a submarine hydrothermal manganese deposit from the south west pacific island arc hydrothermal deposits of a wide variety of types are being found with increasing frequency on or near actively spreading mid ocean ridges1 4 however they also have a potential to occur in other submarine volcanic settings including island arcs5 to follow up indications of mineralization associated with submarine hydrothermal activity in the south west pacific island arc6 a joint new zealand oceanographic institute imperial college research cruise was mounted in may 1981 aboard the rv tangaroa during this cruise over 130 sampling stations were occupied at one of which were dredged manganese deposits with strong hydrothermal affinities this is the first report of such deposits from an island arc setting

a seismic discontinuity in the upper mantle between the eastern alps and the western carpathians constraints from wide angle reflections and geological implications abstract seismic investigation of the lithosphere by means of active source experiments is mostly confined to the crust and the moho structures in the upper mantle are more likely to be discovered by analyses of teleseismic data although these methods are restricted in their resolution capabilities the relatively rare evidence for upper mantle refractors or reflectors in active source data enables challenging and interesting studies of the lower and not so well known part of the lithosphere we present such an example from the tectonically complex region between the eastern alps and the western carpathians this area was covered by several extensive 3d wide angle reflection refraction experiments within the last decade and their layout was designed to illuminate the crustal structure and in particular the moho discontinuity in some areas reflections from below the moho are also recorded these reflections occur at recording offsets between 200 and 500 km and they are particularly strong in cross line recordings we derive a set of travel times from the data and perform a tomographic inversion for the depth and shape of the reflecting interface the inversion makes use of an existing 3d crustal model which also includes the moho topography since the upper mantle velocities are poorly constrained and the azimuthal distribution of the rays is biassed several tests are applied to investigate the reliability of possible solutions the results from the tomographic inversion indicate an overall horizontal and radially dipping reflector the average depth of the reflector is 55 km which is about 25 km below the crust mantle transition and amplitude modelling suggests that the reflecting interface represents a velocity increase the investigated area is further characterised by deep sedimentary basins high heat flow high velocities in the lower crust diffuse moho signature and a strong positive bouguer anomaly nearby xenolith outcrops exhibit a pronounced change in anisotropy and indicate the presence of two distinct layers in the lithospheric mantle whereas the deeper layer is thought to present more juvenile lithosphere derived from thermal relaxation in the post extension phase most likely the upper mantle reflector also represents this change in anisotropy though other scenarios are also possible we conclude that the entire lithosphere is significantly shaped by extensional processes which affect the area since the late oligocene early miocene

ferromanganese oxide deposits from the central pacific ocean i encrustations from the line islands archipelago abstract chemical and mineralogical analyses of a well controlled suite of ferromanganese encrustations from the line islands archipelago central pacific suggest that they represent purely hydrogenous deposits i e they have formed through the slow accumulation of trace metal enriched oxides directly from the water column mineralogically they consist predominantly of mno2 and amorphous feoohxh2o compositionally they are similar to mno2 nodules from adjoining basinal areas but are enriched in both mn mean 20 4 max 29 3 and co mean 0 55 max 1 57 mno2 is the most important trace metal bearing phase strong associations are noted between it and co mo ni zn and cd whilst only be is associated specifically with feooh v sr and pb are partitioned between the authigenic oxide phases whilst ti most probably occurs as tio2xh2o cu is contained in both aluminosilicate contaminant phases and fe oxide phases these relations are considered to reflect the differing scavenging behaviour of mn and fe oxides in the water column crusts from 1 2 km are enriched in mn and the mn related elements and exhibit higher mn fe ratios than deeper crusts which are compositionally constant the higher mn fe ratios may result from a supply of mn from continental borderland sediments at these depths which is transported horizontally by advective diffusive processes since manganophile elements are enriched relative to mn in the 1 2 km crusts it is considered that the supply of mn is scavenged by existing oxides is oxidised and effectively occludes them a higher proportion of oxide particles thus exhibit mn oxide scavenging properties in the 1 2 km depth zone the increased vertical flux of mn resulting from the supply at 1 2 km is not reflected by higher mn fe ratios in deeper crusts so that the vertical flux of oxides is not simply related to the standing crop the mn fe ratios of the crusts thus reflect the composition of suspended oxides at similar depths

permo carboniferous magmatism of the northeast german basin abstract variscan continent continent collision in central europe was followed by a period of syn to late tectonic magmatism involving the extrusion of large volumes of permosilesian age volcanic rocks particularly in the northeast german basin the volcanic suite up to ca 2000 m thick comprises rhyolites acid and basic andesites and tholeiitic and alkali basalts tectono geochemical analysis in conjunction with volume calculations and petrography reveals changes in volcanic activity over time a total of five eruptive stages are recognised beginning in the stephanian and extending up into the upper rotliegend permian these include the stephanian late carboniferous andesite pre ignimbrite stage i lower rotliegend permian age explosive ignimbrite stage ii post ignimbrite stage iii late rhyolite stage iv and upper rotliegend permian age late basalt stage v in addition several volcanic centres including at least two large caldera like structures are recognised the total calculated volume for the northeast german basin volcanics is ca 48 000 km3 comprising rhyolitic rocks and ignimbrites ca 70 andesitic rocks ca 26 and basalts with cogenetic dolerites ca 4 regional variations in composition and geochemistry are related to diverse crust mantle regimes which correlate with differing positions relative to the heterogeneous basement of the variscan foredeep or within the rhenohercynian orogenic zone the crustal thickening and increasing generation depth of the mafic magmas from north to south may reflect the presence of a pre existing subduction influenced basaltic magma source

chemical and isotopic investigations into the origin of clay minerals from the galapagos hydrothermal mounds field abstract a dark green authigenic nontronite is the major component of the galapagos hydrothermal mounds field sediments oxygen isotopic compositions of the chemically purified in situ mounds temperatures of up to 15 c assuming an authigenic origin the fe rich montmorillonite that dominates in the noncarbonate clay fraction of the surrounding pelagic ooze has isotopic formation temperatures of 27 to 39 c compared with measured in situ temperatures of ca 3 5 to 6 5 c the higher isotopic formation temperatures calculated for the hydrothermal nontronite suggest either complex patterns of fluid circulation and nontronite precipitation presently within the mounds or a higher thermal history associated with rapid and episodic periods of deposition during the holocene pleistocene the apparent high isotopic temperature of the fe rich montmorillonite may reflect 1 formation under hydrothermal conditions at spreading centers with subsequent dispersal by bottom currents 2 a detrital origin of the mineral or 3 a mixture of authigenic fe montmorillonite and detrital al montmorillonite in this region

air blast recognition and location using regional seismographic networks abstract regional seismic networks are occasionally useful for purposes other than earthquake studies three such applications which involve surface and atmospheric phenomena are described in this report 1 the detection and location of a missile silo explosion in arkansas 2 analysis of an aircraft sonic boom and 3 attempts to determine the time and location of ground impacts of aircraft and a meteorite when the source of the disturbance is in the air seismic instruments respond to the air coupled surface waves generated in the wake of the atmospheric shock front advancing with the speed of sound in air thus the disturbance9s point and time of origin can be determined using a standard iterative hypocenter program with a crustal velocity model appropriate to the prevailing atmospheric temperature and pressure conditions simplified theory predicts that air coupling is significant only when the speed of sound in air is close to the shear wave velocity of the surface crustal layer this is normally the case only when unconsolidated material is present at the surface the observations discussed here support this prediction seismic network instruments sited on soft ground recorded signals from atmospheric disturbances much more clearly than did higher gain instruments on hard rock

seismic constraints on a large mafic intrusion with implications for the subsidence history of the danish basin 1 seismic refraction data from the estrid 1 profile are used for seismic velocity modeling along the strike of a large mafic intrusion in the norwegian danish basin central denmark the p wave velocity structure identifies a 8 km thick sedimentary succession with velocities between 1 8 and 5 7 km s the top basement is defined by a step to vp 6 2 km s in the middle to lower crust a high velocity body vp 6 7 km s with its top located at about 10 12 km depth is interpreted as a high velocity and high density gabbroic intrusion of permian age this high velocity body explains the large 50 mgal positive gravity anomaly known as silkeborg gravity high the intrusion has a minimum volume of 40 000 km3 which implies that the magma influx and the consequent cooling of the lithosphere from high temperature could have had profound effects on the subsidence of the danish basin in particular because the magma probably intruded during only a few events and other similar structures cover much of the basin an anomalously high velocity gradient from 7 0 km s in the middle crust to 7 7 km s at 30 32 km depth in the central part of the intrusion coincides with an interval without moho reflections indicative of a gradual transition zone between the crust and the mantle this feature may show the location of the feeder dykes of the intrusion

sveconorwegian igneous complexes beneath the norwegian danish basin abstract gravity and magnetic anomalies have previously been interpreted to indicate strongly magnetic permian or even tertiary intrusive bodies beneath the skagerrak waterway such as the skagerrak volcano and beneath silkeborg in denmark our combined modelling of the magnetic and gravity anomalies over these rock bodies indicates that a steep upward magnetisation is required to explain the magnetic anomalies at the surface reminiscent of the magnetic direction in the sveconorwegian rocks of the rogaland igneous province in southern norway the younger rocks of the permian oslo rift region have intermediate and flat magnetisation that is inadequate to explain the observed magnetic field the positive part of the skagerrak aeromagnetic anomaly is continuous with the induced anomalies associated with the eastward extension of the rogaland igneous province this relation also suggests that rocks of the rogaland igneous province and its offshore extension are responsible for the skagerrak anomalies both the negative remanence dominated aeromagnetic anomaly and the positive gravity anomaly can be modelled using constraints from seismic reflection lines and available density data and rock magnetic properties a 7 km thick complex of ultramafic mafic intrusions is located below a southward dipping 1 4 km thick section of mesozoic sediments and 1 2 km of palaeozoic sediments the enormous body of dense ultramafic mafic rocks implied by the modelling could be the residue of the parental magma that produced the voluminous rogaland anorthosites the application of similar petrophysical properties in the forward modelling of the silkeborg source body provides an improved explanation of the observed gravity and magnetic anomalies compared with earlier studies the new model is constrained by magnetic depth estimates from the located euler method ranging between 6 and 8 km forward modelling shows that a model with a reverse magnetic body anorthosite situated above a dense mafic ultramafic body may account for the silkeborg anomalies the anorthosites may have formed by differentiation of the underlying mafic intrusion similar to the intrusive relations in the rogaland igneous province we conclude that there is strong evidence for a sveconorwegian age for both the skagerrak and the silkeborg anomalous rock bodies

seafloor spreading in the north fiji basin southwest pacific abstract the seapso leg 3 cruise december 1985 was carried out in the north fiji basin aboard the ifremer r v jean charcot the main purpose of the cruise was to carry out a geophysical and geological study of the central part of the basin between 16 s and 22 s to this end bathymetric profiles with the help of the seabeam multichannel echosounder single channel seismic reflection profiles and magnetic and gravimetric profiles were made moreover geological samples were taken by dredging and piston coring as well as water samples with the help of a multiprobe one of the aims of the cruise was to explore the present day spreading axis of the north fiji basin we were able to show that accretion the rate of which is about 7 cm yr in the north fiji basin has been constrained in a n s direction for 3 ma very recently between 0 6 and 0 7 ma ago the accretion process changed resulting in the creation of a ridge ridge trench triple junction centered at 173 30 e and 16 40 s water samples taken in the vicinity of this triple junction show significant anomalies of methane and manganese which are probably linked to hydrothermal events in its morphological characteristics transversal as well as longitudinal the ridge of the north fiji basin shows striking similarities to the east pacific rise it presents the same differences in level the same sizes of structures and the same particular features overlapping spreading centers offsets etc another aim of the seapso cruise was to study the complex deformation area located immediately to the west of the fiji islands which we interpret as a distensive type deformation in a strike slip system

geology and chemistry of hydrothermal deposits from active submarine volcano loihi hawaii high resolution bathymetric surveys bottom photography and sample analyses show that loihi seamount at the southernmost extent of the hawaiian hotspot is an active young submarine volcano that is probably the site of an emerging hawaiian island hydrothermal deposits sampled from the active summit rift system were probably formed by precipitation from cooling vent fluids or during cooling and oxidation of high temperature polymetallic sulphide assemblages no exotic benthic fauna were found to be associated with the presently active hydrothermal vents mapped

seismic evidence for late proterozoic orogenic structures below the phanerozoic sedimentary cover in the kattegat area sw scandinavia 1 commercial seismic reflection profiles from sw scandinavia image sveconorwegian orogenic structures in the late precambrian basement beneath the thin phanerozoic cover we interpret 1 pronounced west to wnw dipping reflections as sveconorwegian localized ductile shear which overprint 2 weaker east dipping reflections interpreted as gothian or early sveconorwegian compressional structures folded and laminar seismic fabrics may represent f and c type crystalline thrust sheets emplaced during the gothian and sveconorwegian events some of the dipping reflections are observed to sole into 3 an interval of subhorizontal reflections at 2 5 4 5 s two way travel time which we interpret as a fossil ductile to brittle transition that was unroofed during the gothian orogeny 4 nw se striking thrusts cutting the sveconorwegian structures may indicate a shift in the late sveconorwegian stress field they may represent an early stage in the subsequent phanerozoic basin evolution in the danish area

seismic velocity structure of crustal intrusions in the danish basin abstract we image the east and westward extent of a crustal high velocity body the thickness of a layered sequence around the moho at the flank of the body and the uppermost mantle velocity along the 320 km long refraction and wide angle reflection seismic profile estrid 2007 in the danish basin ray tracing modelling of the seismic data reveals a high velocity body 6 7 7 7 km s between 10 and 30 km depth with a lateral extent of at least 110 km it is interpreted as an intrusive body of gabbroic composition moho depth is variable between 30 and 35 km along the profile the crust is thin in a 180 km wide zone approximately below the western part of the intrusive body and further westward in the basin area the pn velocity in the uppermost mantle is relatively low 7 8 km s beneath the thin crust whereas higher pn velocities 8 0 km s are observed outside this zone the pmp reflection is characterized by reverberations ringing in the seismic signal in the zone with low pn velocity by 2d full waveform modelling these reverberations are explained by a layered transition zone around the moho consisting of intermixed crustal and mantle material the layering may be explained by magmatic intrusion along the moho over a lateral distance of 130 km away from the magmatic source which is located in the middle of the intrusive body these intrusive features may be related to extensional transtensional tectonism in the tornquist fan during the late carboniferous and early permian our new findings suggest a minimum volume of the intrusive body in the danish basin of about 60 000 km3 which is 20 000 km3 larger than interpreted from earlier data such a large intrusion must have affected the temperature field substantially and may have initiated the subsidence of the post permian danish norwegian basin

velocity structure around the baikal rift zone from teleseismic and local earthquake traveltimes and geodynamic implications abstract we present new results on the velocity structure of the baikal rift zone asia deduced from a comparative teleseismic and local tomography analysis the aim of this paper is to better identify the role of deep mantle processes versus that of far field tectonic effects on the occurrence of extensional tectonics within a continental plate we use 36000 traveltimes of p refracted waves from the isc catalogues and pg and pn traveltimes of 578 earthquakes recorded by the russian regional network to determine a velocity model by the use of local and teleseismic inversion procedures the models show that some velocity patterns are continuous from the surface down to at least 400 km among them a narrow negative anomaly goes through mongolia and follows the southern and eastern margins of the siberian craton this structure is interpreted as a thin mantle plume rising beneath the rift axis however our results do not evidence any wide asthenospheric upwarp at this place other velocity anomalies observed near the surface are not deeply rooted in particular a negative anomaly is observed at shallow levels 48 km beneath the northern third of lake baikal which is disconnected from deeper structures it may be explained by the existence of underplated magmatic material at the bottom of the crust by comparing the geometry of deep rooted anomalies to the present day stress field patterns we conclude that the sub lithospheric mantle dynamics is not the main factor controlling extensional processes in the baikal rift however it does contribute to a thermal weakening of the lithosphere along a mechanical discontinuity bounding the siberian shield we finally conclude that three favourable conditions are gathered in the baikal area to generate extension far field extensional stress field mechanical inherited lithospheric weakness and heat supply further studies should help to precise the genetic link between these three factors

finite difference calculation of travel times the travel times of the first arriving seismic waves through any velocity structure can be rapidly computed on a two or three dimensional numerical grid by finite difference extrapolation from point to point wavefronts rather than the traditional rays are tracked head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 03 per cent are found between the results of this technique and raytracing for a complex two dimensional model this scheme is useful for the windowing of finite difference calculations to increase computing speed and promises to aid in earthquake location tomographic inversion and kirchhoff migration in structures that have strong lateral velocity gradients

topo europe studying continental topography and deep earth surface processes in 4d abstract topo europe is a multidisciplinary international research program that addresses the interaction of processes inherent to the deep earth lithosphere mantle with surface processes erosion climate sea level which together shaped the topography of europe the objective of this programme is to assess neotectonic deformation rates and to quantify related geo risks such as earthquakes flooding landslides rock falls and volcanism research incorporating data interactive modeling focuses on the lithosphere memory and neotectonics with special attention on the thermo mechanical structure of the lithosphere mechanisms controlling large scale plate boundary and intraplate deformations anomalous subsidence and uplift and links with surface processes and topography evolution the topo europe natural laboratories in which these processes are analyzed cover a wide range of geodynamic settings these include the post collisional alpine carpathian pannonian basin system the very active aegean anatolian and apennines tyrrhenian orogens and back arc basins the caucasus levant area in the arabia europe collision zone the iberian peninsula caught up between alpine orogens the meta stable west and central european platform the stable east european platform and the seismically active and elevated scandinavian continental margin the topo europe project is a component of the international lithosphere program a european science foundation eurocores project provides funding for part of the research for a total amount of 13 5 m euro creating new research positions for 60 young researchers

3d lithosphere scale density model of the central european basin system and adjacent areas abstract to analyze the first order structural features characterizing the crust and the lithospheric mantle below the central european basin system and adjacent areas a new lithosphere scale 3d structural density model has been derived that integrates published knowledge with 3d gravity modeling with this study we aim to integrate previous results on sub domains of the study area and regional results of limited resolution in conjunction with 3d gravity analysis to derive the configuration of the lower crust and lithospheric mantle that is consistent with the known density distribution of the sediment fill and with observed deep seismic data as well as with observed gravity the derived 3d density model resolves the internal configuration of the crystalline crust below the central european basin system and adjacent areas at the regional scale and shows that the crystalline crust is characterized by a layered structure the upper to middle crystalline crust is characterized by relatively low seismic p wave velocities and densities it shows strong variations in thickness across the study area that partly correlate spatially with the inherited segmentation in response to crustal amalgamation this upper to middle crystalline crust is underlain by a continuous high density high velocity lower crustal layer locally more than 30 km thick beneath the east european craton and generally decreasing in thickness from the older to the younger tectonic domains though the thickness of this high density lower crust is generally small below the permo cenozoic central european basin system it can reach locally more than 18 km below the northeast german and the norwegian danish basins the derived depth to the lithosphere asthenosphere boundary confirms previously proposed geometries and is up to 230 km deep beneath the precambrian domain but less than 100 km deep beneath the phanerozoic domains implying that the average thermal gradient is higher in the phanerozoic lithosphere than in the precambrian part

a re examination of the seismicity of michigan abstract according to reports on felt shocks and those recorded by instruments 105 seismic events have been reported as occurring within michigan from the 1700 s to the present of these 41 instrumental events are due to chemical explosions detonated in experiments during 1964 1966 of the remaining events most of those reported from the upper peninsula are related to mining in the lower peninsula most events can be shown or strongly inferred to be cryoseisms atmospheric shock waves from explosions or aircraft or errors in location or reporting at least four and perhaps up to eight events are thought to be tectonic earthquakes and occur in the southernmost part of the state primarily in western lake erie and the southwestern corner of the state or in lake superior this study indicates that the seismicity level within michigan is very low and that there is no evidence to substantiate either a tectonically active michigan basin seismic zone or the extension of the midcontinent seismic zone into keweenaw peninsula in addition these results suggest that earthquake catalogs for weakly seismic regions may be contaminated with non tectonic and even non seismic events thirty earthquakes occurring outside the state have also been reported in michigan these lie primarily along a line connecting the st lawrence seaway and the new madrid zone of missouri

seismic velocity structure of a large mafic intrusion in the crust of central denmark from project estrid abstract the origin of regional sedimentary basins is being investigated by the estrid project explosion seismic transects around a rift in denmark this project investigates the mechanisms of the formation of wide regional basins and their interrelation to previous rifting processes in the danish norwegian basin in the north sea region in may 2004 a 143 km long refraction seismic profile was acquired along the strike direction of a suspected major mafic intrusion in the crust in central denmark the data confirms the presence of a body with high seismic velocity 6 5 km s extending from a depth of 10 12 km depth into the lower crust there is a remarkable moho relief between 27 and 34 km depth along this new along strike profile as based on ray tracing modelling of pmp reflections the lack of pmp reflections at a zone of very high velocity in the lowest crust 7 3 7 5 km s suggests a possible location of a feeder channel to the batholith the presence of volcanic rocks of carboniferous permian age above the intrusion mafic batholith suggests a similar age of the intrusion an older obliquely crossing profile and two new fan profiles deployed perpendicular to the main estrid profile show that the batholith is about 30 40 km wide the existence of this large mafic batholith supports the hypothesis that the origin of the danish norwegian basin is related to cooling and contraction after intrusion of large amounts of mafic melts into the crust during the late carboniferous and early permian the data and interpretations from project estrid will form the basis for subsidence modelling tentatively we interpret the formation of the danish norwegian basin as a thermal subsidence basin which developed after widespread rifting of the region

deep seismic investigation of crustal extensional structures in the danish basin along the estrid 2 profile summary the crust and uppermost mantle in the danish basin are investigated by modelling the p wave velocity distribution along the north south trending seismic profile estrid 2 seismic tomography and ray inversion modelling demonstrate a variable depth to the top of the crystalline crust from 10 km in the northern part of the profile to 2 km depth in the southernmost part the crystalline crust shows very high p wave velocity in the central part of the profile with 6 7 km s 1 at depths as shallow as 12 km and 7 3 7 5 km s 1 in the lowermost crust these values confirm previous results obtained along the orthogonal estrid 1 profile and the eugeno s profile 2 this high velocity zone in the middle to lower crust is interpreted as a mafic intrusion which explains a positive gravity anomaly of 50 mgal silkeborg gravity high the total length of the intrusion is at least 80 km in the east west direction and 25 35 km in the north south direction the estimated thickness of the intrusion from its top to the moho level is 18 20 km which gives a total minimum volume of 40 50 000 km3 the reflectivity properties of the moho discontinuity are variable along the profile below the intrusion the pmp signal is very weak due to the small velocity contrast between the lowermost crust 7 4 km s 1 and uppermost mantle 7 6 7 7 km s 1 the main moho reflection has a reverberative character to the south of the intrusion this feature is interpreted by layering at the moho level possibly due to magmatic underplating the occurrence of a large crustal mafic intrusion associated with magmatic underplating may be related to extensional transtensional tectonism in the tornquist fan area in the late palaeozoic the extensional event probably caused the opening of a plumbing system for intrusion of mantle derived magma into the crust the ascending magma may have been injected at upper middle crustal levels and during the late phases of the development squeezed laterally along the moho

asymmetric upwarp of the asthenosphere beneath the baikal rift zone siberia in the summer of 1991 we installed 27 seismic stations about lake baikal siberia aimed at obtaining accurately timed digital seismic data to investigate the deep structure and geodynamics of the baikal rift zone and adjacent regions sixty six teleseismic events with high signal to noise ratio were recorded travel time and q analysis of teleseisms characterize an upwarp of the lithosphere asthenosphere boundary under baikal theoretical arrival times were calculated by using the international association of seismology and physics of the earth s interior 1991 earth model and travel time residuals were found by subtracting computed arrival times from observed ones a three dimensional downward projection inversion method is used to invert the p wave velocity structure with constraints from deep seismic sounding data our results suggest that 1 the lithosphere asthenosphere transition upwarps beneath the rift zone 2 the upwarp has an asymmetric shape 3 the velocity contrast is 4 9 in the asthenosphere 4 the density contrast is 0 6 and 5 the p wave attenuation contrast t is 0 1 s

geophysical characteristics of the tornquist fan area northwest trans european suture zone indication of late carboniferous to early permian dextral transtension

patterns of quaternary uplift of the corinth rift southern border n peloponnese greece revealed by fluvial landscape morphometry abstract the rift of corinth is a world class example of young active rifting and as such is an ideal natural laboratory of continental extension however though much investigated for two decades several aspects of the mechanisms at work are still poorly understood the aim of this paper is a detailed morphometric study of the fluvial landscape response to the tectonic uplift of the rift southern shoulder in order to reconstruct the rift s quaternary evolution with special attention to timing location and intensity of uplift episodes based on the use of a large set of catchment and long profile metrics complemented by the new r sr integrative approach of the regional drainage network we identified three distinct episodes of uplift of the northern peloponnese coastal tract of which the intermediate one dated around 0 35 0 4 ma is only recorded in the topography of the central part of the rift shoulder and the youngest one appears to have propagated from east to west over the last 10 20 ka while net uplift remained minimum in the eastern part of the study area during the whole quaternary it shows a clear maximum in the central part of the rift shoulder since 0 4 ma and an eastward shift of this maximum in recent times maximum uplift rates calculated from the morphometric data are of 1 05 and 2 5 mm year 1 for the mid middle pleistocene and holocene uplift episodes respectively the morphometric evidence reveals an onshore uplift history remarkably consistent with the rift evolution reconstructed from other data sets in the long term it shows a stable pattern of maximum activity in the central part of the rift confirming previous conclusions about the absence of rift propagation in the short term it sheds light on a possible e w migration of the zone of recent uplift suggesting that in the near future fault activity and seismic hazard might concentrate in the heliki aegion area at the western tip of this uplift wave

the oslo rift a review the tectonomagmatic history of the oslo rift may be subdivided into 5 main periods stage 1 300 ma development of a shallow depression stage 2 300 295 ma roughly simultaneous onset of widespread basalt volcanism b1 and vertical movement along nnw sse to n s trending faults stage 3 295 275 ma main rifting period accompanied by volcanism dominated by rhomb porphyry lavas latites stage 4 275 240 ma change of magmatic style from basaltic shield volcanism to central volcanoes of mixed compositions many of which went through caldera collapses intrusion of composite batholiths of intermediate to granitic compositions stage 5 240 ma dike intrusions may have continued into triassic time both the crust and total lithosphere under the rift are thinned relative to the precambrian basement on both sides a 12 km thick high density and high velocity body occupies the lower crust along the rift axis and extends under the eastern and western rift shoulder petrological and geochemical data imply that this massive layer represents dense cumulates and gabbroic rocks formed from mantle derived magmas in deep crustal magma chambers two mantle sources one nearly undepleted age corrected nd of about 1 and sr of 10 to 15 the other mildly depleted nd of about 4 sr of 10 contributed melts to the oslo rift magmatism the two sources are interpreted as different parts of a heterogeneous upper mantle which before the rifting event belonged to the subcontinental lithosphere on their way to the surface the mantle derived magmas were retained in deep crustal magma chambers where they underwent extensive fractional crystallization some also suffered moderate contamination anatectic melts and melts formed by mixing between mantle derived and anatectic melts gave rise to syenitic and granitic intrusions

crustal structure and extensional deformation of thinned lithosphere in northern china abstract we herein present an interpretation of a 320 km long wide angle seismic profile between anxin and kuancheng which was obtained in 2002 the profile runs from the north china plain ncp where the lithosphere is just 70 km thick to the yanshan mountain folded belt ymfb where the lithosphere is180 km thick our model shows a crustal thickness that varies from 31 km under the ncp to 36 km under the ymfb the observed thinning of the crust in the ncp is about 14 which compares with an average extension of 24 41 at basin scale and 25 at lithosphere scale this finding suggests that the extensional deformation of the lithosphere in the north china block depends on depth the thin high velocity crust mantle transition zone has most likely originated after a delamination of the bottom of the crust and a concomitant intrusion of materials from the mantle the lower velocity of the lower crust may be attributed to the destruction of the lithosphere which permitted the lateral flow of melting materials above the moho from the ncp to the ymfb the differences found between the crust and the lithospheric mantle help to dispel any of the remaining uncertainty in the extensional factors and they may be attributed to detachment of middle crust and an intrusion of magma that originated in either lithosphere or asthenosphere we infer that the detachment of the middle crust lower crustal flow and magma intrusion probably lead to the underestimation of the crustal scale extensional factor and may represent the crustal response to the thinning of the lithosphere

crustal structure across the three gorges area of the yangtze platform central china from seismic refraction wide angle reflection data abstract we present active source seismic data recorded along a 300 km long profile across the three gorges area of the western yangtze platform central china from west to east the profile crosses the zigui basin huangling dome and jianghan basin the derived crustal p wave velocity structure changes significantly across the tongchenghe fault that lies at the transition from the huangling dome to the jianghan basin west of the tongchenghe fault beneath the zigui basin and the huangling dome we observe a 42 km thick crust of relatively low average velocity 6 3 6 4 km s in contrast east of the tongchenghe fault beneath the jianghan basin the crust is only 30 km thick and has a high average velocity 6 6 6 7 km s a west east variation in crustal composition along the tongchenghe fault is also inferred west of the fault p wave velocities suggest a felsic composition with an intermediate layer at the base of the crust whilst east of the fault felsic intermediate and mafic crustal layers are apparent our results suggest that the crust beneath the jianghan basin has been thinned by rifting accompanied by intrusion of the lower crust by mafic dikes and sills the west to east division of the crust in the three gorges area coincides with first order geophysical contrasts in gravity topography crustal and lithospheric thickness

some geochemical indicators for discrimination between diagenetic and hydrothermal metalliferous sediments abstract two types of metalliferous sediments are known hydrothermal associated with tectonic faults and diagenetic enriched in precipitates from seawater they are similar in their appearance and chemical composition only the first type is an indicator for hydrothermal discharge therefore in exploration criteria for differentiating between the two are needed four possible indicators have been investigated and are discussed a differences in p y correlation b differences in cr zr correlation c different distribution patterns of ree and d different as contents the as content is easy to determine and seems to be at the present time the most suitable criterion for determining the genesis of the metalliferous sediments

hydrothermal clay mineral formation of east pacific rise and bauer basin sediments samples of surface metalliferous sediment recovered from the crest of the east pacific rise at 6 s and 10 s latitudes and from the adjacent bauer basin are characterized by an authigenically formed fe rich montmorillonite that dominates the non carbonate mineralogy of the clay fraction 2 m oxygen isotopic formation temperatures indicate that the fe montmorillonites are created by low temperature hydrothermal processes 30 50 c in the 10 s region of the east pacific rise and bauer basin possibly as a result of cooling and oxidation of unstable high temperature 380 30 c sulfide assemblages or as a result of the percolation of hydrothermally altered seawater solutions through underlying basalt and sediments the widespread sedimentation of the clay mineral is suggested to be caused by colloidal transport possibly as a result of erosion of hydrothermal mounds by bottom currents hydrothermal fe montmorillonite nontronite formation may act as a direct and significant oceanic sink for si and fe released by the high temperature hydrothermal alteration of basalt at ocean spreading centers

layered crust mantle transition zone below a large crustal intrusion in the norwegian danish basin abstract the crust and uppermost mantle below the large positive gravity anomaly in the central part of the norwegian danish basin the silkeborg gravity high sgh is investigated using controlled source seismic data a more than 80 km long and 20 km thick intrusion is interpreted the seismic velocity within the intrusion varies from 6 8 km s at the top 12 km depth to 7 2 7 7 km s at the moho level 30 31 km depth the characteristics of the moho reflectivity vary along the profile below the central part of the intrusion the moho reflectivity is very weak due to a small velocity contrast between the lowermost crust 7 7 km s and the uppermost mantle 7 9 8 0 km s the seismic data show a ringing moho below the western part of the intrusion the coda trailing the main pmp reflection is about 1 0 s long and is composed of 4 5 wavelets we demonstrate that this feature may be explained by a layered transition zone between 30 and 35 km depth where high velocity layers of mantle affinity 7 9 8 05 km s alternate with layers of typical lower crustal velocity 6 7 7 3 km s the characteristics of this layering which causes the pmp coda are modelled by matching synthetic seismograms to the observed data synthetic seismograms calculated using the reflectivity method for 1d models constrain the nominal layer thickness to 100 500 m waveform modelling in 2d constrains the layered internal structure of the transition zone and the length of the heterogeneous zone along the profile we interpret the layered crust mantle transition zone as solidified melts of mantle affinity which were intruded into the lowermost crust the layered zone is proposed to be associated with the extensional tectonism that occurred in the area in late carboniferous early permian which also caused the large crustal intrusion the subsequent cooling may have initiated the formation of the danish basin

crustal structure along the egt profile across the tornquist fan interpreted from seismic gravity and magnetic data abstract the tornquist fan is a northwestward widening splay of late carboniferous to early permian fault zones in the danish area which emanates from the teisseyre tornquist zone in the baltic sea it is located over an area of deep sedimentary basins around denmark in the southwestern corner of the former baltica plate this paper presents an integrated model of seismic velocity density and magnetic properties along the egt profile across the tornquist fan between southern sweden and northern germany the model shows distinct features that may be related to the main tectonic events in the area the caledonian crustal suture between baltica and eastern avalonia is interpreted from changes in physical parameters at all crustal levels most noticeable as an abrupt lateral change in seismic velocity in the lowest crust at moho level from 6 9 km s in baltica to only 6 4 km s in eastern avalonia thick sedimentary successions beneath the mesozoic sequence are interpreted as palaeozoic sedimentary rocks which may have been deposited on the southern margin of the baltica plate in a deep caledonian foreland basin and in late carboniferous early permian rifts and extensional basins rift related magmatism is indicated by the presence of zones of high velocity density and magnetic susceptibility as well as strong remanent magnetization thermal subsidence following the emplacement of these bodies may be the origin of regional subsidence of the danish and north german basins of late paleozoic and mesozoic age late cretaceous to early cenozoic inversion of basin structures is detected at the sorgenfrei tornquist zone stz in the transition between shield and basin areas the stz coincides with a pronounced change in crustal thickness and the southwestern edge of a major magmatic intra crustal body significant abrupt changes in moho depth exist along the profile 26 30 km beneath the deep basins ca 35 km beneath the ringkobing fyn basement high and 48 km beneath the baltic shield around the sorgenfrei tornquist inversion zone the basement in the tornquist fan area is of proterozoic age the moho topography indicates strong thinning of the crust during its phanerozoic evolution the model supports the hypothesis that existing tectonic structures and weakness zones are continuously being reactivated during subsequent tectonic activity

magnetic lineations in marginal basins of the western pacific this paper separates the small oceanic basins around the western perimeter of the pacific ocean into marginal basins that have formed through back arc extension and those that apparently have not and reviews our knowledge of magnetic lineation patterns observed in possible and probable back arc basins magnetic lineations in these basins resemble lineations commonly associated with the world s mid oceanic spreading systems indicating that similar processes of crustal accretion occur in both tectonic environments in some back arc basins of the southwestern pacific magnetic lineation and other evidence suggest that back arc basins can evolve through the interaction and growth of multi plate systems because of the small time and space characteristics of back arc basins compared with the world s major oceanic spreading systems tectonic conditions favourable for the generation of back arc basins are either relaxed rapidly or easily interrupted models proposed to account for back arc basin development include a local models where back arc extension is mechanically driven by the downgoing slab and b global plate kinematics models where conditions favourable for back arc extension are governed by the motion of the overriding plate relative to the trench axis

seismic images of caledonian lithosphere scale collision structures in the southeastern north sea along mona lisa profile 2 abstract the unexposed suture between baltica and eastern avalonia is imaged by coincident normal incidence reflection and wide angle reflection refraction seismic data of the mona lisa project we present new results of the upper lithospheric seismic structure from the n s striking profile 2 across the caledonian deformation front which represents the crustal collision suture between baltica and eastern avalonia that formed after closure of the tornquist sea during the caledonian orogeny in late ordovician times three different crustal types are identified with great similarities to the nearby profile 1 1 a three layered crust typical of shields to the north 2 a transitional crust suture zone in the central part and 3 a two layered crust of caledonian origin to the south characterized by very low velocities throughout the crust the crustal thickness varies from 38 35 km under the northern margin of the ringkobing fyn high baltica crust to 28 27 km beneath the north german basin in the caledonian crust to the south the suture zone is imaged by s dipping crustal reflections from 1 9 to 10 6 s two way travel time twt over a horizontal distance of 70 km within the transitional crust the reflection suture zone terminates in a 60 km wide reflective lens with velocities of 6 6 6 8 km s in the lowermost crust the reflective lens may be interpreted as a remnant of oceanic or island arc crust that was accreted to the leading edge of baltica during closure of the tornquist sea alternatively the reflective lens may represent an indentor of baltica crust into the avalonian terrane the change in lower crustal reflectivity and the abrupt transition from a three to a two layered crust further south suggest that the trans european fault and the elbe lineament continue into the north sea the upper mantle reflectivity is dominated by bands of strong amplitude s dipping reflections from 13 5 to 21 8 s twt and a weaker band of n dipping reflections from 12 to 16 s twt reversed wide angle refractions and reflections indicate the presence of a s dipping high velocity layer 8 65 8 8 km s in the uppermost mantle the sub moho high velocity layer partially coincides with the strong s dipping normal incidence mantle reflections we propose that the s dipping mantle structure may represent a late caledonian or late carboniferous to early permian mantle shear zone

interpretation in statu nascendi of seismic wide angle reflections based on eugeno s data abstract in a new interpretation approach seismic wide angle data are analysed in statu nascendi i e in the state where the interpretational characteristics of the signals are born the approach is complementary to other interpretation methods such as kinematic interpretation of velocity models seismogram sections are qualitatively compared and classified in order to perform the analysis as close as possible to the seismic wavefield correlation of seismic phases and selection of representative reflection characteristics of the phases are required for the analysis the approach has been tested on the methodologically homogeneous eugeno s data set from the danish and surrounding areas together with other data the classifications of intra crustal and pmp reflections show clear regionalizations which correlate with the main geological tectonic and orogenic units the reflection characteristics of the thin crust below the deep basins in denmark the tornquist fan are different from the thick crust of the baltic shield and the ringkobing fyn high the character of moho reflections is ringing in the basin area as opposed to sharp in the latter areas and intra crustal reflections are stronger and more consistent in the basin area than in the latter areas

high velocity breakup related sills in the voring basin off norway multichannel seismic reflection profiles in the hel graben v0ring basin reveal a sill complex at approximately 5 km depth it is associated with exceptionally high 7 4 km s 1 seismic wide angle velocities the existence of observable wide angle arrivals shows that the sills act as efficient waveguides seismic reflection data and amplitude modeling constrain the thickness of individual sills to approximately 100 m sonic logs from sills of similar thickness on the nearby utgard high show an average velocity of 7 0 km s 1 such high velocities require an olivine gabbroic sill composition and emplacement under conditions which allowed growth of relatively large crystal sizes a possible reason for such an emplacement environment is the hei graben s role as an intrusion center during breakup volcanism this would provide the necessary duration of the magmatic activity as well as locally increased melt volumes and cooling times sill complexes of this kind decrease the accuracy of determined velocity fields and crustal geometries below the top of the sill complex affecting depth conversion and gravity modeling furthermore the results question the concept of lower crustal bodies as large scale homogeneous accumulations of mafic melt

geophysical evidence on segmentation of the tancheng lujiang fault and its implications on the lithosphere evolution in east china abstract the north south trending tancheng lujiang tanlu fault belt extends from northeast china to the dabie sulu orogenic belt for a length of more than 3000 km this fault belt probably has close links with the lithosphere evolution seismic activity and mineral resource concentration in east china surface geological mapping and studies on sedimentation and basin formation have indicated segmentation at the southern middle and northern domains of the fault here we employ geophysical constraints to evaluate these fault segments unlike previous geophysical studies focused on laterally varying crust mantle seismic velocity structure across the fault in this study we have integrated a variety of geophysical data sets such as crustal p wave velocity earthquake occurrence and released seismic energy seismogenic layer thickness surface heat flow and geothermal field to understand the deep structure and strength of the lithosphere along the tanlu segmented fault belt the results demonstrate remarkable crustal scale north to south segmentation this major fault the geophysical evidence and some geochemical constraints suggest that the tanlu fault belt probably served as a channel for melt and fluid percolation and exerted a significant control on the lithosphere evolution in east china

the moho depth map of the european plate summary the european plate has a 4 5 gy long and complex tectonic history this is reflected in the present day large scale crustal structures a new digital moho depth map is compiled from more than 250 data sets of individual seismic profiles 3 d models obtained by body and surface waves receiver function results and maps of seismic and or gravity data compilations we have compiled the first digital high resolution map of the moho depth for the whole european plate extending from the mid atlantic ridge in the west to the ural mountains in the east and from the mediterranean sea in the south to the barents sea and spitsbergen in the arctic in the north in general three large domains within the european plate crust are visible the oldest archean and proterozoic crust has a thickness of 40 60 km the continental variscan and alpine crust has a thickness of 20 40 km and the youngest oceanic atlantic crust has a thickness of 10 20 km

geophysical evidence for early permian igneous activity in a transtensional environment denmark abstract integrated interpretation of gravity magnetic and seismic data in the area of the strong silkeborg gravity high in central denmark has led to a structural geological model involving a transtensional rift feature which probably formed in connection with permo carboniferous wrench faulting and the development of the oslo horn graben system reinterpretation of the deep seismic data from profile 2 of the eugeno s project in the area of the silkeborg gravity high detected a new thin high velocity layer 6 25 km s at about 7 km deep within the palaeozoic sedimentary sequence in the crystalline basement a separate body of high velocity 6 9 7 0 km s was found at 11 km deep beneath this body the moho reaches its shallowest position in the area the seismic results compare well with an interpretation based on magnetism of a shallow volcanic or subvolcanic layer at a depth of 6 8 km this layer shows strong remanent and induced magnetization with a southward inclination of about 10 gravity reinterpretation of the area allows the separation of an upper body of density contrast 0 1 g cm3 from a lower body of density contrast 0 2 g cm3 the upper body roughly coincides with the magnetic body the lower body the major one coincides with the deep seismic body and it is responsible for the main gravity anomaly in the area

effects of asthenospheric heat flow in basin modelling exemplified with the danish basin a mathematical model is developed to simulate the process of basin formation by rifting subsidence is driven by crustal thinning and influenced by the transient temperature variation within the lithosphere the model offers possibilities for multiple phases of rifting superimposing non steady state conditions important features in the model are two layered heat production and a definition of the lower boundary in terms of heat flow instead of one of constant temperature as opposed to other models furthermore phase transformations caused by rift induced pressure and temperature changes are suggested as an additional syn rift subsidence mechanism the relevant phase transformations are assumed to be those occurring during an amphibolite to granulite facies transition perturbation of the steady state temperature profile is performed by one or several phases of rifting with finite stretching rates by sill intrusion or by changes in asthenospheric heat flow the greatest deviation from the steady state geotherm occurs at the lower boundary because of the flow definition of the lower boundary the consequence of this is a much longer duration of anomalous heat flow the model is applied to the danish basin north jylland denmark this basin was formed in permian times and since then it has been thermally subsiding apart from a minor event of stretching in the late cretaceous the higher present day heat flow in the basin compared to the adjacent fennoscandian shield area is successfully explained by the model presented in this paper it is concluded that transient temperature conditions still today contribute significantly to present surface heat flow although main rifting took place for more than 240 m y ago this cannot be explained by models using a fixed temperature definition of the lower boundary the modelled contribution by phase transformations to subsidence explains why the syn rift early and middle triassic subsidence was accompanied by insignificant faulting activity as evidenced by the seismic data available

danish basin subsidence by triassic rifting on a lithosphere cooling background i report here the first quantitative study of the entire post palaeozoic subsidence history in the ensemble of north west european sedimentary basins data obtained from wells suggests that the late permian through mesozoic subsidence of the danish basin is a result of lithosphere wide cooling following an igneous event of late carboniferous to early permian age this cooling subsidence was superposed by major triassic rifting and minor mid jurassic uplift both events apparently not having significant thermal consequences the danish basin did not experience the late jurassic to early cretaceous rifting and subsequent cooling which were so important in the development of the central north sea rifts this result suggests that a reappraisal of north sea tectonics should look for pre jurassic subsidence

the horn graben and its relationship to the oslo graben and the danish basin abstract the tectonic development of the horn graben the danish basin and the oslo graben are compared with special emphasis on the timing of their initiation the comparison is based on seismic interpretation and is supported by analysis of tectonic subsidence devonian and carboniferous sedimentary strata and rotliegende volcanics constitutes the pre rift succession in the horn graben in the central part of the graben it rests unconformably on precambrian possibly dalslandian basement to the south on caledonian metamorphic basement whereas lower palaeozoics occur to the north the distribution of these pre devonian assemblages suggests initial early devonian uplift of the ringkobing fyn high as the lower palaeozoic sediments are believed to have covered the high originally further uplift appears to have taken place in the early permian a subdivision of the horn graben into a northern and a southern segment are recognized the rifting was apparently heralded only in the southern segment in the rotliegende and the main rifting phase did not start before the triassic subsidence modelling shows that the thermal anomaly created by rifting of the horn graben was less significant than in the danish basin and resulted in much less post rift thermal subsidence the reduced thermal anomaly may account for an earlier consolidation of the crust thereby rendering the area resistant to late cretaceous early tertiary inversion tectonism the oslo graben was tectonically active during late carboniferous to early permian times and essentially became inactive during triassic times when the main tectonism of the horn graben occurred subsidence modelling shows that the danish basin south of the fjerritslev trough was tectonically active mainly during the zechstein whereas the rapid triassic subsidence is modelled to be caused by phase transformations as indicated by the minor amount of basement attached faulting based on the differing subsidence patterns it is concluded that the three basins did not develop simultaneously and that they are not genetically related

accumulation rates chemical partitioning and q mode factor analysis of metalliferous sediments from the north fiji basin abstract in order to assess recent hydrothermal contributions to pelagic sediments from active back arc spreading surface samples collected from the north central north fiji basin and the vanuatu archipelagic apron were chemically and mineralogically analyzed including phase and size partitioning and q mode factor analysis of the chemical data sediment cores were analyzed for accumulation rates by the excess 230 th method using alpha track autoradiography the north central north fiji basin sediments are largely carbonate oozes in which the non carbonate material is dominated by volcanic detritus in the 2 m fraction and by fe montmorillonite amorphous ferromanganese oxides and fine grained volcanic ash in the m fraction phase and size partitioning indicate that metal concentrations in the m fraction approach those for metalliferous sediments from the eastern pacific and that the metalliferous material in the majority of these sediments is concentrated in the oxide portion of the m fraction suggesting potential widespread colloidal dispersion from the source areas iron accumulation rates are similar to those of metalliferous sediments from the east pacific rise the mean mn accumulation rate is about twice the global authigenic rate whereas al is accumulating about ten times faster than in eastern and equatorial pacific pelagic sediments because of volcanic ash contributions from the vanuatu island arc and possible intra basin volcanism q mode factor analysis of the compositional data produced three major factors hydrothermal authigenic and detrital which together account for 90 of the sample variance in both size fractions of the non carbonate material the aerial distribution of the m fraction factor components reveals a major hydrothermal source associated with the south pandora ridge spreading center in general agreement with the mn and as accumulation rate patterns two smaller hydrothermal sources were revealed by the factor analysis which are not associated with any known active tectonic features suggesting that back arc rifting and associated hydrothermal activity in this portion of the north fiji basin may follow both ordered and complex regional modes

a stretching model for the oslo rift abstract data on the tectonic and magmatic activity crustal thickness seismic velocity and gravity have been integrated to constrain a stretching model for the late carboniferous early permian oslo rift the crustal structure and numerical modelling of temperature and melt generation during rifting suggest that the oslo rift in skagerrak developed in response to crustal stretching with a factor between 1 4 and 1 6 however upper crustal extension by faulting accounts for a stretching factor of only 1 1 1 2 thermal expansion of the lithosphere due to an increased asthenospheric potential temperature the intrusion of melted mantle material into the lower crust and probably a lowering of the erosional base by a fall in sea level all contributed to the erosion of 2 4 km of lower palaeozoic pre rift sediments over large areas the lateral extent of the thermal anomaly reached far beyond the oslo and skagerrak grabens

the hatton bank continental margin ii deep structure from two ship expanding spread seismic profiles summary the continent ocean transition adjacent to hatton bank was studied using a dense grid of single ship and two ship multichannel seismic profiles the interpretation of the explosive expanding spread profiles esps which were shot as part of this survey are discussed here in detail extensive seaward dipping reflectors are developed in the upper crust across the entire margin these seaward dipping reflectors continue northwards on the faeroes and voring margins where they have been shown to be caused by basaltic lavas as well as on the conjugate margin of east greenland the dipping reflectors are an important feature of the rifting history of the margin and show that extensive volcanism was associated with the extension the esps show clear seismic arrivals out to ranges of 100 km wide angle moho reflections can be seen on all the lines as well as good mid and lower crustal arrivals the determination of seismic velocity structure was constrained by ray tracing and by amplitude modelling using reflectivity synthetic seismograms the results from the esps show that there is a thick region of lower crustal material beneath the margin with an unusually high crustal velocity of 7 3 7 4 km s this lower crustal material reaches a maximum thickness of 14 km beneath the central part of the margin and is terminated at depth by the moho the lower crustal lens of high velocity material is interpreted as underplated or intruded igneous rocks associated with the large volumes of extrusive basaltic lavas now seen as dipping reflectors on the margin

moho and lower crustal reflectivity beneath a young rift basin results from a two ship wide aperture seismic reflection experiment in the valencia trough western mediterranean summary we present new images of the lower crust and moho beneath the valencia trough a young rift basin in the western mediterranean these images were obtained from a two ship wide aperture reflection experiment and show several features not distinguishable on previously available conventional single ship reflection profiles the moho which was previously only seen intermittently can now be confidently traced throughout the basin we have constructed a present day depth to moho map and estimated the degree of crustal thinning for the whole basin crustal thinning is at a maximum in the centre of the basin where values reach 3 15 0 25 at the margins of the basin the value decreases to 1 5 0 1 the reflective character of the lower crust and moho is different beneath different parts of the basin we have been able to correlate these differences with the amount of stretching we therefore interpret the variations of the observed lower crustal reflectivity as having been caused by the most recent neogene stretching event that opened the valencia trough along the iberian margin there is well developed lower crustal reflectivity consisting of 1 2s two way time twt of 1 4 km long near horizontal reflectors underlain by a more continuous although not significantly stronger reflector interpreted to be the reflection moho offshore this lower crustal reflective unit thins rapidly such that it is undetectable 40 50 km from the coastline where the crust has been stretched by a factor of 1 7 0 1 as the lower crustal reflectivity becomes undetectable the reflection moho becomes a robust continuous event where exceeds 2 4 0 2 however the moho is a weak event and difficult to trace we infer that either the extension itself or associated melting significantly weakened or even destroyed the lower crustal reflectivity in the centre of the basin and enhanced the moho where extension was moderate the balearic margin is somewhat anomalous in that there appears to have been flexural loading of the crust due to thrusting and folding that occurred at the same time as extension in the valencia trough the lower crust shows evidence of weak but locally variable lower crustal reflectivity it is possible that the lower crustal reflectivity was preserved simply because the moho was flexed downward and so decompression and hence melting of the upper mantle was restricted this suggests that the melting itself rather than the extension is the primary mechanism of lower crustal modification

petrographic investigations of seafloor sediments from the kita bayonnaise submarine caldera shichito iwojima ridge izu ogasawara arc northwestern pacific abstract seafloor sediment samples were recovered by box and gravity corers from the kita bayonnaise submarine caldera shichito iwojima ridge izu ogasawara arc northwestern pacific the samples were separated into heavy light and claysized fractions for assessment of the hydrothermal contribution to the sediments of the submarine caldera and have been determined by microscopy sem edx and xrd heavy fractions are composed of predominant magnetic minerals dominant silicates and common sulfides and sulfate minor sulfosalt rutile phosphates carbonates and mn oxyhydroxides primary hydrothermal phases are composed of barite pyrite sphalerite chalcopyrite galena fahlore marcasite and rutile secondary mineral associations are usually present around galena and chalcopyrite grains the assemblages of barite pellet type pyrite chalcopyrite sphalerite and sphalerite chalcopyrite pyrite with galena or fahlore suggest that hydrothermal mineralization took place in the caldera the assemblages of epidote carbonate or chlorite in heavy transparent fractions and of sericite quartz feldspar in light fractions are indicative of hydrothermal alteration origin because of only dacite sampled from the caldera some of the smectite chlorite sericite and kutnahorite in clay sized fractions could also support the existence of hydrothermal activity other minerals consisting of hypersthene augite hornblende quartz feldspar and biotite are observed in heavy and light fractions as detritus of dacite in the caldera significant amounts of sulfides and barite mixtures ranging from about 1 0 to 3 5 wt in treated fractions 0 063 0 25 mm in gravity core samples suggest that hydrothermal mineralization took place several times in the caldera many lines of evidence indicate the presence of hydrothermal mineralizations associated with ag and th in the kita bayonnaise submarine caldera the mineralogical analyses are powerful for exploring signs of past or present hydrothermal mineralization in submarine calderas

geochemistry of hydrothermal deposits from loihi submarine volcano hawaii abstract dredging across the northeast rim of the summit crater of loihi seamount recovered several morphologically similar but chemically and mineralogically distinct hydrothermal deposits encrusting the surface of fresh pillow lava talus the multicolored deposits suggest a precipitation sequence that may be controlled by an oxidation reduction gradient in which smectite ranging in composition from fe montmorillonite to nontronite has precipitated along with iron oxide under slightly reducing conditions this deposition was apparently followed by amorphous iron oxide and silica precipitation possibly under more oxic conditions oxygen isotope geothermometry indicates formation temperatures in the range of 31 57 c for the loihi smectites trace element enrichments appear to be positively correlated with the isotopic formation temperature of the smectite suggesting either increased trace element solubility within the higher temperature vent fluids or increased smectite and iron oxide scavenging with increased precipitation rates the trace element abundances further suggest the presence of polymetallic sulfides that either are directly associated with the smectites as amorphous phases or occur beneath these deposits in the volcanic pile of the seamount

on the evolution of marginal basins this paper presents a critique of the models proposed to explain the formation of back arc basins those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related several marginal basins are not generically back arc but rather are related to the initiation of subduction continental rifting or plate boundary readjustments models of back arc basin formation must be consistent with the observed characteristics of back arc basins which include the following 1 the crustal structure and magnetic lineation patterns of back arc and major ocean basins are similar implying that the processes of crustal accretion in both tectonic environments are also similar 2 arc trench systems without currently spreading back arc basins are in the majority 3 although petrographically similar to and within the compositional range of midocean ridge basalts morb back arc basin basalts babb show consistent geochemical differences from n type morb and are in many respects transitional toward island arc tholeiites iat 4 distinct babb and iat mantle sources may be in extremely close proximity during the initial rifting of an island arc 5 rifting of the volcanic arc may occur on either side of the line of active volcanoes 50 km and may vary from one side to the other along strike 6 a remnant arc is not always developed 7 back arc spreading may be initiated above subducted lithosphere but with time the center of spreading migrates away from the arc and no longer overlies a benioff zone and 8 there is no simple correlation between the timing of global plate reorganizations and the formation of back arc basins the three main classes of models proposed to explain back arc basin formation are mantle diapirism induced aesthenospheric convection and global plate kinematics because the driving mechanism for the first two classes of models is provided by the local subducting slab these models fail to explain the temporal and spatial distribution of back arc basins the third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line the latter being observed to move only in a seaward direction with respect to a hot spot reference frame however the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions for example some dynamic back arc force seems required to move the new hebrides tonga and south sandwich forearc plates seaward neither the local nor the global models adequately specify the necessary and or sufficient conditions for back arc basin formation a better understanding of the processes associated with the initiation of rifting and of subduction is required arc rifting and back arc spreading are both secondary phenomena to subduction but they are also primary extensional phenomena fundamentally analogous to continental rifting and the initial spreading of major ocean basins

hydrothermal methane plumes in the mariana back arc spreading centre submarine hydrothermal vents were first unequivocally detected by measurement of 3he 4he ratio anomalies coupled with deviations from ambient t s relationships on the galapagos spreading centre1 2 since then the existence of hydrothermal vents along the entire length of the east pacific rise has been demonstrated by shipboard observations of 3he methane and manganese in the effluent plumes3 5 we have extended our studies of vent fields at mid ocean ridge spreading centres to search for effluent plumes in back arc basins which are also thought to be active regions of seafloor spreading where new oceanic crust is formed6 and thus where rifting and assuring along the axis of extension should provide similar opportunities for the downward penetration of sea water and the development of active hydrothermal systems we have detected large plumes of methane enriched water in the mariana trough back arc basin7 and also in the summit crater of loihi seamount7 at the present site of the hawaiian hotspot thus establishing the widespread existence of off ridge hydro thermal vents in the ocean however although the loihi vents produce plumes enriched in both 3he and methane7 we find that surprisingly the mariana vents are enriched in methane without a corresponding enrichment in 3he this is the first time such a decoupling of these two effluent plume tracers has been observed

loihi seamount hawaii a mid plate volcano with a distinctive hydrothermal system the initial discovery of deep sea hydrothermal vents at the galapagos rift1 2 has resulted in a decade of exploration and experimentation subsequent discoveries in the pacific3 8 and atlantic9 ocean basins have established the importance of these features as sources of mantle derived gases and solutes10 2 and as loci for heat dissipation13 14 polymetallic sulphide mineral deposition15 16 and geochemical exchange17 19 the presence of dense communities of bacteria and specialized macrofauna20 22 has called into question the absolute role of sunlight as the principal energy source for marine organisms until recently these deep sea vents were known only at or near plate boundaries but recent dredging23 24 near bottom water sampling25 27 and sea floor camera survey28 programmes have indicated that hydrothermal vents are present at the summit of loihi seamount a mid plate hotspot submarine volcano at the southern end of the hawaiian island chain29 here we report results obtained from sampling two active vent fields the fluid chemistry especially high co2 and fe presence of alkalic lavas and iron depositing bacteria and absence of macrofauna suggest that mid plate vents may differ from the hydrothermal vents found at mid ocean ridges

seismic structure across the caledonian deformation front along mona lisa profile 1 in the southeastern north sea abstract seismic data from the mona lisa marine and onshore north sea acquisition for lithospheric seismic analysis project in the southeastern north sea image the caledonian deformation front cdf which is the collisional suture between baltica to the north and east and avalonia to the south and west the n s trending mona lisa normal incidence reflection profile 1 was recorded to 26 s twt coincident wide angle data were acquired on nine ocean bottom hydrophones and several onshore mobile seismographs along and off the profile the model of compressional seismic velocity shows three different crustal types a a typical three layered shield type crust below the ringkobing fyn high to the north b a highly complex transitional crust in the central part and c a two layered crust of caledonian origin to the south sharp and strong normal incidence and wide angle reflections from moho were recorded south of the caledonian deformation front in contrast to less distinct reflections further north s dipping crustal reflections from 4 to 11 s twt over 70 km horizontal distance terminate at moho and coincide with a change in the seismic velocity structure this indicates northward obduction of avalonian crust non migrated normal incidence seismic sections show crossing weak n dipping and stronger s dipping reflections to 20 s twt from the uppermost mantle we propose a tectonic model where the closure of the tornquist sea took place along a n dipping subduction zone which was later overprinted by a late or post caledonian s dipping shear zone sub moho velocities are 7 8 7 9 km s under 34 35 km thick baltica crust and 8 1 8 3 km s under 25 26 km thick caledonian crust the sub horizontal moho across the caledonian collision zone implies late or post caledonian re equilibration of the seismological moho we interpret the low velocity upper mantle 7 8 8 1 km s to the north as former baltica lower crust in eclogite facies after pressure induced metamorphism as a result of lithospheric flexure during the caledonian orogeny these rocks today appear as upper mantle that was uplifted to their present position during the middle devonian collapse of the north german polish caledonides

volcanic and hydrothermal history of ridge segments near the rodrigues triple junction central indian ocean deduced from sediment geochemistry abstract sediments from the central indian ridge cir and southeast indian ridge seir near the rodrigues triple junction rtj were investigated in order to relate the accumulation rates of sediment components to the volcanic and hydrothermal history of the respective ridge segments the sediments contain 50 90 biogenic carbonates and have been deposited over at least the last 2 5 myr without large hiatuses since there is no early diagenetic element mobilization the main primary geochemical signals are preserved in the sediments a three step multivariate analysis of carbonate free geochemical data showed that the sediments consist of two main endmembers with respect to si ti al fe mg k mn cu ni and zn which are mixed in different ratios in the samples the first endmember comprises volcanic detrital components mainly basalt fragments basaltic glass and their weathering products smectites the second endmember is made up of hydrothermal fe mn oxides the volcanic detritus is distributed via bottom currents the fe mn oxides are formed as particle fallout from hydrothermal plumes the first component dominates in the rift valley the second on the ridge flanks along the seir varying accumulation rates were found to reflect the cyclic magmatic volcanic activity of the intermediate spreading rift axis in cores from the cir the accumulation of both endmembers reflects the evolution of a non transform fault separating the first and second cir segments rather than the spreading axis history