carbon mineralization and oxygen dynamics in sediments with deep oxygen penetration lake superior to understand carbon and oxygen dynamics in sediments with deep oxygen penetration we investigated eight locations 160 318 m depth throughout lake superior despite the 2 4 weight percent organic carbon content oxygen penetrated into the sediment by 3 5 to 12 cm at all locations such deep penetration is explained by low sedimentation rates 0 01 0 04 cm yr 21 high solubility of oxygen in freshwater and a shallow 2 cm bioturbation zone in response mainly to oxygen variations in the bottom waters the sediment oxygen penetration varied seasonally by as much as several centimeters suggesting that temporal variability in deeply oxygenated sediments may be greater than previously acknowledged the oxygen uptake rates 4 4 7 7 mmol m 22 d 21 average 6 1 mmol m 22 d 21 and carbon mineralization efficiency 90 of deposited carbon were similar to those in marine hemipelagic and pelagic sediments of comparable sedimentation rates the reactivity of organic carbon was found to decrease with age similarly to the power law documented in marine environments the burial flux of carbon into the deep sediment 0 7 mmol m 22 d 21 was 2 5 of the previously estimated primary production maximum volume specific carbon degradation rates were 0 3 1 5mmol cm 23 d 21 bioturbation coefficient near the sediment surface was 3 8 cm 2 yr 21 these results indicate that carbon cycling in large freshwater systems conforms

relationship between uplift of the metamorphic core of the southernmost andes and shortening in the magallanes foreland fold and thrust belt tierra del fuego chile rocks containing upper amphibolite facies assemblages reflecting mesozoic cenozoic metamorphism form the 5000 km2 core of the southernmost andes in cordillera darwin approximately 55 s latitude results from mapping along a 50 km continuous transect across the southernmost andes on the main island of tierra del fuego show structural relationships between the late cretaceous to early tertiary uplift of the metamorphic core in northern cordillera darwin and mid cretaceous to paleogene shortening in the adjacent magallanes foreland fold and thrust belt new data indicate that pre late jurassic basement schists exposed along the northeastern edge of cordillera darwin were uplifted along a discrete brittle continent vergent thrust fault this thrust overlies a series of thin skinned thrusts and back thrusts that affect mesozoic tertiary rocks of the magallanes foreland basin the thrust that uplifts basement schists is interpreted to have formed out of sequence with respect to the thin skinned structures because it placed the schists over previously shortened lower cretaceous sedimentary strata of the magallanes basin the geometry of thrusts observed across the transect and their inferred sequence of deformation indicate a thick skinned contractional mechanism for uplift of the pre late jurassic schists of northern cordillera darwin relative to the structural level of metamorphic basement in the magallanes foreland the relative time of out of sequence thrusting and basement uplift inferred from thrust geometries in this paper is consistent with the latest cretaceous to early tertiary episode of rapid exhumation and cooling of the metamorphic core of the southern andean orogen determined in previous structural metamorphic and geochronological studies

impact of suboxia on sinking particulate organic carbon enhanced carbon flux and preferential degradation of amino acids via denitrification fluxes of particulate organic carbon poc through the oxygen deficient waters in the eastern tropical north pacific were found to be relatively less attenuated with depth than elsewhere in the eastern north pacific the attenuation coefficient b for the flux was found to be 0 40 versus the composite value of 0 86 determined by martin et al 1987 to examine this further sinking poc was collected using sediment traps and allowed to degrade in oxic and suboxic experiments using a kinetic model it was found that degradation proceeded at similar rates roughly 0 8 day 1 under oxic and suboxic conditions but a greater fraction of bulk poc was resistant to degradation in the suboxic experiments 61 vs 23 amino acids accounted for 37 of poc collected at 75m but following degradation the value dropped to 17 and 16 in the oxic and suboxic experiments respectively poc collected from 500m was 10 amino acids the non aa component of poc collected at 75m was not degraded under suboxic conditions while under oxic conditions it was these results suggest that microbes degrading oc under suboxic conditions via denitrification preferentially utilize nitrogen rich amino acids this preferential degradation of amino acids suggests that 9 more nitrogen may be lost via water column denitrification than is accounted for when a more redfieldian stoichiometry for poc is assumed

evolution of retreating subduction boundaries formed during continental collision retreating subduction boundaries formed where the rate of subduction exceeds the rate of overall plate convergence appear to be commonly developed features within regions of early or incomplete continent continent collision they are characterized by regional extension within the overriding plate and at their leading edge by thin skinned arcuate thrust belts that are concave towards the overriding plate as is illustrated by examples from the mediterranean region the formation of retreating subduction boundaries is intimately related to the process of continental collision during the early stages of collision retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary retreating plate boundaries can also form before the main collision and the associated thrust belts emplaced as precollisional accretionary assemblages because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab slab pull subduction usually ceases when and only when thick buoyant continental crust enters the subduction zone thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted in some cases retreating subduction boundaries may escape into the open ocean where they form nearly isolated local tectonic systems in these systems the rate of subduction is approximately compensated by the rate of upper plate extension and migration of the system across the oceanic region may be very rapid for example the horseshoe seamounts located about 800 km offshore in the eastern north atlantic may be the active expression of an east dipping westwardly migrating retreating subduction boundary that has evolved from the betic cordillera rif system active in miocene time and may now be progressing across the atlantic at approximately 50 mm yr an analogous situation may be represented by the scotia arc system a westward dipping retreating subduction system located between the south american and antarctic plates which may have escaped into the south atlantic ocean from a zone of crustal shortening in the andes and is now progressing across the atlantic at a rate of about 80 mm yr

recent large fold nucleation in the upper crust insight from gravity magnetic magnetotelluric and seismicity data sierra de los filabres sierra de las estancias internal zones betic cordillera abstract rheological heterogeneities in the upper crust have a close relationship with the fold position where rigid bodies could constitute initial perturbations that allow the nucleation of folds consequently establish the position and geometry of anomalous rocks located in the upper crust by geophysical studies help to understand the folded structure observed on surface new geological observations in the field along with gravity magnetic magnetotelluric and seismicity data reveal the subsurface structure in the sierra de los filabres sierra de las estancias folded region part of the alpine belt in southern spain the geometry of the upper crust is determined by geological field data 2d gravity models 2d magnetic models and 2d mt resistivity model while seismicity evidences the location of the deep active structures these results allow us to propose that a basic rock body at 4 to 9 km depth has determined the nucleation and development of the sierra de los filabres kilometric antiform n vergent large late folds are subjected to a variable present day stress field earthquake focal mechanisms suggest the presence in depth of a regional nw se compressive stress field however most of the seismogenetic structures do not extend up to the surface where nw se and wnw ese outcropping active normal faults are observed thus indicating a ne sw extension in the upper crust simultaneous to orthogonal nw se compression related to reverse faults and minor folds developed in the eastern almanzora corridor and in the nearby huercal overa basin the recent and active tectonic studies of cordilleras hinterland subjected to late folding greatly benefits from the integration of surface observations together with geophysical data

a tomographic study of the lithosphere beneath the pyrenees from local and teleseismic data a tomographic image of the pyrenean lithosphere from the surface down to 200 km is deduced from the propagation of the seismic waves recorded at the regional pyrenean seismograph network crustal and subcrustal models are derived independently from the inversion of two different data sets p and s velocity models for the crust are obtained from the propagation of local earthquakes the upper crust appears very heterogeneous and exhibits sharp velocity contrasts in the uppermost 7 km an east west trending zone with velocities up to 6 higher than the surrounding material is observed all along the pyrenees this zone coincides in most places with the exposure of the paleozoic rocks of the axial zone low velocities are observed on both sides of this high velocity zone at the surface they coincide with well known sedimentary basins but dip down to about 11 km depth toward the axis of the range in the central and eastern pyrenees two large high velocity bodies stand out in the north pyrenean zone and extend down to the base of the upper crust they coincide with large positive gravity anomalies and are interpreted as lower crust or upper mantle blocks uplifted into the upper crust the lower crust is poorly resolved but appears to be less heterogeneous than the upper crust moreover the lower crustal structure is almost uncorrelated with the upper crustal structure the major feature of the lower crust is a low velocity anomaly which stretches along the north pyrenean fault in the western pyrenees and beneath the axial zone in the central pyrenees its location nearly coincides with the distribution of the seismicity in the upper mantle p models are derived from the travel times of the remote events recorded at the pyrenean stations the most important feature is the presence in the eastern and central pyrenees of a vertical low velocity heterogeneity south of the north pyrenean fault down to 80 100 km this structure exhibits a 2 velocity contrast with respect to the surrounding structures and is interpreted as lower crustal material subducted during the pyrenean convergence this heterogeneity is not observed in the western pyrenees suggesting that the crustal shortening has been completely accommodated by several thrusts in the thinned aquitaine crust

seismic tomography of the crust and lithospheric mantle in the betic cordillera and alboran sea abstract the alboran domain lies between the south of spain and the north of morocco and contains a miocene extensional basin which formed during continuous compression between the african and eurasian plates we have performed a 1 d and 3 d passive velocity tomography study of the structure of the crust and lithospheric mantle of the betic cordillera and alboran sea region an important drawback encountered in most passive seismic tomography studies is that the quality of the data set used is inhomogeneous and it is often impossible to measure the error this occurs as the data set needs to be very large which implies using several years worth of data from different seismic networks with different station noise levels and phase travel time picking accuracy we present a simple method to quantitatively determine the errors in the travel time data typically used in passive seismic tomography studies we use this method to determine the standard error of our data set and present the tomography results with quantitatively determined error bars an additional advantage of performing the error analysis is that the data can be weighted in terms of its standard error and hence the contribution of noisy data can be reduced in the inversion resulting in substantially enhanced tomography images the main structural results obtained in the 1 d inversion of the data are 1 the average velocity of the crust in the betic cordillera is v p 6 0 0 05 km s and v s 3 5 0 05 km s indicating an average v p v s ratio of 1 71 for the crust 2 the average moho depth in the betic cordillera is less than 36 km 3 the average crustal velocity in the alboran sea is v p 6 4 0 1 km s and v s 3 4 0 2 km s indicating an average v p v s ratio of 1 88 for the crust 4 the average depth of the moho in the alboran sea is greater than 20 km the 3 d inversion provides new structural information of the alboran basin over the depth range 24 60 km for the depth range 24 31 km we image the centre and north western margin of the basin and observe velocities of the order of 7 2 7 5 0 45 km s which could correspond to lower crust material or else anomalous low velocity mantle material for the depth range 31 40 km we image the centre and the southern margin of the basin and observe p phase velocities of 8 0 8 2 0 55 km s which we interpret as normal mantle material below this layer in the depth range 40 60 km we image the centre and northern margin of the basin and observe low p velocities v p 7 8 0 55 km s at present the preferred model used to explain the formation of the alboran sea basin makes use of the process of lithospheric delamination the low velocity layer observed in the mantle starting at approximately 40 km depth could be interpreted to be hot mantle material or the asthenosphere however the normal mantle velocities observed in the 31 40 km layer above it do not support replacement of the lithosphere by hot asthenospheric material in the lower crust or immediately beneath the crust if this is so then delamination must have occurred within the lithospheric mantle at an approximate depth of 40 km or else not occurred at all

seismic velocity and poisson s ratio tomography of the crust beneath east anatolia abstract eastern anatolia is a region in the early stages of continent continent collision and so provides a unique opportunity to study the early development of continental plateau located within the alpine himalayan fold thrust fault belt the anatolian plateau is geologically very complex with over half of the surface area covered with late cenozoic volcanics of diverse composition the plateau is also seismically active and is dissected by numerous seismogenic faults predominantly of strike slip motion in this study we determine 3 d tomographic images of the crust beneath eastern anatolia by inverting a large number of arrival time data of p and s waves from the obtained p and s wave velocity models we estimated the poisson s ratio structures for a more reliable interpretation of the obtained velocity anomalies our tomographic results are generally consistent with the major tectonic features of the region high p and s wave velocity anomalies are recognized near the surface while at deeper crustal layers low seismic wave velocities are widely distributed poisson s ratio exhibits significant structural heterogeneities compared to the imaged velocity structure the seismic activity is intense along highly heterogeneous zones and is closely associated with pre existing faults in the central and western parts of the study area results of the checkerboard resolution test indicate that the imaged anomalies are reliable features down to a depth of about 40 km the low velocity high poisson s ratio zones in the middle to lower crust are consistent with many geophysical observations such as strong sn attenuation low pn and sn velocity and the absence of mantle lid implying the presence of partial melt in the uppermost mantle

slab rupture and delamination under the betics and rif constrained from receiver functions abstract we map the lithospheric structure under the westernmost mediterranean convergent setting interpreting p receiver functions obtained from a dense seismic network no orogenic root occurs under the eastern and great part of the central betics however the subducted south iberian continental lithosphere is found beneath the western betics where the iberian moho reaches depths of approximately 65 km dipping gently towards the se meanwhile at the rif strong crustal and lithospheric thickness contrasts occur across the nekor nw se sinistral fault that overlies the region of present slab tearing east of the nekor fault there is no orogenic root and the crust has been thinned to approximately 22 km whilst to the west the crust reaches 55 km thickness and the maghrebian continental lithosphere is attached to the lithospheric slab imaged by tomography under the alboran basin these data suggest that subduction rollback under the alboran and algerian basins together with continental slab tearing or detachment producing edge delamination under the betics and rif have been the main tectonic mechanisms driving extension magmatism and regional uplift in the westernmost mediterranean since the late miocene until present the surface expression of edge delamination and slab tearing is marked by regional uplift denudation of hp rocks in elongated core complex type domes late miocene volcanism in the eastern betics and rif and by large ne sw strike slip transfer faults like the alpujarras crevillente torcal or nekor faults that accommodate strong gradients in crustal displacements the iberian slab is still attached to the oceanic slab imaged under the alboran basin at the western betics where intermediate depth seismicity recent dextral strike slip faulting and folding could reflect slab tearing meanwhile active faulting and differential gps measured displacements would mark slab tearing beneath the rif coinciding with the trace of the sinistral nekor fault

a model for the evolution in water chemistry of an arsenic contaminated aquifer over the last 6000 years red river floodplain vietnam aquifers on the red river flood plain with burial ages ranging from 500 to 6000 years show with increasing age the following changes in solute concentrations a decrease in arsenic increase in fe ii and decreases in both ph ca and bicarbonate these changes were interpreted in terms of a reaction network comprising the kinetics of organic carbon degradation the reduction kinetics of as containing fe oxides the sorption of arsenic the kinetics of siderite precipitation and dissolution as well as of the dissolution of caco3 the arsenic released from the fe oxide is preferentially partitioned into the water phase and partially sorbed while the released fe ii is precipitated as siderite the reaction network involved in arsenic mobilization was analyzed by 1 d reactive transport modeling the results reveal complex interactions between the kinetics of organic matter degradation and the kinetics and thermodynamic energy released by fe oxide reduction the energy released by fe oxide reduction is strongly ph dependent and both methanogenesis and carbonate precipitation and dissolution have important influences on the ph overall it is the rate of organic carbon degradation that determines the total electron flow however the kinetics of fe oxide reduction determines the distribution of this flow of electrons between methanogenesis which is by far the main pathway and fe oxide reduction modeling the groundwater arsenic content over a 6000 year period in a 20 m thick aquifer shows an increase in as during the first 1200 years where it reaches a maximum of about 600 g l during this initial period the release of arsenic from fe oxides actually decreases but the adsorption of arsenic onto the sediment delays the build up in the groundwater arsenic concentration after 1200 years the groundwater arsenic content slowly decreases controlled both by desorption and continued further but diminishing release from fe oxide being reduced after 6000 years the arsenic content has decreased to 33 g l the modeling enables a quantitative description of how the aquifer properties the reactivity of organic carbon and fe oxides the number of sorption sites and the buffering mechanisms change over a 6000 year period and how the combined effect of these interacting processes controls the groundwater arsenic content

lateral variation of the crust in the iberian peninsula new evidence from the betic cordillera abstract new results from a seismic refraction wide angle reflection survey carried out in the betic cordillera in autumn 1989 contribute to a better picture of its deep structure one nw se profile cuts across the iberian massif the external and internal betics the structure of the crust in the iberian massif shows characteristics similar to those found in previous experiments the lower crust is found as a distinct layer 12 km thick with an average velocity of 6 8 km s 1 the moho is found at about 35 km depth this structure extends southeastward until a 3 4 km upwelling of the moho about 30 km north of the present day surface boundary between the external and internal betic units further southeast the moho deepens to 38 km and the lower crust is no longer seismically detected the absence of differentiated lower crust beneath part of the external betics may be related with the mesozoic rifting of the south iberian passive margin instead this absence under the internal betics may be caused by rifting in conjunction with the collisional evolution of the orogen a wnw ese profile lying in the internal betics shows the presence of a prominent reflector at 10 12 km depth this seems to be a widespread feature in the internal betics and may be interpreted as a detachment surface the moho is found at 38 km depth rising strongly in the easternmost betics seismic data suggest a thin crust in the offshore area southwest of malaga probably containing a massive zone of high velocity rocks which is also supported by available geophysical and geological data

crustal structure of the central sector of the betic cordillera se spain deep seismic reflection gravimetric and magnetometric data allow the main features of the deep structure of the central sector of the betic cordillera to be established the moho is horizontal or dips slightly toward the s below the mountain chain and its morphology has no direct relationship with the region s topography the deep reflectors are not deformed by the neogene kilometric scale folds that produced the main topographic features probably due to the existence of detachment levels in the crust the crust is slightly thickened in the betic cordillera almost 35 km and has an abrupt transition to the thin crust of the alboran sea 15 km in the central alboran sea along an e w oriented narrow band subparallel to the coast line where the moho dips more than 60 n the neogene evolution of the mountain chain caused compressive deformation in the external zones in the internal zones pre tortonian low angle normal faults developed after the alpine metamorphism and later high and low angle normal faults and strike slip faults occurred up to the quaternary geophysical and field geology data can be used to establish a geological model for the recent evolution of the crustal deformation in this model the recent evolution of the region is probably determined by thrusting of the thin continental crust of the alboran sea toward the nw over the southern margin of the iberian massif this deformation could be a consequence of the oblique convergent movement between the african and eurasian plates which may have uplifted the entire region causing regressions in the sedimentary basins and the formation of folds and extensional structures in the upper part of the thickened crust

seismic tomography from local crustal earthquakes beneath eastern rif mountains of morocco we applied a tomographic method to image an aseismic strike slip fault in north morocco and found that the occurrence of earthquakes is not only controlled by the state of tectonic stress but also by material heterogeneity in the crust we have constructed an integrated model of seismic electric magnetic and heat flow properties across northeastern morocco primarily based on a tomography inversion of local earthquake arrival times the seismic images obtained show a pronounced lowvelocity zone at 5 km depth parallels to the nekor fault coinciding with an anomalously high conductive and low gravity structure which is interpreted as a fault gouge zone and or a fluid filled subsurface rock matrix below 10 km depth a weak positive velocity zone indicates that the fault gouge is stable the seismicity and the seismic velocity results for the alhoceimas region show that the concentrations of earthquakes are confined in the high velocity area this anomaly is interpreted to be a brittle and competent layer of the upper crust that sustains seismogenic stress on the eastern coast line of morocco we infer that a high density high velocity body exists in the shallowest layers of the upper crust probably formed by miocene volcanic rocks d 2003 elsevier science b v all rights reserved

elemental and major biochemical changes across an oxidation front in a relict turbidite an oxygen effect abstract the elemental and major biochemical compositions of the relict f turbidite sampled in two cores from the madeira abyssal plain were determined this fine grained distal sequence occurs at ca 9 m core depth and includes a surficial oxidized horizon defined by a distinct color change oxygen diffused downward through sediments above this interface and in ca 10 kyr destroyed 80 of the organic substances that below the front had survived degradation in the presence of porewater sulfate for ca 140 kyr these deposits provide an opportunity to establish the extent and selectivity of oxic sedimentary degradation under natural conditions without the usual complications of bioturbation and varying sources or sedimentation rates in both cores a sample from the upper oxidized layer was compared to two samples from the underlying unoxidized layers the unoxidized sequences of both turbidities contained 0 93 1 02 wt organic carbon oc and 0 10 0 11 wt total nitrogen tn approximately 20 of the initial oc and 40 of the initial tn remained in the oxidized horizons with a consequent decrease in atomic c n ratio from ca 11 to ca 5 all samples gave very low yields of lignin phenols and comparable oc normalized yields of total aldoses and amino acids and indicated predominantly marine organic matter om and nonselective oxic degradation of these biochemical classes compositions of individual aldoses and amino acids generally were also similar in surface and deep sediments except that the oxidized horizons yielded markedly elevated 3 5x percentages of nonprotein amino acids this study clearly demonstrates that prolonged exposure to oz can lead to organic matter alteration which is far more extensive than that obtained with sulfate alone in comparison to early diagenesis however alteration of the measured biochemicals was largely nonselective such oxidation reactions could control the distribution and composition of organic matter in slowly accumulating continental rise and deep ocean environments

p wave tomographic images in the central betics alboran sea south spain using local earthquakes contribution for a continental collision to investigate the relationship between shallow and intermediate depth earthquakes and the crust and upper mantle structure beneath the betic cordillera and alboran sea region we have applied seismic tomography to 2569 p wave arrival times from 367 microearthquakes recorded by both permanent and temporary seismic stations deployed in the region these earthquakes occurred in the depth range of 0 to 110 km beneath the central betic cordilleras and alboran sea our results have revealed significant structural heterogeneities in the crust and upper mantle beneath the study area in the upper crust there is a high velocity anomaly at sierra gorda western boundary of the granada basin penetrating to 15 km depth which is in good agreement with the aeromagnetic anomaly in the granada basin a low velocity anomaly is located in the middle crust which coincides with a previously detected greater cutoff depth of seismicity and are considered to be associated with a highly fractured zone generated by an intracrustal detachment the northern boundary of the alboran sea malaga coast is well imaged as low velocities from 50 to 90 km depths this low velocity zone in the upper mantle is associated with the intermediate depth seismicity which outline a section of continental crust related to the collision subduction beneath the northern part of the alboran sea southern part of the central betics

tomographic imaging of deep velocity structure beneath the eastern and southern carpathians romania implications for continental collision the vrancea seismic zone located beneath the elbow of the eastern and southern carpathians is characterized by intermediate depth seismicity associated with subduction and plate collision among many strong earthquakes that have occurred in the region the 1977 vrancea earthquake march 4 1977 ms 7 1 depth of 86 km caused extensive damage and fatalities the vrancea seismic zone is defined by a tabular region of seismicity that extends from the surface to a depth of 200 km the small size about 80 km in length and 40 km in width and geometry of the seismic zone have made it difficult to interpret the kinematics of subduction and continental collision in the romanian carpathians to better understand the collision subduction environment body wave seismic tomography was used to image the deep structure beneath the eastern and southern carpathians in romania a total of 7837 p and 2292 s arrival times from 433 shallow and intermediate depth earthquakes recorded by local and regional seismic stations were inverted for the three dimensional 3 d seismic velocity structure the travel times and ray paths were calculated by an efficient 3 d ray tracing technique by utilizing the lsqr algorithm p and s wave velocity perturbations were determined from the surface down to a depth of 200 km the high resolution p wave tomographic images reveal a pattern of broad heterogeneity in the velocity structure of the region one of the most important features is a high velocity body existing beneath the carpathian arc at depths between 100 and 170 km this high velocity body outlines a slab dipping near vertically where most of the intermediate depth earthquakes occur beneath the transylvanian basin a high velocity zone is dipping toward the se and may represent the base of lithosphere which is in convergence with the subducted slab low velocity bodies are found along the forearc of the carpathian foredeep down to depths of 70 to 80 km these low velocity bodies may be correlated with the shallow sedimentary layer in the foreland and may represent crustal materials wedged and underthrust along with the subducted lithosphere

asymmetric delamination and convective removal numerical modeling comparison with evolutionary models for the alboran sea region convective removal and mantle delamination are geodynamical mechanisms proposed to explain the presence of extension in the alboran sea within a regional context of compression using a new thermomechanical algorithm we present here a quantitative evaluation and comparison of conceptual models based on these geodynamical mechanisms in contrast to the in situ convective removal process the laterally propagating delamination mechanism is shown here to be consistent with first order features of the alboran sea such as the thinning thickening distribution intermediate depth seismicity and upper mantle structure imaged by seismic tomography the lower crust is predicted to reach depths of 100 150 km in some areas due to mechanically driven viscous drag of the downwelling mantle

new isotopic ages and the timing of orogenic events in the cordillera darwin southernmost chilean andes abstract the cordillera darwin a structural culmination in the andes of tierra del fuego exposes an orogenic core zone that has undergone polyphase deformation and metamorphism some of the classic problems of orogenic zones have remained unanswered in the cordillera darwin the age of deformed plutonic rocks the distinction of structurally reactivated basement and metamorphosed cover rocks and the timing of orogenic events this study addresses and partially answers these questions a well constrained rb sr isochron age of157 8m y and an initial 87 sr 86 sr ratio of 0 7087 obtained from a pre tectonic granitic suite suggest a genetic relation between this suite and upper jurassic silicic volcanic rocks in the cover sequence tobifera formation and also suggest involvement of continental crust in formation of these magmas a poorly constrained rb sr isochron age of240 40m y obtained from supposed basement schists is consistent with field relations in the area which suggest a late paleozoic early mesozoic metamorphism for these pre late jurassic rocks however because of scatter in the data and the uncertainties involved in dating metasedimentary rocks the significance of the isotopic age is dubious compilation of previously published ages in the area 9 with new mineral ages reported here indicate that early andean orogenic events occurred between 100 and 84 m y ago and that subduction related magmatism has contributed probably discontinuously to the crustal evolution of the region throughout the mesozoic

cretaceous diapiric plutonism in the southern cordillera chile the castores and probably the santa rosa plutons of north west isla navarino southern chile have been emplaced by in situ diapirism into metasedimentary rocks of the upper jurassic lower cretaceous yaghan formation for the former this model is consistent with the concentric foliation paralleling the margin of the pluton and the foliation and stratification planes in the metamorphic aureole only the southern part of the santa rosa pluton is preserved and it has some structures similar to those of the castores pluton which can also be interpreted as produced by an inflating diapir the main intrusive rocks of these plutons are quartz monzodiorites and quartz diorites with synmagmatic foliation they were preceded by minor bodies of hornblende gabbros and followed by dykes and small bodies of non foliated granodiorites non foliated to weakly foliated granodiorites forming the centre of the castores pluton probably represent a younger intrusive pulse twelve k ar mineral dates from 10 specimens of plutonic rocks interpreted as near crystallization ages span the period 80 90 ma these dates do not show the sequence of intrusion of the different rock types which may suggest that all of them were intruded and cooled in a short period of time the timing of emplacement of these plutons in relation to tectonism is difficult to determine however a post tectonic emplacement for at least the castores pluton is proposed

the patagonian batholith s of tierra del fuego chile timing and tectonic implications reconnaissance field studies on the patagonian batholith exposed s of tierra del fuego together with 49 new k ar and rb sr mineral dates render new insights into two main aspects of the late mesozoic geology the geochronology and composition of the batholith and time relations between the emplacement of the plutons and deformation the envelope of the batholith in this area is mainly composed of late jurassic early cretaceous sedimentary and volcanic rocks formed in an island arc marginal basin system three main plutonic groups have been recognized which have yielded mineral dates interpreted as crystallization ages ranging from 141 to 34 ma the earliest is a gabbroic assemblage including gabbros diorites and hornblendites exposed as relicts within later granitoids and with dates ranging from 141 to 103 ma the canal beagle plutonic group is composed of granitoids characterized by a penetrative syn magmatic foliation and with dates of 113 to 81 ma the seno ano nuevo plutonic group is formed of granitoids with dates of 60 to 34 ma and is mainly exposed in the outer islands indicating a local oceanwards shift of plutonism with time lower cretaceous island arc volcanism was contemporaneous with the intrusion of the gabbroic assemblage and in part at least of the canal beagle plutonic group folding of the marginal basin infill which has been related to the closure of the marginal basin occurred from about 100 to 93 ma ago the gabbroic assemblage was apparently mainly pre tectonic whereas the seno ano nuevo plutonic group was clearly intruded after this deformational event a post tectonic or possibly late tectonic emplacement for the youngest plutons of the canal beagle plutonic group and a pre or syn tectonic intrusion for the oldest is inferred

delay time tomography of the upper mantle below europe the mediterranean and asia minor summary the method of linearized delay time tomography is applied to retrieve the p velocity heterogeneity of the upper mantle in central europe the mediterranean and asia minor the upper mantle is parametrized in slowness cell functions with lateral dimensions of 1 the model parametrization includes event mislocation vectors and station delay time corrections in all 20 000 parameters describe the model delay time data selected from the isc data base 1964 1982 from regional events and from stations up to 90 of epicentral distance are used about 18 000 events and 937 stations contributed nearly 500 000 delay times we discuss the tomographic method the data processing the cell model employed the computation of the ray geometry the use of composite rays in tomography and the cell hit count the inversions are performed with two different iterative row action methods lsqr and sirt results obtained with the lsqr method will be shown only the spatial resolution is investigated with sensitivity analyses using two different synthetic velocity models a spike model and a model with harmonically varying velocity anomalies the spatial resolution is estimated to be high on the order of the cell size in the mantle below central and southeastern europe the mapped velocity heterogeneity exhibits primarily low velocities around a depth of 200 km and just above the transition from upper to lower mantle this indicates a systematic deviation of the reference model jeffreys bullen from the radially averaged earth structure which may violate the basic assumptions underlying the linearized approach a posteriori investigation of the data also indicates the presence of a low velocity zone and the existence of strong velocity depth gradients

organic carbon burial efficiency in lake sediments controlled by oxygen exposure time and sediment source we compared the burial efficiency of organic carbon buried oc deposited oc in a diverse set of 27 different sediments from 11 lakes focusing on the potential effects of organic matter source oxygen exposure and protective sorption of oc onto mineral surfaces average oc burial efficiency was high mean 48 and it was particularly high in sediments receiving high input of allochthonous organic matter mean 67 further oc burial efficiency was strongly negatively related to the oxygen exposure time again particularly so in sediments receiving high allochthonous loads on the other hand oc burial efficiency was not related to the mineral surface area which was used as a proxy of the sorption capacity of the mineral phase for oc the high oc burial efficiency in many lake sediments can thus be attributed to the frequent and significant input of allochthonous organic matter to lakes as well as to a strong dependence of oc burial efficiency on oxygen exposure time this study demonstrates that the carbon sink in lake sediments alters the oc export from the continents to the sea and that the fate of oc in lake sediments burial vs mineralization to carbon dioxide and methane is highly sensitive to environmental conditions

holocene denudation rates from the superhumid southernmost chilean patagonian andes 53 s deduced from lake sediment budgets abstract holocene denudation rates and their regional variations in the superhumid and temperate climate of the southernmost patagonian andes are poorly surveyed therefore we have investigated denudation in five small lake catchments 0 11 1 62 km 2 across a precipitation gradient from 600 to 9000 mm yr 1 in southernmost chile at 53 s variations in denudation rates can be defined most precisely by using small catchments and short time spans holocene thus can be related to variable bedrock types vegetation cover elevation and climate minimum physical denudation rates were determined from sediment budgets based on sediment echosounder data and physical properties of sediment cores the holocene denudation rates vary from 0 08 mm kyr 1 in the highly elevated bare granitic rock catchments to 9 01 mm kyr 1 in the low and vegetated catchments with a basement of sedimentary rocks these values are comparable with those from similar areas in norway sweden and nw iceland the high annual precipitation in the study area has only a minor effect on denudation enhanced geomorphic activity during paraglacial conditions caused a faster denudation after the glacier retreat until either the clastic glacial detritus was removed or the catchment was covered with vegetation chemical comparisons between basement rocks and lake sediments indicate a pronounced chemical denudation in catchments with peaty soils and low soil water ph values of 3 to 5 the holocene surface lowering rates are on average 200 fold lower than cenozoic surface lowering rates calculated from fission track data for the southern andes this discrepancy can be explained by significantly higher erosion rates during glacial periods

basic magmas in the early evolution of the southernmost patagonian batholith chile basic magmas played an important role in the early evolution of a segment of the patagonian batholith 141 to 34 m y exposed south of tierra del fuego chile the basic magmatism lasted 40 m y to possibly 60 m y 141 to 84 m y it is represented by early gabbros and hornblendites 141 to 103 m y and by synplutonic dikes and some of the numerous basic inclusions in the granitoids of a cretaceous 113 to 84 m y plutonic group some of the inclusions are interpreted as the disrupted portions of synplutonic dikes

seismic sources on the iberia african plate boundary and their tectonic implications the plate boundary between iberia and africa has been studied using data on seismicity and focal mechanisms the region has been divided into three areas a the gulf of cadiz b the betics alboran sea and northern morocco and c algeria seismicity shows a complex behavior large shallow earthquakes h 30 km occur in areas a and c and moderate shocks in area b intermediate depth activity 30 h 150 km is located in area b the depth earthquakes h 650 km are located to the south of granada moment rate slip velocity andhvalues have been estimated for shallow shocks and show similar characteristics for the gulf of cadiz and algeria and quite different ones for the central region focal mechanisms of 80 selected shallow earthquakes 8 mb 4 show thrust faulting in the gulf of cadiz and algeria with horizontal nnw sse compression and normal faulting in the alboran sea with e w extension focal mechanisms of 26 intermediate depth earthquakes in the alboran sea display vertical motions with a predominant plane trending e w solutions for very deep shocks correspond to vertical dip slip along n s trends frohlich diagrams and seismic moment tensors show different behavior in the gulf of cadiz betic alboran sea and northern morocco and northern algeria for shallow events the stress pattern of intermediate depth and very deep earthquakes has different directions vertical extension in the nw se direction for intermediate depth earthquakes and tension and pressure axes dipping about 45 o for very deep earthquakes regional stress pattern may result from the collision between the african plate and iberia with extension and subduction of lithospheric material in the alboran sea at intermediate depth the very deep seismicity may be correlated with older subduction processes

decompression and high temperature low pressure metamorphism in the exhumed floor of an extensional basin alboran sea western mediterranean leg 161 of the ocean drilling program odp has made a major contribution to our understanding of the origin of the alboran basin by demonstrating that it is underlain by rocks of continental origin that have undergone high temperature metamorphism and melting at exceptionally low pressure after exhumation and decompression basement rocks recovered from site 976 consist of high grade schist and gneiss derived from aluminous sediments and minor amounts of marble granitic dikes and migmatitic segregations of granitic material mineral assemblages and textural relations show that an early assemblage including biotite garnet staurolite plagioclase and rutile is overprinted by a second assemblage of biotite sillimanite plagioclase potassium feldspar and ilmenite both assemblages are overprinted by andalusite potassium feldspar and minor garnet migmatitic gneiss contains relict andalusite overprinted by sillimanite and cordierite coexisting with granitic leucosome preliminary pressure temperature estimates suggest that the metamorphic evolution followed an approximately isothermal decompression path from 7 to 3 kbar at temperatures in the range 580 to 630 c after decompression granitic melts formed at 670 c after andalusite breakdown and within the sillimanite stability field the cored rocks closely resemble high grade metamorphic rocks in the adjacent betic cordillera of southern spain which yield early miocene radiometric dates at odp site 976 they are overlain by middle miocene marine sediments the combination of exhumation in an extensional tectonic environment and the evidence for high and increasing temperature during exhumation provide support for and new constraints on current models for the basin that involve the removal of lithospheric mantle below a zone of continental collision accompanied or followed by extension

contrasting styles of superposed deformation in the southernmost andes brittle fault arrays regional structural patterns and marine multichannel seismic reflection profiles across the magallanes basin near 53 s latitude show contrasting styles of superposed extensional contractional and transtensional deformation below the northeastern end of a ne sw transect of the basin seismic reflection profiles show deepening levels of pre late jurassic basement and upper jurassic to lower cretaceous volcanic horizons toward the southwest this trend is associated with apparent normal fault offsets that may be attributed to a widespread late jurassic early cretaceous extensional event contrastingly below the southwestern end of magallanes basin subsurface depths of basement and volcanic horizons shallow to the southwest integration of seismic reflection data with published seismic refraction data geologic mapping and satellite gravity anomaly data suggest that this shallowing trend is controlled structurally by craton vergent basement involved contractional deformation this style of shortening contrasts with thin skinned styles of thrusting in the northern and central parts of the basin on tierra del fuego structural relationships stratigraphic data and previously published metamorphic data indicate that basement involved contraction occurred during latest cretaceous to early tertiary time overprinting all contractional structures steep tertiary strike slip and oblique slip faults in the central and southern parts of the magallanes basin display oblique normal mostly down to the southwest offsets across wnw ese trending segments of the south american scotia transform plate boundary on the south american continent newly discovered features along the plate boundary segment include sediment filled grabens and half grabens up to 7 km wide that occur between subparallel fault splays in the magallanes fault zone kinematic analyses of fault slip data indicate that sinistral transtensional motion occurred within this fault zone between the eastern arm of the straits of magellan and central tierra del fuego together these data indicate that 1 the southernmost andes were under compression during rapid cooling and exhumation of unique high pressure 8 kbars basement rocks now exposed in the darwin metamorphic complex dmc near 55 s latitude and 2 final exhumation of basement rocks in the dmc occurred during a shift away from compression toward transtension during late tertiary crustal relaxation we suggest that preexisting structural anisotropies formed during basement involved contractional deformation and the initiation of seafloor spreading south of south america during the oligocene controlled late tertiary sinistral transtensional patterns in the magallanes fault zone

shear wave velocity structure of the south eastern part of the iberian peninsula from rayleigh wave analysis in this study we present the lithospheric structure of the south eastern part of the iberian peninsula by means of a set of 2d images of shear velocity for depths ranging from 0 to 50 km this goal will be attained by means of the inversion of the rayleigh wave dispersion for it the traces of 25 earthquakes occurred on the neighbouring of the study area from 2001 to 2003 will be considered these earthquakes have been registered by 11 broadband stations located on iberia all seismic events have been grouped in source zones to get an average dispersion curve for each source station path the dispersion curves have been measured for periods between 2 and 45 s by combination of two digital filtering techniques multiple filter technique and time variable filtering the resulting set of source station averaged dispersion curves has been inverted according to the generalized inversion theory to get s wave velocity models for each source station path later these models have been interpolated using the method of kriging to obtain a 2d mapping of the s wave velocity structure for the south eastern part of iberia the results presented in this paper show that the techniques used here are a powerful tool to investigate the crust and upper mantle structure through the dispersion analysis and its inversion to obtain shear velocity distributions with depth by means of this analysis principal structural features of the south eastern part of iberia such as the existence of lateral and vertical heterogeneity in the whole study area or the location of the moho discontinuity at 30 km of depth with an average s velocity of uppermost mantle of 4 7 km s have been revealed other important structural features revealed by this analysis have been that the uppermost of iberian massif shows higher velocity values than the uppermost of the alpine domain indicating that the massif is old and tectonically stable the average velocity of the crust in betic cordillera is of 3 5 km s while in the iberian massif is 3 7 km s all these features are in agreement with the geology and other previous geophysical studies

jurassic to miocene k ar dates from eastern central patagonian cordillera plutons chile 45 48 s thirty nine k ar and one ar ar radiometric dates from the eastern central meso cenozoic patagonian batholith and eastern satellite plutons of the aysen region of southern chile between latitudes 45 and 48 s combined with previous dating of seven plutons have yielded six age groups 1 middle to late jurassic 2 early cretaceous 3 mid cretaceous 4 late cretaceous 5 oligocene and 6 miocene in general the cretaceous and younger ages correspond to previous reported ages for other parts of the main batholith but for the satellite plutons to the east show a wider age spectrum than the previously accepted late miocene dates these results indicate a relatively continuous late jurassic to mid cretaceous plutonism known to have been coeval with volcanic activity followed by intermittent magmatism biotite k ar dates of c 143 151 and 106 109 ma from cataclastic granitoids may be marking the time of deformation a review of all radiometric data on magmatic rocks from the region between 45 and 48 s in chile shows a gap in palaeocene ages that may correlate with a period of low angle flat slab subduction between 65 50 ma

crustal growth during back arc closure cretaceous exhumation history of cordillera darwin southern patagonia the cordillera darwin metamorphic complex is unique in the andes in exposing kyanite staurolite schist north of the beagle channel in southern patagonia garnet in amphibolite facies pelitic schists from bahia pia has patchy textures whereby some grains consist of clear grossular rich garnet with fine grained s1 inclusion trails truncated by regions of turbid spessartine pyrope rich garnet with biotite muscovite plagioclase and quartz inclusions micron scale aqueous inclusions in turbid garnet are consistent with recrystallization facilitated by fluid ingress s2 inclusion trails indicate this was broadly contemporary with the growth of kyanite and staurolite in the matrix pseudosection modelling in na2o cao k2o feo mgo al2o3 sio2 h2o tio2 fe2o3 nckfmashto is used to infer a p t path dominated by decompression from 12 to 9 kbar at t 620 c coupled with garnet mode decreasing from 5 to 1 u th pb in situ dating of s2 monazite indicates that staurolite and kyanite growth and thus exhumation was underway before 72 6 1 1 ma contact aureoles developed adjacent to late granite intrusions include sillimanite bearing migmatites formed at p 6 kbar after 72 ma metamorphism of southern cordillera darwin induced by continental underthrusting beneath the arc related to closure of the rocas verdes back arc basin was terminated by thrusting controlled exhumation with the rocks at p 9 kbar by c 73 ma and 6 kbar by c 70 ma

a high toc shale in a low productivity world the late mesoproterozoic arctic bay formation nunavut abstract the latest mesoproterozoic arctic bay formation borden basin nunavut canada is up to 1130 m thick and contains a significant proportion of unusually organic rich black shale up to 12 3 wt total organic carbon insofar as increased biological productivity is related to organic matter burial this organic rich succession is seemingly incongruent with the low biological productivity world hypothesised for much of the proterozoic to better understand the conditions leading to development of this organic rich unit we explore the redox geochemistry of the arctic bay formation using a multi proxy approach nitrogen isotopes iron speciation total organic carbon total sulphur and trace metal abundances redox proxy data support a stratified water column with oxic surface waters underlain by intermittently euxinic waters which are in turn underlain by persistently ferruginous deeper waters the highly alkaline restricted marine basin in which the arctic bay formation was deposited may have allowed for rapid sequestration of highly reactive iron in carbonate minerals resulting in an excess of sulphur that resulted in sulphurisation of organic matter estimates for organic matter burial rates during deposition of the arctic bay formation suggest that they were perhaps 5 6 times mid proterozoic average values although there are permissible scenarios in which it was extremely productive underscoring that such organic rich sedimentary rocks could be produced in a low productivity world

fluid inclusion and stable isotopic constraints on fluid sources and evolution of the luojiahe cu deposit in the southern margin of the north china craton abstract the luojiahe cu deposit in the zhongtiaoshan region is located in the southern margin of the north china craton the orebodies are hosted in the mafic volcanic sedimentary sequences of the metamorphosed greenschist facies neoarchean songjiashan group the luojiahe cu mineralization can be divided into the primary volcanogenic massive sulfide vms mineralization stage stage i banded or stockwork ores and the subsequent metamorphic remobilization stage stage ii coarse vein ores three types of quartz selected for fluid inclusion fi studies were collected from the stage i banded q1 and stockwork q2 ores and stage ii coarse vein q3 ores four types of fis were identified 1 liquid rich fis l type 2 pure vapor and vapor rich fis v type 3 daughter mineral bearing fis s type and 4 ch4 h2o fis c type systematical microthermometric and h o isotopic studies show that the stage i ore forming fluids consist predominantly of high salinity evolved seawater 125 220 c 23 9 27 9 wt nacl equiv and some magmatic hydrothermal fluids 249 339 c 34 5 42 2 wt nacl equiv the two fluid end members are represented by the l type fis in q1 and the s and v type fis in q2 the temperature and salinity variation trends of the l type fis in q1 indicate a mixing process between the hot evolved seawater and cold seawater at stage i furthermore the v and s type fi coexistence in q2 and their microthermometric data suggest that fluid unmixing has occurred in original magmatic fluids at stage i in contrast the stage ii ore forming fluids consist of ch4 rich metamorphic fluids 192 350 c 10 6 43 2 wt nacl equiv carbon isotopic analysis of the stage ii calcite 4 58 to 10 83 and graphite 32 01 to 39 16 in the ore hosting chlorite schist indicates that the metamorphic ore forming fluids had exchanged carbon isotope with graphite the generation of ch4 may have resulted from the interaction between h2o released by metamorphic devolatilization and graphite the continuous consumption of h2o in the hydrothermal fluid system may have increased the fluid salinity and triggered fluid unmixing in the ch4 nacl h2o system in addition the vms metallogenic environment is generally favorable for microbial communities it is considered that the graphite at luojiahe may have been derived from sedimentary organic matter formed in seafloor hydrothermal vent systems as also supported by carbon isotopic data we propose that at stage i the main mineralization may have been resulted from 1 fluid mixing of hot evolved seawater and cold seawater in the near surface environment and 2 fluid unmixing caused by the percolation of magmatic fluids into syn volcanic faults forming the stockwork ores at stage ii the interaction between h2o and graphite may have resulted in the reduction of ore forming fluids and cu precipitation and fluid unmixing in the ch4 nacl h2o system may have further promoted the cu mineralization

active tectonics of the pamirs and karakorum the source parameters of moderate to large sized earthquakes and the spatial distribution of earthquake hypocenters are used to investigate the active tectonics and three dimensional configuration of the subducted lithosphere in the pamir karakorum intracontinental convergence zone the continental collision has resulted in intense deformation in the upper crust the deformation is distributed over a broad area and is absorbed by thrust and strike slip faulting along the northern margin of the pamirs where the deformation due to continental collision is most intense the northern pamir thrust and several major fault zones mark the present boundary most of the earthquakes that occurred in these fault zones are shallow crustal events with focal mechanism solutions correlated with the tectonic features of the region along the pamir front thrust faulting dominates while on the western and eastern edges of the pamirs the style of deformation is characterized by oblique thrusting with a component of strike slip motion thrust type events beneath the tadjik depression indicate that both the sedimentary rocks and the basement are involved in shortening right lateral strike slip motion is observed on the eastern edge of the pamirs where they border the tarim basin and in the talas fergana kun lun fault zone a southward dipping seismic zone beneath the pamirs and karakorum indicates that the asian lithosphere has been subducted along the pamir front to a depth of at least 200 km this interpretation is consistent with published geological and geophysical data focal mechanism solutions of some intermediate depth events beneath the pamirs show strike slip faulting with approximately n s horizontally oriented p axes indicating that nearly horizontal compression at the intermediate depth is the predominant mode of deformation a 90 km deep event beneath the karakorum is interpreted as occurring in the subducted asian lithosphere the fact that the p axis of this event is oriented parallel to the descending plate suggests downdip compression within the asian lithosphere the overall tectonics of the region is interpreted as a consequence of the underthrusting asian lithosphere being impinged upon by the shallow northward underthrusting indian lithosphere

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

significant role of organic sulfur in supporting sedimentary sulfate reduction in low sulfate environments abstract dissimilatory sulfate reduction dsr is a major carbon mineralization pathway in aquatic sediments soils and groundwater which regulates the production of hydrogen sulfide and the mobilization rates of biologically important elements such as phosphorus and mercury it has been widely assumed that water column sulfate is the main sulfur source to fuel this reaction in sediments while this assumption may be justified in high sulfate environments such as modern seawater we argue that in low sulfate environments mineralization of organic sulfur compounds can be an important source of sulfate using a reaction transport model we investigate the production of sulfate from sulfur containing organic matter for a range of environments the results show that in low sulfate environments 50 of sulfate reduction in well oxygenated systems porewater sulfate profiles often exhibit sub interface peaks so that sulfate fluxes are directed out of the sediment our measurements in lake superior the world s largest lake corroborate this conclusion offshore sediments act as sources rather than sinks of sulfate for the water column and sediment dsr is supported entirely by the in sediment production of sulfate sulfate reduction rates are correlated to the depth of oxygen penetration and strongly regulated by the supply of reactive organic matter rate co regulation by sulfate availability becomes appreciable below 500 m level the results indicate the need to consider the mineralization of organic sulfur in the biogeochemical cycling in low sulfate environments including several of the world s largest freshwater bodies deep subsurface and possibly the sulfate poor oceans of the early earth

microseismicity and tectonics in the granada basin spain a microseismic experiment carried out in 1994 in the granada basin spain permitted the precise recording of more than 80 local earthquakes the dense distribution of the local network with 40 to 50 instrumental records for each event enabled us to have well controlled hypocenters and also 10 reliable focal mechanisms the above observations are interpreted together with topographic data neotectonics and sub surface information microtectonic observations in sierra elvira padul and zafarraya gave a set of fault planes and striae which were interpreted in terms of the recent regional stress tensor the actual stress tensor obtained from the microseismic campaign data gives a regime in radial extension with r1 vertical and r3 oriented ns to nne microtectonic information is coherent with these orientations but closer to 3 axial extension a set of 64 mechanisms obtained from the permanent andalusian network favors a ns orientation for r3 this results are interpreted in terms of the general model implying the lateral ejection of the betic ranges towards the atlantic d 2002 elsevier science b v all rights reserved

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

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

crustal thickness and images of the lithospheric discontinuities in the gibraltar arc and surrounding areas the gibraltar arc and surrounding areas are a complex tectonic region and its tectonic evolution since miocene is still under debate knowledge of its lithospheric structure will help to understand the mechanisms that produced extension and westward motion of the alboran domain simultaneously withnw se compression driven by africa europe plates convergence we perform a p wave receiver function analysis in which we analyse new data recorded at 83 permanent and temporary seismic broad band stations located in the south of the iberian peninsula these data are stacked and combined with data from a previous study in northern morocco to build maps of thickness and average vp vs ratio for the crust and cross sections to image the lithospheric discontinuities beneath the gibraltar arc the betic and rif ranges and their iberian and moroccan forelands crustal thickness values show strong lateral variations in the southern iberia peninsula ranging from 19 to 46 km the variscan foreland is characterized by a relatively flat moho at 31 km depth and an average vp vs ratio of 1 72 similar to other variscan terranes which may indicate that part of the lower crustal orogenic root was lost the thickest crust is found at the contact between the alboran domain and the external zones of the betic range while crustal thinning is observed southeastern iberia down to 19 km and in the guadalquivir basin where the thinning at the iberian paleomargin could be still preserved in the cross sections we see a strong change between the eastern betics where the iberian crust underthrusts and couples to the alboran crust and the western betics where the underthrusting iberian crust becomes partially delaminated and enters into the mantle the structures largely mirror those on the moroccan side where a similar detachment was observed in northern morocco we attribute a relatively shallow strong negativepolarity discontinuity to the lithosphere asthenosphere boundary this means relatively thin lithosphere ranging from 50 km thickness in southeastern iberia and northeastern morocco to 90 100 km beneath the western betics and the rif with abrupt changes of 30 km under the central betics and northern morocco our observations support a geodynamic scenario where in western betics oceanic subduction has developed into ongoing continental subduction delamination while in eastern betics this process is inactive

3 d crustal structure of the extensional granada basin in the convergent boundary between the eurasian and african plates three dimensional p and s wave velocity models of the crust under the granada basin in southern spain are obtained with a spatial resolution of 5 km in the horizontal direction and 2 to 4 km in depth we used a total of 15407 p and 13704 s wave high quality arrival times from 2889 local earthquakes recorded by both permanent seismic networks and portable stations deployed in the area the computed p and s wave velocities were used to obtain three dimensional distributions of poisson s ratio r and the porosity parameter vpvs the 3 d velocity images show strong lateral heterogeneities in the region significant velocity variations up to 7 in p and s velocities are revealed in the crust below the granada basin at shallow depth high velocity anomalies are generally associated with mesozoic basement while the low velocity anomalies are related to the neogene sedimentary rocks the south southeastern part of the granada basin exhibits high r values in the shallowest layers which may be associated with saturated and unconsolidated sediments in the same area vpvs is high outside the basin indicating low porosity of the mesozoic basement a low velocity zone at 18 km depth is found and interpreted as a weak ductile crust transition that is related to the cut off depth of the seismic activity in the lower crust at 34 km depth a clear slow vp and vs anomalous zone may indicate variations in lithology and or with the rigidity of the lower crust rocks d 2002 elsevier science b v all rights reserved

deep structure of japan subduction zone as derived from local regional and teleseismic events we have determined a detailed three dimensional p wave velocity structure of the japan subduction zone to 500 km depth by inverting local regional and teleseismic data simultaneously we used 45 318 p wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the japan islands the arrival times are recorded by the japan university seismic network which covers the entire japan islands densely and uniformly we also used 4211 travel time residuals from 100 teleseismic events which are read from seismograms recorded by seismic stations in northeastern japan in comparison with the previous results obtained from only local and regional events the present result for the area around the lower plate boundary and the mantle below the plate is determined more reliably because of the addition of 7035 data from 100 teleseismic events and 41 very deep earthquakes in the crust and uppermost mantle low velocity zones are clearly visible beneath active volcanoes in the mantle wedge the low velocity zones generally parallel with the slab and exist continuously to a depth of about 200 km which is consistent with the petrological geochemical and geodynamic studies we consider that the existence of volcanism related low velocity anomalies in the mantle wedge is a general seismological characteristic of subduction zones in light of all the available tomographic results for many subduction zones in the world the pacific slab beneath japan is imaged more clearly than in previous studies as a high velocity zone with a thickness of 80 90 km and a p wave velocity 4 6 higher than the normal mantle lower velocity anomalies are found in the mantle below the slab

mechanism for burgess shale type preservation exceptionally preserved fossil biotas of the burgess shale and a handful of other similar cambrian deposits provide rare but critical insights into the early diversification of animals the extraordinary preservation of labile tissues in these geographically widespread but temporally restricted soft bodied fossil assemblages has remained enigmatic since walcott s initial discovery in 1909 here we demonstrate the mechanism of burgess shale type preservation using sedimentologic and geochemical data from the chengjiang burgess shale and five other principal burgess shale type deposits sulfur isotope evidence from sedimentary pyrites reveals that the exquisite fossilization of organic remains as carbonaceous compressions resulted from early inhibition of microbial activity in the sediments by means of oxidant deprivation low sulfate concentrations in the global ocean and low oxygen bottom water conditions at the sites of deposition resulted in reduced oxidant availability subsequently rapid entombment of fossils in fine grained sediments and early sealing of sediments by pervasive carbonate cements at bed tops restricted oxidant flux into the sediments a permeability barrier provided by bed capping cements that were emplaced at the seafloor is a feature that is shared among burgess shale type deposits and resulted from the unusually high alkalinity of cambrian oceans thus burgess shale type preservation of soft bodied fossil assemblages worldwide was promoted by unique aspects of early paleozoic seawater chemistry that strongly impacted sediment diagenesis providing a fundamentally unique record of the immediate aftermath of the cambrian explosion

geophysical signatures of a basic body rock placed in the upper crust of the external zones of the betic cordillera southern spain 1 the combined use of different geophysical techniques seismic tomography gravity and magnetic research carried out in the external zones of the betic cordillera southern spain points to the existence of a controversial set of anomalies the high resolution seismic tomography clearly indicates a fast anomaly of up to 7 p wave velocity and the surface magnetic and aeromagnetic measurements also show a positive anomaly the negative anomaly of the dipole is very smooth however the estimated bouguer anomaly values are clearly negative this set of anomalies has been interpreted as due to a basic body rock gabbros located in the upper crust the existence of this body suggests that the iberian massif constitutes the basement of the external zones of the betic cordilleras up to near the boundary with the internal zones

organic carbon burial efficiencies in sediments the power law of mineralization revisited the reactivity of marine organic carbon toward oxidation has been described as a simple power law function that decreases with carbon age and is applicable from fresh phytoplankton to nearly kerogen we demonstrate that a correction to this law that accounts for a slowing in carbon mineralization under anoxic conditions makes the law consistent with observed carbon burial efficiencies which are known to be higher in anoxic environments our analyses of data from freshwater lakes reveal that the reactivity of autochthonous organic carbon follows similar trends in freshwater and marine environments the similarity suggests that the power law may be applicable to environments with diverse water chemistries such as the anoxic or low sulfate oceans of the geologic past using this power law we calculate the theoretical burial efficiencies for organic carbon under a range of conditions including those in anoxic ancient oceans the calculations suggest that in abyssal sediments the buried fraction of deposited organic carbon may have been 6 40 times higher in precambrian anoxic oceans than today whereas for coastal areas the difference was much smaller oxygenation of the deep ocean then would have caused a large scale decrease in organic carbon burial with a corresponding negative feedback on rates of atmospheric oxygen accumulation

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

geodynamic implications for the formation of the betic rif orogen from magnetotelluric studies 1 magnetotelluric data from the central betics mountains spain have been used to determine the electrical resistivity of the crust after a three dimensional 3d interpretive approach at shallow levels 3 km the most striking and well resolved feature of the model is an upper middle crust conductive body located at the core of the internal betics antiform this approximately 14 km thick body is interpreted as basic or ultrabasic rocks containing a conducting mineral phase its structural location above the sole thrust of the betic orogen and beneath the nevado filabride complex confirms the presence of a major suture zone between this complex and the autochthonous iberian plate this suture may correspond to an ancient oceanic or transitional domain developed between iberia and the alboran domain during the opening of the tethys ocean partially subducted and closed during the development of the betic orogen the possible geodynamic scenarios for the betics have been reconsidered taking into account this new constraint

effect of sea level on organic carbon preservation in the okinawa trough over the last 91 kyr abstract the burial of marine organic carbon primarily synthesized by marine autotrophic organisms into sediment removes co2 from the ocean atmosphere system however linkages between export productivity and organic carbon burial and the controlling factors remained ambiguous especially for marginal seas in this study we measured redox sensitive trace metal concentrations and planktic foraminifer fauna assemblages for a sediment core collected from the middle okinawa trough spanning the past 91 kyr through compilation with other published data we found that the export productivity was decoupled from organic carbon burial variability of manganese and total sulfur contents suggested that both bottom water and surface sediment had experienced significant redox change in the last glacial cycle by using the ratio of marine organic carbon to reactive phosphorus i e ocmarine preact we reconstructed organic carbon burial efficiency which showed relatively higher values during the last glacial and low values during the middle late holocene after synthesizing our new and reported data we conclude that the middle okinawa trough bottom water redox state was controlled by deep water ventilation induced by sea level change and related kuroshio current intrusion instead of by the oxygen consumption owing to local export productivity

kinematics of the iberia maghreb plate contact from seismic moment tensors and gps observations the iberian peninsula and the maghreb experience moderate earthquake activity and oblique nw se convergence between africa and eurasia at a rate of 5 mm yr coeval extension in the alboran basin and a n35 e trending band of active left lateral shear deformation in the alboran betic region are not straightforward to understand in the context of regional shortening and evidence complexity of deformation at the plate contact we estimate 86 seismic moment tensors mw 3 3 to 6 9 from time domain inversion of near regional waveforms in an intermediate period band those and previous moment tensors are used to describe regional faulting style and calculate average stress tensors the solutions associated to the trans alboran shear zone show predominantly strike slip faulting and indicate a clockwise rotation of the largest principal stress orientation compared to the regional convergence direction 1 at n350 e at the n algerian and sw iberian margins reverse faulting solutions dominate corresponding to n350 e and n310 e compression respectively over most of the betic range and intraplate iberia we observe predominately normal faulting and wsw ene extension 3 at n240 e from gps observations we estimate that more than 3 mm yr of african nubian eurasian plate convergence are currently accommodated at the n algerian margin 2 mm yr in the moroccan atlas and 2 mm yr at the sw iberian margin 2 mm yr is a reasonable estimate for convergence within the alboran region while alboran extension can be quantified as 2 5 mm yr along the stretching direction n240 e superposition of both motions explains the observed left lateral transtensional regime in the trans alboran shear zone two potential driving mechanisms of differential motion of the alboran betic gibraltar domain may coexist in the region a secondary stress source other than plate convergence related to regional scale dynamic processes in the upper mantle of the alboran region as well as drag from the continental scale motion of the nubian plate along the southern limit of the region in the atlantic ocean the 3 5 mm yr westward motion of the gibraltar arc relative to intraplate iberia can be accommodated at the transpressive sw iberian margin while available gps observations do not support an active subduction process in this area

recent tectonic structures in a transect of the central betic cordillera the betic cordillera has undergone recent alpine deformations related to the eurasian african plate interaction boundary most of the present day relief has been built up since tortonian times and is related to the development of folds and faults that are overprinted on older deformations and some of the faults may be considered as out of sequence the combination of geophysical and geological data makes it possible to determine the main features of the recent tectonic structures or those recently active in its central transect the main fault is a crustal detachment that separates a footwall constituted by the iberian massif and a hanging wall formed by the rocks of the betic cordillera while the footwall is practically undeformed the hanging wall has been folded and faulted the folds are mainly e w to ne sw and have larger sizes and higher related relieves towards the south the reverse faults are mainly concentrated in the northern mountain front however normal faults affect the southern part of the cordillera and are associated with the development of large asymmetrical basins such as the granada depression in this setting the slip along the crustal detachment is variable and should increase southwards the model of the recent tectonics in the central transect of the cordillera is compatible with the presence of an active subduction in the alboran sea and contrasts notably with the setting of the eastern betic cordillera mainly deformed by transcurrent faults

back arc extension in the southern andes a review and critical reappraisal the interpretation that the mafic rocas verdes green rocks complex of the southern andes represents part of the uplifted floor of a late jurassic to early cretaceous back arc basin has proved particularly useful in understanding the geological evolution of the southern andes the north scotia ridge and the antarctic peninsula clear field evidence of the back arc setting of the 9rocas verdes gabbro sheeted dyke pillow lava ophiolitic assemblages has encouraged fruitful petrological and geochemical comparison with mid ocean ridge and marginal basin basalts other onshore ophiolite complexes and archaean greenstone belts uncertainty still surrounds estimates of the original width and depth of the basin as well as the proportion of new mafic crust compared with relict sialic crust in the basin floor these questions are unresolved owing mainly to the considerable lower cretaceous turbiditic basin infill and the effects of mid cretaceous compressional deformation while the field relations clearly indicate that the rocas verdes basin is not an older piece of ocean floor trapped behind a volcanic arc it is not yet clear whether the basin is directly subduction related or falls in the category of back arc leaky transforms like the proto gulf of california or apparent rip off features like the andaman sea

the magallanes and deseado fault zones major segments of the south american scotia transform plate boundary in southernmost south america tierra del fuego the 600 km long magallanes fault zone and the newly discovered 100 km long deseado fault zone represent large but little understood segments of the south american scotia sam sco transform on the south american continent 50 56 s latitude kinematic analyses of fault populations near mount hope on tierra del fuego indicate that these fault zones have accommodated sinistral strike slip motion with a probable component of normal slip on vertical subvertical faults during cenozoic time regional geological data and overprinting relationships between the strike slip oblique slip faults and mid cretaceous to oligocene contractional structures suggest that sinistral motion in the mount hope segments of the two fault zones occurred significantly since 60 ma and dominantly since 30 ma with the most recent episode of activity occurring during the quaternary results from mapping show 20 25 km of cumulative sinistral separation of a thrust contact across a well exposed segment of the magallanes fault zone north and east of mount hope although the exact amount of displacement accommodated by this segment of the magallanes fault zone remains uncertain the 20 25 km of cumulative sinistral separation combined with an estimate of 3 km of vertical separation is consistent with small magnitudes of displacement of the order of tens of kilometers this result apparently contrasts with predictions based on plate reconstructions that large amounts of strike slip displacement 200 km have been accommodated internally within southernmost south america since early cenozoic time a low magnitude of displacement along the mount hope segment of the magallanes fault zone suggests that 1 strike slip motion has been partitioned across the continent since about 30 ma and or 2 the mount hope segment of the magallanes fault zone represents a young part of the modern plate boundary that migrated to its present location during the last few million years

velocity structure of the pamir hindu kush region possible evidence of subducted crust the arrival times of compressional p and shear s waves from approximately 580 microearthquakes recorded by a temporary array in the pamir hindu kush region in central asia are used to deduce one and three dimensional velocity structures of this region the results for one dimensional structures imply that the moho is at 70 5 depth also there is a velocity reversal near 160 km depth which is inferred to be the beginning of the low velocity zone this reversal continues to depths of approximately 230 km below 230 km velocities are somewhat higher than those of normal mantle at similar depths 9 3 km s versus 8 4 km s for p waves the outstanding feature of the results for three dimensional velocity structures is a broad 40 km centrally located region with 8 to 10 lower velocities than those in the surrounding regions this low velocity region envelopes the seismic zone at depths between 70 and 150 km the region may actually extend beyond these depths but the results for shallower and deeper structure lack sufficient resolution to decide several tests using both hypothetical and real data were performed to estimate the reliability of the three dimensional solutions the results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks although one must be cautious of the effects of averaging in interpreting the solution the low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust therefore while subduction has occurred in the pamir hindu kush the results of the three dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km appendices are available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 013 1 00 payment must accompany order

calc alkaline and alkaline miocene and calc alkaline recent volcanism in the southernmost patagonian cordillera chile abstract in the southernmost patagonian cordillera south of lat 54 45 previously unrecorded calc alkaline dacites and andesites and adjacent alkali basalts were dated at 21 and 18 ma k ar respectively in the same area post glacial low k calc alkaline andesites identified at isla cook represent the southernmost recent volcanism recorded so far in south america about 400 km south of monte burney previously known as the southernmost recent volcano the association of contemporaneous calc alkaline and alkali volcanics suggests a subduction related environment for the miocene volcanism and the recent calc alkaline volcanics indicate that currently there is a small component of subduction of the antarctic plate beneath the south american part of the scotia plate

constraints on deformation of the southern andes since the cretaceous from anisotropy of magnetic susceptibility the southernmost segment of the andean cordillera underwent a complex deformation history characterized by alternation of contractional extensional and strike slip tectonics key elements of southern andean deformation that remain poorly constrained include the origin of the orogenic bend known as the patagonian orocline here renamed as patagonian arc and the exhumation mechanism of an upper amphibolite facies metamorphic complex currently exposed in cordillera darwin here we present results of anisotropy of magnetic susceptibility ams from 22 sites in upper cretaceous to upper eocene sedimentary rocks within the internal structural domain of the magallanes fold and thrust belt in tierra del fuego argentina ams parameters from most sites reveal a weak tectonic overprint of the original magnetic fabric which was likely acquired upon layer parallel shortening soon after sedimentation magnetic lineation from 17 sites is interpreted to have formed during compressive tectonic phases associated to a continuous n s contraction our data combined with the existing ams database from adjacent areas show that the early cretaceous late oligocene tectonic phases in the southern andes yielded continuous contraction variable from e w in the patagonian andes to n s in the fuegian andes which defined a radial strain field a direct implication is that the exhumation of the cordillera darwin metamorphic complex occurred under compressive rather than extensional or strike slip tectonics as alternatively proposed if we agree with recent works considering the curved magallanes fold and thrust belt as a primary arc i e no relative vertical axis rotation of the limbs occurs during its formation then other mechanisms different from oroclinal bending should be invoked to explain the documented radial strain field we tentatively propose a kinematic model in which reactivation of variably oriented jurassic faults at the south american continental margin controlled the late cretaceous to cenozoic evolution of the magallanes fold and thrust belt yielding the observed deformation pattern

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

the relative locations of multiplets in the vicinity of the western almeria southern spain earthquake series of 1993 1994 summary we have analysed 721 earthquakes 1 5jmbj5 0 of the 1993 1994 western almero a southern spain series and the following seismicity in the area until 1998 among the data there are several multiplets events characterized by very similar seismograms at the short period stations of the local network we detected similar seismograms using crosscorrelation analysis of the p and s arrivals and classified similar events into families or clusters we found 39 multiplet clusters of 3 33 events within each cluster relocations relative to a master event have been calculated by using the interpolated cross correlation maxima for the precise relative timing of p and s phases at each station relative arrival times have been compared for all the possible selections of the master event and adjusted by forming the mean value after removing the outliers the distribution of the stations does not permit a satisfactory resolution of focal depths but relative epicentres have been determined with an accuracy of a few tens of metres typically they draw well defined lineaments and show two dominant strike directions n120u 130ue and n60u 70ue these directions are coincident with known fault systems in the area and with the source parameters of three of the largest events mw 4 8 3 6 and 4 9 which were estimated from waveform modelling of near field acceleration records at a single station consistent with previous studies distances within multiplets typically several tens of metres are for the most part clearly smaller than the fracture radii of these events this indicates repeated slip on the same fault segment it was possible to obtain precise relative locations between several nearby clusters thereby imaging a very heterogeneous seismotectonic fine structure of the source area i e the positions of adjacent active fault segments and the fragmentation of the crust into small approximately 1 km tectonic blocks

effect of oxygen on degradation rate of refractory and labile organic matter in continental margin sediments in order to study the effect of oxygen on degradation rate of bulk sedimentary organic matter we developed a new incubation method in contrast to most previous experiments 1 all of the sediment undergoing decomposition was maintained under oxygenated or oxygen free conditions and 2 organic matter of varying lability was studied the production of co2 during incubations of sediment in glass test tubes corrected for dissolution precipitation of calcium carbonates was used as a measure of degradation rates the laboratory experiments using surficial and buried continental shelf and slope sediment from the open skagerrak northeastern north sea demonstrated that the effect of oxygen on the degradation rate of sedimentary organic matter is a function of the lability of the decomposing material fresh material was degraded with little difference in rates in the presence or absence of oxygen whereas old material was decomposed significantly up to 3 6 times faster with oxygen than without oxygen an implication of these findings is that bioturbation by exposing old buried material to oxygen may enhance integrated organic carbon oxidation in marine sediments this constitutes a previously unexplored mechanism by which faunal reworking may stimulate carbon degradation the anoxic decomposition rates of organic material buried at 20 cm depth in sediment were the lowest measured we found however that the extent of oxidation of this buried old sediment was considerably larger than that of surficial sediment under oxygenated conditions which indicated that the oxic anoxic oxic redox transitions deposition under oxic conditions burial under anoxia and reexposure to oxygen promoted degradation our results therefore also suggest that the extent of long term decomposition of sedimentary organic material is smaller under oxygenated or anoxic conditions alone than when the material is exposed to the repeated activities of both oxic and anoxic microorganisms