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

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

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

critical flow in rockbed streams with estimated values for manning s n the writer presented interesting experimental observations the discusser would like to comment on some aspects of the paper regarding critical flow or more accurately near critical flow critical slope calculations and boundary shear stress distributions in undular flows he hopes that the information will assist in refining the author s work

the educational value of the history of geomorphology abstract the history of geomorphology can be a valuable tool for educating students of geomorphology the largest and most available record of what geomorphologists thought and did in the past is their original published work also available are some fine papers on the history of geomorphology that review and analyze primary sources and past events in the discipline in order to generalize explain trends and reveal historical associations interest in the history of the discipline varies widely of course among geomorphologists nevertheless several educational benefits in addition to explaining the academic affiliation of geomorphology with two separate disciplines can be derived from incorporating the history of geomorphology into university level geomorphology courses history is a popular subject and presenting some geomorphic concepts using a historical approach can help to maintain or stimulate student interest because of the tendency for older literature to contain more qualitative description and methodological detail than more recent literature undergraduate students may comprehend some concepts better from older papers by reading the original literature students determine for themselves what previous practitioners accomplished rather than relying solely on the interpretations of others reading original literature also helps students realize that older does not mean less intelligent and that like the critical reading of recent geomorphic literature the critical reading of historic geomorphic papers can provide a wealth of new research ideas a thematic set of the historic literature can be used to demonstrate to students the scientific method the origin testing and evolution of hypotheses how explanations develop in this field and that science proceeds through individuals working in a sociological context including history of geomorphology in the curriculum helps to place contemporary research in the perspective of the past as well as the perspective of the future

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

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

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

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

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

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

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

crustal structure of atlantic fracture zones iii the tydeman fracture zone summary an extensive seismic survey was performed in march 1982 on two different parts of the tydeman fracture zone this survey consisted of two separate experiments a reversed seismic refraction experiment centred on 36 n 26 w using ocean bottom seismometers and free floating sonobuoys which was shot with explosives and a two ship multichannel experiment of the expanding spread type also shot with explosives at approximately 36 n 23 30 w these experiments are the first and to date only wide angle seismic surveys of fracture zone crust older than 25 myr slope intercept and tau p inversion of the obs sonobuoy travel time data indicates low crustal and mantle seismic velocities within the fracture zone these results are confirmed and refined by a more detailed analysis using raytracing and synthetic seismogram modelling for laterally varying structures which implies velocities for the upper mantle beneath the fracture zone of 7 3 7 5 km s 1 similar analysis of a line perpendicular to the fracture zone reveals the transition from normal mantle velocities to the low values found beneath the fracture zone and suggests that the width of the anomalous region is approximately 12 km the geometry of the obs experiment permitted a delay time analysis and a further simple tomographic inversion of the traveltime data which produced an approximate image of the low velocity mantle region associated with the fracture zone the width of the region of anomalous crust is the same as the width of the sediment filled trough which marks the fracture zone location at the line intersection analysis of the two ship seismic data from a site some 240 km to the east also by synthetic seismogram modelling gave similar results to the obs sonobuoy experiment with a mantle velocity of 7 2 km s 1 detected beneath the crust which possessed lower than normal seismic velocities the close seismogram spacing of less than 0 1 km allowed us to transform the dataset into the tau p domain and enabled us to detect arrivals obscured in the x t domain by the reflected water wave both experiments indicate low crustal seismic velocities within the fracture zone together with unusually low velocities of 7 2 7 5 km s 1 in the upper mantle comparison with other fracture zone surveys reveals that the seismic velocity in the upper mantle beneath fracture zones decreases with increasing age we attribute this to serpentinization of mantle peridotite brought about by hydrothermal circulation to the base of the fractured and faulted crust beneath the fracture zone

analogue modelling of brittle shear zone propagation across upper crustal morpho rheological heterogeneities abstract crustal shearing of pre tectonic weak bodies e g salt pillows igneous pockets associated with basement strike slip fault systems is common in nature yet the structural pattern arising from the interference between a brittle shear zone and such weak rheological heterogeneities is still not fully explained in the present work different sand box analogue modelling experiments were carried out to better understand not only the shear zone perturbation effect caused by a strictly rheologic viscous weak anomaly but also the one arising from a combined morphological perturbation i e traverse topographic crest obtained results show that the development of different shear zone deformation patterns is in each case essentially determined by the interplay between i ductile wrenching strain accommodation in the viscous weak body and ii coincidence vs mismatch between main brittle viscous rheological boundaries and crest related topographic steps comparison of the experimentally obtained results with complying natural examples is further discussed and essentially focused on the ne atlantic tectono magmatic interference between the gloria transform fault and the tore madeira submarine ridge

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

rockbed wear at a flow convergence zone in fifteen mile creek niagara peninsula ontario sculpted bedforms developed in rock channels must be considered within the context of temporally and spatially persistent flow dynamics a shallow meandering channel 14 m long 30 50 cm wide 3 4 cm deep with raised margins has been eroded into resistant irondequoit limestone forming the streambed of fifteen mile creek ontario potholes 18 cm deep 25 cm wide 35 cm long within the channel increase in size downstream subaerial solution of the limestone and fracture patterns within the irondequoit limestone are not responsible for this large dolostone blocks within the channel stabilize patterns of vorticity that form in the lee of the blocks during highflows when flow is about 1 3 m deep vortices migrating downstream along a major flow junction have eroded the distinctive channel small irregularities in the bed may enlarge to potholes by attracting vortices and undergoing erosional wear as they pass the feature may have developed within several hundred to several thousand years but likely d

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

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

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

development of glacial valley cross sections under conditions of spatially variable resistance to erosion abstract a simulation model that examines the development of valley cross profiles as a result of glacial erosion has been constructed by placing a finite element model for ice flow within an iterative program that modifies channel cross section form on the basis of an erosion equation the simulation model has previously been used to examine how different flow laws erosion laws and ice discharge histories affect medium scale landform development this allows us to better understand the development of glacial landforms and also provides a critical test for erosion and flow laws within the erosion component of the simulation program it is possible to specify spatial variations in bedrock erodibility in order to examine the effect of spatially variable lithologies on form development as an initial test of form development under these conditions a central area of enhanced erodibility is placed in the valley center in this case active glacial channels develop that are narrower and deeper than those which occur with homogenous bedrock this change in form occurs as long as the weaker zone is small enough that the active channel does not eventually narrow to a stage such that it is entirely contained within the weaker material and as long as it is large enough that it continues to have a significant impact on overall discharge through the section form variation occurs because the weaker central zone enhances the lateral gradient of the erosion rate which is critical for form development a central area of more erodible bedrock changes not only the form of the active glacial channel but also the convexity of side slopes that are left above the glacier as it erodes downwards into the landscape the absence of a classic u shaped cross section should not be taken a priori to suggest either absence of glacial erosion or ineffective glacial erosion rather the combination of active glacial and hillslope processes in a landscape with spatially and temporally variable ice discharge and rock characteristics can create a diverse set of cross section forms

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

imaging active faulting in a region of distributed deformation from the joint clustering of focal mechanisms and hypocentres application to the azores western mediterranean region abstract the matching between linear trends of hypocentres and fault planes indicated by focal mechanisms fms is frequently used to infer the location and geometry of active faults this practice works well in regions of fast lithospheric deformation where earthquake patterns are clear and major structures accommodate the bulk of deformation but typically fails in regions of slow and distributed deformation we present a new joint fm and hypocentre cluster algorithm that is able to detect systematically the consistency between hypocentre lineations and fms even in regions of distributed deformation we apply the method to the azores western mediterranean region with particular emphasis on western iberia the analysis relies on a compilation of hypocentres and fms taken from regional and global earthquake catalogues academic theses and technical reports complemented by new fms for western iberia the joint clustering algorithm images both well known and new seismo tectonic features the azores triple junction is characterised by fms with vertical pressure p axes in good agreement with the divergent setting and the iberian domain is characterised by nw se oriented p axes indicating a response of the lithosphere to the ongoing oblique convergence between nubia and eurasia several earthquakes remain unclustered in the western mediterranean domain which may indicate a response to local stresses the major regions of consistent faulting that we identify are the mid atlantic ridge the terceira rift the trans alboran shear zone and the north coast of algeria in addition other smaller earthquake clusters present a good match between epicentre lineations and fm fault planes these clusters may signal single active faults or wide zones of distributed but consistent faulting mainland portugal is dominated by strike slip earthquakes with fault planes coincident with the predominant nne ssw and wnw ese oriented earthquake lineations clusters offshore sw iberia are predominantly strike slip or reverse confirming previous suggestions of slip partitioning

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

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

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

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

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

active transform faults in the gulf of guinea insights from geophysical data and implications for seismic hazard assessment the seismotectonics of western africa show the occurrence of major earthquakes e g 1636 southwestern ghana 1855 offshore monrovia 1939 offshore accra and 1983 gaoual guinea and prominent off

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

monte carlo approach to assess the uncertainty of wide angle layered models application to the santos basin brazil abstract in the santos basin brazil two parallel wide angle refraction profiles show different crustal structures one shows moderate crustal velocity gradient and a clear moho with topography the other has an anomalous velocity zone and no clear moho reflections this has large implications on the geological and geodynamical interpretation of the basin model uncertainties must be excluded as a source of these differences we developed vmontecarlo a tool to assess model uncertainty of layered velocity models using a monte carlo approach and simultaneous parameter perturbation using all picked refracted and reflected arrivals it gives insights into the acceptable geological interpretations allowed by data and model uncertainty through velocity depth plots that provide a the velocity depth profile range that is consistent with the travel times b the random model that provides the best fit keeping most of the observations covered by ray tracing c insight into valid models dispersion d main model features unequivocally required by the travel times e g first order versus second order discontinuities and velocity gradient magnitudes e parameter value probability distribution histograms vmontecarlo is seamlessly integrated into a rayinvr based modelling work flow and can be used to assess final models or sound the solution space for alternate models and is also capable of evaluating forward models without the need for inversion thus avoiding local minima that may trap the inversion algorithms and providing information for models still not well parametrised results for the brazilian models show that the imaged structures are indeed geologically different and are not due to different interpretations of the same features within the model uncertainty bounds these differences highlight the strong heterogeneity of the crust in the middle of the santos basin where the rift is supposed to have failed

paleohydrology of late pleistocene superflooding altay mountains siberia cataclysmic flooding is a geomorphological process of planetary significance landforms of flood origin resulted from late pleistocene ice dammed lake failures in the altay mountains of south central siberia peak paleoflows which exceeded 18 x 10 6 cubic meters per second are comparable to the largest known terrestrial discharges of freshwater and show a hydrological scaling relation to floods generated by catastrophic dam failures these seem to have been earth9s greatest floods based on a variety of reconstructed paleohydraulic parameters

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

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

strike slip faults mediate the rise of crustal derived fluids and mud volcanism in the deep sea we report on newly discovered mud volcanoes located at 4500 m water depth 90 km west of the deformation front of the accretionary wedge of the gulf of cadiz and thus outside of their typical geotectonic environment seismic data suggest that fluid flow is mediated by a 400 km long strike slip fault marking the transcurrent plate boundary between africa and eurasia geochemical data cl b sr 87 sr 86 sr d 18 o dd reveal that fluids originate in oceanic crust older than 140 ma on their rise to the surface these fluids receive strong geochemical signals from recrystallization of upper jurassic carbonates and clay mineral dehydration in younger terrigeneous units at present reports of mud volcanoes in similar deep sea settings are rare but given that the large area of transform type plate boundaries has been barely investigated such pathways of fluid discharge may provide an important yet unappreciated link between the deeply buried oceanic crust and the deep ocean

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

deep water seamounts a potential source of tsunami generated by landslides the hirondelle seamount ne atlantic abstract submarine mass failures represent one of the most significant marine geo hazards their importance as a major contributor to tsunami generation and hazard has been recognized over the last 20 30 years this study investigates a newly mapped submarine landslide the south hirondelle landslide shl and its potential to generate a tsunami and to threat the surrounding coasts the shl is located 150 km offshore south west iberia along the southern flank of the hirondelle seamount here available swath bathymetry and one multichannel seismic profile show the presence of large geometrically well constrained deep water landslide deposit of about 500 km 3 and its associated scar the failure likely occurred in one single event and according to a detailed numerical modelling of the landslide dynamics and of the resulting water propagation the mass failure generated a mega tsunami with significant impact along the surrounding coastal areas of iberia and morocco this result strongly supports the inclusion of tsunami induced by deep water submarine landslides in the marine geo hazard assessment of the north east atlantic region

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

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

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

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

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

palaeoclimate and palaeoenvironmental responses in the western mediterranean over the last 140 ka evidence from mallorca spain climatic change in the western mediterranean basin over the last 140 ka has been investigated from sediment and soil sequences along the coast of the northeastern part of mallorca spain palaeoclimate and palaeoenvironments are reconstructed from sediment and soil structures marine and terrestrial molluscan fauna grain size distributions mineral magnetics sediment and soil chemistry sem and oxygen isotope values dating is by osl and amino acid geochronometry along with marine molluscan faunal assemblages all oxygen isotope stages and substages from ois 6 to 1 are recorded mean annual temperatures m a t of c 19 2 c for the climatic optimum of the last interglacial compare with c 17 3 c for the present and are associated with high sea level extensive woodland and stable soils although wide temperature ranges are detected during this stage with values as low as c 10 3 c through ois 5d to 5a m a t fluctuated from 8 2 c to 17 9 c low sea level open vegetation effective river activity and aeolian sand and dust deposition characterize 5d and 5b while relatively high sea level and soil development on stable landscapes dominate ois 5c and 5a ois 4 is represented by m a t of c 4 9 c with aeolian sand and loess deposition limited river activity and weak soil development loess deposition persisted into ois 3 but at this time the m a t was c 13 c and with higher humidity and vegetation growth during the lgm ois 2 m a t was c 8 1 c and effective river slope and aeolian processes along with associated with limited vegetation cover were responsible for the most extensive changes in the landscape major rates of change occur during periods of climatic deterioration when the high levels of geomorphic energy were imposed on a landscape undergoing a breakdown of vegetation cover

origin and geochemical evolution of the madeira tore rise eastern north atlantic the madeira tore rise located 700 km off the nw african coast forms a prominent ridge in the east atlantic the age and origin of the rise are controversial this study presents major and trace element sr nd pb hf isotope and 40ar 39ar age determinations from volcanic rocks dredged from different sites along the rise in addition isotopic compositions of rock samples from great meteor seamount in the central atlantic are presented the new radiometric and paleontologically constrained ages identify two major episodes of volcanism the first is the base of the rise circa 80 to 95 ma and the second is seamounts on the rise 0 5 16 ma it is proposed that interaction of the canary hot spot with the mid atlantic spreading center formed the deep basement of the madeira tore rise and the j anomaly ridge west of the atlantic spreading center in the mid cretaceous age and geochemical data and plate tectonic reconstructions suggest however that the recovered late cretaceous volcanic rocks represent late stage volcanism from the time when the madeira tore rise was still close to the canary hot spot long after moving away from the influence of the canary hot spot the madeira tore rise was overprinted by late cenozoic volcanism miocene to pleistocene volcanism at the northern end of the rise can be best explained by decompression mantle melting beneath extensional sectors of the azores gibraltar fracture zone african eurasian plate boundary the geochemical compositions of these volcanic rocks suggest that the magmas were variably contaminated by enriched material within or derived by melting of enriched material underplated at the base of the lithosphere possibly originating from the cretaceous canary plume alternatively these late cenozoic volcanic rocks may have derived from decompression melting of enriched pyroxenitic eclogitic material in the upper mantle isotopically more depleted pliocene to pleistocene volcanism at the southern end of the madeira tore rise may be related to the nearby madeira hot spot

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

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

are subduction zones invading the atlantic evidence from the southwest iberia margin subduction initiation at passive margins plays a central role in the plate tectonics theory however the process by which a passive margin becomes active is not well understood in this paper we use the southwest iberia margin sim in the atlantic ocean to study the process of passive margin reactivation currently there are two tectonic mechanisms operating in the sim migration of the gibraltar arc and africa eurasia convergence based on a new tectonic map we propose that a new subduction zone is forming at the sim as a result of both propagation of compressive stresses from the gibraltar arc and stresses related to the large scale africa eurasia convergence the gibraltar arc and the sim appear to be connected and have the potential to develop into a new eastern atlantic subduction system our work suggests that the formation of new subduction zones in atlantic type oceans may not require the spontaneous foundering of its passive margins instead subduction can be seen as an invasive process that propagates from ocean to ocean

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

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