good neighbours shaped by vigorous currents cold water coral mounds and contourites in the north atlantic abstract over the recent years two research fields addressing continental slope sedimentary systems have gained significant momentum contourites and cold water coral mounds being both related to intense bottom currents and being both dependent on sediment supply they share some commonalities resulting in their widespread co occurrence however as cold water coral mounds predominantly occur in shallow to intermediate water depths this co occurrence is restricted to these depth levels for both systems the available knowledge is derived basically from north atlantic ocean settings indicating the potential to discover such systems hidden in the other ocean basins as well as still hidden in the atlantic contourites and cold water coral mounds have the capability of providing high resolution palaeo environmental records although with often differing temporal ranges and or temporal resolutions applying the concept of a joint exploitation of sedimentary records obtained from mounds and contourites offers the unique possibility of analysing contourite and mound development as well as the related palaeo environmental settings much more comprehensively than done when only considering one of these archives thus after two decades of intense research providing significant knowledge about contourites and cold water coral mounds from the north atlantic new ideas and concepts may arise from a close collaboration of scientists working in these two fields

reefs of the deep the biology and geology of cold water coral ecosystems coral reefs are generally associated with shallow tropical seas however recent deep ocean exploration using advanced acoustics and submersibles has revealed unexpectedly widespread and diverse coral ecosystems in deep waters on continental shelves slopes seamounts and ridge systems around the world advances reviewed here include the use of corals as paleoclimatic archives and their biogeological functioning biodiversity and biogeography threats to these fragile long lived and rich ecosystems are mounting the impacts of deep water trawling are already widespread and effects of ocean acidification are potentially devastating

aftershocks of the 1999 chi chi taiwan earthquake the first hour within the hour immediately after the 1999 chi chi mainshock only 40 aftershocks were located by the taiwan central weather bureau seismic network cwbsn because of the power outage in more than half of the island and the limited dynamic range of the cwbsn high gain instruments here by analyzing 20 near field on scale records from the taiwan strong motion instrumentation program tsmip we determined a catalog of 296 aftershocks with m l 3 4 within the first hour after the chi chi mainshock focal mechanisms were also determined for 24 of these aftershocks the frequency magnitude relation obtained from the 296 aftershocks indicates that the catalog is complete above magnitude m l 4 3 most of the aftershocks occurred in a small slip region of the mainshock immediately to the north of the mainshock epicenter spatially the aftershocks appear to migrate downward from the mainshock hypocenter later in the hour the aftershocks began to concentrate in the fringe area of the main rupture during the first hour the b value as defined by events in the 4 3 m l

looking for clues to paleoceanographic imprints a diagnosis of the gulf of cadiz contourite depositional systems a new morphosedimentary map of the gulf of cadiz is presented showing the contourite depositional system on the gulf s middle slope this map is constructed from a broad da tabase provided by the spanish research council and the u s naval research laboratory our map shows that this contourite depositional system comprises five morphosedimentary sectors 1 proximal scour and sand ribbons 2 overflow sedimentary lobe 3 channels and ridges 4 contourite deposition and 5 submarine canyons the gulf of cadiz con tourite depositional system stems directly from the interaction between mediterranean out flow water and the seafloor its morphosedimentary sectors are clearly related to the sys tematic deceleration of the mediterranean outflow waters westward branches bathymetric stress on the margin and the coriolis force the slope s depositional system can be consid ered as a mixed contourite and turbidite system i e a detached combined drift and fan

contourite erosive features caused by the mediterranean outflow water in the gulf of cadiz quaternary tectonic and oceanographic implications abstract contourite depositional systems have been the focus of much recent research but still relatively little is known about contourite erosive features and their associated processes based on multibeam bathymetry side scan imagery and different resolution seismic records a detailed description and classification of the major erosive submarine valley features of the contourite depositional system of the gulf of cadiz middle slope is presented for the first time four types of erosive features have been differentiated including contourite moats contourite channels marginal valleys and large isolated furrows and interpreted in terms of their tectonic and oceanographic implications during the quaternary the study of the distribution and characteristics of erosive features is essential to better understand the present and past interaction of the mediterranean outflow water mow with the middle slope seafloor and allows us to propose a new and more precise scheme for the mow circulation patterns this scheme includes a main along slope circulation responsible for the excavation of the contourite moat and channels and a secondary down slope circulation responsible for the erosion of marginal valleys and isolated furrows three evolutionary stages have been observed in the development of the erosive system during the quaternary that can be related to changes in the distribution and splitting of the mow as a consequence of the segmentation of the ne sw diapiric ridges by neotectonic effects 1 early pleistocene to mid pleistocene linear diapiric ridges 2 mid pleistocene to late pleistocene diapiric reactivation and 3 late pleistocene to holocene main recent phase of diapiric ridges segmentation and rotation this study provides important regional clues for establishing the evolution of the erosive features in relation with neotectonic effects and represents a good example of the potential of erosive features as evidences for the reconstruction of the paleoceanography and recent tectonic changes

influence of the atlantic inflow and mediterranean outflow currents on late quaternary sedimentary facies of the gulf of cadiz continental margin abstract the late quaternary pattern of sedimentary facies on the spanish gulf of cadiz continental shelf results from an interaction between a number of controlling factors that are dominated by the atlantic inflow currents flowing southeastward across the cadiz shelf toward the strait of gibraltar an inner shelf shoreface sand facies formed by shoaling waves is modified by the inflow currents to form a belt of sand dunes at 10 20 m that extends deeper and obliquely down paleo valleys as a result of southward down valley flow a mid shelf holocene mud facies progrades offshore from river mouth sources but atlantic inflow currents cause extensive progradation along shelf toward the southeast increased inflow current speeds near the strait of gibraltar and the strong mediterranean outflow currents there result in lack of mud deposition and development of a reworked transgressive sand dune facies across the entire southernmost shelf at the outer shelf edge and underlying the mid shelf mud and inner shelf sand facies is a late pleistocene to holocene transgressive sand sheet formed by the eustatic shoreline advance the late quaternary pattern of contourite deposits on the spanish gulf of cadiz continental slope results from an interaction between linear diapiric ridges that are oblique to slope contours and the mediterranean outflow current flowing northwestward parallel to the slope contours and down valleys between the ridges coincident with the northwestward decrease in outflow current speeds from the strait there is the following northwestward gradation of contourite sediment facies 1 upper slope sand to silt bed facies 2 sand dune facies on the upstream mid slope terrace 3 large mud wave facies on the lower slope 4 sediment drift facies banked against the diapiric ridges and 5 valley facies between the ridges the southeastern sediment drift facies closest to gibraltar contains medium fine sand beds interbedded with mud the adjacent valley floor facies is composed of gravelly shelly coarse to medium sand lags and large sand dunes on the valley margins by comparison the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand because of the complex pattern of contour parallel and valley perpendicular flow paths of the mediterranean outflow current the larger scale bedforms and coarser grained sediment of valley facies trend perpendicular to the smaller scale bedforms and finer grained contourite deposits of adjacent sediment drift facies radiocarbon ages verify that the inner shelf shoreface sand facies sedimentation rate 7 1 cm kyr mid shelf mud facies maximum rate 234 cm kyr and surface sandy contourite layer of 0 2 1 2 m thickness on the cadiz slope 1 12 cm kyr have deposited during holocene time when high sea level results in maximum water depth over the gibraltar sill and full development of the atlantic inflow and mediterranean outflow currents the transgressive sand sheet of the shelf and the mud layer underlying the surface contourite sand sheet of the slope correlate respectively with the late pleistocene sea level lowstand and apparent weak mediterranean outflow current

emplacement of a passive margin evaporitic allochthon in the betic cordillera of spain the triassic section of the external domain of the betic cordillera is rootless and thus allochthonous the frontal accretionary wedge of the guadalquivir allochthon consists dominantly of triassic evaporites and red beds forming a melange with upper cretaceous paleogene deep water sedimentary rocks throughout the unit jurassic rocks are absent it is here proposed that the widespread triassic evaporites of the guadalquivir allochthon were originally emplaced as gravitational allochthonous masses in a passive margin setting much like the widespread allochthonous salt of the texas louisiana gulf of mexico thus on the betic passive margin allochthonous evaporites were first emplaced during the late cretaceous paleogene later during neogene time these evaporites were overthrusted as an accretionary wedge to form the guadalquivir allochthon a schematic reconstructed position of the guadalquivir allochthon places the original passive margin allochthon in its continental slope setting

dynamic triggering by strong motion p and s waves evidence from the 1999 chi chi taiwan earthquake high frequency band pass filtering of acceleration records from the 1999 chi chi taiwan earthquake m w 7 6 resolves the continuous signal into a series of relatively short duration discrete energy bursts we hypothesize that these bursts originate near the individual stations as small shallow events that have been dynamically triggered by the p and s waves generated by the chi chi mainshock however we cannot rule out the possibility that they originate from some nonseismic source associated with the surface such as buildings or trees bursts are observed only during the seismic signal and not in the pre or postsignal noise the bursts are not likely to be generated by the instruments because groupings of three colocated individual instruments record identical bursts also we show that the bursts are not due to the band pass filtering of instrumentally generated step functions the hypothesis that they are local events is supported by the observation of bursts in the 40 hz frequency band at distances up to 170 km from the epicenter if the bursts originated on the chi chi fault plane as hypothesized by chen et al 2006 based on their analysis of recordings within 20 km from the chelungpu fault then they should not be observable at this distance assuming any reasonable value of crustal attenuation assuming a local origin we estimate an average local event magnitude of m w 0 2 and a source receiver distance of approximately 1 km we extended our analysis to lower stress levels by analyzing records from a smaller m w 5 3 event that was recorded by many of the same instruments used in the chi chi analysis for this event bursts are observed only on the accelerograms from stations relatively close to the mainshock hypocenter analysis of the combined data set from both mainshocks suggests a stress threshold for triggering in the range 0 03 0 05 mpa for s wave triggering and 0 0013 0 0033 mpa for p wave triggering consistent with prior observations of surface wave triggering

crustal deformation in taiwan results from finite source inversions of six mw 5 8 chi chi aftershocks 1 we inverted strong motion data for the finite source parameters of six large aftershocks of the 1999 chi chi taiwan earthquake to investigate seismogenic structure in taiwan for each event we derived a preferred model by testing different focal mechanisms hypocenters rupture velocities and dislocation risetimes as well as different combinations of stations in more than 1000 inversions we documented how the fits between the waveforms and the corresponding synthetics deteriorated as the hypocenter and focal mechanism deviate from those of the preferred model if the deviation in focal mechanisms and hypocenters is less than 20 and 5 km respectively we generally recovered 80 of the preferred model s synthetic waveform fit unlike the dislocation risetime the rupture velocity used in the inversion had a strong influence on the waveform fits in this study we also used the slip models to study fault geometry two of the aftershocks ruptured on the southern extension of the main shock slip area one strike slip aftershock nucleated within the basement but ruptured mainly within the overlying sedimentary strata suggesting that seismogenic deformation in the basement can influence shallow structures p axes of the derived models have azimuths consistent with current plate motion finally gps displacement derived from the six slip models can explain 80 of the postseismic deformation observed in the aftershock regions indicating that studies of postseismic deformation must take into account the cumulative effects of large shallow aftershocks

new seismogenic source and deep structures revealed by the 1999 chia yi earthquake sequence in southwestern taiwan summary in a tectonically active setting large earthquakes are always threats however they may also be useful in elucidating the subsurface geology instrumentally recorded seismicity is therefore widely utilized to extend our knowledge into the deeper crust especially where basement is involved it is because the earthquakes are triggered by underground stress changes that usually corresponding to the framework of geological structures hidden faults therefore can be recognized and their extension as well as orientation can be estimated both above are of relevance for assessment on seismic hazard of a region since the active faults are supposed to be re activated and cause large earthquakes in this study we analysed the 1999 october 22 earthquake sequence that occurred in southwestern taiwan two major seismicity clusters were identified with spatial distribution between depths of 10 and 16 km one cluster is nearly vertical and striking 032 corresponding to the strike slip meishan fault msf that generated the 1906 surface rupture another cluster strikes 190 and dips 64 to the west which is interpreted as west vergent reverse fault in contrast to previous expectation of east vergence our analysis of the focal solutions of all the larger earthquakes in the 1999 sequence with the 3 d distribution of all the earthquakes over the period 1990 2004 allows us reinterpret the structural framework and suggest previously unreognized seismogenic sources in this area we accordingly suggest 1 multiple detachment faults are present in southwestern taiwan coastal plain and 2 additional seismogenic sources consist of tear faults and backthrust faults in addition to sources associated with west vergent fold and thrust belt

structure of the 1999 chi chi earthquake rupture and interaction of thrust faults in the active fold belt of western taiwan abstract we summarize the structural characteristics of the surface ruptures of the 1999 mw 7 6 chi chi earthquake in western taiwan and discuss the geological interaction of the chelungpu fault with adjacent faults based on geological investigations seismological analyses and gps measurements of surface co seismic displacements we describe the regional 3 d fault plane and slip distribution of the chi chi earthquake and compare these to the geological features of the chelungpu fault we find that one key feature of the chelungpu fault is the stratigraphy controlled slip surface at the level of the uppermost few kilometers the chelungpu fault slip plane generally follows the bedding plane of the pliocene chinshui shale the second key feature of the chelungpu fault is the difference in structurally geometric configuration between its northern and southern segments the northern fengyuan segment shows a bedding parallel thrust fault within east dipping strata in both footwall and hanging wall in contrast the southern tsaotun segment exhibits east dipping strata are overthrust onto flat lying recent alluvial deposits these two features not only explain a hinterland imbricate thrusting on the hanging wall of the fengyuan segment but also explain the change in strike of the chi chi surface ruptures at the northern end the southern end of the 1999 chi chi rupture is interpreted to be linked to a series of nw trending strike slip faults in particular we propose that the luliao strike slip fault served as the lateral ramp of the chelungpu fault and the gukeng strike slip fault acted as a barrier to end the southern propagation of the 1999 rupture geomorphic features and paleoseismological data indicate that the range front chelungpu fault has generated large earthquakes during the last several thousand years alternatively in the miaoli area to the north and the chiayi area to south historical earthquakes as well as active geomorphic features are not restricted on the range front thrust faults instead more complicated structures including tightly spaced folds duplex structures and strike slip faults are involved in seismogenic processes a more detailed investigation of regional structural characteristics is needed for mitigation against the seismic hazards in the 300 km long active fold belt in western taiwan where several damaging large earthquakes have been documented during the last century

the importance of sub mesoscale processes for the exchange of properties through the strait of gibraltar abstract this article presents a detailed analysis of the sub mesoscale transport processes in the strait of gibraltar the interest is focussed on the camarinal sill region and special attention is paid to the across strait transport processes the divergences and convergences in the central zone and the small scale circulation patterns along the northern coastal margin the analysis is based on high resolution 7 m sst images acquired by an air borne hyper spectral scanner and has been complemented with a rhodamine release experiment continuous thermo salinograph records acoustic doppler current adcp profiles from both moorings and vessel mounted experiments and numerical modelling it is deduced from the analysis that the coupling between the upwelling processes induced by the internal tide and the generation of large amplitude internal waves and the cyclonic eddies formed on the coastal margin seems to be the mechanism that explains the chlorophyll maxima frequently found on the coastal margin of the studied area further as a consequence of the small scale patterns of circulation induced by the internal waves the suspended substances are displaced from the coastal margins toward the central zones and later are carried by the westward current toward the convergence zones created by the internal waves where they may be retained and accumulate then in the eastward phase of the tidal current over the camarinal sill these nuclei of concentrated substances nutrients chlorophyll and plankton are transported toward the alboran sea where they must contribute in part to the primary productivity there

multidisciplinary study of mud volcanoes and diapirs and their relationship to seepages and bottom currents in the gulf of cadiz continental slope northeastern sector abstract the seabed morphology type of sediments and dominant benthic species on eleven mud volcanoes and diapirs located on the northern sector of the gulf of cadiz continental slope have been studied the morphological characteristics were grouped as i fluid escape related features ii bottom current features iii mass movement features iv tectonic features and v biogenic related features the dominant benthic species associated with fluid escape hard substrates or soft bottoms have also been mapped a bottom current velocity analysis allowed the morphological features to be correlated with the benthic habitats and the different sedimentary and oceanographic characteristics the major factors controlling these features and the benthic habitats are mud flows and fluid escape related processes as well as the interaction of deep water masses with the seafloor topography mud volcano eruptions give rise to mud flows and or aqueous fluid seepage these processes sustain chemosynthesis based communities closely associated with fluid seepage large depressions in the nearby area are influenced by collapse related phenomena where active fluid escape and the erosive effect of bottom currents have been identified when the extrusion activity of the mud volcano is low and the seepage is diffuse authigenic carbonates form within the edifice sediments the bottom current sweeps the seafloor from the se to the nw when the velocity is moderate sedimentary contourite processes take place on both sides of the edifices at high velocities the authigenic carbonates may be exhumed and colonised by species associated with hard substrates small carbonate mounds are found at the summits of some volcanoes and diapirs living corals have been found on the tops of the shallowest mud volcanoes revealing different oceanographic conditions and strong bottom currents that favour the availability of nutrients and organic particles the edifices affected by very high current velocities are located in the channels where erosive processes dominate

seabed morphology and hydrocarbon seepage in the gulf of cadiz mud volcano area acoustic imagery multibeam and ultra high resolution seismic data abstract extensive mud volcanism mud diapirism and carbonate chimneys related to hydrocarbon rich fluid venting are observed throughout the spanish portuguese margin of the gulf of cadiz all the mud volcanoes and diapirs addressed in this paper lie in the region of olistostrome accretionary complex units which were emplaced in the late miocene in response to nw directed convergence between the african and eurasian plates the study area was investigated by multibeam echo sounder high and ultra high resolution seismic profiling dredging and coring the structures observed on multibeam bathymetry at water depths between 500 and 1300 m are dominated by elongate mud ridges mud cones mud volcanoes and crater like collapse structures ranging in relief from 50 to 300 m and size from 0 8 to 2 km in diameter the main morphotectonic features named the guadalquivir diapiric ridge gdr and the cadiz diapiric ridge cdr are longitudinally shaped diapirs which trend ne sw and consist of lower middle miocene plastic marly clays the gdr field and the tasyo field which consist of mud volcanoes and extensive fluid venting related to diapiric ridge development are described in this paper the gdr field is characterised by numerous single sub circular mud volcanoes and mud cones the single mud volcanoes are cone shaped features with relatively gentle slopes of 3 6 consisting of several generations of mud breccia deposition with indications of gas saturation degassing structures and the presence of h2s the mud cones have asymmetric profiles with steep slopes of up to 25 and contain large surficial deposits of hydrocarbon derived carbonate chimneys and slabs the tasyo field is characterised by an extensive concentration of small sub circular depressions oval and multi cone mud volcanoes and large sediment slides mud volcanoes in this area are characterised by moderate slopes 8 12 have bathymetric relief ranging from 100 to 190 m and consist of sulphide rich mud breccia calcite chimneys carbonate crusts and chemosynthetic fauna pogonophora tube worms we propose that all these hydrocarbon seepage structures are related to lateral compressional stress generated at the front of the olistostromic accretionary wedge this stress results in the uplifting and squeezing plastic marly clay deposits additionally the compressional stress at the toe of the olistostrome forms overpressurised compartments which provide avenues for hydrocarbon enriched fluids to migrate

structure and evolution of the olistostrome complex of the gibraltar arc in the gulf of cadiz eastern central atlantic evidence from two long seismic cross sections reflection profiles characterize the structure and the upper mesozoic to cenozoic deposits of the gulf of ca diz region two long ene wsw multichannel seismic lines ca 400 500 km long are analyzed to study the evolution of the area from the continental shelf to the horseshoe and seine abyssal plains the huge allochthonous deposits emplaced in this region the socalled olistostrome of the gulf of cadiz are described in terms of three different domains on the basis of the seismic architecture the main tectonic features and the nature of the basement oceanic or continental the eastern domain extends along the continental shelf and upper and middle slope and corresponds to the offshore extension of the betic rifean external front it is characterized by salt and shale nappes later affected by extensional collapses the central domain develops along the lower slope between the betic rifean front and the abyssal plains and is characterized by a change in dip of the allochthonous basal surface and the basement the allochthonous masses were emplaced by a combined gravitational and tectonic mechanism the northern boundary of this domain is marked by the occurrence of an outstanding wnw ese trending thrust fault with a strike slip component termed here as the gorringe horseshoe fault the westernmost domain corresponds to the abyssal plains where the distal emplacement of the allochthonous body takes place it is characterized by thrust faults affecting both the sedimentary cover and the oceanic basement the allochthonous masses show a less chaotic character and the thickness decreases notably these domains represent different evolutionary steps in the mechanisms of emplacement of the allochthonous units the eastern domain of the allochthonous units was emplaced as part of the pre messinian orogenic wedge related to the collision that gave rise to the betic rifean belt whereas the allochthonous wedge of the central and western domains were emplaced later as a consequence of the ne sw late miocene compression that continues in present times

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

the determination of uniaxial compressive strength from point load strength for pyroclastic rocks abstract the point load test has often been reported as an indirect measure of compressive strength it has been used widely in practice due to easy testing the simplicity of specimen preparation and possible field applications many researchers have investigated the relation between uniaxial compressive strength ucs and point load index is for different rock types however there are limited studies in the literature for soft rocks such as pyroclastic rocks in this study the relation between ucs and is was investigated for pyroclastic rocks having ucs values are less than 50 mpa very strong exponential relations were found for dry rocks saturated rocks and both dry and saturated rocks the results were also compared to the studies in the literature

contourite characterization and its discrimination from other deep water deposits in the gulf of cadiz contourite depositional system this project was funded through the joint industry project supported by bp eni exxonmobil total wintershall dea and tgs within the framework of the drifters research group at royal holloway university of london rhul in conjunction with the projects ctm 2012 39599 c03 cgl2015 66835 p ctm2016 75129c3 1 r cgl2016 80445 r aei feder ue and b rnm 072 ugr18 this research used data and samples collected by iodp expedition 339 aboard the joides resolution data was acquired at the xrf core scanner lab at marum center for marine environmental sciences university of bremen germany our thanks to dr javier dorador rhul uk for the high resolution core imaging treatment to dr david roque icman csic spain for his help in the hydrographic panels from fig 2 and to dr angel puga bernabeu university of granada spain for revision that helped to improve the original version of the manuscript before submission this study significantly benefited from discussions with all who attended the iodp exp 339 mow meeting in heidelberg germany in january 2019 we would like to thank associate editor dr adam mcarthur and the three reviewers dr rachel brackenridge dr giancarlo davoli and dr adriano viana for their positive and constructive comments that helped us improve the manuscript

spatio temporal slip and stress level on the faults within the western foothills of taiwan implications for fault frictional properties we use preseismic coseismic and postseismic gps data of the 1999 chi chi earthquake to infer spatio temporal variation of fault slip and frictional behavior on the chelungpu fault the geodetic data shows that coseismic slip during the chi chi earthquake occurred within a patch that was locked in the period preceding the earthquake and that afterslip occurred dominantly downdip from the ruptured area to first order the observed pattern and the temporal evolution of afterslip is consistent with models of the seismic cycle based on rate and state friction comparison with the distribution of temperature on the fault derived from thermo kinematic modeling shows that aseismic slip becomes dominant where temperature is estimated to exceed 200 at depth this inference is consistent with the temperature induced transition from velocity weakening to velocity strengthening friction that is observed in laboratory experiments on quartzo feldspathic rocks the time evolution of afterslip is consistent with afterslip being governed by velocity strengthening frictional sliding the dependency of friction l on the sliding velocity v is estimated to be ol o ln v 8 10 3 we report an azimuthal difference of about 10 20 between preseismic and postseismic gps velocities which we interpret to reflect the very low shear stress on the creeping portion of the decollement beneath the central range of the order of 1 3 mpa implying a very low friction of about 0 01 this study highlights the importance of temperature and pore pressure in determining fault frictional sliding

stress before and after the 2002 denali fault earthquake 1 spatially averaged absolute deviatoric stress tensors along the faults ruptured during the 2002 denali fault earthquake both before and after the event are derived using a new method from estimates of the orientations of the principal stresses and the stress change associated with the earthquake stresses are estimated in three regions along the denali fault one of which also includes the susitna glacier fault and one region along the totschunda fault estimates of the spatially averaged shear stress before the earthquake resolved onto the faults that ruptured during the event range from near 1 mpa to near 4 mpa shear stresses estimated along the faults in all these regions after the event are near zero 0 1 mpa these results suggest that deviatoric stresses averaged over a few tens of km along strike are low and that the stress drop during the earthquake was complete or nearly so

very large subaqueous sand dunes on the upper continental slope in the south china sea generated by episodic shoaling deep water internal solitary waves abstract very large subaqueous sand dunes were discovered on the upper continental slope of the northern south china sea the dunes were observed along a single 40 km long transect southeast of 21 93 n 117 53 e on the upper continental slope in water depths of 160 m to 600 m the sand dunes are composed of fine to medium sand with amplitudes exceeding 16 m and crest to crest wavelengths exceeding 350 m the dunes apparent formation mechanism is the world s largest observed internal solitary waves which generate from tidal forcing on the luzon ridge on the east side of the south china sea propagate west across the deep basin with amplitudes regularly exceeding 100 m and dissipate extremely large amounts of energy via turbulent interaction with the continental slope suspending and redistributing the bottom sediment while subaqueous dunes are found in many locations throughout the world s oceans and coastal zones these particular dunes appear to be unique for two principal reasons their location on the upper continental slope away from the influence of shallow water tidal forcing deep basin bottom currents and topographically amplified canyon flows and their distinctive formation mechanism approximately 60 episodic extremely energetic large amplitude events each lunar cycle

kinematic and dynamic fault slip analyses implications from the surface rupture of the 1999 chi chi taiwan earthquake the 1999 chi chi earthquake created a 100 km long surface rupture on the chelungpu thrust fault in taiwan due to convergence between the phillipine sea and eurasian plates fault slip measurements were made by several researchers from the entire length of the rupture directly following the earthquake and can thus be attributed to a single tectonic event conventional fault slip analyses are applied to these data and the results are compared with independent seismological and kinematic observations unlike many fault slip analyses complications due to multiple deformations can be categorically excluded and the results can be evaluated from the seismological and plate movement data kinematic analyses of fault slip data that are weighted by displacement show sub horizontal nw se shortening that is parallel to the plate convergence vector a single fault plane solution satisfies almost all the data right dihedra and trihedra solutions also satisfy almost all the surface rupture measurements and give 1 in a nw se orientation that is similar to the results of stress inversion and to inversion from earthquakes in the chi chi earthquake sequence despite criticisms of fault slip analysis methods these results show that fault slip analyses from data collected along major faults which have not witnessed multiple deformation events can be valid homogeneous strain and stress states exist in the sense that kinematic and dynamic solutions can be found that fit essentially all the data and these solutions have tectonic significance

response of the san andreas fault to the 1983 coalinga nunez earthquakes an application of interaction based probabilities for parkfield 1 the parkfield cholame section of the san andreas fault site of an unfulfilled earthquake forecast in 1985 is the best monitored section of the world s most closely watched fault in 1983 the m 6 5 coalinga and m 6 0 nunez events struck 25 km northeast of parkfield seismicity rates climbed for 18 months along the creeping section of the san andreas north of parkfield and dropped for 6 years along the locked section to the south right lateral creep also slowed or reversed from parkfield south here we calculate that the coalinga sequence increased the shear and coulomb stress on the creeping section causing the rate of small shocks to rise until the added stress was shed by additional slip however the 1983 events decreased the shear and coulomb stress on the parkfield segment causing surface creep and seismicity rates to drop we use these observations to cast the likelihood of a parkfield earthquake into an interaction based probability which includes both the renewal of stress following the 1966 parkfield earthquake and the stress transfer from the 1983 coalinga events we calculate that the 1983 shocks dropped the 10 year probability of a m 6 parkfield earthquake by 22 from 54 22 to 42 23 and that the probability did not recover until about 1991 when seismicity and creep resumed our analysis may thus explain why the parkfield earthquake did not strike in the 1980s but not why it was absent in the 1990s we calculate a 58 17 probability of a m 6 parkfield earthquake during 2001 2011 index terms 7223 seismology seismic hazard assessment and prediction 7230 seismology seismicity and seismotectonics 7260 seismology theory and modeling keywords coalinga earthquake parkfield stress change earthquake probability seismicity rate

contourite processes associated with the mediterranean outflow water after its exit from the strait of gibraltar global and conceptual implications we characterize the eastern gulf of cadiz proximal to the strait of gibraltar using a multidisciplinary approach that combines oceanographic morphosedimentary and stratigraphic studies two terraces upper and lower were identified along the middle slope they are composed of several associated morphologic elements including two large erosive channels which allow us to determine a new and more detailed understanding of the mediterranean outflow water mow pathway and its deceleration upon exiting the strait of gibraltar there is evidence for along slope circulation and additional secondary circulation oblique to the main flow the present upper core of the mow flows along the upper terrace and the lower core flows along the lower terrace however the lower terrace shows larger and better defined erosive features on the seafloor than does the upper terrace we attribute this to a denser deeper and faster mow circulation that prevailed during past cold climates development of the present features started ca 3 8 3 9 ma but the present morphology was not established until the late pliocene early quaternary 3 2 to older than 2 0 ma when the mow was enhanced coeval with global cooling a sea level fall and an increase in thermohaline circulation we propose a direct link between the mow and the atlantic meridional overturning circulation and therefore between the mow and both the northern hemisphere and global climate our results have enabled a better understanding of a major overflow related to an oceanic gateway and are of broad interest to geologists climatologists oceanographers and petroleum geologists

the chi chi earthquake sequence active out of sequence thrust faulting in taiwan we combined precise focal depths and fault plane solutions of more than 40 events from the 20 september 1999 chi chi earthquake sequence with a synthesis of subsurface geology to show that the dominant structure for generating earthquakes in central taiwan is a moderately dipping 20 to 30 thrust fault away from the deformation front a second subparallel seismic zone lies about 15 kilometers below the main thrust these seismic zones differ from previous models indicating that both the basal decollement and relic normal faults are aseismic

large scale slope failure involving triassic and middle miocene salt and shale in the gulf of cadiz atlantic iberian margin sheets of salt and ductile shale advancing beyond the thrust front of the gibraltar arc iberian moroccan atlantic continen tal margin triggered downslope movements of huge allochth onous masses these allochthons represent the cadiz nappe which detached from the gibraltar arc along low angle normal faults and migrated downslope from the iberian and moroccan continental margins towards the atlantic ocean extensional tectonics initiated upslope salt withdrawal and downslope diapirism during large scale westward mass wasting from the shelf and upper slope low angle salt and shale detachments bound by lateral ramps link extensional structures in the shelf to folding thrusting and sheets of salt and shale in the gulf of cadiz from backstripping analyses carried out on the depo centres of the growth fault related basins on the shelf we infer two episodes of rapid subsidence related to extensional collapses these were from late tortonian to late messinian 200 400 m myr 1 and from early pliocene to late pliocene 100 150 m myr 1 the extensional events that induced salt movements also affected basement deformation and were probably associated with the westward advance of frontal thrusts of the gibraltar arc as a result of the convergence between africa and eurasia the complexities of salt and or shale tectonics in the gulf of cadiz result from a combination of the deformations seen at convergent and passive continental margins

response of seismicity to coulomb stress triggers and shadows of the 1999 mw 7 6 chi chi taiwan earthquake 1 the correlation between static coulomb stress increases and aftershocks has thus far provided the strongest evidence that stress changes promote seismicity a correlation that the chi chi earthquake well exhibits several studies have deepened the argument by resolving stress changes on aftershock focal mechanisms which removes the assumption that the aftershocks are optimally oriented for failure here one compares the percentage of planes on which failure is promoted after the main shock relative to the percentage beforehand for chi chi we find a 28 increase for thrust and an 18 increase for strike slip mechanisms commensurate with increases reported for other large main shocks however perhaps the chief criticism of static stress triggering is the difficulty in observing predicted seismicity rate decreases in the stress shadows or sites of coulomb stress decrease detection of sustained drops in seismicity rate demands a long catalog with a low magnitude of completeness and a high seismicity rate conditions that are met at chi chi we find four lobes with statistically significant seismicity rate declines of 40 90 for 50 months and they coincide with the stress shadows calculated for strike slip faults the dominant faulting mechanism the rate drops are evident in uniform cell calculations 100 month time series and by visual inspection of the m 3 seismicity an additional reason why detection of such declines has proven so rare emerges from this study there is a widespread increase in seismicity rate during the first 3 months after chi chi and perhaps many other main shocks that might be associated with a different mechanism

the shaping of continental slopes by internal tides the angles of energy propagation of semidiurnal internal tides may determine the average gradient of continental slopes in ocean basins 2 to 4 degrees intensification of near bottom water velocities and bottom shear stresses caused by reflection of semi diurnal internal tides affects sedimentation patterns and bottom gradients as indicated by recent studies of continental slopes off northern california and new jersey estimates of bottom shear velocities caused by semi diurnal internal tides are high enough to inhibit deposition of fine grained sediment onto the slopes

tectonics and mud volcano development in the gulf of cadiz many structures associated with fluid escape including mud volcanoes mud carbonate mounds pockmarks and slides have been identified and characterized in the gulf of cadiz most of the mud volcanoes following ne sw and nw se main trends are found at 350 2000 m depth in the eastern domain of the gulf of cadiz which corresponds to the betic rifean margin scattered mud volcanoes have also been recognized on the lower slope at 2300 3900 m depth the major tectonic structures are thrust faults extensional faults strikeslip faults and diapirs all these tectonic structures have provided escape pathways for overpressured material and fluids or have favoured upward fluid movement along the sedimentary column and eventually the build up of mud volcanoes in this work we present images of the mud volcano plumbing systems and the relationship between regional tectonics and mud volcano development seismic profiles acquired during the tasyo 2000 and mvseis ttr 15 cruises are used to image and interpret the link between the mud volcano edifices and the subsurface tectonic structures

significance of mud volcanism 1 mud volcanism and diapirism have puzzled geoscientists for 2 centuries they have been described onshore and offshore in many places on earth and although they occur in various tectonic settings the majority of the features known to date are located in compressional tectonic scenarios this paper summarizes the main thrusts in mud volcano research as well as the various regions in which mud volcanism has been described mud volcanoes show variable geometry up to tens of kilometers in diameter and several hundred meters in height and a great diversity regarding the origin of the fluid and solid phases gas predominantly methane water and mud may be mobilized at subbottom depth of only a few meters but in places can originate from several kilometers depth with minor crustal or mantle input the possible contribution of mud extrusion to global budgets both from quiescent fluid emission and from the extrusive processes themselves is important in regions where mud volcanoes are abundant such as the collision zones between africa and eurasia fluid flux through mud extrusion exceeds the compaction driven pore fluid expulsion of the accretionary wedge also quiescent degassing of mud volcanoes may contribute significantly to volatile budgets and hence to greenhouse climate

stress tensor analysis in the taiwan area from focal mechanisms of earthquakes abstract we produce a map of the stress pattern in and around taiwan based on 200 earthquake focal mechanism solutions these solutions were determined by using data from taiwan telemetered seismographic network microearthquake surveys and wwssn the stresses are derived through a minimization of angles between the slip vector and the shear stress on each nodal plane considered as a fault employing appropriate weighting factors the whole set of focal mechanisms is divided into several groups mainly according to apparent clustering of the event locations the results show that the direction of maximum principal stress in taiwan area is nearly horizontal and se nw on average this is in good agreement with the direction of relative motion between the philippine sea plate and the eurasian plate in western taiwan the fan shaped distribution of the maximum principal stress is consistent with the direction of philippine sea eurasian plate convergence through a simple model of viscous material indented by a rigid wedge in the northeastern part of taiwan a nearly horizontal minimum principal stress oriented n s is found for shallow depths it occurs in a region of low seismic velocities probably related to the back arc activity of the okinawa trough down dip compressional and down dip extensional stresses have been identified in different depth ranges within the subducting slab of the philippine sea plate in the northern taiwan this may reflect the slab characteristics in this area a complex stress pattern prevails in the hualien area at the junction between the ryukyu subduction system and the taiwan collision zone

stress evolution following the 1999 chi chi taiwan earthquake consequences for afterslip relaxation aftershocks and departures from omori decay summary we explore how coulomb stress transfer and viscoelastic relaxation control afterslip and aftershocks in a continental thrust fault system the 1999 september 21 m w 7 6 chi chi shock is typical of continental ramp dsystems throughout the world and so inferences drawn from this uniquely well recorded event may be widely applicable first we find that the spatial and depth distribution of aftershocks and their focal mechanisms are consistent with the calculated coulomb stress changes imparted by the coseismic rupture some 61 per cent of the m 2 aftershocks and 83 per cent of the m 4 aftershocks lie in regions for which the coulomb stress increased by 0 1 bars and there is a 11 12 per cent gain in the percentage of aftershocks nodal planes on which the shear stress increased over the pre chi chi control period second we find that afterslip occurred where the calculated coseismic stress increased on the fault ramp and dsubject to the condition that friction is high on the ramp and low on the d ecollement third viscoelastic relaxation is evident from the fit of the post seismic gps data on the footwall fourth we find that the rate of seismicity began to increase during the post seismic period in an annulus extending east of the main rupture the spatial extent of the seismicity annulus resembles the calculated 0 05 bar coulomb stress increase caused by viscoelastic relaxation and afterslip and we find a 9 12 per cent gain in the percentage of focal mechanisms with 0 01 bar shear stress increases imparted by the post seismic afterslip and relaxation in comparison to the control period thus we argue that post seismic stress changes can for the first time be shown to alter the production of aftershocks as judged by their rate spatial distribution and focal mechanisms

mediterranean undercurrent sandy contourites gulf of cadiz spain abstract the pliocene quaternary pattern of contourite deposits on the eastern gulf of cadiz continental slope results from an interaction between linear diapiric ridges that are perpendicular to slope contours and the mediterranean undercurrent that has flowed northwestward parallel to the slope contours and down valleys between the ridges since the late miocene opening of the strait of gibraltar coincident with the northwestward decrease in undercurrent speeds from the strait there is the following northwestward gradation of sediment facies associations 1 upper slope facies 2 sand dune facies on the upstream mid slope terrace 3 large mud wave facies on the lower slope 4 sediment drift facies banked against the diapiric ridges and 5 valley facies between the ridges the southeastern sediment drift facies closest to gibraltar contains medium fine sand beds interbedded with mud the adjacent valley floor facies is composed of gravelly shelly coarse to medium sand lags and large sand dunes on the valley margins compared to this the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand further northwestward sediment drift grades to biogenous silt near the faro drift at the portuguese border because of the complex pattern of contour parallel and valley perpendicular flow paths of the mediterranean undercurrent the larger scale bedforms and coarser grained sediment of valley facies trend perpendicular to the smaller scale bedforms and finer grained contourite deposits of adjacent sediment drift facies the bottom current deposits of valleys and the contourites of the cadiz slope intervalley areas are distinct from turbidite systems the valley sequences are not aggradational like turbidite channel levee complexes but typically exhibit bedrock walls against ridges extensive scour and fill into adjacent contourites transverse bedform fields and bioclastic lag deposits both valley and contourite deposits exhibit reverse graded bedding and sharp upper bed contacts in coarse grained layers low deposition rates and a regional pattern of bedform zones textural variation and compositional gradation the surface sandy contourite layer of 0 2 1 2 m thickness that covers the gulf of cadiz slope has formed during the present holocene high sea level because high sea level results in maximum water depth over the gibraltar sill and full development of the mediterranean undercurrent the late pleistocene age of the mud underlying the surface sand sheet correlates with the age of the last sea level lowstand and apparent weak mediterranean undercurrent development thus the cyclic deposition of sand or mud layers and contourite or drape sequences appear to be related to late pliocene and quaternary sea level changes and mediterranean water circulation patterns since its pliocene origin the contourite sequence has had low deposition rates of

surface deformation and tectonic setting of taiwan inferred from a gps velocity field 1 we have determined the present day surface deformation of taiwan by computing the velocity gradient field and fault slip from 143 gps velocity vectors in southern taiwan the derived strain and rotation rates and fault slips are indicative of lateral extrusion toward the south in northern taiwan we infer the onset of gravitational collapse which is induced by the on land extension of the okinawa trough in the eastern central range the observed inverted nw se extension is consistent with geological observations and high heat flow measurements this could be the result of exhumation of crustal material the model further shows a significant decrease in slip rate northward along the longitudinal valley fault at 23 7 n the northern coastal range shows high strain rates and two oppositely rotating blocks by combining the surface deformation model with seismicity data and seismic tomography we are able to propose a coherent model for the present day tectonic activity both seismicity and tomography show further evidence for active southward propagating exhumation of a crustal slice in the eastern central range offshore east taiwan we deduce strong evidence of a southward propagating crustal tear fault accommodating most of the philippine sea plate eurasian plate convergence the tear is the crustal response to incipient northwestward subduction of the philippine sea plate thus the ryukyu trench is bending southward becoming almost perpendicular to the convergence direction while subduction of the philippine sea plate continues in this setting a sudden rapid southward propagation of the afore mentioned tear is conceivable

source geometry and slip distribution of the april 21 1935 hsinchu taichung taiwan earthquake abstract the disastrous hsinchu taichung earthquake magnitude mgr 7 1 occurred on april 21 1935 the earthquake was known to be associated with two faults the tuntzuchiao oblique right lateral strike slip fault and the shihtan reverse fault preliminary studies for this earthquake have been done at present however the source geometry and slip distribution are still not clear in this study the geodetic observations of coseismic ground deformation for this earthquake are modeled using the finite element method to infer its source parameters and to deduce the mechanism of its fault rupture we determined the length width and offset of the fault the seismic moment and the stress drop of this shock series and proposed that the hsinchu taichung earthquake sequence was actually associated with three main faults the shihtan fault the tuntzuchiao fault and the fault of the july 17 aftershock the shihtan fault is a high angle reverse fault of a curved plane geometry with a dip angle decreasing continuously with depth the major fault slips occurred in the central part of the fault plane the fault length width and average slip are 25 km 11 km and 1 94 m respectively the tuntzuchiao fault is a high angle right lateral strike slip fault dipping southeast the fault length width and average slip are 15 km 6 2 km and 1 9 m respectively the july 17 aftershock was associated with a pure left lateral strike slip fault dipping at 60 toward southwest having a fault length of 12 5 km a width of 5 8 km and a uniform slip of 1 0 m the estimated seismic moments of these faults are 3 2 1026 dyne cm for the shihtan fault 1 1 1026 dyne cm for the tuntzuchiao fault and 4 4 1025 dyne cm for the july 17 aftershock the stress drops are 162 bar for the tuntzuchiao fault 90 bar for the shihtan fault and 88 bar for the july 17 aftershock our results indicated that the hsinchu taichung earthquake is a special case of complex fault geometry and heterogeneous slip distribution furthermore from the fault rupture pattern and regional stress direction we deduced that the motions on these faults were consistent with those under the regional tectonic stress in the se nw direction and the ruptures of the tuntzuchiao fault and these aftershocks along the houlung river including the july 17 aftershock seem to indicate conjugate faulting

velocity field of gps stations in the taiwan area abstract the 131 stations of the taiwan gps network were surveyed 4 6 times from 1990 to 1995 with dual frequency geodetic receivers the standard deviation of an observed baseline length with its linear trend removed is in the range of 6 10 mm for a 3 120 km long baseline the average rates of length change for all baselines of the network and those from nine continuously monitoring permanent stations are used in a least squares adjustment to estimate the velocities of the gps stations relative to paisha penghu situated at the chinese continental margin to the south of fengping in the northern coastal range the velocity vectors of stations in lanhsu lutao and the coastal range trend in the directions of 306 322 with rates of 56 82 mm yr in contrast there is a dramatic decrease in the rates to the north of fengping this may be caused by the motion along the ne sw trending thrusts which obliquely cut the northern coastal range a discontinuity of about 30 mm yr in the rates along with a remarkable change in the directions of station velocity is observed across the longitudinal valley then the moving directions gradually shift to the west for the stations in the western foothills in the kaohsiung pingtung coastal area the station velocities are even directed toward the southwest to the north of the peikang high the velocity vectors of the stations change direction from the west gradually to the north and finally to the east and southeast significant nw se extensional deformation is found in the ilan plain and northern taiwan in general the pattern of the velocity field for gps stations in the taiwan area is quite consistent with the directions of present day tectonic stress

surface rupture of 1999 chi chi earthquake yields insights on active tectonics of central taiwan the 1999 chi chi earthquake was caused by rupture of the chelungpu fault one of the most prominent active thrust faults of taiwan this largest of taiwan s historical fault ruptures broke the surface for over 90 km at the western base of the rugged mountain range a short right lateral tear extended southwestward from the southern end of the chelungpu fault and a complex assemblage of shallow folds and faults ran northeastward from the northern end vertical offsets averaged about 2 m along the southern half of the chelungpu fault and about 4 m along the northern half and offsets of 5 to 7 m were typical along the northern part of the major thrust the sinuous nature of the surface trace is consistent with seismographic data that indicate a dip of about 30 the 1999 rupture draws attention to the fact that this active fault system is highly segmented and that this segmentation influences the characteristics of seismic ruptures active faults to the south north and west of the chelungpu fault have distinctly different characteristics faults to the south and north broke the surface during earthquakes in 1906 and 1935 the active changhua fault to the west a blind thrust similar in length to the chelungpu has not ruptured in the historical period and should be considered a prime candidate for generating a future earthquake manuscript received 4 october 2000

three dimensional seismic analysis of sediment waves and related geomorphological features on a carbonate shelf exposed to large amplitude internal waves browse basin region australia this study analyses the three dimensional geometry of sedimentary features recorded on the modern sea floor and in the shallow subsurface of a shelf to upper slope region offshore australia that is characterized by a pronounced internal wave regime the data interpreted comprise an extensive 12 500 km2 industrial three dimensional seismic reflection survey that images the northern part of the browse basin australian north west shelf the most prominent seismic morphological features on the modern sea floor are submarine terrace escarpments fault scarps and incised channels as well as restricted areas of seismic distortion interpreted as mass wasting deposits besides these kilometre scale sea floor irregularities smaller bedforms were discovered also including a multitude of sediment waves with a lateral extent of several kilometres and heights up to 10 m these sedimentological features generally occur in extensive fields in water depths below 250 m mostly at the foot of submerged terraces along the scarps of modern faults and along the shelf break between the outer shelf and the upper continental rise additional bedforms that characterize the more planar regions of the outer shelf are elongate north west south east oriented furrows and ridges the formation of both sediment waves and furrow ridge systems requires flow velocities between 0 3 m sec 1 and 1 5 m sec 1 which could be generated by oceanic currents gravity currents or internal waves in the studied setting these velocities can be best explained as being generated by bottom currents induced by internal waves an interpretation that is discussed against oceanographic background data and modelling results in addition to the documentation of three dimensional seismic geomorphological features of the modern sea floor it was also possible to map kilometre scale buried sediment wave fields in the seismic volume down to ca 500 ms two way time below the present sea floor indicating the general potential for the preservation of such bedforms in the sedimentary record

integrating borehole breakout dimensions strength criteria and leak off test results to constrain the state of stress across the chelungpu fault taiwan article the paper describes the computation of the maximum horizontal stress h magnitude in the vicinity of the chelungpu fault taiwan host of the slip zone during the chi chi earthquake mw 7 6 1999 the scientific hole b intercepts the chelungpu fault at 1136 m at the depths of logged breakouts 940 1310 m the vertical stress v as estimated from density logs increases linearly with depth from 22 to 31 mpa a series of leak off tests yielded two reliable shut in pressures 23 7 mpa at 1085 m and 29 8 mpa at 1279 m which are lower than the estimated v albeit by only 2 1 and 0 6 mpa respectively in our analysis the shut in pressures were considered to represent estimates of the least horizontal principal stresses h at the respective depths and consequently the test induced fractures were assumed to have been vertical principal stress directions had been previously determined by others 105 155 for the maximum horizontal stress h

strain and stress field in taiwan oblique convergent system constraints from gps observation and tectonic data abstract this paper focuses on the stress and strain rate field of the taiwan area the strain rate field in the taiwan region is studied qualitatively and quantitatively based on the gps observation in 1990 1995 it reflects the accommodation of the ongoing lithospheric deformation within the seismogenic portion of lithosphere and exhibits zones with contrasting deformation modes and amounts we then compare the obtained strain rate field with the tectonic information provided by studies of borehole breakouts and earthquake focal mechanisms for the present and by fault slip data analyses for the quaternary period in the first approximation the stress and strain rate fields show spatial similarity the orientation of principal shortening is generally consistent with the compressive stress orientation that reflects the oblique indentation of the luzon arc into the eurasian continental margin in more detail significant anomalies in the deformation pattern deserve consideration in that they may reveal ongoing stress accumulation despite the short term variations related to the earthquake cycle some major features of the strain rate field including the distribution of extension and compression highlight the long term tectonic behavior of the mountain belt at the lithospheric scale the time and space variations of strain should be a function of local heterogeneity and be transferred between interseismic and coseismic periods

high resolution stratigraphy of the mediterranean outflow contourite system in the gulf of cadiz during the late pleistocene the impact of heinrich events abstract a detailed high resolution stratigraphic analysis of the mediterranean outflow contourite system at the continental slope of the gulf of cadiz has been carried out through the correlation between a dense network of seismic reflection profiles sparker airgun 3 75 khz and parametric echosounder topas calypso giant piston and standard gravity cores from such correlation we determine a stacking pattern constituted by four main seismic units a d that are internally structured into ten subunits each subunit shows a single sequence formed by transparent seismic facies at the base to smooth parallel reflectors of moderate to high amplitude facies at the top being well correlated in the cores with a coarsening upward sequence the latest pleistocene holocene deposits form glacial interglacial depositional sequences related to cycles with a frequency range below the milankovitch band that corresponds to millennial timescale climatic changes such as dansgaard oeschger 1 5 ka and bond cycles 10 15 ka oxygen isotope records of planktonic foraminifera and the occurrence of ice rafted debris ird in the most recent contourite subunits show clear evidence of the influence of the north atlantic climatic conditions especially the climatic heinrich events h in the slope sedimentation of the gulf of cadiz and then in the circulation of the mediterranean outflow water mow the coarser contourite deposits are mostly associated with the last glacial maximum younger dryas and heinrich events on the central area of the middle slope during globally cooler conditions the mow was denser so that it was more active in deeper areas than today on the other hand during warm periods the mow became less dense favoring an increased intensity of the mow on the distal area of the upper slope therefore spatial and vertical fluctuations of the mow contourite system are strongly affected by global climate and oceanographic changes being clearly influenced by iceberg discharges and probably also by the resumption of thermohaline circulation in the north atlantic ocean during ice melting periods

carbonate mounds in a mud volcano province off north west morocco key to processes and controls abstract this paper presents a new cluster of carbonate mounds discovered in 2002 in the gulf of cadiz off morocco r v belgica 2002 in water depths of 500 to 600 m amidst a field of giant mud volcanoes multibeam bathymetry side scan sonar imagery and 2d seismics are analyzed to present four mound provinces 1 the pen duick mound province on the pen duick escarpment 2 the renard mound province on the renard ridge 3 the vernadsky mound province on the vernadsky ridge and the al idrisi mound province on the gas blanked sediments above the buried al idrisi ridge video imagery and surface samples are described to ground truth the different mound areas the paradox is that nearly no live corals are presently being observed at the surface of the mounds while the mound cores display throughout a high number of reef forming cold water coral fragments scleractinians in association with numerous associated fauna formerly inhabiting the econiches provided by the coral framework environmental and oceanographic conditions during the recent past glacials stadials were probably more favourable for cold water coral growth pore water analyses in on mound cores at the south eastern edge of pen duick escarpment give evidence of focused higher methane fluxes and sulphate reduction rates on mounds than in the surrounding sediments cores from several mounds display horizons of strong corrosion and dissolution of the coral fragments a three phase model for carbonate mound evolution in these settings is proposed 1 in a first stage external controls positive oceanographic and environmental conditions the presence of an active planktonic food chain based on a high primary production and a suitable substrate are responsible for the initiation of cold water coral growth 2 once the cold water corals established an initial framework sedimentation becomes an important factor controlling mound growth the cold water corals baffle the sediments 3 throughout mound growth the mound may episodically be affected by diagenetic processes responsible for aragonite dissolution and probably carbonate precipitation

observations of internal waves and associated mixing phenomena in the portimao canyon area internal wave activity and induced mixing phenomena in the portimao canyon area southern portuguese coast are analysed observations of current velocities and ctd measurements were taken during a 24 h period at two fixed stations located along the main axis of the canyon the station nearest shore was located close to the canyon head and the other station was 5 km seaward these basic measurements are complemented with vertical sections of temperature acquired by xbt casts along the main axis of the canyon and also with time series of temperatures recorded by a thermistor chain moored on the continental shelf near the canyon head the time sequences of velocities and density profiles recorded at the fixed stations were analysed using a procedure based on empirical orthogonal function eof analysis and dynamical modes decomposition dmd techniques by this method two different sources for internal wave activity are identified a clear internal tide signal on the one hand and a shorter than tidal period internal waves which exhibit greater current velocities and density oscillation amplitudes than internal tide on the other it is suggested that these shorter period internal waves are responsible for the vertical mixing affecting the water column over the slope and shelf waters in this region finally it is also suggested that the evacuation of these mixed water masses formed around the continental shelf break may be related to a core of relatively cold water flowing over the continental slope toward west

spatial heterogeneity of tectonic stress and friction in the crust new evidence from earthquake focal mechanisms in taiwan we performed inversions of earthquake focal mechanisms in central taiwan to investigate the heterogeneity of the stress field and fault strength and temporal variations of stress parameters friction and pore pressure associated with the 1999 chi chi earthquake we divided the focal mechanism data into two groups before and after the chi chi earthquake and analysed them separately with the assumption of a uniform stress field the friction coefficient is mostly within a range of 0 2 0 4 in central taiwan which is lower than the commonly quoted laboratory result 0 6 0 85 the low friction coefficient is also inferred by the rotation of principal stress axes after the chi chi earthquake by contrast if we assume that the friction is constant and failures occur on optimally oriented planes we find that the resulting stress orientations must be spatially variable however a large dispersion of stress orientations is not seen in borehole breakouts and fault slip data implying a constant friction model might be ruled out our analysis suggests that either the distribution of the coefficient of friction or pore pressure changed during the 1999 chi chi earthquake we infer that the pore pressure probably rose in the chi chi rupture area and northern longitudinal valley and dropped in the areas south of the coseismic rupture area after the main shock

exchange through the strait of gibraltar to measure the exchange between the atlantic and mediterranean through the strait of gibraltar an array of current meter moorings was deployed for a year in the strait during 1985 1986 a novel aspect of these measurements is the inclusion of conductivity as well as temperature and pressure sensors on each current meter so that the salinity of the flows could be monitored continuously these salinity measurements determine the water mass characteristics of the flows crossing the sill they allow definition of the 37 psu isohaline as the interface between inflowing fresher atlantic water and outflowing saltier mediterranean water and they enable time series to be developed for the depth of this interface for the upper layer inflow and for the lower layer outflow from these measurements the time averaged outflow of mediterranean water is estimated to be 0 68sv 1sv 1 106m3s 1 and the outflow salinity transport defined to be the outflow times the salinity excess above a basic atlantic water salinity of 36 1 psu is estimated to be 1 50 103m3s 1 1sv 1psu 1 103m3s 1 equivalent to a net evaporation over the mediterranean basin of 52cm y 1 extrapolated measurements of the inflow from current meters generally deployed below 100m depth yield an estimate for the time averaged inflow of 0 93sv which is believed to be unrealistically high in view of the better measured outflow and net evaporation thus a more realistic estimate of the inflow is 0 72sv equal to the sum of the outflow and net evaporation as required by the mass budget for the mediterranean sea such estimates of the exchange are smaller by almost a factor of 2 than previous values for the exchange by lacombe and richez 1982 the exchange across the gibraltar sill is found to be due in nearly equal parts to the mean currents and to the tidal fluctuations the mean currents are smaller than had been expected reaching a peak value of only about 60 cm s 1 in the deep outflow over the sill the tidal exchange is due to a strong correlation over the tidal period between the depth of the interface and the strength of the inflowing currents for the m2 tide at the sill the amplitude of the interface depth is 51 m and the amplitude of the tidal currents is 1 2m s 1 furthermore the inflow and interface depth have similar phases as a consequence the upper layer is deep on the inflowing tide so that a large slug of atlantic water crosses the sill into the mediterranean on the outflowing tide the interface is shallow so that a large slug of mediterranean water crosses the sill into the atlantic similar processes occur for the s2 o1 and k1 tides though the amplitude are smaller in this manner tidal oscillations lead to a time averaged exchange of water masses across the gibraltar sill the inflow and outflow defined to be the instantaneous transports above and below the 37psu isohaline interface exhibit m2 tidal amplitudes of 2 3 sv and 1 3sv respectively thus the tides are large enough to reverse the mean upper layer inflow and lower layer outflow daily averaged inflow and outflow transports exhibit low frequency fluctuations with standard deviations of 0 37sv and 0 22sv respectively such low frequency fluctuations have been shown previously to be associated with barotropic flows through the strait of gibraltar compensating for sea level variations over the mediterranean due to atmospheric pressure fluctuations candela winant and bryden 1989 finally from these measurements there appear to be little fortnightly or annual period fluctuations in the exchange through the strait of gibraltar

mud diapirism and subcretion in the barbados ridge accretionary complex the role of fluids in accretionary processes mud diapirism has recently been recognized in several modern accretionary wedges it provides an important means of dewatering accretionary wedges and should be regarded as an important process for producing the melanges found in both modern and ancient accretionary terranes mud diapirism affects a large area of the barbados ridge accretionary complex the distribution of the mud diapirs appears to be primarily controlled by the presence of underconsolidated terrigenous submarine fan deposits that are being accreted to the complex the frequency of diapir occurrence decreases northward as the fan becomes thinner mud diapirs are absent from the very eastern most part of the complex formed from sediments accreted at its toe with the exception of a few mud volcanoes on the ocean floor in front of the complex the initiation of diapirism appears to be spatially coincident with the onset of subcretion or underplating of sediment to the base of the complex at a ramp between two levels of decollement it is proposed that the release of mud and pore water from the subcreted sediments is a direct or indirect cause of most of the mud diapirism in the accretionary complex there is a range of diapiric form dependent on the viscosity of the mud from mud volcanoes fed by low viscosity mud to higher viscosity mud ridges the diapirs in the eastern areas of the complex are generally mud volcanoes with narrow conduits feeding a surface mound mud ridges are prominent in the western parts of the complex this is interpreted as reflecting a general westward decrease in the fluid content of the accretionary complex bottom simulating seismic reflectors formed by gas hydrate are commonly developed in the areas of mud volcano occurrences the presence of the hydrate indicates that large volumes of methane are being generated at depth in these regions the generation of methane may be contributing to zones of overpressuring in the wedge methane may also be partly responsible for driving the diapiric material to the surface to form mud volcanoes ridges in the subducting oceanic crust beneath the accretionary complex locally enhance diapirism above their crests and southern flanks faults formed later in the development of the complex are more commonly associated with diapirism than those resulting from accretion at the toe of the wedge these later faults play an important role in controlling the sites of individual mud volcanoes chains of mud volcanoes and mud ridges

temporal and spatial variation of stress field in taiwan from 1991 to 2007 insights from comprehensive first motion focal mechanism catalog article i nfo the total amount of 4 761 focal mechanisms was determined based on p wave first motion polarities from 1991 to 2007 in taiwan region this dataset offers us a good opportunity to examine temporal and spatial variability of the stress field we find that the spatial variations of stress axes are mainly controlled by tectonic structures while the temporal changes are greatly influenced by the chi chi earthquake the orientation of the maximum horizontal compressive stress axes sh shows a general agreement with the direction of plate motion between a depth range of 0 30 km the 20 anticlockwise rotation of sh from the longitudinal valley lv to western taiwan is probably caused by the left lateral motion on the longitudinal valley fault lvf that has consumed part of the oblique motion of plate convergence on the other hand part of the oblique convergence is transferred into the central range and the hsuehshan range judging from counterclockwise rotation of sh from east to west and strike slip faulting in the hsuehshan range most events with a depth greater than 30 km occur offshore eastern taiwan and the azimuth of sh is close to e w directed different from nw se directed at shallow depths this may infer the existence of the transition of lithosphere rheology in offshore eastern taiwan the trends of sh in the depth of 0 10 km are strongly affected by the coseismic stress change of the chi chi earthquake in the northern half of the chi chi rupture area the trends of sh rotate 30 clockwise and the stress ratio increased by a factor of six after the mainshock the orientations of sh still differ by 30 in 2007 comparing to that in the period before the chi chi earthquake the variation of sh trend is more diverse in the southern half of the rupture area showing 20 counterclockwise rotation immediately after the 1999 mainshock followed by a clockwise rotation the trend of sh returns to the pre seismic direction of 110 in 2001 these notable changes of sh before and after the chi chi mainshock suggests that the magnitude of background stress in the rupture area is close to the coseismic stress drop we also recognize a significant 10 counterclockwise rotation of sh in the entire chi chi rupture area between 1991 and 1999 before the earthquake took place however to the south of the chi chi rupture the trends of sh remain little changed before and after the chi chi earthquake

glacial cold water coral growth in the gulf of cadiz implications of increased palaeo productivity a set of 40 uranium series datings obtained on the reef forming scleractinian cold water corals lophelia pertusa and madrepora oculata revealed that during the past 400 kyr their occurrence in the gulf of cadiz goc was almost exclusively restricted to glacial periods this result strengthens the outcomes of former studies that coral growth in the temperate ne atlantic encompassing the french iberian and moroccan margins dominated during glacial periods whereas in the higher latitudes irish and norwegian margins extended coral growth prevailed during interglacial periods thus it appears that the biogeographical limits for sustained cold water coral growth along the ne atlantic margin are strongly related to climate change by focussing on the last glacial interglacial cycle this study shows that palaeo productivity was increased during the last glacial this was likely driven by the fertilisation effect of an increased input of aeolian dust and locally intensified upwelling after the younger dryas cold event the input of aeolian dust and productivity significantly decreased concurrent with an increase in water temperatures in the goc this primarily resulted in reduced food availability and caused a widespread demise of the formerly thriving coral ecosystems moreover these climate induced changes most likely caused a latitudinal shift of areas with optimum coral growth conditions towards the northern ne atlantic where more suitable environmental conditions established with the onset of the holocene

oceanographic processes and morphosedimentary products along the iberian margins a new multidisciplinary approach abstract our understanding of bottom currents and associated oceanographic processes e g overflows barotropic tidal currents including intermittent processes e g vertical eddies deep sea storms horizontal vortices internal waves and tsunamis is rapidly evolving many deep water processes remain poorly understood due to limited direct observations but can generate significant depositional and erosional features on both short and long term time scales this paper represents a review work which describes for the first time these oceanographic processes and examines their potential role in the sedimentary features along the iberian continental margins this review explores the implications of the studied processes given their secondary role relative to other factors such as mass transport and turbiditic processes and highlights three major results a contourite depositional and erosional features are ubiquitous along the margins indicating that bottom currents and associated oceanographic processes control the physiography and sedimentation b the position of interfaces between major water masses and their vertical and spatial variation in time specifically appears to exert primary control in determining major morphologic changes along the slope gradient including the contourite terraces development and c contourites deposits exhibit greater variation than the established facies model suggests therefore a consistent facies model however faces substantial challenges in terms of the wide range of oceanographic processes that can influence in their development an integrated interpretation of these oceanographic processes requires an understanding of contourites seafloor features their spatial and temporal evolution and the near bottom flows that form them this approach will synthesize oceanographic data seafloor morphology sediments and seismic images to improve our knowledge of permanent and intermittent processes around iberia and evaluate their conceptual and regional role in the margin s sedimentary evolution given their complexes three dimensional and temporally variable nature integration of these processes into sedimentary oceanographic and climatological frameworks will require a multidisciplinary approach that includes geology physical oceanography paleoceanography and benthic biology

time spatial variability observed in velocity of propagation of the internal bore in the strait of gibraltar 1 some aspects of the time spatial variability of the phase speed of the internal bore generated almost every tidal cycle in camarinal sill are revised using a set of high resolution experimental data collected in two different positions of the strait during may 2003 this variability is mainly driven by the intense tidal currents comparable with the intrinsic propagation velocity of the first mode baroclinic bore it is shown that the importance of the diurnal tide in the strait of gibraltar induces a considerable diurnal inequality on the bore velocity with an observed maximum difference of 0 7 ms 1 between the speed of two consecutives bores propagating along the eastern part of the strait a regularly spatial pattern has been also found the internal bore reaches its maximum velocity in tarifa narrows a theoretical estimation predicts an extreme phase speed of 2 6 ms 1 during our period of study

possible stress states adjacent to the rupture zone of the 1999 chi chi taiwan earthquake abstract we explore regional stress states in the vicinity of the rupture area of the 1999 chi chi taiwan earthquake by reconciling data from the taiwan chelungpu fault drilling project tcdp in situ measurements and earthquake focal mechanisms given the background deviatoric stress in the range of 10 50 mpa and the horizontal nw se directed maximum principal stress axis the predicted fault types show strike slip and normal faulting near the coseismic surface rupture and thrust and strike slip faulting in central taiwan such predictions are able to fit tcdp in situ observations in a local scale and aftershock earthquake focal mechanisms in a regional scale additionally the proposed stress state explains remarkable rotations of the maximum stress axes observed near the northern segment of the chelungpu fault this result provides key information for forecasting of consequent earthquakes and evaluation of focal mechanisms after the occurrence of a large earthquake

the mediterranean ridge and related mud diapirism a background abstract the mediterranean ridge stretching from the calabrian rise to the florence rise is the largest structural unit of the eastern mediterranean sea it is directly related to ongoing convergence and collision of the african and eurasian plates starting in the oligocene and is considered to be a giant accretionary complex consisting of intensively folded and faulted rocks of the african margin since its origin in the late miocene the ridge continued to grow up and outward experiencing more deformation because of the developing collision the mud diapirism and mud volcanism are usual and wide spread phenomena for the mediterranean ridge that developed as a result of an intensive tectonic overburden due to stacking of rock units by thrusting and strong lateral compressional stress pressing up and squeezing plastic sedimentary series out onto the seafloor

development of sediment drifts approaching an active plate margin under the sw pacific deep western boundary current the abyssal pacific ocean is fed by a 1000 km wide deep western boundary current dwbc that flows northward along the continental margin east of new zealand between the passive margin of chatham rise and the subduction zone of kermadec trench a distance of 1200 km the dwbc has formed a suite of sediment drifts over a depth range of 2200 5700 m airgun and 3 5 khz profiles record a variety of drift types that reflect regional variations in bathymetry sediment supply and the tectonic volcanic framework on chatham rise the dwbc has deposited a sinuous linear body along the south flank 3000 m an extensive apronlike drift on the north flank 2200 4500 m and a ridgelike drift about the rise base 4500 5200 m the flow has also deposited a body of sediment over 400 km long within a moat at the base of the nearby louisville seamount chain further downcurrent the 250 km long rekohu drift 3600 4190 m has developed northward to 39 s south of this latitude drifts comprise mainly reworked pelagic hemipelagic material and sediment transported from distant southerly sources in contrast drifts north of 39 s have received a major injection of terrigenous sediment from hikurangi channel which runs 1400 km from new zealand eastward across the hikurangi plateau to disgorge on to the abyssal floor at the plateau edge en route turbidity current overspill from the channel has moved north under the influence of the shallow dwbc to contribute to a series of small ridge and patch drifts among the numerous seamounts on the plateau at 3500 4200 m off hikurangi channel mouth a large fan has accumulated the dwbc has extended the fan into a drift running over 250 km along the base of hikurangi plateau 5150 5770 m toward kermadec trench here drift sediment becomes increasingly disrupted by mass wasting associated with the active subduction in this area the seismic stratigraphy reveals the drifts to rest mainly on a widespread erosional surface that is interpreted to mark the inception of the dwbc in the region with the late oligocene opening of the australian antarctic seaway drift construction commenced during the miocene but was punctuated in the late miocene by another period of erosion that coincided with increased bottom water production in antarctica deposition resumed in plio pleistocene times when large quantities of sediment from the rapidly rising landmass of new zealand were injected into the boundary current the modern flow continues to affect drift deposition as manifest by an active boundary channel along the foot of hikurangi plateau and widespread scour zones and sediment wave fields

thousands of cold water coral mounds along the moroccan atlantic continental margin distribution and morphometry abstract coral mounds formed by framework forming cold water corals pierce the seabed along most continental margins of the atlantic ocean and new sites are continuously being discovered here we describe an extremely high accumulation of coral mounds at the nw moroccan atlantic margin between 35 n and 35 5 n within an area of only 1440 km2 3400 mounds were found exposed at the seabed the coral mounds are nowadays characterized by an almost complete lack of living cold water corals in addition numerous buried mounds were identified in hydroacoustic sub bottom profiles and are estimated to be 3 7 times more frequent than the exposed mounds consequently a total of 16 000 buried and exposed mounds is estimated for the entire study area the exposed mounds are rather small with a mean height of 18 m and show a conspicuous arrangement in two slope parallel belts that centre in water depths between 720 and 870 m and 890 980 m respectively putting them among the deepest mound occurrences discovered so far in the atlantic the mostly elongated mounds largely stretch downslope pointing to a significant influence of internal waves in the mound formation process moreover based on their average dimensions the entire coral mound volume can be estimated as 1 3 km3 which means the mounds store a considerable amount of coral carbonate highlighting their potentially important role as regional carbonate factories in combination with further occurrences of coral mounds along the moroccan margin both in the mediterranean and in the atlantic ocean these new findings underline morocco s role as a hotspot for the occurrence of cold water coral mounds

contourite drift types and their distribution in the north and south atlantic ocean basins abstract patterns of sediment accumulation under the control of deep bottom currents depend on interacting factors intensity of the regional deep circulation morphology of the sea floor coriolis effects abundance of terrigenous and biogenic supplies subsequent turbidity of deep nepheloid layers and depth of the ccd based on many examples of bottom current deposits from the atlantic and from other oceans we recognise three major types of contourite accumulation 1 giant elongate drifts 2 contourite sheets and 3 channel related drifts they generally occur in distinct topographic settings and show different geometries facies and rates of deposition in the north atlantic strong bottom circulation an open basin system and high sediment supply have led to the construction of large elongate contourite drifts which have striking relief 200 1000 m above the surrounding sea floor and are composed of muddy contourites rich in calcareous pelagic material in the south atlantic strong bottom currents are often trapped in gyre like circulation patterns around topographically closed basins or are accelerated through deep narrow connecting passages the resulting deposits are mainly extensive contourite sheets covered by fields of large sediment waves and channel related drifts including contourite fans constructed at the downstream exit of deep passages

morphobathymetric analysis of the large fine grained sediment waves over the gulf of valencia continental slope nw mediterranean abstract detailed analysis of recently acquired swath bathymetry together with high resolution seismic profiles and bottom sediment samples revealed the presence of large scale fine grained sediment waves over the gulf of valencia continental slope as many other deep water sediment waves these features were previously attributed to gravitational slope failure related to creep like deformation and are here reinterpreted as sediment wave fields extending from 250 m depth to the continental rise at 850 m depth geometric parameters were computed from the high resolution multibeam dataset sediment wave lengths range between 500 and 1000 m and maximum wave heights of up to 50 m are found on the upper slope decreasing downslope to minimum values of 2 m high sediment waves on the lower part of the slope are quasi stationary vertically accreting whereas they show an upslope migrating pattern from the mid slope to the upper part of the continental slope high resolution seismic profiles show continuous internal reflectors with sediment waves merging down section and sediment wave packages decreasing in thickness downslope these sediment packages are thicker on the crest of each individual sediment wave and thinner on the downslope flank 210pb analyses conducted on sediment cores collected over the sediment wave fields also indicate slightly higher sediment accumulation rates on the wave crests sediment wave formation processes have been inferred from contemporary hydrodynamic observations which reveal the presence of near inertial internal waves interacting with the gulf of valencia continental slope internal wave activity is suggested to be the preferential mechanism for the transport and deposition of sediment and the maintenance of the observed sediment wave fields

on the mediterranean water composition abstractthe mediterranean outflow water mow spills from the mediterranean sea east north atlantic basin west off the strait of gibraltar as mow outflows it entrains eastern north atlantic central waters enacw and intermediate waters to form the neutrally buoyant mediterranean water mw that can be traced over the entire north atlantic basin its high salinity content influences the thermohaline properties of the intermediate deep water column in the north atlantic and its dynamics here the composition of mw in its source region the gulf of cadiz west off strait of gibraltar is investigated on the basis of an optimum multiparameter analysis the results obtained indicate that mixing of mow 34 1 0 3 occurs mainly with overlying enacw 57 1 0 8 in a process broadly known as central water entrainment a diluted form 80 of dilution of the antarctic intermediate water aaiw reaches the region and also takes part in mw formation 8 3 0 5 finally the underlying labrador sea wat

multiprocess interaction shaping geoforms and controlling substrate types and benthic community distribution in the gulf of cadiz abstract the iberian margin in the gulf of cadiz hosts a fluid venting area embedded into a contourite depositional drift and is a natural laboratory to study how multiple interacting processes shape the seafloor a high resolution morpho sedimentary analysis based on geophysical data sediment samples and submarine imagery has been carried out the onset and evolution of the main geoforms are discussed and the influence of geological oceanographic and biogenic processes conditioning substrate types and benthic community distribution are evaluated the interplay of geological e g salt and shale diapirism mud volcanism oceanographic e g water mass circulation secondary circulation vertical eddies internal waves and biogenic e g methane derived authigenic carbonates formation coral mound aggradations processes drive the formation of a large variety of geoforms including among others contourite drifts channels diapiric ridges mud volcanoes pockmarks and coral mounds at a smaller scale the interaction between massive and fluid extrusive biogenic and bottom current related processes both depositional and erosive determines the distribution of substrate types and benthic communities which have been classified as belonging to different biotopes

anomalous sea floor backscatter patterns in methane venting areas dnepr paleo delta nw black sea the relation between acoustic sea floor backscatter and seep distribution is examined by integrating multibeam backscatter data and seep locations detected by single beam echosounder this study is further supported by side scan sonar recordings high resolution 5 khz seismic data pore water analysis grain size analysis and visual sea floor observations the datasets were acquired during the 2003 and 2004 expeditions of the ec funded crimea project in the dnepr paleo delta area northwestern black sea more than 600 active methane seeps were hydroacoustically detected within a small 3 96 km by 3 72 km area on the continental shelf of the dnepr paleo delta in water depths ranging from 72 m to 156 m multibeam and side scan sonar recordings show backscatter patterns that are clearly associated with seepage or with a present dune area seeps generally occur within medium to high backscatter areas which often coincide with pockmarks high resolution seismic data reveals the presence of an undulating gas front i e the top of the free gas in the subsurface which domes up towards and intersects the sea floor at locations where gas seeps and medium to high backscatter values are detected pore water analysis of 4 multi cores taken at different backscatter intensity sites shows a clear correlation between backscatter intensity and dissolved methane fluxes all analyzed chemical species indicate increasing anaerobic oxidation of methane aom from medium to high backscatter locations this is confirmed by visual sea floor observations showing bacterial mats and authigenic carbonates formed by aom grain size analysis of the 4 multi cores only reveals negligible variations between the different backscatter sites integration of all datasets leads to the conclusion that the observed backscatter patterns are the result of ongoing methane seepage and the precipitation of methane derived authigenic carbonates mdacs caused by aom the carbonate formation also appears to lead to a gradual self sealing of the seeps by cementing fluid pathways horizons followed by a relocation of the bubble releasing locations

sedimentary environment of the faroe shetland and faroe bank channels north east atlantic and the use of bedforms as indicators of bottom current velocity in the deep ocean in the northeast atlantic much of the deep cold water flow between the norwegian sea and the main north atlantic basin passes through the faroe shetland and faroe bank channels generating strong persistent bottom currents capable of eroding and transporting sediment up to and including gravel a large variety of sedimentary bedforms including scours furrows comet marks barchan dunes sand sheets and sediment drifts is documented using sidescan sonar images seismic profiles seabed photographs and sediment cores from the floor of the channel published information on current velocities associated with the various bedforms has been used to reconstruct the pattern of bottom currents acting on the channel floor the results broadly reflect the current pattern predicted on the basis of regional oceanographic observations but add considerable detail the internal consistency of the results suggests that the methods used are robust giving confidence in the fine detail of the observed bottom current structure bottom current velocities in the range 1ae0 m s 1 are indicated by the range of observed bedforms with the strongest currents associated with south west transport of norwegian sea deep water nsdw at water depths of 800 1200 m the main nsdw flow forms a relatively narrow core that follows the base of the faroes slope this core follows the 90 change in trend of the faroes slope at the junction between the faroe shetland and faroe bank channels the strongest currents within the nsdw core are found over the shallowest sill in the faroe shetland channel and in the narrowest part of the channel immediately downstream of the sill and are generated by topographic constriction of the flow eastward flow of deep water along the northern flank of the wyville thomson ridge suggests a complex current pattern with some recirculation of deep water within the deep faroe bank channel basin the observations suggest that coriolis force is the main agent controlling the westward deflection of the nsdwinto the faroe bank channel contradicting a previous suggestion that this was controlled by the topography of the wyville thomson ridge

crustal stress field in southern california and its implications for fault mechanics we present a new high spatial resolution image of stress orientation in southern california based on the inversion of earthquake focal mechanisms we use this image to study the mechanics of faulting in the plate boundary region the stress field contains significant spatial heterogeneity which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause temporal changes in the stress field are also observed primarily related to major earthquakes the observed 15 10 rotation of the stress axes due to the 1992 m7 3 landers mainshock implies that the deviatoric stress magnitude in the crust is low of the order of 10 mpa this suggests that active faults in southern california are weak the maximum principal stress axis near the san andreas fault is often at 50 to the fault strike indicating that the shear stress on the fault is comparable to the deviatoric stress the san andreas in southern california may therefore be a weak fault in a low strength crust

vast fields of hydrocarbon derived carbonate chimneys related to the accretionary wedge olistostrome of the gulf of cadiz we report the first discovery and sampling of vast fields of hydrocarbon derived carbonate chimneys along the gulf of cadiz continental slope at depths between 500 and 1200 m a large variety of carbonate chimneys more than 200 samples were recovered from four different areas named the diasom tasyo gbf guadalquivir basin and esf east moroccan fields observations from an underwater camera revealed a spectacular high density of pipe like chimneys some of them longer than 2 m lying over the sea floor and some protruding from muddy sediment local fissures and alignment of isolated chimneys were also observed suggesting that their distribution is controlled by fault planes chimneys collected show a wide range of morphological types spiral cylindrical conical mushroom like and mounded with numerous nodule protuberances and ramified fluid channelways the chimneys are mainly composed of authigenic carbonates ankerite fe bearing dolomite and calcite with abundance of iron oxides forming agglomerates of pseudo pyrite framboids n 18 o isotopic values vary from 0 7 to 5 5x whereas n 13 c values indicate that chimney carbonates are moderately depleted in 13 c ranging from 346x to 320x pdb interpreted as formed from a mixture of deep thermogenic hydrocarbons and shallow biogenic methane these vast fields of carbonate chimneys imply new considerations on the importance of hydrocarbon fluid venting in the gulf of cadiz and moreover on the active role of the olistostrome accretionary wedge of the gibraltar arc b 2003 elsevier science b v all rights reserved

neogene to recent contraction and basin inversion along the nubia iberia boundary in sw iberia the sw of iberia is currently undergoing compression related to the convergence between nubia and iberia multiple compressive structures and their related seismic activity have been documented along the diffuse nubia iberia plate boundary including the gorringe bank west of the gulf of cadiz and the betic rif orogen to the east despite seismic activity indicating a dominant compressive stress along the algarve margin in the gulf of cadiz the structures at the origin of this seismicity remain elusive this paper documents the contractional structures that provide linkage across the gulf of cadiz and play a major role in defining the present day seismicity and bathymetry of this area the structures described in this paper caused the neogene inversion of the jurassic oblique passive margin that formed between the central atlantic and the ligurian tethys this example of a partially inverted margin provides insights into the factors that condition the inversion of passive margins

preseismic deformation and coseismic displacements associated with the 1999 chi chi taiwan earthquake the destructive 1999 chi chi taiwan earthquake m w 7 5 produced an approximately 100 km long surface rupture mostly along the previously recognized north south trending chelungpu fault preseismic deformation in central taiwan is realized from annually repeated global positioning system gps data acquired during the 1992 1999 period the total wnw ese shortening rate in the vicinity of the epicentral region that is from the west coast to the western boundary of the central range is up to 25 mm yr the crustal deformation before the chi chi earthquake was essentially a uniaxial compressional strain of 0 36 strain yr in the direction of 114 the gps measurements taken 0 2 2 7 yr before and within 3 months after the mainshock were utilized to estimate the coseismic displacements horizontal movements of 1 1 9 1 m in the nw nnw directions are observed on the hanging wall eastern side of the fault there is a northward increasing trend in the magnitude of the displacement vectors and a dramatic change in the direction of about 50 toward the east along the fault strike in contrast much smaller se see movements of 0 1 1 5 m are found on the footwall western side of the fault the gps data show 2 4 10 1 m of total horizontal offsets across the chelungpu fault vertical offsets of 1 2 4 4 m with the eastern side up are also observed along the surface rupture the uplift on the hanging wall decreases rapidly toward the east it becomes subsidence at sun moon lake and in the puli wushe area the stations on the footwall show subsidence of 0 02 0 26 m the width of the uplift zone increases from about 10 km in the south to approximately 30 km in the north manuscript received 13 october 2000

mixing and spreading of the mediterranean outflow abstract hydrographic and current profiler data taken during the 1988 gulf of cadiz expedition have been analyzed to diagnose the mixing spreading and descent of the mediterranean outflow the s properties and the thickness and width of the outflow were similar to that seen in earlier surveys the transport of pure mediterranean water i e water with s 38 4 psu was estimated to be about 0 4 106 m3 s 1 which is lower than historical estimates most of which were indirect but comparable to other recent estimates made from direct velocity observations the outflow transport estimated at the west end of the strait of gibraltar was about 0 7 106 m3 s 1 of mixed water and the transport increased to about 1 9 106 m3 s 1 within the eastern gulf of cadiz this increase in transport occurred by entrainment of fresher north atlantic central water and the salinity anomaly of the outflow was consequently reduced the velocity weighted salinity decreased to 36 7 psu within 60 km of the strait and decre

plate collision and paleostress trajectories in a fold thrust belt the foothills of taiwan abstract field analyses of compressional faulting and folding in the foothills of western taiwan enable us to reconstruct paleostress trajectories over a large area and to establish the relative chronology of tectonic events two main compressional events have contributed to the present structure of the fold thrust belt stratigraphic data show that these events are plio pliocene in age older normal faulting indicates nnw sse extension across the chinesse passive continental margin during the neogene the two main compressional events of the taiwan collision correspond to similar fan shaped distributions of maximum compressive stress trajectories with a counterclockwise shift of 30 50 between the two events using the relationship between recent stress trajectories and the direction of recent plate motion as a guide we reconstruct the direction of plate convergence for the older event we suspect that the relative motion philippine sea plate eurasia has rotated counterclockwise of at least 35 45 in taiwan during collision this conclusion is in agreement with independent plate tectonic reconstructions several problems provide objectives to further tectonic and paleomagnetic studies including the duration and diachronism of compressional events as well as possible clockwise rotation of northernmost taiwan

bedform velocity matrix the estimation of bottom current velocity from bedform observations a wide variety of bedforms both depositional and erosional in origin has been recognized on the deep seafloor and attributed to the influence of bottom currents these range in scale from those visible in bottom photographs centimeter to decimeter to those recorded with seafloor bathymetric imaging meter to kilometer in many cases it has been possible to provide some quantification of substrate grain size and flow velocity responsible for each bedform type we have synthesized both our own and published data in order to present a bedform velocity matrix which facilitates the estimation of bottom current velocity based on bedform type despite imperfections we believe this to be a valuable model for assessing strength and variability of bottom currents that can have a significant influence on the siting of submarine cables pipelines and other seafloor installations

on the transition between 2d and 3d dunes sediment transport in sand bedded alluvial channels is strongly conditioned by bedforms the planimetric morphology of which can be either two or threedimensional experiments were undertaken to examine the processes that transform the bed configuration from two dimensional 2d dunes to threedimensional 3d dunes a narrowly graded 500 lm size sand was subjected to a 0ae15 m deep non varying mean flow ranging from 0ae30 to 0ae55 m sec 1 in a 1 m wide flume changes in the planimetric configuration of the bed were monitored using a high resolution video camera that produced a series of 10 sec time lapsed digital images image analysis was used to define a critical value of the non dimensional span sinuosity of the bedform crestlines that divides 2d forms from 3d forms significant variation in the non dimensional span is observed that cannot be linked to properties of the flow or bedforms and thus appears random images also reveal that once 2d bedforms are established minor transient excesses or deficiencies of sand are passed from one bedform to another the bedform field appears capable of absorbing a small number of such defects but as the number grows with time the resulting morphological perturbations produce a transition in bed state to 3d forms that continue to evolve but are pattern stable the 3d pattern is maintained by the constant rearrangement of crestlines through lobe extension and starving downstream bedforms of sediment which leads to bifurcation the experiments demonstrate that 2d bedforms are not stable in this calibre sand and call into question the reliability of bedform phase diagrams that use crestline shape as a discriminator

interaction of tectonic and depositional processes that control the evolution of the iberian gulf of cadiz margin abstract this study provides an integrated view of the growth patterns and factors that controlled the evolution of the gulf of cadiz continental margin based on studies of the tectonic sedimentologic and oceanographic history of the area seven sedimentary regimes are identified but there are more extensive descriptions of the late cenozoic regimes because of the larger data base the regimes of the mesozoic passive margin include carbonate platforms which become mixed calcareous terrigenous deposits during the late cretaceous early tertiary the oligocene and early miocene terrigenous regimes developed in contrast over the active and transcurrent margins near the african iberian plate boundary the top of the gulf of cadiz olistostrome emplaced in the late miocene is used as a key horizon to define the post orogenic depositional regimes the late miocene progradational margin regime is characterized by a large terrigenous sediment supply to the margin and coincides with the closing of the miocene atlantic mediterranean gateways the terrigenous drift depositional regime of the early pliocene resulted from the occurrence of high eustatic sea level and the characteristics of the mediterranean outflow currents that developed after the opening of the strait of gibraltar the late pliocene and quaternary regimes are dominated by sequences of deposits related to cycles of high and low sea levels deposition of shelf margin deltas and slope wedges correlate with regressive and low sea level regimes caused by eustasy and subsidence during the highstand regimes of the holocene inner shelf prograding deltas and deep water sediment drifts were developed under the influence of the atlantic inflow and mediterranean outflow currents respectively a modern human cultural regime began 2000 years ago with the roman occupation of iberia human cultural effects on sedimentary regimes may have equalled natural factors such as climate change interplay of tectonic and oceanographic controls dominated the evolution of the cadiz margin during the cenozoic depositional sequences formed where the tectonic setting provided the accommodation space and the shape of the deposits has been greatly influenced by the strong unidirectional atlantic inflow currents on the shelf and mediterranean outflow currents on the slope the entire cycle of the inflow and outflow deposition along the margin has been controlled first by the tectonic evolution of the betic and rif gateways which become closed during the late miocene and after the messinian by the opening of the strait of gibraltar strong current development during eustatic sea level highstands of the pliocene and quaternary has controlled deposition because of maximum sill depths at gibraltar for water circulation lowstand sea levels slowed circulation and resulted in mud drapes over the slope and regressive stratigraphic sequences over the shelf more recently the human industrial revolution has caused heavy metal contamination of sediment and water over the cadiz margin human activity also has affected sedimentation rates because of deforestation that caused increased depositional rates near undammed rivers and decreased rates where rivers have been dammed future research efforts will need to focus on 1 the effect of increased mediterranean outflow caused by river damming plus global warming and the increased outflow as a potential trigger for new ice ages 2 assessments of geologic hazards for planning man made shoreline structures developing offshore petroleum resources and maintaining undersea communications cables and 3 confirmation of the general geologic history of the cadiz margin

internal waves and short scale distribution patterns of chlorophyll in the strait of gibraltar and alboran sea 1 a selection of asar images have been analyzed together with instantaneous images of surface chlorophyll recorded with meris and modis in order to study the relationship between the physical and biological processes associated with internal waves in the strait of gibraltar and alboran sea the images show peak levels of chlorophyll at the coastal edges to the north and south of the camarinal sill cs during the generation of internal waves and peak levels of chlorophyll associated with the wave fronts as they travel into the alboran sea the images have been compared with in situ data the results seem to indicate that during the generation of the internal waves a suction process takes place by which coastal waters rich in chlorophyll are drawn towards the center of the channel and then accompany the internal waves as they move towards the alboran sea

the influence of oceanographic processes on contourite features a multidisciplinary study of the northern south china sea abstract this study examines how multiscale oceanographic processes interact with the seafloor to influence the development of contourite features in northern part of the south china sea the multidisciplinary approach used in this study combines physical oceanographic monitoring numerical simulation multibeam echosounding seismic reflection profiling and sediment core interpretation the contourites detected and interpreted herein include depositional drifts and sediment waves erosional channels moats and furrows and mixed erosional and depositional features terraces and irregular depressions this study finds that climatic variation influences the intensity of quasi steady along slope regional water circulation which in turn influences the long term development of primary 10 100 km contourite depositional systems energetic but intermittent oceanographic processes such as eddies and internal waves generate smaller secondary features within the contourite depositional systems resulting in a complex local seafloor morphology westward to south westward migrating mesoscale eddies locally enhance regional water mass circulation and trigger sub mesoscale flows which can generate 1 10 km irregular depressions along their paths westward propagating internal solitary waves form sedimentary waves with the aid of numerical modelling results a sedimentary model is proposed to explain how oceanographic processes influence sedimentary processes and determine a hierarchy of associated feature formation this model can help advance understanding of how bottom currents influence sedimentary processes along continental margins and abyssal plains

slip history and dynamic implications of the 1999 chi chi taiwan earthquake we investigate the rupture process of the 1999 chi chi taiwan earthquake using extensive near source observations including three component velocity waveforms at 36 strong motion stations and 119 gps measurements a three plane fault geometry derived from our previous inversion using only static data ji et al 2001 is applied the slip amplitude rake angle rupture initiation time and risetime function are inverted simultaneously with a recently developed finite fault inverse method that combines a wavelet transform approach with a simulated annealing algorithm ji et al 2002b the inversion results are validated by the forward prediction of an independent data set the teleseismic p and sh ground velocities with notable agreement the results show that the total seismic moment release of this earthquake is 2 7 10 20 n m and that most of the slip occurred in a triangular shaped asperity involving two fault segments which is consistent with our previous static inversion the rupture front propagates with an average rupture velocity of 2 0 km s 1 and the average slip duration risetime is 7 2 s several interesting observations related to the temporal evolution of the chi chi earthquake are also investigated including 1 the strong effect of the sinuous fault plane of the chelungpu fault on spatial and temporal variations in slip history 2 the intersection of fault 1 and fault 2 not being a strong impediment to the rupture propagation and 3 the observation that the peak slip velocity near the surface is in general higher than on the deeper portion of the fault plane as predicted by dynamic modeling

the imprint of methane seepage on the geochemical record and early diagenetic processes in cold water coral mounds on pen duick escarpment gulf of cadiz abstract the diagenetic history and biogeochemical processes in three cold water coral mounds located in close proximity to each other on pen duick escarpment in the gulf of cadiz were examined the influence of ascending methane rich fluids from underlying sediment strata delineated two mound groups alpha and beta mound showed evidence for the presence of a sulfate methane transition zone smtz at shallow depth whereas gamma mound appeared to lack a shallow smtz in the methane influenced alpha and beta mound upward diffusion of hydrogen sulfide from the shallow smtz caused extensive pyritization of reactive iron phases as indicated by values for the degree of pyritization 0 7 this secondary pyritization overprinted the sulfur isotope composition of sulfides formed during organoclastic sulfate reduction the almost complete consumption of reactive iron phases by upward diffusing sulfide limited dissimilatory iron reduction to the top layer in these mounds while organic matter in the pyritized zones below was primarily degraded by organoclastic sulfate reduction hydrogen sulfide produced during sulfate reduction coupled to the anaerobic oxidation of methane aom diffused upward and induced aragonite dissolution as evidenced in strongly corroded corals in alpha mound this mound has been affected by strong fluctuations in the depth of the smtz as observed by distinct layers with abundant diagenetic high mg calcite with a 13 c depleted carbon isotope composition in the non methane influenced gamma mound low sulfate reduction rates elevated concentrations of dissolved iron and solid phase iron speciation indicated that organic matter mineralization was driven by dissimilatory iron reduction and organoclastic sulfate reduction coupled to oxidative sulfur cycling the latter process led to 34 s depletion in pyrite of up to 70 relative to pore water sulfate