seismicity and active tectonics in the alboran sea western mediterranean constraints from an offshore onshore seismological network and swath bathymetry data seismicity and tectonic structure of the alboran sea were derived from a large amphibious seismological network deployed in the offshore basins and onshore in spain and morocco an area where the convergence between the african and eurasian plates causes distributed deformation crustal structure derived from local earthquake data suggests that the alboran sea is underlain by thinned continental crust with a mean thickness of about 20 km during the 5 months of offshore network operation a total of 229 local earthquakes were located within the alboran sea and neighboring areas earthquakes were generally crustal events and in the offshore domain most of them occurred at crustal levels of 2 to 15 km depth earthquakes in the alboran sea are poorly related to large scale tectonic features and form a 20 to 40 km wide nne ssw trending belt of seismicity between adra spain and al hoceima morocco supporting the case for a major left lateral shear zone across the alboran sea such a shear zone is in accord with high resolution bathymetric data and seismic reflection imaging indicating a number of small active fault zones some of which offset the seafloor rather than supporting a well defined discrete plate boundary fault moreover a number of large faults known to be active as evidenced from bathymetry seismic reflection and paleoseismic data such as the yusuf and carboneras faults were seismically inactive earthquakes below the western alboran basin occurred at 70 to 110 km depth and hence reflected intermediate depth seismicity related to subducted lithosphere

regolith breccia allan hills a81005 evidence of lunar origin and petrography of pristine and nonpristine clasts mno feo ratios in pyroxene texture abundant brown swirly glass in general typical of lunar regolith breccias and overall composition 75 plagioclase all indicate that allan hills a81005 is of lunar origin it is presumably from a heretofore unsampled region of the moon it differs in detail from other regolith samples e g it has exceptionally low contents of na and kreep a pristine clast contains exceptionally coarse augite compared to similar apollo samples alha81005 is not perceptibly more shocked than typical apollo regolith breccias therefore its discovery on earth lends further credence to suggestions that snc achondrites were derived by impact ejection from mars

constraints on the frontal crustal structure of a continental collision from an integrated geophysical research the central western betic cordillera sw spain 1 melange rocks outcrop widely in the central and western frontal sectors of the betic cordillera as in many other collisional orogens where they form part of the accretionary wedges extrusion of triassic plastic clays and evaporites was favored by the progressive accretion of the betic external zones mixing rocks of different provenance and forming a synorogenic frontal melange unit mt data coupled with gravity data are a valid combined methodology to characterize the geometry of these melange units since the characterization of plastic rocks geometry is usually uncertain using seismic techniques the results correlate well with known geological features sedimentary basins calcareous ranges evaporitic rocks and reveal the deep geometry a resistive body slightly dipping toward the se points to the continuity of the iberian massif below the guadalquivir basin 2 and the external zones 6 8 to the south gravity models show the iberian continental crust subducting below the internal zones with a roughly 20 35 slope the main conductive bodies are related to the location of evaporitic rocks involved in the frontal melange they overlie the iberian massif and southwards the frontal jurassic and cretaceous limestone sequences of the external and median subbetics in this setting thick plastic rock units placed above the foreland could act as a lubricant facilitating continental subduction and being progressively accreted toward a frontal melange

heat flow heat production and lithospheric thermal regime in the iberian peninsula abstract the first heat flow and heat production maps of the iberian peninsula and its margins using previously acquired data are presented the surface heat flow map includes 553 determinations carried out on water and mining exploration wells oil exploration wells and on the seafloor the surface heat flow varies noticeably from the iberian mainland 65 10 mw m 2 to the atlantic and the mediterranean margins where the heat flow reaches values of about 40 50 mw m 2 and 80 100 mw m 2 respectively the heat production map consists of 664 determinations carried out on rock samples from the variscan iberian massif and the betics the higher values are obtained for granitic rocks 2 5 3 5 w m 3 whereas metasediments and basic rocks reach values of 1 2 5 w m 3 and nearly zero respectively the lithospheric structure deduced for the iberian peninsula by combining heat flow heat production and elevation data indicates that most of the iberian mainland is characterized by a lithospheric thickness of 110 5 km this value is maintained across the west atlantic margin whereas towards the mediterranean margin the lithospheric thickness decreases down to 60 40 km this study also suggests that the heat production in the southern variscan iberian massif must be noticeably higher 3 7 0 5 w m 3 than in the rest of the areas in order to fit the measured heat flow crustal thickness and elevation data

geodynamic evolution of the lithosphere and upper mantle beneath the alboran region of the western mediterranean constraints from travel time tomography an edited version of this paper was published by the american geophysical union copyright 2000 agu see also http www agu org pubs crossref 2000 2000jb900024 shtml http atlas geo cornell edu morocco publications calvert2000 htm

heat flow in the balearic and tyrrhenian basins western mediterranean we present the results of three detailed heat flow surveys which are used to investigate the variations of heat flow and age of the balearic and tyrrhenian basins in the western mediterranean analysis of 12 measurements within a 10 km radius from 40 01 4 55 e in the balearic abyssal plain shows a mean heat flow of 92 10 mw m 2 after correction for the effects of sedimentation this value agrees well with the predictions of plate cooling and high extension stretching models for crust of the late oligocene age proposed for the basin a similar survey with 18 measurements around 40 16 n 11 19 e in the western tyrrhenian gives a flux of 134 8 mw m 2 while the third survey of 26 measurements in the southern tyrrhenian abyssal plain at 37 17 n 12 58 e yields a slightly higher heat flow of 151 10 mw m 2 these values are within the range predicted by simple plate cooling models for the late miocene ages of the deep tyrrhenian basin thus our observations suggest that although the mode of crustal formation of these deep marginal basins is less well defined than that of the major ocean basins the thermal signature is similar also the trend of increasing heat flow from west to east through the balearic and tyrrhenian basins is in agreement with models of the formation of the western mediterranean behind an eastwardly migrating trench system in all three areas the measured flux shows significant local variability in the two westernmost surveys this can be attributed to the presence of buried high conductivity salt structures but in the third area se tyrrhenian the magnitude of the variations is several times greater than can be attributed to steady state thermal refraction alone instead evidence exists for localized hydrothermal activity linked to the small topographical relief within the survey area

mineralogy of coexisting pyroxenes in magnesian ureilites and their formation conditions abstract the occurrence of orthopyroxene in two magnesian ureilites found in antarctica yamato y 791538 and lew85440 has been confirmed by electron probe microanalysis and by single crystal x ray diffraction the first coexisting equilibrium pigeonite orthopyroxene pair has been found in y791538 the lew85440 orthopyroxene coexists with minor augite the orthopyroxenes and olivines in y791538 y74659 and lew85440 have almost identical compositions but the ca contents of their coexisting clinopyroxenes differ considerably these phase assemblages can be explained by crystallization of mafic silicates with a similar composition oxygen fugacity and oxygen isotope anomaly at different temperature above 1220 c the high temperature crystallization of refractory mg rich silicates and the possible preservation of the oxygen isotope anomaly of precursor carbonaceous chondrites like materials may be consistent with their formation processes probably through local melting of more matrices than mg rich silicates and crystal growth induced by the planetesimal scale collision

comparisons of antarctic and non antarctic achondrites and possible origin of the differences abstract in order to discuss differences between antarctic and non antarctic achondrites we investigated pairing of mainly the yamato achondrites by mineralogical techniques in addition to five paired specimens of the y74159 type polymict eucrites three new groups distinct from the y74159 type have been recognized one group y791960 of howarditic polymict eucrites is similar to those of non antarctic origin some antarctic monomict eucrites differ in texture and chemistry from the non antarctic specimens more than 50 diogenite specimens recovered from the yamato bare ice field can be grouped into two falls of separate locations compared with 8 from the usa collection and 9 from the non antarctic groups thirty antarctic ureilites are separate falls 2 pairings and their individual masses are small these observations suggest that discussion of differences between the antarctic and non antarctic achondrites largely depends on detection of pairings of the antarctic specimens the pairings of the specimens from victoria land have not been as well characterized as those of the yamato mountains comparisons with non antarctic meteorites show that the antarctic heds and ureilites differ from non antarctic specimens by the following criteria 1 1 polymict eucrites excluding those with howarditic affinity have not been found in the non antarctic collections 2 2 magnesian ureilites and augite bearing ureilites were found only in antarctica the differences may not be attributed to just one factor the above evidence of two unique diogenite falls preserved on the restricted ice fields suggests that the yamato diogenites may represent falls on restricted areas in the distant past the discovery of many small unique eucrites and ureilites suggests that within the large number of small specimens on a bare ice field there is a higher chance of finding unique varieties

new insights from seismic tomography on the complex geodynamic evolution of two adjacent domains gulf of cadiz and alboran sea 1 in this study we present a three dimensional p wave upper mantle tomography model of the southwest iberian margin and alboran sea based on teleseismic arrival times recorded by iberian and moroccan land stations and by a seafloor network deployed for 1 year in the gulf of cadiz area during the european commission integrated observations from near shore sources of tsunamis towards an early warning system ec nearest project the three dimensional model was computed down to 600 km depth the tomographic images exhibit significant velocity contrasts as large as 3 confirming the complex evolution of this plate boundary region prominent high velocity anomalies are found beneath betics alboran sea off shore southwest portugal and north portugal at sublithospheric depths the transition zones between high and low velocity anomalies in southwest and south iberia are associated to the contact of oceanic and continental lithosphere the fast structure below the alboran sea granada area depicts an l shaped body steeply dipping from the uppermost mantle to the transition zone where it becomes less curved this anomaly is consistent with the results of previous tomographic investigations and recent geophysical data such as stress distribution gps measurements of plate motion and anisotropy patterns in the atlantic domain under the horseshoe abyssal plain the main feature is a high velocity zone found at uppermost mantle depths this feature appears laterally separated from the positive anomaly recovered in the alboran domain by the interposition of low velocity zones which characterize the lithosphere beneath the southwest iberian peninsula margin suggesting that there is no continuity between the high velocity anomalies of the two domains west and east of the gibraltar strait

finite frequency rayleigh wave tomography of the western mediterranean mapping its lithospheric structure 1 we present a 3 d shear wave velocity model for the crust and upper mantle of the western mediterranean from rayleigh wave tomography we analyzed the fundamental mode in the 20 167 s period band 6 0 50 0 mhz from earthquakes recorded by a number of temporary and permanent seismograph arrays using the two plane wave method we obtained phase velocity dispersion curves that were inverted for an isotropic vs model that extends from the southern iberian massif across the gibraltar arc and the atlas mountains to the saharan craton the area of the western mediterranean that we have studied has been the site of complex subduction slab rollback and simultaneous compression and extension during african european convergence since the oligocene the shear velocity model shows high velocities beneath the rif from 65 km depth and beneath the granada basin from 70 km depth that extend beneath the alboran domain to more than 250 km depth which we interpret as a near vertical slab dangling from beneath the western alboran sea the slab appears to be attached to the crust beneath the rif and possibly beneath the granada basin and sierra nevada where low shear velocities 3 8 km s are mapped to 55 km depth the attached slab is pulling down the gibraltar arc crust thickening it and removing the continental margin lithospheric mantle beneath both iberia and morocco as it descends into the deeper mantle thin lithosphere is indicated by very low upper mantle velocities beneath the alboran sea above and east of the dangling slab and beneath the cenozoic volcanics

lithospheric structure beneath the alboran basin results from 3d gravity modeling and tectonic relevance a three dimensional gravity modeling combined with integrated heat flow and elevation modeling is conducted to map out the crustal and lithospheric mantle thickness in the alboran basin in the westernmost mediterranean a sediment corrected bouguer anomaly has been derived using a depth to the basement map and densities determined from well logs and seismic data the gravity effect of the base of the lithosphere has been removed from the sediment corrected bouguer anomaly to obtain a crustal bouguer anomaly which has been inverted for crustal thickness the resulting lithospheric structure is further constrained by elevation data under the assumption of local isostasy the low residual elevation anomalies obtained 100 m in average suggest that the area is in local isostasy particularly the medium and long wavelength topography features variations in crustal thickness range from 36 km underneath the betic and rif chains to 12 km beneath the easternmost part of the alboran sea basin in the transition to the south balearic basin in western alboran the moho lies at a rather constant depth of 18 km deepening sharply toward the gibraltar strait down to 30 32 km the base of the lithosphere shallows from 140 km depth in the gibraltar strait to 45 km depth in the easternmost alboran sea lithospheric thinning penetrates to the southeastern side of the iberian margin crosscutting topographic highs of the central and eastern betic chain our results favor mantle delamination produced by detachment and subsequent peeling away of the lithospheric mantle rather than convective removal of the lithospheric mantle either by orogenic collapse or detachment and sinking of a lithospheric slab

a rapid method to map the crustal and lithospheric thickness using elevation geoid anomaly and thermal analysis application to the gibraltar arc system atlas mountains and adjacent zones we present a method based on the combination of elevation and geoid anomaly data together with thermal field to map crustal and lithospheric thickness the main assumptions are local isostasy and a four layered model composed of crust lithospheric mantle sea water and the asthenosphere we consider a linear density gradient for the crust and a temperature dependent density for the lithospheric mantle we perform sensitivity tests to evaluate the effect of the variation of the model parameters and the influence of rms error of elevation and geoid anomaly databases the application of this method to the gibraltar arc system atlas mountains and adjacent zones reveals the presence of a lithospheric thinning zone sw ne oriented this zone affects the high and middle atlas and extends from the canary islands to the eastern alboran basin and is probably linked with a similarly trending zone of thick lithosphere constituting the western betics eastern rif rharb basin and gulf of cadiz a number of different even mutually opposite geodynamic models have been proposed to explain the origin and evolution of the study area our results suggest that a plausible slab retreating model should incorporate tear and asymmetric roll back of the subducting slab to fit the present day observed lithosphere geometry in this context the lithospheric thinning would be caused by lateral asthenospheric flow an alternative mechanism responsible for lithospheric thinning is the presence of a hot magmatic reservoir derived from a deep ancient plume centred in the canary island and extending as far as central europe

integrated lithospheric modeling combining thermal gravity and local isostasy analysis application to the ne spanish geotransect a two dimensional algorithm to determine the steady state thermal structure of the lithosphere that integrates thermal gravity and local isostasy analyses is presented gravity analyses together with seismic data are used to constrain spatial variations in density and crustal structure while absolute elevation is used to determine the lithospheric mantle thickness the calculation is performed using a finite element technique that links the different physical equations the program optionally calculates the temperature at any material boundary and with given rheological parameters the strength distribution and the total lithospheric strength in selected columns we apply the algorithm to the northeastern spanish geotransect which extends from the pyrenees to the balearic promontory and along which a strong variation in crustal and lithospheric thickness is evident we assess the use of two different inferred density models for the lithospheric mantle the first assumes a linear decrease in density with increasing temperature using the asthenospheric density as a reference the second model assumes a constant density for the whole lithospheric mantle although conceptually the two hypotheses differ substantially the results obtained do not show significant differences lithospheric thicknesses of 120 130 km below the pyrenees 60 65 km in the valencia trough and 65 75 km below the balearic promontory are deduced in all cases the mean lithospheric mantle density has to be 40 60 kg m 3 higher than the asthenospheric density the algorithm is shown to be a powerful tool in lithospheric thermal modeling especially in areas where surface heat flow is poorly constrained because of the temperature density elevation relationship

laboratory measurements of seismic p wave velocities on rocks from the betic chain southern iberian peninsula abstract ultrasonic laboratory measurements of seismic compressional velocities vp carried out on rock samples encompasing a wide range of metamorphic grades and lithologies from the betic chain are presented measurements were performed at a high confining pressure on a total of 12 representative samples of the alboran domain internal betics p wave mean velocities referred to pressure conditions of 280 10 mpa range from 5 82 to 7 13 km s 1 the maximum values corresponding to high amphibolite facies metapelites garnet gneiss under these pressure conditions vp anisotropies range from 1 2 to 16 1 with the lowest values being associated with granitic rocks and the highest values associated with metapelites from the upper structural units p wave velocities were calculated for prevailing p t conditions at depths of 10 20 30 and 40 km we discuss the significance of the analyzed rock types in terms of metamorphic grade and derived vp at different crustal levels p wave velocities and densities measured on low to middle and middle to high grade metamorphic rocks are compatible with values obtained from previous seismic and gravity studies for the upper and middle crust respectively at lower crustal levels the integration of our measurements with other geophysical data suggests the presence of partial melting of high grade metamorphic rocks the estimation of the impedance contrast from the measured physical properties suggests that the reflective character of the mid lower crust is associated with shear mylonitic zones and or fine scale lithological layering enhanced by partial melting

a high electrical conductive zone at lower crustal depth beneath the betic chain spain abstract magnetotelluric data with periods of up to 2000 s were collected and the conductivity structure of the crust and upper mantle was imaged in the central betic chain interpretation of the impedance tensor components and the geomagnetic transfer functions was made in terms of a two dimensional resistivity model along a nw se profile from the guadalquivir foreland basin and across the boundary between the external and the internal betics elongated high conductivity zones in the upper and middle crust related to fluid circulation along both sediments and faults were detected the conductors in the upper crust coincide with the areal distribution of the cenozoic and mesozoic sedimentary rocks from the guadalquivir basin and the external betics and the conductors in the middle crust with the major miocene extensional detachments affecting the basement rocks of the internal betic zone a high conductivity zone at lower crustal levels beneath the outer internal betics was also detected next to an area where earlier seismic reflection data had imaged a duplication of the reflective moho this conductor was interpreted as partial melting of a southeast dipping subducted iberian lower crust the geodynamic significance of this subduction depends mainly on whether the overthrusting lower crust belongs to the alboran domain or to the iberian plate in the former case it would correspond to a major plate boundary and in the latter to an intraplate feature

delamination in collisional orogens thermomechanical modeling 1 modes of mantle delamination in collision zones are discussed in the light of 2d numerical modeling freely evolving thermomechanical models take into account phase changes in the mantle and melting in the crust distinct modes are identified in which orogens undergo mantle delamination concurrently or after collision delamination propagates along the moho of the subducted plate together with the retreating trench provided that slab pull is sufficient and that the meta stability of the crust lithospheric mantle interface is overcome topography is an instantaneous response to delamination and migrates with the focused and localized separation between crust and lithospheric mantle delamination front early exhumation of high pressure rocks is followed by exhumation of high temperature partially molten rocks convective stabilization of delamination outlasts slab break offs and impedes renewed build up of mechanically strong lithospheric mantle by static cooling parameter studies investigate the sensitivity of delamination to the age of the subducted oceanic plate the collision rate the mantle rheology and explore the effects of melting and sedimentation mantle flow patterns that form around delamination and subduction are sensitive to and often limited by the semi permeable 670 km spinel perovskite transition demonstrating that phase changes are essential factors delamination may account for the formation of complex boundary zones between continents such as between africa and europe in the mediterranean where transient compression and extension the lack of mantle lithosphere and the exhumation of high grade metamorphic core complexes are features also obtained in our modeling results

lithospheric structure of the gorringe bank insights into its origin and tectonic evolution 1 the gorringe bank is a 5000 m high seamount near the atlantic coast of iberia characterized by a 9 m high geoid anomaly and a 120 mgal bouguer anomaly relative to the surrounding abyssal plains it has been linked to a nw directed thrust carrying exhumed upper mantle rocks and transitional crust on top of flexed down eurasian oceanic crust along the tagus abyssal plain however estimations of crustal shortening have yielded dissimilar results and the deep structure of the ridge remains highly unknown we present a restored cross section and a new model of the lithospheric structure based on gravity geoid elevation and the presence of serpentinized peridotites at least 20 km of shortening took place along a flat ramp flat thrust fault and the density structure of the lithosphere is consistent with mantle serpentinization varying from 70 at the surface to 20 at 14 km depth and 0 at 40 km the topographic relief and gravity anomalies are explained by assuming a flexural isostatic model with an elastic thickness te of 30 km the evolution of the gorringe bank since the late jurassic is interpreted in relation to eurasia africa north america plate motion in four stages 1 transtension between newfoundland iberia and africa which generated small oceanic basins and mantle exhumation 2 opening of the north atlantic and seafloor spreading at the nw side of the exhumed gorringe which produced gabbro intrusions and serpentinization 3 a quiescent tectonic period dominated by subsidence and sediment accumulation and 4 a transpressional plate boundary between eurasia and africa with nw directed subcrustal thrusting and generation of the present gorringe relief

heat flow in the western mediterranean thermal anomalies on the margins the seafloor and the transfer zones the western mediterranean basin has been formed by miocene back arc extension and is underlain by a thin and young lithosphere this young lithosphere is warm as testified by an overall elevated offshore heat flow heat flow within the western mediterranean is however highly variable and existing data are unevenly distributed and poorly studied in the central part of the liguro provencal and algero balearic basins this central part is floored by a young oceanic crust bordered by different continental margins cut by transform faults and filled by up to 8 km of sediments we present a total of 148 new heat flow data collected during the medsalt and westmedflux cruises in 2015 and 2016 and aligned along seven regional profiles that show an important heat flow variability on the basin scale but also locally on the margins a new heat flow map for the western mediterranean outlines the following regional features 1 a higher average heat flow in the algero balearic basin compared to the liguro provencal basin 94 13 mw m2 and 78 16 mw m2 respectively and 2 a regional thermal asymmetry in both basins but with opposed heat flow trends up to 20 of this heat flow difference can be explained by sediment blanketing but age and heterogeneity of ocean crust due to an asymmetric and polyphased opening of the basins are believed to have given the major thermal imprint estimates of the age of the oceanic crust based on the new heat flow suggest a considerably younger west algerian basin 16 23 ma compared to the east algerian basin and the west sardinia oceanic floor 31 37 ma on the margins and ocean continent transitions of the western mediterranean the new heat flow data point out the existence of two types of local anomalies length scale 5 30 km 1 locally increased heat flow up to 153 mw m2 on the gulf of lion margin results from thermal refraction of large salt diapirs and 2 the co existing of both low 110 mw m2 heat flow areas on the south balearic margin suggests a heat redistribution system we suspect the lateral heat advection is resulting from a regional fluid circulation in the sediments associated to the widespread plio pleistocene volcanism on the south balearic margin

morphogenesis of the sw balearic continental slope and adjacent abyssal plain western mediterranean sea we present the seafloor morphology and shallow seismic structure of the continental slope south east of the balearic promontory and of the adjacent algero balearic abyssal plain from multibeam and chirp sonar data the main purpose of this research was to identify the sediment pathways from the balearic promontory to the algero balearic deep basin from the early pliocene to the present the morphology of the southern balearic margin is controlled by a sw ne structural trend whose main expressions are the emile baudot escarpment transform fault and a newly discovered wsw ene trend that affects the sw end of the escarpment and the abyssal plain we relate the two structural trends to right lateral simple shear as a consequence of the miocene westward migration of the gibraltar arc newly discovered steep and narrow volcanic ridges were probably enabled to grow by local transtension along the transform margin abyssal plain knolls and seahills relate to the subsurface deformation of early stage halokinetic structures such as salt rollers salt anticlines and salt pillows the limited thickness of the overburden and the limited amount of deformation in the deep basin prevent the formation of more mature halokinetic structures such as diapirs salt walls bulbs and salt extrusions the uppermost sediment cover is affected by a dense pattern of sub vertical small throw normal faults resulting from extensional stress induced in the overburden by subsurface salt deformation structures shallow gas seismic character and the possible presence of an active polygonal fault system suggest upward fluid migration and fluid and sediment expulsion at the seafloor through a probable mud volcano and other piercement structures one large debris flow deposit named formentera debris flow has been identified on the lower slope and rise of the south formentera margin based on current observations we hypothesize that the landslide originating the formentera debris flow occurred in the holocene perhaps in historical times

the structure of the alboran sea an interpretation from seismological and geological data abstract almost all the earthquakes included in the catalog for the iberian maghrebian area up to 2000 have been used in order to know the structure of the alboran sea area after the verification of the aleatory nature of the errors in their spatial localization a new focal mechanisms catalog has also been used as well as many of the available geological data during the mesozoic and till the oligocene the betic rif internal zone was situated further e but with the opening of the algero provencal basin in the early miocene the betic rif internal zone moved to the w contemporary the alboran sea was created as the western prolongation of the algero provencal basin the betic rif internal zone overthrused part of the iberian and african plates producing the partial sinking of both plates and being responsible of the intermediate seismicity existing in the western sector of the alboran sea the intermediate earthquakes in the atlas towards the ne and wsw are not related to lithospheric sinking but to significant deep faults limiting a subplate in nw africa in the atlantic the intermediate earthquakes between the gorringe sector and gibraltar are produced in the contact between the iberian and african plates and by the important faults crossing it the existence of four very deep earthquakes is related to the previous sinking of the lithosphere originally associated to the domain in which the betic rif internal zone was situated

seismic properties of lower crustal xenoliths from el hoyazo se spain experimental evidence up to partial melting abstract seismic techniques provide unique tools to investigate the structure and in combination with petrological geochemical and petrophysical study the composition of the lower crust controversies can be solved with comparative study of metamorphic terrains or xenoliths that occur adjacent to areas where seismic refraction reflection data are available xenoliths represent a direct sampling of the inaccessible lower crust at the time of the volcanism whilst exposed crustal sections can only be used as analogue of present day lower crust the present study is focused on the measurements of compressional wave velocities up to conditions exceeding the beginning of melting 950 c at 500 mpa confining pressure on three garnet biotite sillimanite metapelitic xenoliths recovered from the neogene dacites of el hoyazo se spain they preserve widespread interstitial rhyolitic glass as evidence of primary melt extraction and represent the best example of partially molten lower crust in the alboran domain the influence of glass on vp is primarily reflected by anomalous positive dvp dt while heating with velocity increasing at 500 mpa from 4 98 to 5 50 km s 1 at room temperature to 5 85 6 79 km s 1 at 650 700 c this corresponds to the glass transition where all the grain boundaries and most of the pores within the glass are closed after this point the velocity decreases to 6 2 6 5 km s 1 at 950 c where re melting of the glass is achieved and additional partial melt produced on cooling the behavior is normal with negative dvp dt after the thermal treatment velocities are 30 higher 6 07 7 21 km s 1 and reveal that in the presence of intergranular melt velocity measurements at room temperature cannot be extrapolated to high temperatures p waves measured at melting conditions are in agreement with deep seismic refraction data and tomography in the area and corroborate the hypothesis that partial melts are actually present in alboran lower crust

the polymict eucrites the polymict eucrites are a suite of about 14 polymict achondritic meteorites that contain at least 90 vol of eucritic material and are found both in antarctica and elsewhere they are part of a continuum of polymict breccias that includes howardites and polymict diogenites rare gas data suggest that some are gas rich regolith breccias the clasts contained in polymict eucrites sample a variety of lithologies from their parent body and include eucritic clasts alkali enriched eucrites pyroxene cumulate eucrites two pyroxene mafic rocks possibly feldspar cumulate eucrites diogenites fayalitic lithologies and a variety of other mineral clasts the eucritic clasts are probably samples of near surface lithologies on the parent body but the presence of two pyroxene mafic rocks is considered to be evidence of magmatic activity at pressures in excess of 1 kbar in the parent body partial melt models may explain some of the diversity among the clasts but cumulus processes are believed to have been important if pressures greater than 1 kbar are confirmed then the basaltic achondrite parent body bap must have a diameter in excess of 200 km the variety of lithologies suggests that not all basaltic achondrites are samples of igneous rocks formed by single stage melting of a chondritic asteroid but are instead samples of an evolved and diverse crust produced by serial magmatism

evidence for the multi stage formation of the south western valencia trough a basin evolution model for the south western valencia trough is established using thermal and palaeo thermal data bottomhole temperature and drillstem test data from oil wells have been used to determine the geothermal gradient in the study area a combination of well log data and x ray diffractometry analyses carried out on rock samples from the ibiza marino well has allowed an estimation of the bulk thermal conductivity and heat flow density integrating these data with previous seafloor heat flow measurements yields a regional surface heat flow density of about 90 100 mw m 2 vitrinite reflectance data available from the ibiza marino well have been used as a palaeo thermal constraint for the proposed tectonic models two different modelling approaches are tested a single rift phase model and a multi phase formation model the single rifting phase which considers different eroded thicknesses is ruled out as it does not match the observed data a multi stage evolution coherent with the geological and geophysical data is proposed and consists of three mesozoic rifting events palaeogene compression to which erosion of about 5 km of late jurassic and cretaceous sediments is related and a neogene rifting event that gave rise to the present day valencia trough surface heat flow vitrinite data and eroded thickness are successfully integrated in the proposed model

the depth of the earthquake activity in the central betics southern spain in this study we used relatively velocity independent methods wadati diagrams to estimate and check the focal depth of the seismic activity in a sector of the betic cordilleras alboran sea in southern spain to understand the behaviour of the seismic activity a total of 335 earthquakes have been analyzed the most interesting feature found in the analyzed sector is the strong variation of the focal depth on average the focal depth distribution in the granada basin is restricted to h 20 km the lower cut off in the seismic activity which is interpreted to mark the brittle ductile transition this cut off is coincident with a midcrustal reflector detected in the zone by previous seismic refraction and deep reflection experiments that was interpreted as an extensional detachment the seismic activity in the malaga zone reaches depths up to 100 km in this case the seismic activity affect to the crust and upper mantle the deepest activity coincides with low values of heat flow possibly thus indicating a thicking of the lithosphere towards the w in the centre of the alboran sea the activity is shallower and corresponds with a higher heat flow we think that our results support models that have proposed active delamination towards the west such a model may explain the increase in the lithosphere thickness in this direction which would be congruent with the depth of the earthquakes

determination of thermal conductivity and formation temperature from cooling history of friction heated probes summary the formation temperature below the sea or lake floor is frequently measured with a sensor probe that is inserted into the unconsolidated sediment and the thermal conductivity of sediment is measured in situ with an independent heating experiment friction during the insertion raises the temperature of a probe this paper presents the method and the test results of using the cooling history of friction heated probes for the determination of equilibrium formation temperature and thermal conductivity the inverse modelling im for the desired parameters is formulated together with the finite element simulation of the cooling history the starting parameter values for im are generated with a genetic algorithm ga that searches for them in the likely parameter ranges the im is constrained in an objective function by a commonly used asymptotic relation between the declining temperature and the inverse time in addition to satisfying the root mean square misfit the results are assessed by the equality between the model conductivity and the conductivity obtained through the asymptotic relation and by how well the models can predict the cooling behaviour at time beyond the record period for im the results of testing synthetic data with random noise up to 0 005 k and twenty 8 11 min long data sets from four sites in lake baikal indicate that the desired parameters can be determined from the first 2 5 min of the cooling records by comparison with independent measurements or through repeated ga im equilibrium temperatures can be consistently determined to within 0 002 k and thermal properties typically to 5 per cent the method has also been successfully applied to seven sets of 5 min long data sets at one marine heat flow station

a survey of the shallow structure beneath the alboran sea using rg waves and 3d imaging abstract a dispersion analysis of rg waves generated by local earthquakes surrounding the alboran sea western mediterranean has been undertaken to obtain its shear wave velocity structure at shallow depths we have used single component short period stations belonging to the regional seismic network of andalucia and the spanish national seismic network our database includes seismograms generated by 18 earthquakes that occurred in the area during the period of 1992 95 the seismic events with magnitudes between 2 9 and 4 9 and focal depths ranging from 2 7 to 12 km were recorded at 13 stations at distances between 91 and 245 km from the source we have analysed a total of 20 source station rg wave paths these seismic trajectories across the alboran sea provide irregular path coverage due to their geographical location despite sparse sampling the data collected have been processed with the help of digital filtering and group velocity dispersion curves have been obtained the path group velocities vary from 1 21 to 1 91 km s 1 within the 1 7 5 8 s period interval then using a generalised inversion approach we have obtained 1d shear wave velocity models for all source station paths inversion for shear velocity structure gives velocity profiles down to a depth of 5 km the shear wave velocities range between 0 7 and 3 2 km s 1 within the standard deviation band of 0 05 0 17 km s 1 and show significant variations from each other in the region of the alboran basin we have applied an imaging technique aimed at volumetric modelling and data visualisation and we hereby present the tomographic results based on ray path seismic velocities that were obtained for the basin we have got an overall view of the sedimentary cover of the area regarding its geometry and s wave velocity structure at very shallow depths 0 5 km although the relationship between lateral change in rg dispersion and geologic structure may not be straightforward we have observed a good correlation between those changes and surface geology

heat flow in the alboran sea western mediterranean abstract the results of the first regional heat flow survey carried out in the alboran basin are presented the survey consists of 98 heat flow measurements obtained using a violin type probe 697 nautic miles of gravity profiles 1446 nautic miles of bathymetric survey and 22 gravity cores a remarkable difference in heat flow patterns exists between the western wab and eastern eab parts of the alboran basin the average heat flow in the wab is 69 6 mw m 2 with a generally increasing trend towards the centre and to the east in contrast the heat flow pattern in the eab shows an average value of 124 8 mw m 2 and it is maintained rather constant for the overall area superimposed on this general pattern there are some local thermal anomalies associated with hydrothermal activity which have been detected in the central wab up to 123 mw m 2 in the south alboran basin sab up to 153 mw m 2 and in the djibouti bank db up to 254 mw m 2 after corrections for thermal refraction sedimentation and cooling of volcanic bodies the resulting heat flow distribution in the wab is smoother but still shows the increasing trend towards the centre and to the east in the eab the application of these corrections did not lead to any noticeable changes a 1 d approach that combines heat flow data crustal structure and elevation shows a dramatic decrease in lithospheric thickness from the wab 50 90 km to the eab 38 40 km likewise the resulting crustal thickness is around 14 16 km in the central part of the wab increasing towards the borders of the basin whereas in the eab the crustal thickness varies between 12 5 and 14 5 km in its western part and between 10 and 11 5 km in its eastern part

orogens and slabs vs their direction of subduction abstract subduction zones appear primarily controlled by the polarity of their direction i e w directed or e to nne directed probably due to the westward drift of the lithosphere relative to the asthenosphere the decollement planes behave differently in the two end members in the w directed subduction zone the decollement of the plate to the east is warped and subducted whereas in the e to nne directed it is ramping upward at the surface there are w directed subduction zones that work also in absence of active convergence like the carpathians or the apennines w directed subduction zones have shorter life 30 40 ma than e or ne directed subduction zones even longer than 100 ma the different decollements in the two end members of subduction should control different ptt paths and therefore generate variable metamorphic assemblages in the associated accretionary wedges and orogens these asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast eastward migrating structural wave along w directed subduction zones whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones the magmatic pair calc alkaline and alkaline tholeiitic volcanic products of the island arc and the back arc basin characterise the w directed subduction zones magmatic rocks associated with e or ne directed subduction zones have higher abundances of incompatible elements and mainly consist of calc alkaline shoshonitic suites with large volumes of batholithic intrusions and porphyry copper ore deposits the subduction zones surrounding the adriatic plate in the central mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab the western margin of the adriatic plate contemporaneously overridden and underthrust europe toward the west to generate respectively the alps and the apennines while the eastern margin subducted under the dinarides hellenides these belts confirm the characters of the end members of subduction zones as a function of their geographic polarity similarly to the pacific subduction zones

the diogenite eucrite links and the crystallization history of a crust of their parent body aspects of the crystallization history of the primitive crust of the hed howardites eucrites diogenites achondrite parent body have been deciphered from the microtextures and chemical compositions of pyroxene and plagioclase in some antarctic hed achondrites intermediate to diogenite and eucrite five new specimens related to yamato 75032 and three eucritic clasts in y791186 and y791208 have been investigated by electron microprobe analytical transmission electron microscope and x ray diffraction techniques their pyroxene compositions cover the entire range from fe rich diogenite to eucrite the pyroxene textures of the y75032 type achondrites indicate that many of them crystallized as low ca pigeonite and some as orthopyroxene later shock events modified the original inversion and exsolution textures the chemical compositions and the interstitial occurrence of plagioclase crystals in pyroxene indicate crystallization from intercumulus trapped liquid the pyroxene crystallization and the relation between the anorthite content an of plagioclase and the mg mg fe number of the coexisting pyroxene in these specimens reveal two crystallization trends one trend is intermediate to that of the mg rich suites of the lunar highland crust and that of ferroan anorthosite and the other is a trend with a steep decrease in the an content while the mg mg fe of the pyroxene remains effectively constant a model involving crystal fractionation in a shallow magma ocean or intrusion in the parent body has been proposed

micromorphological evidence of clay translocation in poorly dispersible soils abstract clay translocation is an important pedogenic process field and laboratory studies are used herein to evaluate the process in some ultisols and oxisols having horizons with poor dispersion characteristics surface areas measured by ethylene glycol monoethyl ether egme and micromorphological studies confirmed the presence of argillic horizons in the ultisols two of the oxisols also have illuviation argillans but most of these are below oxic horizons staining and cementation by fe and al oxides in soils in the oxidic families appear to have masked evidence of clay accumulation except in lower horizons because overlying horizons do not disperse well we conclude that either environmental conditions changed or else weathering became more intense following development of the argillic horizons climatic shifts and continued intense weathering apparently are responsible for formation of the low activity clays and the iron and aluminum oxide cementation causing the poor dispersion

structure of the mantle beneath the alboran basin from magnetotelluric soundings we present results of marine mt acquisition in the alboran sea that also incorporates previously acquired land mt from southern spain into our analysis the marine data show complex mt response functions with strong distortion due to seafloor topography and the coastline but inclusion of high resolution topography and bathymetry and a seismically defined sediment unit into a 3d inversion model has allowed us to image the structure in the underlying mantle the resulting resistivity model is broadly consistent with a geodynamic scenario that includes subduction of an eastward trending plate beneath gibraltar which plunges nearly vertically beneath the alboran our model contains three primary features of interest a resistive body beneath the central alboran which extends to a depth of 150 km at this depth the mantle resistivity decreases to values of 100 ohm m slightly higher than those seen in typical asthenosphere at the same depth this transition suggests a change in slab properties with depth perhaps reflecting a change in the nature of the seafloor subducted in the past two conductive features in our model suggest the presence of fluids released by the subducting slab or a small amount of partial melt in the upper mantle or both of these the one in the center of the alboran basin in the uppermost mantle 20 30km depth beneath neogene volcanics and west of the termination of the nekkor fault is consistent with geochemical models which infer highly thinned lithosphere and shallow melting in order to explain the petrology of seafloor volcanics

evidence of submarine hydrothermal discharge to the northwest of guadeloupe island lesser antilles island arc abstract during the soviet french survey carried out on board of the r v akademik n strakhov between guadeloupe and montserrat island in the central lesser antilles evidence of submarine hydrothermal activity was discovered on the southeastern tip of the shoe rock escarpment the latter is part of a large 130 km long transverse structure the montserrat marie galante fault which crosses the guadeloupe archipelago another locus of activity probably of subdued importance nowadays is represented by the d entrecasteaux dome a small faulted seamount located about 25 km to the sw of the former area the evidence for hydrothermal activity is a a wide range of values of conductive heat flow q 6 11 mw m 2 and regular vertical variation at some sites with extreme values located close to the two above mentioned features average regional value q 105 mw m 2 s d 32 b occurrence of secondary mineralizations todorokite nontronite etc replacing locally the primary matrix of basal upper pliocene deposits c geochemical anomalies in seawater near the sea bottom showing enrichment in mantle derived 3 he 3 he up to 10 2 correlated with a sharp zn anomaly content up to 523 ppb further investigations along the submerged segments of the montserrat marie galante fault should probably lead to other discoveries of hydrothermal venting and or mineralizations

comparison of yamato and victoria land polymict eucrites a view from mineralogical and isotopic studies new interior samples of four yamato polymict eucrites y74159 y74450 y75011 and y75015 have been studied by electron microprobe transmission electron microscope and x ray diffraction techniques and compared with several samples of the victoria land polymict eucrites these same samples have been analysed using rb sr and nd sm isotopic systematics several grains of inverted pigeonite with blebby augite similar to those in binda and moama have been identified in all four yamato eucrites coarse grained mesostasis rich subophitic basalts which contain mg rich pigeonite with fe rich olivine veinlets zoned outward to a subcalcic ferroaugite rims have also been found these unique clasts were not found in alh76005 77302 78040 78158 and 78165 and eet eucrites the tight grouping of rb sr and sm nd ratios and similar modal compositions of the yamato group indicate that they are most likely to be pieces from a single fall and distinct from the alh and eet groups however the yamato allan hills and elephant moraine groups may sample a few distinct magmas or similar but different source regions on the same parent body

rb sr and sm nd internal isochron ages of a subophitic basalt clast and a matrix sample from the y75011 eucrite the yamato polymict eucrite y75011 contains several coarse grained mesostasis rich basaltic clasts with subophitic texture the preservation of extensive mg fe chemical zoning in pyroxenes and the presence of a dark mesostasis shows that these clasts are representative of pristine lava like basalts extruded on the surface of the howardite eucrite diogenite hed parent body rubidium strontium analyses of a large clast y75011 84b yielded a precise internal isochron age of 4 60 0 05 b y for 87rb 0 0139 b y 1 or 4 50 0 05 b y for 87rb 0 0142 b y 1 the rb sr systematics of a matrix sample are nearly identical to those of the pristine clast showing that the matrix is dominantly comminuted mesostasis rich basalt the rb sr internal isochron age of the matrix is 4 56 0 06 b y for 87rb 0 0139 b y 1 or 4 46 0 06 b y for 87rb 0 0142 b y 1 these ages are within error limits of the rb sr whole rock age of chondrites and are in agreement with 206pb 207pb and sm nd ages of ordinary monomict eucrites for 87rb 0 01402 b y 1 based on a comparison of the rb sr and u th pb ages of chondrites minster et al 1982 the good precision obtained for the rb sr ages of both clast and matrix is due to the presence of a mesostasis phase in which the rb sr ratio is enriched 3 4 times above that of any eucrite phase previously analysed the initial 87sr 86sr ratios of clast and matrix are 0 69894 2 and 0 69896 3 respectively and are in satisfactory agreement with the babi value papanastassiou and wasserburg 1969 there is evidence of isotopic disturbance in both the rb sr and sm nd systems of the most magnetic separate analysed a sm nd internal isochron of 4 55 0 14 b y for the combined data from both clast and matrix exclusive of the most magnetic separate is in agreement with the more precise rb sr age the corresponding initial 143nd 144nd 0 50587 19 normalized to 148nd 144nd 0 24308 the clast data alone exclusive of the most magnetic separate define an isochron age of 4 52 0 16 b y and initial 143nd 144nd 0 50593 19 the isochron error limits quoted here for the sm nd data are 2 from the york 1966 program and include the factor s n 2 used in that program to compensate for geological error however this factor is not appropriate for the rb sr isochrons and has been omitted from the quoted error limits the initial 143nd 144nd of the pristine clast at 4 56 b y ago is 0 505877 25 in agreement within error limits with the chur value jacobsen and wasserburg 1984 clast y75011 84 represents the oldest pristine unambiguously basaltic lava as yet recovered from a solar system object rare earth element abundance measurements shimizu and masuda 1985 show it to be one of the most differentiated eucrites thus moderately evolved basaltic lavas were produced very early in the history of the hed parent body their preservation as clasts in the polymict eucrites suggests derivation from a different geologic setting on the parent body than that occupied by the more thermally metamorphosed monomict eucrites we suggest that the preservation of pristine clasts in polymict eucrites may be due to their impact excavation to the surface of the parent body away from heat sources concentrated within impact craters the monomict eucrites may be derived from the floors of craters where they were thermally metamorphosed by shock heating and by the presence of overlying impact melts and nearly molten breccias

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

geothermal structure of the tyrrhenian sea over 200 heat flow measurements have been carried out in the tyrrhenian sea mostly since 1977 as part of the heat flow map for italy and surrounding seas italian program as well as part of the french italian tyrrhenia program all data have been corrected for the influence of the plio quaternary sedimentation measured heat flow values were evaluated according to the sedimentary environment following criteria outlined by sclater et al 1980 in this way areas affected mainly by shallow thermal effects in the zones with maximum density of data have been displayed filtering out these shallow contributions the major regional geothermal trends were obtained the tyrrhenian basin can be divided in two distinct regions with different geothermal characteristics the northern part of the basin appears to be connected with the continental structures involved in the interaction between the corsica and tuscan margins the maximum heat flow area located offshore civitavecchia is related to the tensile structures of the tuscan geothermal province the most important thermal features of the central and southern part are located along the central belt trending nw se the lowest heat flow values are located in the peri tyrrhenian areas and are related to the continental margins of the basin while the maxima are connected with the stretched and thinned area localized in the bathyal plain taking into account the heat flow data as well as other geological and geophysical information an evolutionary model for the tyrrhenian sea is presented suggesting a generalized stretching of an initially continental crust accompanied by intensive rifting phases which may have favoured the onset of oceanic type crust

heat flow through the sea bottom around the yucatan peninsula heat flow studies were conducted in january february 1987 off the atlantic coast of mexico on board the r v akademik nikolai strakhov two areas were surveyed one transecting the salt dome province and the campeche canyon in the gulf of mexico and the other on the eastern flank of the yucatan peninsula conductive heat flow through the bottom sediments was determined as the product of vertical temperature gradient and in situ thermal conductivity measured with a thermal probe using a multithermistor array and real time processing capabilities forward two dimensional modeling allows us to estimate heat flow variations at both sites from local disturbances and to obtain average heat flow values of 51 mw m2 for the transect within the gulf of mexico and 38 and 69 mw m2 for two basins within the yucatan area sea bottom relief has a predominant effect over other environmental factors in the scatter of heat flow determination in the gulf of mexico

formation of ureilites by nebular processes oxygen isotopic data are presented for 23 ureilites including 14 antarctic meteorites they show a much greater range in isotopic composition than those previously studied most noteworthy is the fact that unlike other achondrite groups the ureilites do not fall on a mass dependent isotopic fractionation trend but rather fall along an extrapolation of the 16o mixing line defined by components of allende and other c3 chondrites correlations between isotopic and chemical data imply that the isotopic and chemical variations are due primarily to processes in the solar nebula and not to igneous processes within a parent body the ureilite parent body accreted with isotopic and chemical heterogeneities which were not much modined by subsequent parent body processes the precursor materials were related in some way to the c3 carbonaceous chondrites

mineralogy of antarctic ureilites and a working hypothesis for their origin and evolution abstract mineralogical and crystallographic data and electron microprobe analyses of silicate minerals for antarctic ureilites y74123 y74130 y74659 y790981 alha77257 alha78019 and alh82106 extend the chemical and textural variations among ureilites beyond those known for non antarctic ureilites low ca pyroxenes possibly orthopyroxene poikilitically enclosed in olivine and coexisting with mg rich pigeonites have been identified in y74659 and alha77257 coexisting low ca pyroxene pigeonite and augite together with the very fe rich olivine in ureilites were observed in y74130 almost fe free pigeonite and diopside coexist in alh82106 no systematic differentiation trend has been reconstructed from these data but the pyroxene pairs yield formation temperatures above 1200 c a weak anticorrelation between mno and feo delineated by these pyroxenes suggests that a reduction reaction is responsible for the chemical variations a working hypothesis for ureilite formation involves high temperature episoded and a planetesimal scale collision which might have facilitated compaction and recrystallization of mafic silicates during the episodes fe ni s eutectic melt and a partial melt rich in ca al and fe might have been lost from a carbonaceous chondrite like source material

variable bottom water in the western mediterranean basin the deep water of the western mediterranean has long been considered to have practically constant values of s and between 1909 1910 the thor expedition and the 1970 cruise of the jean charcot the deep water at more than 1800 m depth showed very little variation the small changes which appeared through the years could be due in part to changes in instrumentation analysis and methods of sampling in 1972 and 1973 during the medoc cruises a bottom layer of 200 300 m thickness was found its temperature and salinity showed a small gradient producing at the bottom an increase of potential temperature of about 0 01 to 0 02 c and an increase in salinities of about 0 010 00 relative to a minimum of and s in the deep water at 1800 2000 m these minima of and s were found to be homogeneous horizontally at all stations occupied in the medoc area and in the ligurian sea in 1973 interestingly the bottom layer was not present in 1974 however in november 1981 during the cruise mediprod iv the variable bottom water was again present extending from the ligurian sea and medoc areas along the western coasts of corsica and sardinia to the eastern alboran sea in most cases the maximum values of and s were recorded at the bottom and were higher than the 1973 values by about 0 03 c and up to 0 0150 00 but with a rather high spatial variability by february 1982 cruise dyome ii the situation had become more complicated not only was a layer of minimum and s present but it was underlain by a layer of maximum and s below which a decrease appeared producing a new minimum at the very bottom in addition a number of stations displayed higher values of by about 0 05 c and of s by about 0 0150 00 in the layers of minimum and maximum characteristics as well as on the bottom in contrast to the situation in 1973 high spatial variability was found in the deep layering the variability may have been due to patchy deep convention activity possibly in the ligurian sea the phenomenon of variable bottom water and the processes which produce it may be relevant to our understanding of water mass formation

thermal evolution rate of exhumation and tectonic significance of metamorphic rocks from the floor of the alboran extensional basin western mediterranean high grade metamorphic rocks drilled at ocean drilling program site 976 in the alboran sea show a pt path characterized by decompression from about 1050 mpa 40 km depth to 350 mpa 13 km depth accompanied by an increase in temperature from about 550 50 c to 675 25 c the ar ar dating on muscovite and apatite fission track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20 5 18 ma which coincides with the initiation of sedimentation in the alboran sea basin the alboran sea formed by miocene extension on the site of a late cretaceous to paleogene contractional orogen and extension coincided with thrusting in the peripheral parts of the betic rif arc which surrounds the basin on three sides thermal modeling of the pt path was carried out with the aim of constraining geodynamic models for the formation of the basin variables considered in the modeling included 1 the thickness and thermal gradient of the postorogenic lithosphere 2 the radiogenic heat production in the thickened crust 3 the time gap pause between the end of contractional tectonics and the start of extension 4 removal of lithospheric mantle below 125 75 or 62 5 km and 5 the rate of extension the only combinations of variables that produce modeled pt paths with the observed characteristics involve high radiogenic heat production combined with a significant postcontractional pause to produce high temperatures in rocks initially at 40 km depth removal of lithosphere below 62 5 km to produce further heating during decompression extension by a factor of 3 in 6 m y to delay the attainment of the maximum temperature until the rocks reached shallow depths and final exhumation and cooling in about 3 3 m y to satisfy radiometric and petrological constraints this gives a maximum of about 9 m y for exhumation from 40 km depth to the surface lithospheric stretching in response to plate boundary forces such as trench rollback without removal of lithosphere cannot explain the late onset of heating and the high temperatures reached by these rocks removal of lithosphere at depths significantly greater than 62 5 km cannot explain the combination of high temperatures reached by these rocks and the shallow depth at which they attained the maximum temperature only a combination of significant postcollisional radiogenic heating then wholesale removal of lithospheric mantle below the orogenic crust followed by rapid stretching can explain the observed pt path these results appear to support some form of lithospheric delamination as the primary cause for the formation of the alboran sea basin

e w opening of the algerian basin western mediterranean we present geophysical and geological evidence of fast 2 cm yr 1 in the east and 5 cm yr 1 in the west e w spreading between 16 and 8 ma in the region behind the gibraltar arc in the algerian basin

propagation of regional seismic phases lg and sn and pn velocity structure along the africa iberia plate boundary zone tectonic implications we used over 1000 regional waveforms recorded by 60 seismic stations located in northwest africa and iberia to map the efficiency of l g and sn wave propagation beneath the gulf of cadiz alboran sea and bounding betic rif and atlas mountain belts crustal attenuation is inferred from the tomographic inversion of l g pg amplitude ratios upper mantle attenuation is inferred from maps of sn propagation efficiency derived by inversion of well defined qualitative efficiency assignments based on waveform characteristics regions of l g attenuation correlate well with areas of thinned continental or oceanic crust significant sedimentary basins and lateral crustal variations comparison of the sn efficiency results with velocities obtained from an anisotropic pn traveltime inversion shows a fairly good correlation between regions of poor sn efficiency and low pn velocity a low pn velocity 7 6 7 8 km s 1 and significant sn attenuation in the uppermost mantle is imaged beneath the betics in southern spain in sharp contrast to the relatively normal pn velocity 8 0 8 1 km s 1 and efficient sn imaged beneath the alboran sea slow pn velocity anomalies are also imaged beneath the rif and middle atlas in morocco we do not identify any conclusive evidence of lithospheric scale upper mantle attenuation beneath the rif although the crust in the gibraltar region appears highly attenuating making observations at stations in this region ambiguous paths crossing the gulf of cadiz eastern atlantic and the moroccan and iberian mesetas show very efficient sn propagation and are imaged with high pn velocities 8 1 8 2 km s 1 the spatial distribution of attenuation and velocity anomalies lead us to conclude that some recovery of the mantle lid beneath the alboran sea must have occurred since the early miocene episode of extension and volcanism we interpret the low velocity and attenuating regions beneath the betics and possibly the rif as indicating the presence of partial melt in the uppermost mantle which may be underlain by faster less attenuating mantle in the light of observations from other geophysical and geological studies the presence of melt at the base of the betic crust may be an indication that delamination of continental lithosphere has played a role in the neogene evolution of the alboran sea region

p and s velocity anomalies in the upper mantle beneath europe from tomographic inversion of isc data we present a new tomographic model for p and s velocity anomalies beneath europe 30 n 55 n 5 w 40 e extending in depth up to 700 km and constrained by inversion of data from the international seismological center isc catalogue the algorithm uses the traveltimes from events located in the study area recorded by all available worldwide stations as well as times from teleseismic events recorded by european stations the events from the isc catalogue have been relocated and combined into composite events all the traveltimes were corrected for crustal structure using the reference model eucrust 07 the resulting velocity anomalies show similar large scale patterns as observed in previous studies but have a higher resolution which allows detection of some features in more detail for example it is now possible to assess the depth extension of the small slow velocity body beneath the eifel region and eger graben the p and the s model show a good consistency in the uppermost 200 km below most of the european area and in some parts even in the deeper layers e g beneath the apennines and the hellenic arc the new model provides clear images of some principal features which were previously detected in a limited number of studies while the comparison between p and s velocity anomalies provides novel constraints to address on their nature e g the gap in the adriatic plate subducted below the central southern apennines in this paper we pay special attention to testing the reliability of the results the random noise effect is evaluated using a test with independent inversion of two data subsets with odd even events the spatial resolution is estimated using different checkerboard tests furthermore we present a synthetic model with realistic patterns which reproduces after performing forward and inverse modelling the same shape and amplitudes of the anomalies as in the case of the real data inversion in this case the parameters of the model can be used to assess the amplitudes of p and s anomalies that is critical for evaluation of other petrophysical parameters temperature density composition etc in the upper mantle 2009 the authors journal compilation 2009 ras