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

classification of sea floor features associated with methane seeps along the gulf of cadiz continental margin abstract based on recently gathered swath bathymetry high to ultra high resolution seismic and underwater camera data along with dredging and coring samples this paper examines the relationship between sea floor features and the nature of hydrocarbon enriched fluid and gas leaks from degassing of deeply buried sediments along the continental margin of the gulf of cadiz eastern central atlantic a classification into three main groups is proposed on the basis of the morphology and nature of deposits 1 mud volcanoes 2 methane derived authigenic carbonates mdac mounds and 3 crater like pockmarks mud volcanoes are topographically cone shaped sea floor edifices built up from catastrophic mud and fluid degassing intercalated with periods of inactivity so far more than 25 mud volcanoes have been discovered in the gulf of cadiz named in memory of deceased colleagues e g ginsburg and baraza or researchers birth places e g faro cibeles almazan san petersburgh yuma rabat bonjardim coruna gades these structures range from 800 to 2500 m in diameter and tower 150 300 m above the seabed the volcanoes consistently feature a well defined outer ring or circular terrace and an inner dome all mud volcanoes are built up of episodes of mud breccia flows intercalated with deep current deposits with evident indications of gas saturation degassing structures a strong h2s smell and chemosynthetic fauna such as pogonophora sp tube worms and calyptogena sp commonly observed carbonate crusts and slabs overlying some mud volcanoes are thought to have been formed by slow diffuse venting during periods of inactivity or slower rates of fluid venting following the ejection of mud a fermentation process the result of microbial mediated oxidation of hydrocarbon enriched fluids seems to play an important role in the growth of large deep water carbonate mounds and chimneys during periods of low methane seep fluid pressure more than 400 crater like pockmarks a common sea floor feature in the tasyo field have been observed their seabed diameter ranges from 500 to 3000 m with steep slopes ranging from 12 to 35 according to their internal structure pockmarks can be grouped into 1 pockmarks with low central mounds and 2 crater like pockmarks apparently collapse structures lacking central mounds with a flat to slightly undulating sea bottom surrounded by steep slopes up to 25

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

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

impact of basin structure and evaporite distribution on salt tectonics in the algarve basin southwest iberian margin abstract the sw iberian margin developed as a passive margin during mesozoic times and was later inverted during the mainly cenozoic alpine orogeny the initial syn rift deposits include a lower jurassic evaporite unit of variable thickness in the onshore this unit is observed to thicken basinward i e southward in fault controlled depocenters and salt related structures are only present in areas of thick initial evaporites in the offshore multiple salt structures cored by the lower jurassic evaporites are interpreted on seismic reflection data and from exploratory drilling offshore salt structures include the allochthonous esperanca salt nappe which extends over an area roughly 40 60 km the abundance of salt related structures and their geometry is observed to be controlled by the distribution of evaporite facies which is in turn controlled by the structure of rift related faulting this paper presents a comprehensive study of salt tectonics over the entire onshore and offshore sw iberian passive margin southern portugal and gulf of cadiz covering all aspects from initial evaporite composition and thickness to the evolution of salt related structures through mesozoic extension and cenozoic basin inversion

mud volcanism in the gulf of cadiz results from the ttr 10 cruise a new deep water mud volcano field between 2000 and 3500 m water depth was discovered in the deep south portuguese margin as well as several new mud volcanoes in the south spanish and western moroccan margins of the gulf of cadiz during the ttr 10 training through research unesco ioc cruise in july august 2000 this work followed the discovery of a large mud volcano field in the gulf of cadiz first investigated during the ttr 9 cruise gardner 2001 geophys res lett 28 339 342 kenyon et al 2000 ioc technical series no 56 the discoveries were made based on a seamap side scan sonar mosaic and multibeam bathymetry seabeam collected in the area by the naval research laboratory nrl washington dc usa in 1992 kindly released for this purpose single channel seismics long range side scan sonar okean tv controlled grab hull mounted 3 5 khz profiler and coring were used to investigate several seafloor features observed on the side scan sonar imagery in the south iberia spanish and moroccan margins of the gulf of cadiz which were confirmed to be mud volcanoes the typical structures related to fluid venting in the gulf of cadiz are essentially represented by conical mud volcanoes with diameters ranging from several tens of meters to 4 km and heights that can reach 200 m some of these structures appear to be aligned along major conjugate ne sw and nw se trending faults that can be identified on the side scan sonar imagery the new field discovered in the south portuguese margin is the deepest in the gulf of cadiz area and includes three new mud volcanoes bonjardim olenin and carlos ribeiro which seem to be quite active with near surface gas hydrate occurrence and a high saturation in h2s and hydrocarbon gases mainly methane in the mud breccia and overlying pelagic sediments gas hydrates were recovered from the bonjardim mud volcano an intensely gassified mud breccia with one fragment of semi consolidated claystone with a thin bituminous veneer at the surface was recovered from the carlos ribeiro mud volcano the fauna recovered consists mainly of pogonophoran worms belonging to several species and undetermined species of foraminifera three new mud volcanoes were also discovered in the nw moroccan margin rabat student and jesus baraza these show a richer fauna that includes several species of molluscs polychaetes pogonophoran worms crustaceans echinoderms and some fragments of dead coral madrepora and lophelia carbonate crusts were recovered from the student mud volcano the ginsburg mud volcano discovered during the previous ttr 9 cruise was revisited and gas hydrates recovered once again a new mud volcano was also discovered in the spanish margin tasyo where evidence was found of coral build ups on the hard substratum of the mud volcanic edifice

cenozoic deformation and mesozoic abandoned spreading centre in the tagus abyssal plain west of portugal results of a multichannel seismic survey the seismic stratigraphy of the sedimentary section that fills in the tagus abyssal plain is specified by means of new multichannel seismic data six sedimentary sequence units are identified these ranging from late jurassic to recent have been dated by extrapolation of deep sea drilling project dsdp and ocean drilling program odp drilling results and from sampling by dredging and submersible surveys based on this stratigraphic calibration and other geophysical data a geodynamic evolution can be established for the tagus abyssal plain faulting and folding in these sedimentary units clearly indicate that both the portuguese margin and the adjacent oceanic crust were deformed by a miocene compressive event an oxfordian kimmeridgian rifting in the lusitanian basin and on the portuguese margin may herald a sea floor spreading in the tagus abyssal plain the continent ocean boundary is marked by landward dipping reflectors similar to those observed along the canadian margin the oceanic structure of

an interpretation of a prominent magnetic anomaly near the boundary between the eurasian and african plates gulf of cadiz sw margin of iberia abstract the gulf of cadiz is located at the boundary between the eurasian and african plates in the southwestern margin of iberia and it straddles a place of major tectonic events since the mesozoic the gibraltar belt migrated westward into the gulf of cadiz during the miocene while a north south trending margin was built upon allochthonous formations at the western end of the betic rif orogen the geological setting of the gulf allows a study of both the mesozoic and cenozoic evolution of the contact between these two major plates this study reports a regional geophysical survey with an interpretation of the magnetic results using a 2d modelling constrained by recently acquired multichannel seismic profiling mcs and gravity data from the magnetic pattern the area can be subdivided into three sectors which may be the expression of different geological provinces in the eastern sector we find relatively small anomalies 50 nt interpreted as the magnetic expression of the morphological ridges and valleys intersecting the central gulf of cadiz in the southern sector where data are scarce we find a smooth magnetic zone with large wavelength anomalies the western sector dominated by the guadalquivir bank is where the largest magnetic anomalies are observed the guadalquivir bank is a structural high which represents the offshore extension of the hercynian iberian massif we have interpreted a large amplitude magnetic anomaly of more than 300 nt peak to peak with a very steep gradient trending northward to be caused by an intrusive material with an average estimated susceptibility distribution of 0 035 si from the model studies we believe that the magnetic source lies southwest of the guadalquivir bank and is mainly composed of crustal material the material rises up to 4 km depth and probably was uplifted prior to the initial opening extensional stages of the central north atlantic during the triassic

source analysis of the february 12th 2007 mw 6 0 horseshoe earthquake implications for the 1755 lisbon earthquake 1 while very large earthquakes are generally confined to subduction zones the sw iberian margin setting of the famous mw 8 5 8 7 1755 lisbon tsunami earthquake may be an exception to this rule evidence for active subduction is not conclusive here but instead plate convergence in old oceanic lithosphere with large brittle layer thickness can account for the occurrence of great earthquakes along moderate length faults we estimate the source parameters of the february 12th 2007 horseshoe earthquake regional moment tensor inversion yields an mw 6 0 reverse to strike slip faulting source in the upper mantle modelling teleseismic surface reflected body waves pp pwp sp indicates a source depth of 40 km beneath the seafloor analysing apparent source time functions allows identifying the preferred fault plane strike n245 e dip 55 rake 50 and estimating rupture area 53 km2 and average slip 0 27 m scaling the source characteristics to the size of the 1755 earthquake suggests a fault length of 230 315 km being compatible with the length of mapped faults in the area

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

crustal thinning in the southwestern iberia margin the mode of crustal thinning in the southwestern margin of the iberian peninsula is investigated along a transect that extends from onshore iberia to the eastern end of the horseshoe abyssal plain on onshore areas the crustal structure has been deduced using wide angle seismic reflection data whereas offshore we have used coincident steep and wide angle reflection data along a ne sw oriented seismic profile that extends from cape san vicente to the horseshoe abyssal plain in addition 2d gravity modelling has been performed to validate the crustal structure deduced from seismic data our model results reveal that the crust undergoes a strong but continuous thinning from 31 km onshore iberia to less than 15 km in the horseshoe abyssal plain and that thinning occurs over horizontal distances of about 120 km

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

perched lobe formation in the gulf of cadiz interactions between gravity processes and contour currents algarve margin southern portugal abstract the gulf of cadiz is swept by the strong saline mediterranean outflow water mow on the algarve margin south portugal this current has constructed fine grained contourite drifts this margin is dissected by the portimao canyon and three short channels that only incise the upper slope and are absent on a terrace located at mid slope depths along the algarve margin high resolution seismic profiles and sediment cores highlight the original architecture of the sedimentary deposits on this terrace coarse grained lenticular chaotic bodies formed during major relative sea level lowstands are intercalated within the drift the lobate shape and sandy nature of the lenticular chaotic bodies and their location at the mouths of the three short channels suggest they are gravity generated deposits that are perched on the middle continental slope in the gulf of cadiz the interaction between contour current and gravity processes is strongly controlled by climatic variations and relative sea level changes during the late quaternary during cold periods when sea level was low erosion intensified on the continental shelf and the deepest part of mow was active sediment was transported downslope through the channels and deposited on sedimentary lobes perched on the mid slope terrace during warm periods when relative sea level was high the supply of sediment from the shelf was shut off and the shallowest part of mow was more active contourite drifts fill the channels and bury the sandy lobes

effects of mass waste events on thrust wedges analogue experiments and application to the makran accretionary wedge 1 olistostromes cover large portions of active thrust wedges like makran gulf of cadiz and offshore borneo olistostrome emplacement by submarine mass flow represents an instantaneous and massive mass redistribution that may influence thrust wedge mechanics different scenarios are experimentally tested they show that the postolistostrome wedge development depends on the thickness and extent of the added load these results are discussed after the example of the iranian makran wedge situated between the arabian and eurasian plates mass redistribution caused by a late miocene mass flow may explain a change in deformation style from intense folding and thrusting to gentle folding and eventually a jump of thrust imbrication toward the frontal offshore part of this active accretionary wedge

morphotectonic characterization of major bathymetric lineaments in gulf of cadiz africa iberia plate boundary insights from analogue modelling experiments abstract new high resolution multi beam bathymetry data allowed the recognition of several bathymetric lineaments ca 100 km long trending wnw ese in seafloor sediments of the gulf of cadiz offshore sw iberia the interpretation of multi channel mcs profiles crosscutting these lineaments showed that they are controlled by underlying deep seated faults which have endured a polyphase reactivation history to get insights on the recent tectonic evolution of these structures we performed two sets of analogue modelling experiments assuming 1 right lateral strike slip basement faulting and coupled passive shearing affecting an overlying soft cover and 2 low angle transpressive deformation along a narrow shear band overlying the fault our results show a good correlation between the experimentally obtained structural patterns and the natural morphotectonic lineaments allowing the use of some of the observed natural features as strain gauges based on this we conclude that the study lineaments correspond to the bathymetric expression of ongoing dextral wrenching reactivation of wnw ese pre existing faults and we estimate the age of this tectonic reactivation as being ca 1 8 ma i e form late pliocene to present day these characteristics agree with the most recent kinematic models derived from geodetic observations indicating that present day convergence between nubia and iberia is subparallel to the newly identified lineaments and occurs at a 4 mm yr rate

an evaporite bearing accretionary complex in the northern front of the betic rif orogen the guadalquivir accretionary complex forms a largely oblique prism at the northern edge of the betic rif orogen where miocene sediments plus allochthonous evaporite bearing units were accreted during the displacement of the alboran domain towards the west traditional interpretations end the tectonic structuring of the betic cordillera at the present topographic front beyond which gravitational and or diapiric processes would predominate however this study shows pervasive tectonic deformation in the outer prism with coherent oblique shortening kinematics which is achieved through an alternation of roughly n s arcuate thrust systems connected by e w transfer fault zones these structures accord well with the geophysical models that propose westward rollback subduction the main stage of tectonic activity occurred in the early middle miocene but deformation lasted until the quaternary with the same kinematics evaporite rocks played a leading role in the deformation as evidenced by the suite of ductile structures in gypsum distributed throughout the area s and l gypsum tectonites scaly clay fabrics and brittle fabrics coexist and consistently indicate westward motion top to 290 with subordinate n s contraction almost perpendicular to the transfer zones this work reveals ductile tectonic fabrics in gypsum as a valuable tool to elucidate the structure and deformational history of complex tectonic melanges involving evaporites above the decollement level of accretionary wedges

crustal structure of the sw iberian passive margin the westernmost remnant of the ligurian tethys abstract at present the sw iberian margin is located along the convergent iberia nubia plate boundary in mesozoic times the margin was located at the triple junction of the ligurian tethys central atlantic and northern atlantic the characterization of its crustal structure has allowed us to propose a configuration for this triple junction and to determine the role that this transform margin played within the plate kinematic system in this paper we present an integrated study based on the interpretation of a 2d regional multichannel seismic survey consisting of 58 profiles tied with onshore geology and exploratory wells and on gravimetric modeling performed over four nw se trending profiles integrated interpretation of mcs data combined with 2d gravity modeling reveals a complex pattern in the southward crustal thinning of sw iberia and supports the possible presence of oceanic crust under the gulf of cadiz the tapering of iberian crust is characterized by steps with rapid changes in the thickness of the crust and thinning to

tethys atlantic interaction along the iberia africa plate boundary the betic rif orogenic system abstract initial se dipping slow subduction of the ligurian tethys lithosphere beneath africa from late cretaceous to middle oligocene twisting to a later faster e dipping subduction of the subcrustal lithosphere is proposed as an efficient geodynamic mechanism to structure the arcuate betic rif orogenic system this new subduction related geodynamic scenario is supported by a kinematic model constrained by well dated plate reconstructions tectonic sedimentary and metamorphic data sets the slow initial se dipping subduction of the ligurian tethys realm beneath the malaguide upper plate unit is sufficient to subduct alpujarride and nevado filabride rocks to few tens of kilometers of depth in middle eocene times the shift from se to e dipping subduction during latest oligocene early miocene was possibly caused by both the inherited geometry of the highly segmented ligurian tethys domain and by the fast roll back of the subducted lithospheric slab the early miocene rather synchronous multiple crustal and subcrustal processes comprising the collision along the betic front the exhumation of the hp lt metamorphic complexes the opening of the alboran basin its flooring by hp alpujarride rocks and subsequent ht imprint can be explained by the fast nw and w directed roll back of the ligurian tethys subcrustal lithospheric slab the w retreat of the ligurian tethys lithosphere in middle late miocene times could partly explain the initiation of its lateral tear and consequent subcrustal processes from latest miocene onward the betic rif system evolved under both the northerly push of africa resulting in tightening at crustal and subcrustal levels and by the distinct current dynamics of the steep lithospheric slab the sw directed scape of the rif fold belt is one of the most striking evidences linked to the recent evolution of the squeezed betic rif system between africa and iberia

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

iberian plate kinematics a jumping plate boundary between eurasia and africa the rotation of iberia and its relation to the formation of the pyrenees has been difficult to decipher because of the lack of detailed sea floor spreading data although several models have been proposed1 7 here we use detailed aeromagnetic measurements from the sea floor offshore of the grand banks of newfoundland to show that iberia moved as part of the african plate from late cretaceous to mid eocene time with a plate boundary extending westward from the bay of biscay when motion along this boundary ceased a boundary linking extension in the king s trough to compression along the pyrenees came into existence finally since the late oligocene iberia has been part of the eurasian plate with the boundary between eurasia and africa situated along the azores gibraltar fracture zone

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

bottom currents and their influence on the sedimentation pattern in the el arraiche mud volcano province southern gulf of cadiz abstract the completion of iodp expedition 339 within the cadiz contourite depositional system along the southern iberian margin which was created and maintained by the mediterranean outflow water has brought increased amounts of attention to this natural contourite laboratory in contrast a lot less attention is given to the southern gulf of cadiz despite the ubiquitous presence of tectonic ridges small contourite deposits mud volcanoes and cold water coral mounds the el arraiche mud volcano province eamvp is located in the southern gulf of cadiz and is characterized by an extensional regime creating two tectonic ridges named renard and vernadsky also nine mud volcanoes and numerous cold water coral mounds are present in this area the northward flowing bottom currents are deflected by the topographic obstacles and flow in a westward direction at the foot of the nw se oriented renard ridge calculations indicate that this bottom current is capable of turning around the tip of the renard ridge and continues its path along its northern edge the locations of the contourite deposits at the foot of the renard ridge are controlled by the steepness of the ridge slopes of more than 12 are associated to contourite deposits while less steep ones merely show hemipelagic deposits the moats around the mud volcanoes originate due to a combination of subsidence and the action of bottom currents as the seismic data show separated mounded drift deposits perpendicular to the moats as well as subsidence rims some mud volcanoes have a less incised northern and a deeper southern moat which indicates eastward flowing bottom currents this orientation is consistent with the shoreward component of the internal tides which flow vigorously in this area with peaks of speed over 30 cm s the integration of geophysical and hydrographic datasets in the eamvp sheds new light on the dynamic nature of the interaction of bottom currents and topographic features

neogene through quaternary tectonic reactivation of sw iberian passive margin southwest portugal the gulf of cadiz and morocco are under the potential threat of natural hazards linked to seismicity and tsunami generation we report the results of two multi channel seismic mcs surveys carried out in 1992 and 1998 along the continental margin and oceanic crust of sw iberia this mcs data set shows the evidence of the compressional deformation which involves both the continental and the oceanic crust of the study area the area of deformation extends from the southern border of the tagus abyssal plain to the seine abyssal plain encompassing the continental margin of sw portugal most of the structures observed are probably related to a mid miocene phase of africa europe plate convergence in this paper we discuss the recent advances on the identification of the tectonic structures that are still active and that may generate great earthquakes and tsunamis the tectonic structures identified are located respectively at the guadalquivir bank along the eastern border of the horseshoe abyssal plain and along the southern continental slope of sw portugal

underpinning tectonic reconstructions of the western mediterranean region with dynamic slab evolution from 3 d numerical modeling no consensus exists on the tectonic evolution of the western mediterranean since 35 ma three disparate tectonic evolution scenarios are identified each portraying slab rollback as the driving mechanism but with rollback starting from strongly different subduction geometries as a critical test for the validity of each tectonic scenario we employ thermomechanical modeling of the 3 d subduction evolution from each tectonic scenario we configure an initial condition for numerical modeling that mimics the perceived subduction geometry at 35 ma we seek to optimize the fit between observed and predicted slab morphology by varying the nonlinear viscoplastic rheology for mantle slab and continental margins from a wide range of experiments we conclude that a tectonic scenario that starts from nw dipping subduction confined to the balearic margin at 35 ma is successful in predicting present day slab morphology the other two scenarios initial subduction from gibraltar to the baleares and initial subduction under the african margin lead to mantle structure much different from what is tomographically imaged the preferred model predicts slab rotation by more than 180 east west lithosphere tearing along the north african margin and a resulting steep east dipping slab under the gibraltar strait the preferred subduction model also meets the first order temporal constraints corresponding to mid miocene 16 ma thrusting of the kabylides onto the african margin and nearly stalled subduction under the rif gibraltar betic arc since the tortonian 8 ma our modeling also provides constraints on the rheological properties of the mantle and slab and of continental margins in the region

crustal architecture and tectonic evolution of the gulf of cadiz sw iberian margin at the convergence of the eurasian and african plates 1 the gulf of cadiz located at the southwestern iberian margin is characterized by widespread seismicity compressional and strike slip fault plane solutions and by a large elongated positive free air gravity anomaly the gulf of cadiz gravity high gcgh multichannel seismic profiles across and along gcgh together with bathymetric and gravity data allow us to study in detail the tectonic architecture and crustal structure of the gulf of cadiz the upper shelf and slope of the gulf of cadiz includes the main structural domains of the betic fold and thrust belt in the middle part of the gulf the paleozoic basement crops out on the shallow guadalquivir bank and is associated with the largest signature of the gcgh whereas toward the outer part of the gulf the basement deepens progressively a large nw se normal fault and conjugate ne sw faults define a prominent basement high associated with the gcgh modeling of the gcgh suggests localized crustal thinning of 10 km along the central part of the gulf of cadiz probably generated during the mesozoic rifting episode between the iberian and african plates concentric wedges of fold and thrust belts and large allochthonous masses were emplaced in the gulf of cadiz during the neogene compressional phase the final emplacement of these units becomes progressively young from the se pre early langhian toward the foreland in the nw late tortonian seafloor surface ruptures pockmarks and submarine landslides provide evidence of active faulting in the gulf of cadiz to accommodate the present day convergence between the african and eurasian plates previously extensional faults have probably been reactivated and inverted at depth as suggested by the intermediate depth seismicity

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

3d effects of strain vs velocity weakening on deformation patterns in accretionary wedges abstract shear zones are weaker than surrounding rocks whether weakening depends on accumulated strain or strain rate is debated we used a three dimensional numerical code with a visco plastic brittle rheology to investigate the influence of strain and strain rate weakening often referred to as velocity weakening on the evolution of thin skinned fold and thrust belts two model setups are used i a uniform setup to recognize the effects of each weakening mode on the structural evolution and dynamics of fold and thrust belts and ii a dual setup with two adjacent domains of decollement strength to investigate the structural response of laterally varying systems strain weakening and velocity weakening lead to remarkably different structural patterns in particular strain weakening favours out of sequence shear zones whilst some amount of finite plastic strain has to be achieved before the structural development initiates weakening in contrast velocity weakening starts with the onset of high strain rate bands and does not favour out of sequence deformation we also tested separately the influences of shortening velocity cover sequence thickness and weakening style and amount on the structural evolution of the dual systems the shortening velocity has a minor effect in strain weakened models velocity weakening and a thinner cover sequence enhance the occurrence of strike slip shear zones we conclude that the weakening mode strongly influences the fault patterns and the dynamics of thrust wedges inversely natural fault patterns are keys to discriminate weakening processes

holocene earthquake record offshore portugal sw iberia testing turbidite paleoseismology in a slow convergence margin the sw margin of the iberian peninsula hosts the present day boundary between the eurasian and african plates convergence 4 5 mm yr is accommodated through a wide deformation zone characterized by moderate magnitude seismic activity this zone has also been the source of the most important seismic events in western europe such as the 1755 lisbon earthquake and tsunami and 1969 horseshoe earthquake despite efforts to identify active seismogenic structures in the gulf of cadiz in the last ten years little is known about its paleoseismic history the turbidite paleoseismology approach was applied for the first time in a low rate convergent margin to determine the recurrence interval of large earthquake events that occurred in sw iberia during the holocene four sediment cores collected at strategically located sites offshore portugal i e tagus abyssal plain infante don henrique basin and horseshoe abyssal plain reveal that these deep sea basins preserve a record of episodic deposition of turbidites in the sw iberian margin excluding special climatic events earthquakes are the most likely triggering mechanism for synchronous widely spaced distributed turbidites during the holocene when the sea level was relatively stable age correlation together with textural mineralogical physical properties and geochemical signatures of the new cores complemented by pre existing multicores and gravity cores reveals a total of 7 widespread turbidite events for the holocene precise dating of the most recent turbidite event e1 based on

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

compressive episodes and faunal isolation during rifting southwest iberia abstract evidence for three short lived compressive episodes of late carixian late callovian early oxfordian and tithonian berriasian ages that lasted 5 m yr and occurred during the process of rifting of the algarve basin is presented these tectonic inversion episodes are described at outcrop and cartographic scales and have been dated with the accuracy provided by the ammonoid scale an uplift event of late toarcian aalenian age of undetermined tectonic origin is also described we show that these four tectonic episodes coincide in time with important ecological events such as the onset of migration and or the segregation of boreal and tethyan ammonite species and the confinement of the algarve basin stratigraphic and paleoecological data from the algarve and lusitanian basins are compared and discussed together with eustatic and tectonic information we propose that the tectonic inversion episodes that caused uplift are the origin of the mesozoic sedimentary gaps and the intermittent opening and

gorringe ridge gravity and magnetic anomalies are compatible with thrusting at a crustal scale summary the main features of the deep structure of the gorringe ridge are analysed on the basis of gravity and magnetic measurements as well as seismic profiles drill holes rock dredges submersible observations and seismicity data the gravity and magnetic models of the gettysburg and ormonde seamounts which form the gorringe ridge suggest that the moho is approximately flat and the upper part of the ridge corresponds to a northwestwards vergent fold this structure is the result of a northwestward vergent thrust that deformed the oceanic crust with a minimum slip of approximately 20 km the activity of the thrust probably started 20 myr and produced the recent stages of seamount uplift the seamount is mainly composed of gabbros of the oceanic crust serpentinized rocks and alkaline basalts the large antiform located in the hangingwall of the thrust is probably deformed by minor faults this oceanic ridge is a consequence of the oblique convergence between the african plate and the overlapping eurasian plate

lateglacial and holocene variation in aeolian sediment flux over the northeastern tibetan plateau recorded by laminated sediments of a saline meromictic lake predominantly laminated lake sediments from a saline closed basin lake on the northeastern tibetan plateau were investigated using a multi proxy approach 14c accelerator mass spectrometry dating smear slide analysis loss on ignition grain size x ray diffraction elemental concentration ostracod assemblages stable isotopes of ostracod shells to trace the regional environmental and climatic history in the lateglacial and holocene before about 15 cal ka bp small saline water bodies probably filled the basin under unstable cold and harsh environmental conditions soon after about 14 9 cal ka bp a relatively deep saline lake was established probably as a result of runoff from melting snow ice and frozen ground in the lake s catchment large changes in flux of aeolian material to the lake were recorded during this initial period of formation of lake kuhai highest lake levels a low sediment accumulation rate and less saline conditions were maintained between about 12 8 and 7 1 cal ka bp when the aeolian influx diminished significantly after about 7 1 cal ka bp the aeolian influx remained at a moderate level apart from a strongly increased dust delivery to the lake between about 6 1 and 5 4 cal ka bp and a minor short lived period of slightly enhanced aeolian influx at about 2 7 cal ka bp the strongly enhanced dust input to the lake between 6 1 and 5 4 cal ka bp represented the largest influx of aeolian material to lake kuhai during the entire holocene however evidence for climatic deterioration during this period is not seen at most other palaeoclimate sites on the tibetan plateau but instead a significant increase in aridity has been recorded at numerous sites in the northern foreland of the tibetan plateau and on the chinese loess plateau the large dust input to lake kuhai between 6 1 and 5 4 cal ka bp probably did not result from a severe climate deterioration on the tibetan plateau itself but from the pronounced aridity in its northern and eastern foreland in contrast the increase in dust influx about 2 7 cal ka bp seems to correspond to a brief warming spell recorded at other sites on the tibetan plateau too a slight increase in lake level and decrease in salinity after about 0 6 cal ka bp suggests a slightly higher effective moisture during the final lake stage accompanied by a somewhat larger dust influx this apparent contradiction possibly results from enhanced human activities on the northeastern tibetan plateau during the last 600 years copyright 2009 john wiley sons ltd

evolution of the late miocene mediterranean atlantic gateways and their impact on regional and global environmental change marine gateways play a critical role in the exchange of water heat salt and nutrients between oceans and seas as a result changes in gateway geometry can significantly alter both the pattern of global ocean circulation and associated heat transport and climate as well as having a profound impact on local environmental conditions mediterranean atlantic marine corridors that pre date the modern gibraltar strait closed during the late miocene and are now exposed on land in northern morocco and southern spain the restriction and closure of these miocene connections resulted in extreme salinity fluctuations in the mediterranean leading to the precipitation of thick evaporites this event is known as the messinian salinity crisis msc the evolution and closure of the mediterranean atlantic gateways are a critical control on the msc but at present the location geometry and age of these gateways are still highly controversial as is the impact of changing mediterranean outflow on northern hemisphere circulation here we present a comprehensive overview of the evolution of the late miocene gateways and the nature of mediterranean atlantic exchange as deduced from published studies focussed both on the sediments preserved within the fossil corridors and inferences that can be derived from data in the adjacent basins we also consider the possible impact of evolving exchange on both the mediterranean and global climate and highlight the main enduring challenges for reconstructing past mediterranean atlantic exchange

deep structure recent deformation and analog modeling of the gulf of cadiz accretionary wedge implications for the 1755 lisbon earthquake the gulf of cadiz spans the plate boundary between africa and eurasia west of the betic rif mountain belt a narrow east dipping subduction zone descends beneath the gulf of cadiz and the straits of gibraltar the deep crustal structure of the gulf and the adjacent sw iberian and moroccan margins is constrained by numerous multi channel seismic reflection and wide angle seismic surveys a compilation of these existing studies is presented in the form of depth to basement sediment thickness depth to moho and crustal thickness maps these structural maps image an e w trending trough with thin 10 km crust beneath the gulf of cadiz this trough is filled by an eastward thickening wedge of sediments reaching a thickness of 10 15 km in the eastern gulf these sediments are tectonically deformed primarily along a series of westward vergent thrust faults and represent a 200 250 km wide accretionary wedge the northern and especially the southern limits of the accretionary wedge are marked by sharp morphological lineaments showing evidence of recent deformation these tectonic limits are situated in an internal position with respect to the miocene deformation front external betic and rif allocthons which has been abandoned at the western boundary of the accretionary wedge near the adjacent seine and horseshoe abyssal plains an e w trending basement high coral patch ridge can be seen indenting the deformation front in an asymmetric manner analog modeling is performed using granular materials accreted against a semicircular backstop representing the basement of the rif and betic mountain belts the modeling initially produces a symmetric arcuate accretionary wedge the ensuing collision of an oblique rigid indenter retards accretion on one side resulting in an embayment and a locally steeper deformation front the deformation pattern observed in morphology and high resolution seismic profiles suggests the accretionary wedge and underlying subduction system is still active the implications of active subduction for the source region of the 1755 lisbon earthquake and the regional seismic hazard assessment are discussed

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

sediment failures and flows in the gulf of cadiz eastern atlantic several types of sediment failures in the gulf of cadiz were observed using multibeam bathymetry acoustic imagery and high resolution seismic these instabilities are mainly sediment failures and flows their width and length vary from 1 to more than 10 km the failures are mainly related to high sedimentation rates particularly in places where the mediterranean outflow water mow spills over such as channel bends and the outer side of the giant contourite levee steep slopes are also a trigger for failure at the continental shelf slope transition on valley sides on canyon flanks and on the sides of bathymetric highs other mass movements are related to fluid escape mud volcanoes and earthquakes in areas where the mow flows along the seafloor the constant shearing and related erosion can add to the overall stresses the frequency of failures can be estimated using the deposits resulting of their distal transformations into turbidites

structural analysis and miocene to present tectonic evolution of a lithospheric scale transcurrent lineament the sciacca fault sicilian channel central mediterranean sea abstract seismo stratigraphic and structural analysis of a large number of multichannel seismic reflection profiles acquired in the northern part of the sicilian channel allowed a 3 d reconstruction of a regional ns trending transfer zone which displays a transcurrent tectonic regime and that is of broad relevance for its seismotectonic and geodynamic implications it is constituted of two major transcurrent faults delimiting a 30 km wide mostly undeformed basin the western fault capo granitola does not show clear evidence of present day tectonic activity and toward the south it is connected with the volcanic area of the graham bank the eastern fault sciacca is structurally more complex showing active deformation at the sea floor particularly evident along the nerita bank the sciacca fault is constituted of a master and splay faults compatible with a right lateral kinematics sciacca fault is superimposed on an inherited weakness zone a mesozoic carbonate ramp which borders to the east a 2 5 km thick plio quaternary basin and that was reactivated during the pliocene a set of scaled claybox analogue models was carried out in order to better understand the tectonic processes that led to the structural setting displayed by seismic data tectonic structures and uplift subsidence patterns generated by the models are compatible with the 3 d model obtained from seismic reflection profiles the best fit between the tectonic setting deriving from the interpretation of seismic profiles and the analogue models was obtained considering a right lateral movement for the sciacca fault nevertheless the stress field in the study area derived from gps measurements does not support the present day modelled right lateral kinematics along the sciacca fault moreover seismic events along this fault show focal mechanisms with a left lateral component we ascribe the slip change along the sciacca fault from a right lateral transcurrent regime to the present day left lateral kinematics to a change of principal horizontal stress direction starting from late pliocene

linked halokinesis and mud volcanism at the mercator mud volcano gulf of cadiz mud volcanoes are seafloor expressions of focused fluid flow that are common in compressional tectonic settings new high resolution 3 d seismic data from the mercator mud volcano mmv and an adjacent buried mud volcano bmv image the internal structure of the top 800 m of sediment at both mud volcanoes revealing that both are linked and have been active episodically the total volumes of extruded mud range between 0 15 and 0 35 km3 and 0 02 0 05 km3 for the mmv and the bmv respectively the pore water composition of surface sediment samples suggests that halokinesis has played an important role in the evolution of the mud volcanoes we propose that erosion of the top of the vernadsky ridge that underlies the mud volcanoes activated salt movement triggering deep migration of fluids dissolution of salt and sediment liquefaction and mobilization since the end of the pliocene since beginning of mud volcanism in this area the mud volcanoes erupted four times while there was only one reactivation of salt tectonics this implies that there are other mechanisms that trigger mud eruptions the stratigraphic relationship of mudflows from the mmv and bmv indicates that the bmv was triggered by the mmv eruptions this may either be caused by loading induced hydrofracturing within the bmv or due to a common feeder system for both mud volcanoes this study shows that the mud volcanoes in the el arraiche mud volcano field are long lived features that erupt with intervals of several tens of thousands of years

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

reconstruction of the glacial ice covers of greenland and the canadian arctic islands by three dimensional perfectly plastic ice sheet modelling a three dimensional perfectly plastic ice sheet model developed for determining the surface elevations and the flow pattern of an ice sheet with given bottom topography and ice margin positions is applied to the reconstruction of the glacial ice covers of greenland and the canadian arctic islands in the northern regions two different reconstructions have been performed with ice margins along the present 600 and 200 m sea depth contours respectively in central greenland the ice margin is considered to be at the outermost ice margin deposits on the coastal shelf to the west and at the present 200 m sea depth contour to the east the main conclusions to be drawn from the reconstructions are 1 the flow pattern of the glacial ice cover of greenland shows a great resemblance to the present one the central ice divide being displaced less than 50 km from its present position and being no more than 200 m higher than today 2 the main ice divide of the ice sheet covering the canadian arctic islands the innuitian ice sheet was located over the highlands of eastern ellesmere island with local domes positioned over the present ice caps indicating that even the deep ice of wisconsin age in these ice caps is of local origin this is also the case for the devon island ice cap 3 even in the not very likely case of a rather extensive glacial ice cover in north west greenland the ice flow pattern upstream of the camp century deep drill site would not have changed radically compared to the present flow pattern thus it is concluded that even advanced ice margins in late wisconsin time could at most have resulted in an elevation of the deposition site of the late wisconsin ice at camp century 600 m higher than at present the consequences of this conclusion are discussed

modeling the evolution of the guadalquivir foreland basin southern spain 1 previous quantitative studies dealing with the origin of foreland basins have focused primarily either on the rheological basis of the lithosphere mechanical response or on the relationship between orogenic loading and sediment geometry to link the evolution of the guadalquivir foreland basin south iberia with the thermomechanical stratification of the iberian lithosphere we combine quantitative approaches to deep and shallow processes thrust loading lithospheric flexure thickness changes of the crust and the lithospheric mantle and surface mass transport a planform flexural analysis of the present day load distribution shows that basement subsidence is related not only to upper crustal thrusting but also to a deep seated additional load on the basis of the observed gravity and geoid fields we propose this additional load to be related to a lithospheric mantle thickening larger than the coeval crustal thickening further modeling of the evolution of a basin cross section reveals that the architecture of the sedimentary basin is additionally related to the lithosphere rheological response the quantitative study of the evolution of basement faulting and the forebulge uplift of sierra morena leads us to conclude that viscous stress relaxation and or plastic yielding within the lithosphere are key processes to explain the flexural evolution of the basin

geophysical evidence of gas hydrates in shallow submarine mud volcanoes on the moroccan margin 1 gas hydrates inside mud volcanoes have been observed in several locations but are generally found at water depths of 1000 m and deeper we present the first observation of the base of a gas hydrate stability zone within a shallow mud volcano in the el arraiche mud volcano field on the moroccan atlantic margin the mud volcano base is located at about 475 m and is over 125 m high on high resolution seismics we observed an anomalous but coherent reflection under the slopes of the mud volcano the event was interpreted as the base of a gas hydrate stability zone because of its inverse polarity and its morphology far from the crater the event is nearly parallel to the seafloor closer toward the crater the event shallows inside the mud volcano crater no event is observed a stability model using thermogenic gas compositions is applied to local p t conditions indicating that thermogenic gas hydrates can be stable at this depth the high modeled heat flow in the crater of the mud volcano indicates a focused flow of warm fluids below the slopes of the mud volcano the inferred heat flow is also elevated but less high in areas of thermogenic gas production gas hydrates can occur at shallow water depths even in areas with high heat flow this also suggests that dewatering of the accretionary wedge complex is mainly focused along fault surfaces and through seafloor structures such as mud volcanoes

the alboran sea western mediterranean revisited with a view from the moroccan margin this paper presents a new depth to acoustic basement map based on a grid of seismic profiles in the western alboran basin the basement is more than 10 km deep an examination of a seismic profile of this region suggests that late oligocene early miocene 5 km thick synrift formation has been accumulated in this basin this west alboran basin has been formed in an extensional setting but in a fore arc position south of the balearic islands then westwards transported behind a subduction zone

seismic crustal structure in the southwest of the iberian peninsula and the gulf of cadiz abstract the crust under the southwestern iberian peninsula and the gulf of cadiz has been sampled by 1200 km of deep seismic refraction wide angle reflection profiles together with many seismic reflection lines and bore holes wide angle seismic data were collected during the last three decades commercial multichannel data provide a detailed image of the uppermost crust improving the confidence about the models of the deeper structures p wave velocities within the thick column of sediments in the gulf of cadiz range from 2 0 to 3 8 km s while the algarve and the sines areas have higher velocities of 4 3 to 4 8 km s the top of the palaeozoic basement rises to the northwest outcropping in the south portuguese zone of the iberian massif and is characterized by p wave velocities of 5 7 5 9 km s high velocities of 6 4 km s have been found at shallow depths of 7 to 10 km in the south portuguese zone that could be related to the mafic and ultramafic rocks in the beja acebuches zone lower crustal velocities are in the range of 6 7 6 9 km s the crustal thickness shows important lateral changes from 29 km beneath the guadalquivir basin iberian massif contact to 35 km in the southeastern part of the south portuguese zone from the interpretation of these seismic data a geodynamic model of the evolution of the crust in southwestern iberia and the gulf of cadiz is proposed the guadalquivir basin and its continuation at sea the gulf of cadiz is a flexure area of the crust that could be related to the overloading due to the overthrusting of the alboran domain over the iberian plate

structural development of the jurassic magnetic quiet zone off morocco and identification of middle jurassic magnetic lineations 1 geophysical investigations carried out during two meteor cruises have revealed weak linear magnetic anomalies in those parts of the jurassic magnetic quiet zone jmqz off morocco that are not affected by cenozoic igneous activity the linear magnetic anomalies are not correlated with variations in relief or structure of oceanic crust using the reversal sequence m25 m41 the anomalies of the jmqz can be modeled with a half spreading rate of 2 2 cm yr extrapolation of the weak lineations back to the slope anomaly s1 results in a breakup age of 170 ma according to this interpretation the crust of the jmqz off morocco has formed during the time 170 155 ma bajocian to oxfordian however much lower spreading rates around 1 cm yr cannot be excluded s1 coincides with a poorly developed seaward dipping reflector sequence sdrs which is most likely its source the sdrs corroborates the earlier claim that s1 marks the ocean continent transition seaward of s1 is a 70 km wide strip of horizontal reflectors that become landward dipping to the west in the uppermost part of the basement it parallels s1 over a distance of 200 km and may indicate excessive magma supply within the first 2 m y of seafloor spreading similar landward dipping reflectors are observed also in the conjugate sohm abyssal plain off nova scotia the average magnetization of the landward dipping reflectors is much lower than that of the sdrs a lower crustal body with a seismic velocity of 7 3 km s which formed during the neogene beneath the eastern part of the essaouira rise has only a weak magnetization 0 5 a m

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

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

isotopic evidence 87sr 86sr δ7li for alteration of the oceanic crust at deep rooted mud volcanoes in the gulf of cadiz ne atlantic ocean the chemical and isotopic composition of pore fluids is presented for five deep rooted mud volcanoes aligned on a transect across the gulf of cadiz continental margin at water depths between 350 and 3860 m generally decreasing interstitial li concentrations and sr 87 sr 86 ratios with increasing distance from shore are attributed to systematically changing fluid sources across the continental margin although highest li concentrations at the near shore mud volcanoes coincide with high salinities derived from dissolution of halite and late stage evaporites clayey terrigenous sediments are identified as the ultimate li source to all pore fluids investigated light delta li 7 values partly close to those of hydrothermal vent fluids delta li 7 11 9 parts per thousand indicate that li has been mobilized during high temperature fluid sediment or fluid rock interactions in the deep sub surface intense leaching of terrigenous clay has led to radiogenic sr 87 sr 86 ratios similar to 0 7106 in pore fluids of the near shore mud volcanoes in contrast non radiogenic sr 87 sr 86 ratios similar to 0 7075 at the distal locations are attributed to admixing of a basement derived fluid component carrying an isotopic signature from interaction with the basaltic crust this inference is substantiated by temperature constraints from li isotope equilibrium calculations suggesting exchange processes at particularly high temperatures 200 degrees c for the least radiogenic pore fluids of the most distal location advective pore fluids in the off shore reaches of the gulf of cadiz are influenced by successive exchange processes with both oceanic crust and terrigenous fine grained sediments resulting in a chemical and isotopic signature similar to that of fluids in near shore ridge flank hydrothermal systems this suggests that deep rooted mud volcanoes in the gulf of cadiz represent a fluid pathway intermediate between mid ocean ridge hydrothermal vent and shallow marginal cold seep due to the thicker sediment coverage and slower fluid advection rates the overall geochemical signature is shifted towards the sediment diagenetic signal compared to ridge flank hydrothermal environments c 2009 elsevier ltd all rights reserved

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

carbonate formation by anaerobic oxidation of methane evidence from lipid biomarker and fossil 16s rdna carbonate chimneys and other carbonate structures occur widespread in the gulf of cadiz and probably reflect the presence of cold seeps and associated release of methane in the geological past possibly in the early pleistocene but it is unclear under what conditions and by which processes these carbonates were formed we studied a fossil methane related carbonate crust collected from the kidd mud volcano in the gulf concentrations of microbial lipids their stable carbon isotope composition sequences of fossil 16s rrna genes of anaerobic methanotrophic archaea in combination with mineralogical and carbon and oxygen isotopic composition of carbonate were obtained for seven different horizons of the crust this combination of organic and inorganic geochemical techniques with molecular ecological methods gave a consistent view on processes resulting in the formation of the crust and indicated that it took place in two phases and in a downward direction archaeal lipid biomarkers and fossil 16s rrna gene sequence data revealed the dominance of archaeal anme 2 group and elevated methane partial pressures during the formation of the top part of the crust the lower part of the carbonate was likely formed in an environment with reduced methane fluxes as revealed by the dominance of fossil remains of anme 1 archaea the combination of these methods can be used as an effective tool to reconstruct in unprecedented detail the palaeo biogeochemical processes resulting in the formation of carbonate fabrics this interdisciplinary strategy may also be applied for other fossil methane derived carbonates generating new concepts and knowledge about past methane related carbonate systems

sediment supply from the betic rif orogen to basins through neogene abstract we present a quantification of total and partial divided by time slices sedimentary volumes in the neogene basins of the betic rif orogen these basins include the alboran sea the intramontane basins the guadalquivir and rharb foreland basins and the atlantic margin of the gibraltar arc the total volume of neogene sediments deposited in these basins is 209 000 km3 and is equally distributed between the internal alboran basin and intramontane basins and the external basins foreland basins and atlantic margin the largest volumes are recorded by the alboran basin 89 600 km3 and the atlantic margin 81 600 km3 the guadalquivir and rharb basins amount 14 000 km3 and 14 550 km3 respectively whereas the intramontane basins record 9235 km3 calculated mean sediment accumulation rates for the early middle miocene show an outstanding asymmetry between the alboran basin 0 24 mm yr and the foreland basins 0 06 0 07 mm yr and the atlantic margin 0 03 mm yr during the late miocene sedimentation rates range between 0 17 and 0 18 mm yr recorded in the alboran basin and 0 04 mm yr in the intramontane basins in the pliocene quaternary the highest sedimentation rates are recorded in the atlantic margin reaching 0 22 mm yr sedimentary contribution shows similar values for the inner and outer basins with a generalized increase from late miocene to present from 3500 to 6500 km3 my interestingly the alboran basin records the maximum sedimentary contribution during the late miocene 5500 km3 my whereas the atlantic margin does during the pliocene quaternary 6600 km3 my the spatial and time variability of the sediment supply from the betic rif orogen to basins is closely related to the morphotectonic evolution of the region the high sedimentation rates obtained in the alboran basin during the early middle miocene are related to active extensional tectonics which produced narrow and deep basins in its western domain the highest sedimentary contribution in this basin as well as in the foreland and intramontane basins is recorded during the late miocene due to the uplift of wide areas of the betics and rif chains the analysis of the sedimentary supply also evidences strong relationships with the post tortonian crustal thickening and coeval topographic amplification that occurred in the central betics and rif with the concomitant evolution of the drainage network showing the fluvial capture of some internal basins by rivers draining to the atlantic ocean the ancestral guadalquivir

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

molecular and carbon isotopic variability of hydrocarbon gases from mud volcanoes in the gulf of cadiz ne atlantic investigations of molecular and carbon isotopic variability of hydrocarbon gases from methane through butanes pentanes have been performed on six mud volcanoes from two fluid venting provinces located in the gulf of cadiz ne atlantic the main aims were to define the basic gas types to describe their geochemical characteristics in relationship to their sources and to determine the secondary effects due to migration mixing and microbial alteration hydrocarbon gas data reveal two groups of gases despite the different maturation characteristics both gas groups are allochthonous to the erupted mud breccia and represent a complex of redeposited secondary migrated mixed and microbially altered hydrocarbons it may possibly imply the presence of hydrocarbon accumulations in the deep subsurface of the gulf of cadiz

sources of mud volcano fluids in the gulf of cadiz indications for hydrothermal imprint mud volcanism in the gulf of cadiz occurs over a large area extending from the shelf to more than 3500 m water depth and is triggered by compressional stress along the european african plate boundary affecting a deeply faulted sedimentary sequence of locally more than 5 km thickness the investigation of six active sites shows that mud volcano mv fluids on average are highly enriched in ch4 li b and sr and depleted in mg k and br the purity of the fluids is largely controlled by the intensity of upward directed flow flow rates could be constrained by numerical modelling and vary between 0 05 and 15 cm yr 1 application of d 18o systematics identifies clay mineral dehydration most likely within mesozoic and tertiary shales and marls as the major source of fluids hence cl and na in the pore fluids are mostly depleted below seawater values following a general trend of dilution however deviations from this trend occur and are likely caused by the dissolution of halite in evaporitic deposits other secondary processes overprinting the original fluid composition may occur along the flow path such as dissolution of anhydrite or gypsum and or the formation of calcite and dolomite different sources of fluids are also indicated by variations in 87sr 86sr which range from 0 7086 to 0 7099 at the different sites dehydration may be induced primarily by overburden and tectonic compression however very high concentrations of li and b specifically at captain arutyunov mv camv indicate additional leaching at temperatures above 150 c which could be explained by the injection of hot fluids along deep penetrating major e w strike slip fault systems this hypothesis is supported by the occurrence of generally thermogenic but significantly ch4 enriched light volatile hydrocarbon gases at camv which cannot be explained by shallow microbial methanogenesis li and li b ratios from different types of hot and cold vents are used to infer that high temperature signals seem to be preserved at various cold vent locations and indicate a closer coupling of both systems in continental margin environments than outlined in previous studies

crustal types and tertiary tectonic evolution of the alboran sea western mediterranean multichannel seismic reflection images across the transition between the east alboran and the algero balearic basins show how crustal thickness decreases from about 5 s two way traveltime twtt 15 km thick in the west east alboran basin to 2 s twtt typical of oceanic crust 6 km thick in the east algero balearic basin we have differentiated three different crustal domains in this transition mainly on the basis of crustal thickness and seismic signature boundaries between the three crustal domains are transitional and lack evidence for major faults tilted blocks related to extension are very scarce and all sampled basement outcrops are volcanic suggesting a strong relationship between magmatism and crustal structure stratigraphic correlation of lithoseismic units with sedimentary units of southeastern betic basins indicates that sediments onlap igneous basement approximately at 12 ma in the eastern area and at 8 ma in the western area linking seismic crustal structure with magmatic geochemical evidence suggests that the three differentiated crustal domains may represent from west to east thin continental crust modified by arc magmatism magmatic arc crust and oceanic crust middle to late miocene arc and oceanic crust formation in the east alboran and algero balearic basins respectively occurred during westward migration of the gibraltar accretionary wedge and shortening in the betic rif foreland basins arc magmatism and associated backarc oceanic crust formation were related to early to middle miocene subduction and rollback of the flysch trough oceanic basement subduction of this narrow slab beneath the alboran basin was coeval with collision of the alboran domain with the iberian and african passive margins and subsequent subcontinental lithosphere edge delamination along the betic rif margins

seismotectonics of the ibero maghrebian region abstract a study of the seismotectonics of the ibero maghrebian region is presented as deduced from main geological features seismicity and focal mechanism data the hypocentral distribution of shallow intermediate depth and deep earthquakes is analyzed the direction of the stress distribution is inferred from the focal mechanism of 24 selected events a seismotectonic framework summarizing these results together with the main geological features shows that a nw se general direction of horizontal compressive stresses is present in the region with a localized zone of horizontal tensions in the betics and the alboran sea the presence of intermediate depth and deep earthquakes is tentatively explained as caused by two consecutive processes of subduction

the contourite depositional system of the gulf of cadiz a sedimentary model related to the bottom current activity of the mediterranean outflow water and its interaction with the continental margin the present paper is mostly a synthesis of work conducted on the continental margin of the gulf of cadiz using a broad database collected by several cruises and projects supported by the spanish research council and the nrl and naval oceanographic office usa including bathymetry sidescan sonar imagery seismic profiles sediment cores submarine photographs and physical oceanographic data these data have enabled us to establish a detailed understanding of the morphologic development of the margin its pliocene and quaternary stratigraphy and a full characterization of the contourite depositional system cds generated by the mediterranean outflow water mow the northern margin of the gulf of cadiz shows the following distinct features a an active compressive framework where the cadiz allocthonous unit provides an unstable substratum for late miocene pliocene and quaternary sedimentation b a relative lack of submarine canyons except in the western area of the algarve margin c a very broad continental slope that lacks a marked continental rise d a middle slope dominated by along slope processes driven by the mow which has generated a complex cds during the pliocene and quaternary and e an irregular lower slope and abyssal plain region dominated by down slope processes that is partly detached from an upper slope source region the cds is composed of both depositional and erosive features the main depositional features are characterised by sedimentary wave fields sedimentary lobes mixed drifts plastered drifts elongated mounded and separated drifts and sheeted drifts the main erosive features are contourite channels furrows marginal valleys and moats these various depositional and erosive features have a specific location along the margin and their detailed distribution is essential to understand the present and past interaction of the mow with the middle slope based on this distribution five morphosedimentary sectors have been identified within the cds which from east to west are 1 proximal scour and sand ribbons 2 overflow sedimentary lobe 3 channels and ridges 4 active contourite drifts and 5 submarine canyons the development of the cds has been controlled in general by the pliocene and quaternary environmental and paleoceanographic changes and by the morphology of the margin but in detail the development of each of these sectors is related to systematic deceleration of the mow as it flows westwards to the interaction with the margin bathymetry and to the effects of coriolis force our comprehensive sedimentary model for the cds defines the gulf of cadiz margin as a mixed contourite turbidite system with a detached combined drift fan morphology this is different from many other contourite influenced margins where the contourite processes are dominant on the middle slope and separated from the down slope processes which are characteristic on the lower slope and abyssal plains

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

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

very coarse grained beaches as a response to generalized sea level drops in a complex active tectonic setting pleistocene marine terraces at the cadiz coast sw spain abstract the studied pleistocene deposits at the cadiz coast are located on the current intertidal zone or at cliffs parallel to the actual coast 6 50 m a s l from the localities of el puerto de santa maria to chiclana five sequences of very coarse grained beaches have been recognized and dated they unconformably overlie pliocene units and are formed by clast supported conglomerate and coarse grained sandstone chronological ages 87 sr 86 sr and ams 14 c analysis and stratigraphic correlation stand that the oldest marine terrace is the one placed topographically higher and towards the se of cadiz city dated as early pleistocene 1 3 ma after it the sea retreatment produces subaerial exposure of sequence 1 and the development of several calcrete profiles and a paleokarst subsequent sea level rises allow the sedimentation of sequences 2 3 and 4 during early middle pleistocene 1 to 0 8 ma in areas close to cadiz city with subaerial exposure recorded at the top of sequences 3 and 4 during the very upper pleistocene sedimentation of sequence 5 occurs towards the se of cadiz city based on the spatial position of sequence 5 we can conclude that this area constituted a downthrown block during the latest pleistocene in relation to neotectonic processes and diapirism sandstone framework hybrid arenites reveals that main source areas for these marine terraces include units from the betic orogene betic external zones triassic and jurassic rocks and the gibraltar arc the guadalquivir basin passive margin iberian massif the guadalquivir foreland basin marine miocene and pliocene record and local inputs of pleistocene units the siliciclastic supplies from sources were mainly constant during the sedimentation of all set of sequences suggesting maturation processes as reworking and recycling of homogenized sediments during transport early diagenetic processes have been identified as carbonate cementation neomorphism and dissolution outlining the deduced sea level fluctuations during deposition of pleistocene sequences the marine terraces arrangement and its comparison with regionally adjacent areas allows to deduce that the southern and central portuguese coast iberian massif and of the southern and southeastern spanish coast betic orogene were predominantly tectonically raising during pleistocene however the cadiz gulf coast formerly guadalquivir foreland basin was mainly sinking during the same time interval

changes in dip and frictional properties of the basal detachment controlling orogenic wedge propagation and frontal collapse the external central betics case thin skinned fold and thrust belts ftbs have been extensively studied through both field examples and modelling the overall dynamics of ftbs are therefore well understood one less understood aspect is the combined influence of across strike changes in the detachment properties and the basement topography on the behaviour of an orogenic wedge in this paper we use field data together with reflection seismic interpretation from the external zones of the central betics ftb s spain to identify a significant increase in the wedge basal dip a basement threshold coinciding with the pinch out of a weak substrate this induced both changes to the wedge geometry and to the basal friction which in turn influenced the wedge dynamics the changing dynamics led to a transient stagnation of the ftb propagation topographic build up and subsequent collapse of the ftb front this in turn fed an important langhian depocenter made up of mass transport deposits coevally with the ftb propagation extension took place both parallel and perpendicular to the orogenic trend this case study illustrates how across strike changes in wedge basal properties can control the detailed behaviour of a developing ftb front but questions remain regarding the time space interaction and relative importance of the basal parameters

the sines contourite depositional system along the sw portuguese margin onset evolution and conceptual implications abstract the sines contourite depositional system located in the southwest portuguese margin is a central segment of the iberian contourite depositional complex built under the influence of the mediterranean outflow water mow this work presents the onset and evolution of this system using multibeam bathymetry multichannel seismic reflection lines sediment cores and well data six seismic units of late miocene through holocene age have been identified which defined three evolutionary stages for drift construction a an onset and initial stage from the latest miocene to the late pliocene 5 33 2 5 ma where a sheeted drift was built under a weak flowing mow b a growing stage from the early to middle quaternary 2 5 0 7 ma characterized by the deposition of a mounded drift in the south and a sheeted drift in the north which is correlated with a strengthening of the mow and c a maintenance stage between the middle pleistocene and holocene 0 7 ma present where the modern depositional and erosional contourite features developed associated with three intensification periods of the mow at 0 7 ma 0 4 ma and 20 ka the development of the sines contourite depositional system was constrained in the long term by seafloor paleomorphologies inherited from the mesozoic rifting phases of the southwest portuguese margin the nnw sse horsts and grabens built during the mesozoic rifting provided accommodation for drift growth with limited lateral migration and locally enhanced the alongslope bottom current in short term environmental climate and sea level fluctuations modulated changes in bottom current density and intensity determining sedimentary cycles in the depositional record especially during the quaternary our findings emphasize the occurrence of three main plastered morphologies throughout the seismic record which were compared with other plastered drifts in different continental margins to establish a conceptual model

hydrocarbon derived ferromanganese nodules in carbonate mud mounds from the gulf of cadiz mud breccia sediments and clasts as nucleation sites abstract more than 500 fe mn nodules were sampled during the anastasya 01 cruise tasyo project along the continental margin of the gulf of cadiz eastern central atlantic at the confluence of the mediterranean sea with the atlantic ocean where extensive nodule fields were discovered based on wide previous studies that included swath bathymetry multi channel and very high resolution seismic reflection gravimetry magnetism and underwater photography surveys nodules were collected at water depths ranging from 850 to 1000 m associated with hydrocarbon derived ankerite and dolomite chimneys and crusts thirty six selected samples among the various morphological types were used for the laboratory analysis of physical properties morphology colour surface texture sphericity weight and size mineralogy xrd optical and electronic microscopy and geochemistry xrf aas icp ms epma gc ms s c and o isotopes nodules show a wide range of sizes densities weights and morphologies nodules are formed of multiple millimetre thick layers of fe and mn oxyhydroxides surrounding the nucleus composed of early middle miocene plastic marl the textures developed by the fe and mn oxyhydroxides layers are massive laminated detrital and mottled to dendritic goethite lepidocrocite mn oxides 7 a manganates and 10 a manganates quartz and phyllosilicates are the main components accessory minerals are calcite dolomite siderite rhodochrosite kutnohorite pyrite chalcopyrite potassium feldspar zircon rutile ilmenite and chlorite fe mn carbonates from the siderite rhodochrosite continuous series are forming part of the marl nuclei framboidal filamentous and globular textures are observed in fe mn oxides and pyrite suggesting biogenic origin nodules show a high mean abundance of fe 38 6 moderate mn 6 0 and low contents of trace metals and rees compared to the average content of deep seabed polymetallic nodules from other oceanic areas the mn fe ratio ranges from 0 07 to 0 25 the studied nodules hold in their oxide layers hydrocarbons n alkanes derived from marine bacterial activity aromatic hydrocarbons in the form of phenanthrene are also present which is characteristic of mature petroleum the structure mineralogy and chemical composition in the studied nodules are similar to those of diagenetic hydrogenetic continental margin nodules rather than deep sea nodules we suggest that the formation of this type of nodule occurred through a combined diagenetic hydrogenetic growth process involving fluid venting from deep seated hydrocarbon reservoirs bio mineralization processes and bottom current erosion

the northwestern maghreb boundary of the nubia africa plate abstract a study of the present compressional deformation of the northwestern maghreb nubia africa margin is derived from the analysis of more than 20 000 km of seismic profiles in the western part the compression is distributed in a large zone with on land compression in algeria mainly strike slip deformation on the algerian margin and folds and strike slip faulting in eastern spain in the middle of the algerian margin around algiers the evidences of compression become more obvious in this area a ridge trending n s that is interpreted as a middle to late miocene spreading center interacted with the transpressional margin that trends e w north of the location of the boumerdes zemmouri earthquake the oceanic crust is deformed by blind thrusts up to 60 km from the coast these thrusts are south dipping and with the northward dipping thrusts located onshore form a wedge that maybe a positive flower structure at a crustal scale related to the right lateral transpression of the margin in the eastern part of the northwestern maghreb nubia africa deformed belt off eastern algeria and tunisia the deformation is more intense but limited to the north by the continental slope large late miocene tortonian folds are cut by the messinian erosional surface but the present deformation is also evident it is suggested that the deformation with a double vergence may be followed up to the north of sicily after the docking 18 ma of the kabylies to the africa plate the crust has been thinned and the algerian basin opened during the middle late miocene with an e w direction from the late miocene to the present the margin has been rethickened by transpression and uplifted

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

tracing organic compounds in aerobically altered methane derived carbonate pipes gulf of cadiz sw iberia abstract the primary geochemical process at methane seeps is anaerobic oxidation of methane aom performed by methanotrophic archaea and sulfate reducing bacteria srb the molecular fingerprints biomarkers of these chemosynthetic microorganisms can be preserved in carbonates formed through aom however thermal maturity and aerobic degradation can change the original preserved compounds making it difficult to establish the relation between aom and carbonate precipitation here we report a study of amino acid and lipid abundances in carbonate matrices of aerobically altered pipes recovered from the seafloor of the gulf of cadiz sw iberian peninsula this area is characterized by a complex tectonic regime that supports numerous cold seeps studies so far have not determined whether the precipitation of carbonate pipes in the gulf of cadiz is a purely chemical process or whether microbial communities are involved samples from this site show signs of exposure to oxygenated waters and of aerobic alteration such as oxidation of authigenic iron sulfides in addition the degradation index calculated from the relative abundance of preserved amino acids indicates aerobic degradation of organic matter although crocetane was the only lipid identified from methanotrophic archaea the organic compounds detected n alkanes regular isoprenoids and alcohols are compatible with an origin from aom coupled with bacterial sulfate reduction bsr and subsequent aerobic degradation we establish a relation among aom bsr and pipe formation in the gulf of cadiz through three types of analysis 1 stable carbon and oxygen isotopic composition of carbonate minerals 2 carbonate microfabrics and 3 mineralogical composition our results suggest that carbonate pipes may form through a process similar to the precipitation of vast amounts of carbonate pavements often found at cold seeps our approach suggests that some organic compound patterns in combination with additional evidence of aom and bsr may help indicate the source of altered methane derived carbonates commonly occurring in ancient and modern deposits

eastern segment of the azores gibraltar line central eastern atlantic an oceanic plate boundary with diffuse compressional deformation new seismic reflection images across the eastern segment of the azores gibraltar line west of the collisional area between the african and iberian plates have revealed a complex pattern of compressional deformation involving the mesozoic oceanic lithosphere the compressional deformation developed in a region of slow plate convergence and is diffused at different lithospheric levels across an area spanning 200 km from the gorringe ridge to the seine plain the convergence between the african and iberian plates has been active since tertiary time and our results indicate that no subduction zone exists across this part of the plate boundary

paleogeographic evolution of a segmented oblique passive margin the case of the sw iberian margin the sw iberian margin is an oblique passive margin developed during the mesozoic as part of the opening of the westernmost ligurian tethys the algarve basin developed on the sw iberian margin and its sedimentary record was strongly controlled by tectonics the algarve basin was dominated during the mesozoic by wsw ene trending extensional faults and their associated depocenters segmented by oblique nw se transfer zones two major nw se trending transfer zones connected the algarve basin to the gorringe bank in the west and to the betic basin in the east the sw iberian margin was later inverted during late cretaceous to cenozoic times due to convergence between africa and iberia inversion of the algarve basin and the sw iberian margin was controlled by both the wsw ene and nw se trending structures formed during the passive margin phase the evolution of the algarve basin during the mesozoic and up to the paleogene is documented in this paper through paleogeographic reconstructions based on field geology and reinterpretation of offshore seismic and well data

application of lipid biomarkers to detect sources of organic matter in mud volcano deposits and post eruptional methanotrophic processes in the gulf of cadiz ne atlantic abstract the gulf of cadiz is known as an active mud volcano mv and fluid venting region discovered during the training through research ttr expeditions in 1999 ttr 9 here we report a study of lipid biomarkers in the mud breccia matrix from three mvs in this region the lipid biomarker composition revealed strong compositional resemblance as well as similar thermal maturity properties for the studied mvs this indicates that the primary source of the erupted material for these mvs is located in similar litho stratigraphic units in both areas upward migrated fluid went through the sedimentary series of the allochthonous olistrostrome and through the upper cretaceous horizons the relatively immature characteristics of organic matter from both mud breccia rock clasts and matrix together with the more mature properties of hydrocarbon gases from the same sampling locations indicate that source strata for the initial fluid are located deeper in the subsurface than the source strata for the erupted sedimentary material the presence of microbial lipid biomarkers derived from archaea and sulfate reducing bacteria in the mud breccias revealed anaerobic oxidation of methane aom as a dominant microbial process in these habitats our data suggest that the aom derived lipids are at least in part fossil in origin i e they do not reflect active aom communities and thus reflect aom activity in these mvs integrated over time the low concentrations of aom related biomarkers in these mvs suggest relatively low intensities of aom in the studied mv deposits the distribution of biomarkers suggests an abrupt and brief cold seep activity in the past and the absence of a continuous methane influx in the studied locations

a pliocene mixed contourite turbidite system offshore the algarve margin gulf of cadiz seismic response margin evolution and reservoir implications abstract a buried mixed contourite turbidite system has been identified in the pliocene succession of the algarve basin in the northern gulf of cadiz this margin is currently dominated by the mediterranean outflow water and associated contourite deposition analysis of seismic data along with careful geographical and oceanographic reconstructions of the margin show the gradual move from a turbidite dominated towards a contourite dominated margin and the subsequent birth of an impressive elongate mounded contourite drift system the faro albufeira drifts the contourite drift can be distinguished from down slope turbiditic and mass transport deposits based on the acoustic character distribution analysis and through careful margin reconstruction in the earliest pliocene seismic unit p1 has been interpreted as a dominant down slope most likely turbidite system sourced mainly from the northeast there is clear evidence of contourite reworking at seismic unit p2 where upslope progradation and a sheeted morphology are observed high amplitude reflections are thought to be a result of more vigorous bottom currents in the early upper pliocene that were capable to redistributing coarser sediments however in the northeast of the study area a thick sequence of chaotic seismic facies has been interpreted as mass transport deposits sourced from the north indicate that the bottom currents were unable to dominate over the entire margin due to high down slope clastic influx semi transparent seismic unit p3 indicates that the upper pliocene initially experienced a reduction in bottom current intensity however upslope progradation shows that a mixed system was maintained above the base quaternary discontinuity ca 2 6 ma highly erosive discontinuities and high amplitude seismic reflections are evidence of pronounced intensification of the mediterranean outflow water and a move to a fully contourite dominated slope mixed turbidite contourite systems such as the one identified in the algarve basin could provide impressive petroleum potential where downslope clastics are winnowed and reworked by bottom water currents to leave good reservoir properties here we present a conceptual model for sheeted drifts as hydrocarbon reservoirs in the subsurface

the betic orogen and the iberian african boundary in the gulf of cadiz geological evolution central north atlantic abstract the study of the gulf of cadiz on the basis of multichannel seismic profiles and wells illustrates the stratigraphy and tectonics the evolution of the southern iberian margins was more complex than in most north atlantic margins since it entailed several phases of rifting convergence and strike slip motions three main tectonic provinces surround the internal zones of the gibraltar arc orogenic belt these include in the iberian margin of the gulf of cadiz the flysch units of the campo de gibraltar complex the betic external zones and the neogene basins of the guadalquivir valley fault bounded blocks of flysch and subbetic units crops out over large areas of the southeastern iberian shelf the basement of the northwestern area in contrast is represented by the paleozoic rocks of the hercynian massif of iberia half graben structures determined the main structural trends of the margin during the mesozoic which were affected by inversion structures during the neogene compressional stages the mesozoic and lower cenozoic units are best observed in wells and seismic profiles from the northern area these units are either obscure below a thick olistostrome deposit or are absent in most of the rest of the gulf of cadiz seven lithoseismic units from triassic to upper oligocene and another seven neogene and quaternary units are identified based on the relationship to the depositional sequence and the emplacement of the olistostrome the first tectonic phase was characterized by a passive margin which was controlled by the development of half graben extensional structures and carbonate platforms this evolution comprises the mesozoic and early cenozoic ocean spreading in the north atlantic induced extensional tectonics which deformed the cretaceous syn tectonic post aptian deposits increased amounts of terrigenous materials were supplied to the margin from aptian to albian times controlling depositional patterns while terrigenous siliciclastic facies replaced the jurassic carbonate platforms from middle eocene to early miocene the margin was influenced by the relative motions of iberia and africa and the development of the alpine orogeny the iberian african boundary in the gulf of cadiz experienced transpression and the mesozoic basins probably underwent subduction the emplacement of an olistostrome took place in the gulf of cadiz towards the central atlantic basin plains during the tortonian the end of the olistostrome emplacement during the late miocene coincides with accelerated tectonic subsidence while thick progradational and aggradational depositional sequences were developed the occurrence of closely juxtaposed regions of compression and extension during the miocene may reflect the westwards progression of the gibraltar arc mountain front over a subducting thinned tethys crust when the motion between iberian and africa was n s to nnw sse oriented the migration of the arcuate mountain front into the eastern gulf of cadiz is attributed to a mechanism of collision induced delamination of the mantle lithosphere in contrast to the betic rif belts however there is no geophysical evidence to postulate that continental collision took place in the gulf of cadiz during the cenozoic

authigenic barite records of methane seepage at the carlos ribeiro mud volcano gulf of cadiz submarine mud volcanoes mvs are dynamic features that episodically expel gas charged fluids and mud onto the seafloor transferring various chemical constituents into the overlying water column the temporal variability in mv activity is however poorly understood so their importance as a source of methane ch4 and higher hydrocarbons for the oceanic carbon budget although thought to be significant cannot be properly constrained in this study the history of fluid and gas seepage at the carlos ribeiro mv gulf of cadiz is assessed via geochemical analyses and transport reaction modelling of pore fluids and barium ba rich layers ba fronts in sediment cores recovered along a transect from the eye to the periphery of the mv x ray fluorescence data reveal that ba fronts are absent at the eye while a single front with up to 1740 ppm ba is present at the margin of the summit three ba fronts occur at 45 85 and 130 cm depth within a mudflow to the southeast of the crater spectrometric analyses indicate that barite is the ba rich mineral in these layers upward advecting pore fluids are enriched in barium but depleted in calcium ca2 relative to seawater modelling of the ba2 and ca2 pore fluid profiles indicates that the positions of the ba fronts reflect both the present day hydrodynamic conditions as well as higher fluxes of methane in the past fluid advection appears to have decreased since 340 cal yr before present but degassing of the mudflow is ongoing and is potentially an important source of ch4

the giant chaotic body in the atlantic ocean off gibraltar new results from a deep seismic reflection survey abstract the central eastern atlantic continental slope and parts of the horseshoe and seine plains west of the gibraltar arc are occupied by a seismically chaotic body this paper reports new deep seismic reflection data that allows a first assessment of the three dimensional geometry of the body and discussion of its evolution the seismically chaotic body is huge in area and volume and complex in origin comprising mainly gravity deposits debris flows olistostromes in the northern sector and tectonic melanges in the southern where the body forms part of a south verging accretionary prism related to africa iberia convergence submarine extension of the rifian system of n africa the northern part up to several tens of thousands of cubic kilometers in volume results from the superposition of a few endoolistostromes probably of miocene age centripetally discharged onto the eastern horseshoe plain and adjacent slope from structural highs generated during the late miocene paroxismal phase of iberia africa convergence submarine flows were triggered along regional continuous slopes up to 100 km in length their volume largely exceeds previously reported marine occurrences of gravity flow deposits the outstanding examples of which occur in intraplate settings hot spots old passive margins at the boundaries lateral or frontal ramps of the southern tectonic melange complex smallscale olistostromes of pliocene age also occur recalling the so called precursory olistostromes of collisional chains the gravity and tectonic structures occur in a convergent plate setting with low shortening rates and their eventual fate will be incorporation with disruption in a collisional chain their volume and extent appear comparable to those that can be calculated for huge chaotic units in orogens such as the argille scagliose liguride complex of the apennines in which disrupted olistostromes and tectonic melanges coexist

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