core of aabw flows west through the narrow vema gap creating erosional furrows along the border between the southwest bermuda rise and the vema gap current velocities greater than 20 cm s 1 are inferred from the bedforms in this region farther north along the southwestern edge of the bermuda rise sediment waves become more prevalent this transition from erosional to more depositional bedforms results from diminished current velocities 5 15 cm s 1 and increased sediment supply although some of these bedforms on the southwest bermuda rise appear to be relict their orientation is consistent with current meter data and abyssal current direction inferred from bottom photographs

high resolution acoustic characteristics of epicontinental sea deposits central eastern yellow sea abstract large amounts 45390 line km of closely spaced 2 2 4 4 km high resolution subbottom profiles chirp 2 7 khz made it possible to identify detailed echo types and their distribution in an epicontinental shelf environment on the basis of seafloor morphology surface bedforms and subbottom acoustic characters 10 echo types were identified in the uppermost sedimentary sequence of the central eastern yellow sea flat seafloor with sharp bottom echoes echo types 1 1 1 2 and 1 3a transgressive sediment sheets or relict sands is widespread in the offshore area and underlain to the west by an acoustically transparent wedge echo type 1 3b highstand muds mounded seafloor with either smooth surface or superposed bedforms echo types 2 1 2 2 and 2 3 tidal ridges and flat seafloor with regularly spaced wavy bedforms echo type 1 4 large scale dunes are dominant in the eastern nearshore area large scale mounds with continuous inclined internal reflectors echo type 2 4 giant mud bank occur in the southeastern nearshore area various scale eroded seafloor echo types 3 1 and 3 2 channels and flat seafloor with regularly spaced wavy bedforms echo type 1 4 large scale dunes are present in the northern part of the sea the distribution pattern of echo types in the central eastern yellow sea reflects depositional processes and sediment dispersal systems during the holocene transgression and highstand period 1 development of tidal ridges and large scale dunes in response to strong tidal currents and waves in the eastern nearshore area 2 construction of transgressive to highstand mud bank huksan mud belt by deposition of muds derived from the keum river in the southeastern nearshore area 3 active erosion due to intensified currents in the northern part and 4 highstand mud deposition derived from the huanghe river on the transgressive sediment sheets in the offshore area

seismic attenuation in finely layered porous rocks effects of fluid flow and scattering scattering caused by fine layering is known to play an important role in seismic attenuation if the rock is also porous and permeable additional attenuation can be caused by a flow of the pore fluid induced by the passing wave see e g white 1983 at frequencies higher than 100 khz standard biot s attenuation biot 1962 bourbie et al 1987 may also contribute to the overall attenuation in a layered poroelastic rock while standard biot s attenuation has been well known for decades the effects of scattering by fine layering and interlayer flow have been studied extensively both theoretically and numerically in recent years however in all theoretical approaches proposed thus far these effects have been treated separately scattering was studied for purely elastic rocks burridge and chang 1989 shapiro et al 1994 while attenuation caused by interlayer flow was considered either for periodically stratified media that causes no scattering norris 1993 or for frequencies at which scatterin

acoustical characterization of sediments by parasound and 3 5 khz systems related sedimentary processes on the southeastern weddell sea continental slope antarctica subbottom profiling was performed during four polarstern expeditions to the southeastern weddell sea continental slope northwest of lyddan ice rise antarctica analogue records from high resolution seismic reflection systems 3 5 khz and parasound were used to classify sediment echo types a comparison between the two systems shows the advantage of the parasound system the smaller beam angle produces a higher spatial resolution which reduces the diffraction hyperbolae considerably the higher vertical resolution and deeper penetration provide more detailed information concerning seismic stratification using four basic echo categories p prolonged l layered d diffraction hyperbolae w wedging subbottoms ten discrete sediment echo patterns were distinguished their regional distribution and the depth of acoustic signal penetration into the sediment were mapped sw ne trending sediment ridges with channels running parallel on the southeastern side of the ridges were traced in the eastern part of the study area the prolonged and highly reflective echo types of the channel bottoms indicate deposition of predominantly coarse grained sediments affected by erosional processes these channels are interpreted as drainage systems for bottom water currents derived from the shelf and steep upper slope the flow direction is towards the northeast counter to the weddell gyre in contrast the sediment ridges show up to 80 m subbottom penetration and an echo type of distinct multi layered reflectors running parallel to subparallel with the surface reflector the sediments of the ridges are interpreted to be levee sediments overspilled from the channels and preferentially deposited on the northwestern flank due to the coriolis force sedimentary units separated by wedge shaped reflectors are widespread on the gently inclined terrace in the middle part of the continental slope these sediments are presumed to be produced by syn sedimentary slumping and proximal debris flows having a northeastern direction of movement

late weichselian depositional processes fluxes and sediment volumes on the margins of the norwegian sea 62 75 n abstract quaternary continental margin sedimentation in the norwegian greenland sea has been a response to a combination of tectonic oceanographic and glacial activity using geophysical survey data 6 5 khz side scan sonar and 3 5 khz penetration echo sounding in association with bathymetric datasets the magnitude and frequency of the main forms of margin sedimentation into the lofoten and norwegian basins of the norwegian sea during the late weichselian and holocene are estimated by comparing these geophysically determined estimates of sediment fluxes with numerically modelled ice sheet sediment delivery the role of ice sheets in shaping the processes acting on this margin is assessed sediment flux at the shelf edge during the late quaternary is dominated by ice sheet delivery of sediments focused at the mouths of fast flowing ice streams formed in bathymetric troughs interglacials are by contrast characterised by comparatively little sediment accumulation submarine fans at the margins of cross shelf troughs are major depocentres on this glacier influenced margin however occasional large scale failures of the continental slope account for approximately 75 of the basin sediment accumulation during the past 30 000 yr by eroding substantial quantities of pre quaternary deposits trough mouth fan and a combination of hemipelagic and glacimarine processes are responsible for the accumulation of the remaining 15 and 7 of basin sediments respectively

signatures of the mediterranean outflow from a north atlantic climatology 1 salinity and density fields using historical data from the national oceanic data center the climatology of the eastern north atlantic basin has been investigated for the purpose of detailing the mediterranean outflow water in terms of its salinity density and flow patterns part 1 of this work is a descriptive analysis of the fate of the mediterranean water once it flows out of the strait of gibraltar tracing the salinity and density signatures high resolution maps of the climatological outflow are presented with an emphasis on the continuity of the water from its source from the climatological fields a continuous signal of mediterranean water is tracked northward to 50 20 n yet its westward advection is limited to the tagus basin recirculations of mediterranean water in the gulf of cadiz and in the bay of biscay deduced from property signals are also detailed iorga and lozier this issue present absolute velocity fields from a diagnostic model constrained by geostrophic dynamics conservation of mass and no flux boundary conditions

recent sedimentary processes in the prestige site area galicia bank nw iberian margin evidenced by high resolution marine geophysical methods an echo character analysis of the oil tanker prestige wreck area was undertaken using high resolution marine geophysical methods topas and airgun seismic reflection profiles multibeam echosounder and tobi sidescan sonar integration and comparison of the results using all these methods is presented given some practical applications for indirect near surface and seafloor interpretations ten different echo types were identified and grouped into four main classes i distinct ii indistinct iii irregular and iv hyperbolic echo character distribution enabled us to determine recent sedimentary processes in the area two major depositional systems can be found through the identification of these sedimentary processes a a slope depositional system sds located in the eastern and central area where mass movement processes are dominant and b a turbiditic main channel depositional system tds located in the western area where channel related processes are dominant both of these interact over the half graben structure of the southwestern sector of the galicia bank where the prestige wreck is located within the sds erosive and depositional mass movement processes characterised a complex depositional system erosive processes occur on the fault scarp channels head inter lobe channels and distal part of the sedimentary lobes moreover depositional processes take place on the top of the fault scarp sedimentary wedges sedimentary lobes and on the west flank of the main channel both depositional systems interact but the sds should be more active during fault scarp reactivation periods through relief rejuvenation and new exposed deposits microearthquake activity would favour the available materials being weathered eroded and transported by mass movements in such a situation the tds acts as the main collector of eroded sediment derived from the fault scarp throughout the sds being responsible for its evacuation into the iberian abyssal plain however outside of fault reactivation periods the sds is less active such as during the present situation

fracture channel waves the properties of guided waves which propagate between two parallel fractures are examined plane wave analysis is used to obtain a dispersion equation for the velocities of fracture channel waves analysis of this equation demonstrates that parallel fractures form an elastic waveguide that supports two symmetric and two antisymmetric dispersive rayleigh channel waves each with particle motions and velocities that are sensitive to the normal and tangential stiffnesses of the fractures these fracture channel waves degenerate to shear waves when the fracture stiffnesses are large to rayleigh waves and rayleigh lamb plate waves when the fracture stiffnesses are low and to fracture interface waves when the fractures are either very closely spaced or widely separated for intermediate fracture stiffnesses typical of fractured rock masses fracture channel waves are dispersive and exhibit moderate to strong localization of guided wave energy between the fractures the existence of these waves is examined using laboratory acoustic measurements on a fractured marble plate this experiment confirms the distinct particle motion of the fundamental antisymmetric fracture channel wave a sub 0 mode and demonstrates the ease with which a fracture channel wave can be generated and detected copyright 1999 american geophysical union

single fractures under normal stress the relation between fracture specific stiffness and fluid flow abstract fracture specific stiffness and fluid flow through a single fracture under normal stress are implicitly related through the geometry of the void space and contact area that comprise the fracture data from thirteen different rock samples each containing a single fracture show that relationships between fracture specific stiffness and fluid flow through a fracture fall into two general classes of behavior fractures either fall on a loosely defined universal curve relating fluid flow to fracture specific stiffness or else the flow is weakly dependent on fracture specific stiffness the second relationship shows that flow decreases slowly with increasing fracture specific stiffness the first relationship shows that flow decreases rapidly for increases in fracture specific stiffness to understand this behavior computer simulations on simulated single fractures were performed to calculate fluid flow fracture displacement and fracture specific stiffness as a function of normal stress simulated fractures with spatially correlated and uncorrelated aperture distributions were studied fractures with spatially uncorrelated aperture distributions tend to exhibit a weak dependence of fluid flow on fracture specific stiffness because these fractures tend to have multiple connected paths across the sample which can support flow with uniformly distributed contact area thus an increment in stress will increase the stiffness of the fracture without greatly reducing the amount of fluid flow on the other hand fractures with spatially correlated aperture distributions tend to belong to the universal relationship because correlated fractures tend to have only one or two dominant flow paths and the contact area is limited to a few regions resulting in a compliant fracture thus an increment in stress on a spatially correlated fracture will result in an increase in stiffness and rapid decrease in fluid flow these spatial correlations in fracture void geometry can be differentiated in the laboratory based on the observed fracture specific stiffness fluid flow relationship for a single fracture under normal loading

geomorphology of the iberian continental margin abstract the submarine features and processes around the iberian peninsula are the result of a complex and diverse geological and oceanographical setting this paper presents an overview of the seafloor geomorphology of the iberian continental margin and the adjacent abyssal plains the study covers an area of approximately 2 3 million km 2 including a 50 to 400 km wide band adjacent to the coastline the main morphological characteristics of the seafloor features on the iberian continental shelf continental slope continental rise and the surrounding abyssal plains are described individual seafloor features existing on the iberian margin have been classified into three main groups according to their origin tectonic and or volcanic depositional and erosional major depositional and erosional features around the iberian margin developed in late pleistocene holocene times and have been controlled by tectonic movements and eustatic fluctuations the distribution of the geomorphological features is discussed in relation to their genetic processes and the evolution of the margin the prevalence of one or several specific processes in certain areas reflects the dominant morphotectonic and oceanographic controlling factors sedimentary processes and the resulting depositional products are dominant on the valencia catalan margin and in the northern part of the balearic promontory strong tectonic control is observed in the geomorphology of the betic and the gulf of cadiz margins the role of bottom currents is especially evident throughout the iberian margin the galicia portuguese and cantabrian margins show a predominance of erosional features and tectonically controlled linear features related to faults

biot slow wave effects in stratified rock the transmission of p waves through the stratified layers of a sedimentary basin is modeled numerically using biot theory the effects on the transmissivity of frequency angle of incidence layer thickness permeability and elastic compliance of the rocks are all considered consistent with previous analytical work it is found that the equilibration of fluid pressure between the fine layers of a sedimentary sequence can produce significant p wave attenuation at low frequencies for this attenuation mechanism to act within the surface seismic band say 3 300 hz we find that there must be layering present at the scale of centimeters to tens of centimeters if the layering is restricted to layers of roughly 1 m thickness or greater then for typical sandstone formations the attenuation caused by the interlayer flow occurs below the seismic band of interest such low frequency interlayer flow is called biot slow wave diffusion in the context of biot theory and is likely to be the dominant source of low

mapping active faults offshore portugal 36 n 38 n implications for seismic hazard assessment along the southwest iberian margin swath bathymetry and acoustic backscatter data from the southwest iberian margin which hosts the present day boundary between the european and african plates reveal the surficial expression of several fault structures 100 km offshore of portugal high resolution and multichannel seismic reflection profiles collected perpendicular to these structures show folding and reverse faulting of the quaternary units suggesting present day tectonic activity successive submarine landslide deposits at the base of the scarps provide evidence of cyclic fault activity the location and dimension of these newly identified structures agree with the modeled source suggested for the a d 1755 lisbon earthquake and tsunami possibly the most destructive event in western europe during historical time these fault escarpments and deformed seafloor sediments associated with a cluster of shallow seismicity suggest that these thrusts are active and may pose a significant earthquake and tsunami hazard to the coasts of portugal spain and morocco

finite difference modeling of faults and fractures for the purposes of seismic propagation a slip fault may be regarded as a surface across which the displacement caused by a seismic wave is discontinuous while the stress traction remains continuous the simplest assumption is that this slip and the stress traction are linearly related such a linear slip interface condition is easily modeled when the fault is parallel to the finite difference grid but is more difficult to do for arbitrary nonplanar fault surfaces to handle such situations we introduce equivalent medium theory to model material behavior in the cells of the finite difference grid intersected by the fault virtually identical results were obtained from modeling the fault by 1 an explicit slip interface condition fault parallel to the grid and 2 using the equivalent medium theory when the finite difference grid was rotated relative to the fault and receiver array no additional computation time is needed except for the preprocessing required to find the relevant cells and their associated moduli the formulation is sufficiently general to include faults in and between arbitrary anisotropic materials with slip properties that vary as a function of position

recent sedimentary events in the western gulf of lions western mediterranean abstract analysis of very high resolution data 3 5 khz profiles and em12d acoustic imagery collected during the calmar catalono languedocian margin cruise 1997 highlighted the occurrence of two very recent but previously unknown erosional sedimentary events on the continental rise between the rhone fan and the pyreneo languedocian ridge these events preserved as unconformable very thin lobes observable on acoustic imagery but not on 3 5 khz echo soundings relate to erosional processes linked to near bottom currents that seem to characterize the holocene hydrodynamical conditions in the deep gulf of lions they follow a pleistocene regime when gravity sedimentation dominated on the gulf of lions margin either through aggradational turbidite deposition rhone fan and pyreneo languedocian ridge or through remobilization of sediments by regional mass movements processes

microtopography and surficial sediment patterns in the central gulf of maine a 3 5 khz survey and interpretation abstract the seafloor of the gulf of maine is dominated by ice contact glaciomarine and postglacial sedimentary deposits based on 2085 km of 3 5 khz seismic profiling hummocky topography interpreted as morainal in origin is present over about half of the seafloor in water depths of 100 300 m small exposures 2 of overlying multilayered glaciomarine reflectors extend with the hummocky surface under much of an acoustically transparent layer covering about one third of the survey area this postglacial marine silty clay unit is up to 25 m thick other less common facies include an acoustically opaque smooth bed 10 and bedrock 6 the continuous seismic records reveal the sedimentary sequence and the complex patchy distribution of deposits accumulated during and after the retreat of the late wisconsinan laurentide ice sheet from the gulf of maine over the bedrock and morainal hummocks the meltwaters of glacial retreat locally deposited the glaciomarine multilayered acoustic reflectors the still younger acoustically transparent marine silts and clays display a smooth nearly level surface except for apparent scour around bedrock promontories and v shaped depressions suggestive of pockmark formation in four locations similarly postglacial to contemporary reworking of the seafloor produced the sand waves of georges shoals and smooth sandy seabed areas that are acoustically opaque

elastic interface waves along a fracture non welded interfaces can be treated as a displacement discontinuity characterized by elastic stiffnesses applying this boundary condition to a generalized rayleigh wave it is shown that a fast and a slow dispersive wave can propagate along the fracture even when the seismic properties of the rock on each side are identical

sediment facies determination using acoustic techniques in a shallow water carbonate environment dry tortugas florida abstract two high frequency acoustic seafloor classification systems 12 and 15 khz were used in conjunction with sediment core analysis to characterize sediment facies at a study site near garden key in dry tortugas florida the acoustic system uses echo return amplitude to compute acoustic impedance that is then correlated with sampled sediment impedance values calculated from wet bulk density and compressional wave velocity several bottom provinces were identified using the 15 khz data to construct a real time map of ship tracks in colors that represent the surficial sediment facies type sediment facies over the entire study site 36 km 2 range from sandy silts to exposed limestone rock and coral reef structures color contour maps using the 12 khz data created after correlating acoustic impedance predictions with core measured sediment properties validates the initial facies pattern predictions made in real time the sediment facies patterns indicate a long term pattern of deposition of fine grained silt sized surficial sediments in an area adjacent to the emergent carbonate embankment two dimensional acoustic profiles along survey tracklines also provide cross sectional views of seafloor and subbottom stratigraphy that confirm the buildup of these fine sediments in the northwest corner of the study site a generous supply of sediment resulting from an abundance of benthic green algae halimeda sp on adjacent shallow platforms form a thick sequence of fine sandy silt at the base of the southeastern edge of the embankment and fringing reef sediment cover over the limestone bedrock thins and becomes coarse southeast of garden and bush keys suggesting the likely existence of a dominant flow around the shallow carbonate embankment that restricts export of fine sediments out of the area

sedimentation processes in the northwest pacific basin revealed by echo character mapping studies bottom echoes recorded from the northwest pacific basin on high frequency 3 5 and 12 khz echograms can be classified into five general types four types are related to sedimentation processes the two most widely recorded echo types roughly correlate with the sediment type of the uppermost sub sea floor type 1 a and b echoes distinct with continuous parallel subbottoms are generally recorded from the northwest portion of the basin where sediments consist of siliceous ooze with inter beds of volcanic ash type ii echoes distinct with continuous subbottoms in the top 10 to 20 m and semitransparent with discontinuous subbottoms below characterize the southeastern portion of the basin where brown red clays with interbeds of chert predominate type iii echoes are characterized by unconformable migrating or truncated subbottoms occasionally with zones of regular overlapping hyperbolas they represent erosional depositional bed forms created by the flow of thermohaline induced bottom currents that is contour currents presumably the pacific bottom water pbw most of the regions returning type iii echoes are of local extent and are at the bases of seamounts ridges and other topographic obstructions that locally accelerate current flow a few zones of type iii echoes are of regional extent one extensive type iii zone is along the southern boundary of the north pacific basin where the pbw enters from the south through wide gaps in the topography another extensive zone is west of the emperor seamount chain the timing and duration of episodes of the sediment reworking indicated by the type iii echoes are uncertain hydrographic measurements and bottom photographs indicate only weak or sluggish pbw flow throughout the basin at present perhaps most of the sediment reworking occurred during pleistocene and older glacial phases when oceanic water masses may have circulated faster than at present type iv echoes are prolonged incoherent or fuzzy echoes returned from small 5 to 25 km localized slump debris flow deposits these deposits often are observed at the bases of very gentle knolls 100 m relief the majority of the deposits occur in areas of siliceous sediment type i echoes apparently the physical properties of these sediments render them relatively unstable and hence more susceptible to failure than the brown clays

late pleistocene and holocene sedimentary facies on the sw galicia bank atlantic nw iberian peninsula abstract five main pleistocene holocene lithofacies are defined in three different sedimentary environments fault scarp sedimentary lobe and inter lobe channel of the sw galicia bank 1 turbidites biogenous and terrigenous 2 hemipelagites 3 pelagites 4 debrites and 5 heinrich sediments in the sedimentary lobe and inter lobe channel the stratigraphic record consists mainly of turbidites interbedded with debrites and heinrich sediments and hemipelagites that are covered by hemipelagites or pelagites in the fault scarp the stratigraphy comprises hemipelagites and turbidites covered by pelagites frequency of turbidite events has varied between 1 1 2 ka and 1 3 1 ka since 31 3 ka bp at least four turbidite events 1 to 4 between 9 1 and 31 3 ka bp have been correlated between the different sedimentary environments the downslope of turbidity flows prevailed until 9 1 ka after which vertical settling and slow lateral advection have controlled sedimentation the source area of turbidites and debrites is the fault scarp erosion of the slope near surface pelagites hemipelagites and ancient outcropping deposits could explain the presence of biogenous turbidites and terrigenous turbidites respectively the rhythmic development of turbidites interrupted by hemipelagites could represent the manifestation of different pulses of sedimentary instability induced by a combination of oversteepening up to 29 and occurrence of earthquakes

transmission of seismic waves across single natural fractures fractures and other nonwelded contacts are important mechanical and hydrological features of rock masses their effects on seismic wave propagation can be modeled as a boundary condition in the seismic wave equation seismic stress is continuous across such a boundary but seismic particle displacement and seismic particle velocity are not the complete solutions for seismic wave reflection conversion and transmission across a displacement and velocity discontinuity between two half spaces with different densities and elastic properties are derived for all angles of the incident wave the ratio between the seismic stress across this boundary and the seismic particle displacement and velocity are described by a specific stiffness and a specific viscosity respectively a displacement discontinuity results in frequency dependent reflection and transmission coefficients and a frequency dependent group time delay the velocity discontinuity results in frequency independent coefficients and zero delay results of laboratory experiments on compressional and shear wave transmission across three different natural fractures in a quartz monzonite are described measurements were made at different effective stresses under dry and saturated conditions at room temperature it is shown that the effect of these fractures on the spectral amplitudes for compressional and shear pulses transmitted across these fractures are described well by a displacement discontinuity for compressional pulses under dry and saturated conditions and by a combined displacement and velocity discontinuity for shear wave pulses under dry and saturated conditions values of specific stiffness and specific viscosity vary between fractures and increase with increasing effective stress as does the static specific stiffness of these fractures changes in the spectral amplitudes of transmitted pulses are also analyzed in terms of attenuation using the seismic quality factor q which is found to be a function of frequency

seafloor morphology and acoustic facies of the george v land shelf to study the seafloor morphology on the george v land shelf east antarctica over 2000km of high frequency 3 5 27 khz echo sounder data were collected between february and march 2000 the shelf can be divided into four acoustic facies a type ia 2 seabed is ice keel turbate found on mid to outer shelf banks on seafloor less than 500m deep b type ib seabed is siliceous mud and diatom ooze drift drape and fill deposits within the george v basin between 750 and 850m depth c type iib seabed is smooth diamicton below 500m depth and occasionally has lowrelief megaflutes or ridge and swale features d type iiic seabed is high relief ridges and canyons from the coast to the deepest part of the george v basin the acoustic facies are explained in terms of glacial and oceanographic influences on the shelf since the last glacial maximum