mineralogy and chemistry of massive sulfide deposits from the juan de fuca ridge six hydrothermal vent sites and associated benthic communities were located in the axial valley of the southern juan de fuca ridge using transponder navigated bottom photography the hydrothermal deposits form ledges and shallow mounds within a central zone characterized by a linear bathymetric depression and numerous collapse features the flat valley floor adjacent to the central zone consists of ferrobasalt lobate flows and sheet flows sediment cover is minimal vent sites are characterized by concentrations of tube worms clams benthic siphonophores and several unidentified fauna two types of massive sulfide were dredged from one of the vent sites type a samples are angular slabs of dark gray zn rich sulfide with interlayers and a thin partly oxidized crust of fe sulfide these layered sulfide aggregates appear to be fragments of a sulfide wall enclosing an active hydrothermal vent the outer sulfide wall is composed mainly of colloform fe sulfide and fe poor sphalerite deposited under lower temperature conditions whereby sea water and hydrothermal fluid mix above the discharge point with continued sulfide deposition the wall inhibits mixing of sea water and hydrothermal fluid inside the wall the intensifying hydrothermal system deposits a higher temperature assemblage of granular fe rich sphalerite wurtzite pyrite and minor cu fe sulfide the zonation in wurtzite from fe rich cores to fe poor rims may result from a late stage cooling of the hydrothermal fluid and or a change in fluid chemistry the sulfide wall grows outward where a rupture in it permits the escape of high temperature fluid and then deposition of a secondary shell over the breakthrough point as temperature increases earlier formed minerals dissolve and zn fe and pb migrate toward the outer sulfide wall tube worms flourished in the outer wall of type a samples and abandoned tube structures served as conduits for the late stage hydrothermal fluids type b sulfide samples are subrounded spongy textured fragments composed almost entirely of dendritic aggregates of pale fe poor colloform sphalerite and opaline silica this type of sulfide is deposited in open space by moderate to low temperature fluid discharging at a slow but variable rate the fluid becomes increasingly oxidizing resulting in late stage deposition of hematite barite and sulfur type b samples show little evidence of burrowing animals this type of sulfide may be deposited in settings peripheral to sites of focused discharge

extension subsidence and thermal evolution of the los angeles basin a two dimensional model abstract the los angeles basin is a narrow deep basin containing large oil fields its formation began in the middle miocene and its subsidence has been extraordinarily rapid we suggest that although the basin is in a predominantly strike slip tectonic environment the subsidence is largely a result of crustal extension simple one dimensional extension models ignoring the lateral flow of heat are inadequate to explain the rapidity of the subsidence we use a two dimensional numerical method to model the thermal evolution of the basin based on our model studies the central southwest northeast and palos verdes blocks are inferred to have been extended by 178 69 20 and 27 respectively the average crustal extension for the whole basin is 66 the lateral flow of heat out of the central block into adjacent cooler blocks accounts for its rapid subsidence during the early stage the deepest sediments are predicted to have been heated to about 210 c hydrocarbon maturity calculations indicate that the upper miocene sediments reached oil maturity a little over 4 m y b p no sediment is predicted to have achieved natural gas maturity

the juan de fuca ridge hot spot propagating rift system new tectonic geochemical and magnetic data underway geophysics deep tow camera work and preliminary geochemical and magnetic studies from a 1980 cruise to the juan de fuca ridge allow the following interpretations 1 the central third between 45 5 n and 46 5 n of the ridge is now actively spreading from a zone which is 10 to 20 km west of the axis bisecting the brunhes matuyama magnetic reversal boundary 2 the apparent terminus of surficial igneous activity at the northern tip of the active cobb propagator has been identified by dredging and camera work the tip appears curved or offset slightly to the west does not correspond to the northernmost tip of the central magnetic anomaly elvers et al 1974 and spreading has been initiated beneath sediments farther north and 3 iron and titanium in fresh glasses from the active volcanic zone exhibit a progressive enrichment northward then a pronounced reversal to more normal concentrations in the vicinity of the cobb offset

geomagnetism and age study of tyrrhenian seamounts summary we discuss the results of measurements of the magnetic anomaly field of several seamounts located on the ocean floor and on the bordering continental slopes of the tyrrhenian sea this geomagnetic study of seamounts is based on joint 3 d analyses of the magnetic anomalies and the bathymetry on measurements of the magnetic properties of rock specimens and on geochronological data the 3 d analysis of the magnetic anomalies of large volcanoes emplaced in the small oceanic basins of vavilov and marsili serves to shed light on the spatial distribution of the magnetized bodies by eliminating the effect of topography a new magnetic anomaly map of the southern tyrrhenian sea is obtained by the merging of shipborne local data measured on topographic highs with data of regional type the study of the magnetic patterns and age of the oceanic seamounts indicates that the volcanic activity mainly developed prior to chron c2 2 02 ma bp on the vavilov and magnaghi edifices vavilov basin whereas the volcanism of a big oceanic seamount emplaced in the small basin of marsili took place mainly during the period of chron c1 0 0 78 ma bp

hydrothermal activity in the se tyrrhenian sea an overview of 30 years of research subduction related volcanic activity in the southeast tyrrhenian back arc basin has resulted in a large variety of calcalkaline rocks ranging from arc tholeiitic to shoshonitic and rhyolitic composition which were emplaced in both oceanic and continental crust hydrothermal systems in the se tyrrhenian sea have leached these magmatic rocks generating a wide range of oxyhydroxide and sulfide deposits all the hydrothermal occurrences formed at much shallower depths than those from the mid ocean ridges oxyhydroxide deposits metalliferous sediments crusts and nodules are almost pure hydrothermal formations with insignificant hydrogenetic and diagenetic contributions they exhibit strong separation of mn rich and fe rich end members with mn rich occurrences dominant their geochemistry suggests a relative enrichment of mn in the hydrothermal systems of the se tyrrhenian the sulfides are zn and pb rich but poor in cu and relatively poor in fe this suggests either deep seated magma chambers or boiling of the hydrothermal fluids beneath the seafloor as a consequence 1 cu and a large proportion of the fe are deposited as sulfides in the basement rocks and sediment cover 2 the hydrothermal fluids become relatively enriched in mn zn and pb a number of accessory sulfosalts and selenides not typical of mid ocean ridge massive sulfides are present

tectono stratigraphic modelling of the north sicily continental margin southern tyrrhenian sea abstract a two dimensional numerical modelling that simulate the kinematic and thermal response of the lithosphere to thinning was used for the quantitative reconstruction of the late neogene to recent times tectonic and stratigraphic evolution of the north sicily continental margin southern tyrrhenian sea the numerical study of the evolution of the north sicily margin builds on the crustal image and kinematic interpretation of the margin obtained by pepe et al tectonics 19 2000 241 on the basis of seismic data and gravity modelling tectonic modeling indicate that different segments of the margin were undergoing different vertical movements which are mainly expression of the rifting and thinning of the lithosphere occurred during tectonic evolution of the southern tyrrhenian sea a prediction of the pre rift basement topography and the moho along the margin converges to a value of 6 5 km for the depth of necking and a temperature dependent eet 500 isotherm the model fails to reproduce the morphology of the solunto high confirming its non extensional origin a polyphase evolution is required to reproduce the observed syn and post rift stratigraphy during the first rifting stage between 9 and 5 ma crustal thinning factors reach maximum values of 1 27 in the cefalu basin a similar value is predicted for the subcrustal thinning around 60 km nne of the profile margin crustal thinning factors increase during the second rifting stage from 4 to 2 ma and reach values of 2 and up to 3 5 in the cefalu basin and in the continent oceanic transition zone respectively similarly subcrustal lithospheric thinning factors reach values up to 2 5 in the distal sector of the margin an uplift of more than 100 m is predicted for the north sicily shelf and surrounding onshore areas during the post rift stage the evolution of thermal structure with time is very sensitive to the partial thinning factors describing the evolution of the thinning itself during time the lithosphere preserved part of its strength during extension the effective elastic thickness eet along the margin through time is 24 km at the onset of rifting and reaches values less to 8 km during the second rifting stage in the northeastern end of the margin

two dimensional modeling of the p t t paths of regional metamorphism in simple overthrust terrains quantitative modeling of metamorphic p t t paths in thrust belts has so far been limited to one dimensional simplification we have developed a two dimensional time dependent finite element procedure capable of tracing and updating fault and strata boundaries during faulting for modeling thermal evolution of overthrust terrains we show that the two dimensionality of an overthrust and the speed and duration of thrusting have significant effects on the p t t paths of rocks in orogenic belts the results suggest that petrological studies of p t paths of rocks from different lateral positions in an orogenic belt along with two dimensional modeling procedure can put strong constraints on models of regional tectonic processes

seismic gravity and magnetic surveys in the central part of the red sea their interpretation and implications for the structure and evolution of the red sea abstract while there is unanimous agreement that the axial part of the red sea has witnessed seafloor spreading since the early pliocene opinions diverge considerably with regard to the situation in the main trough and shelves new high precision data comprising palaeomagnetic seismic gravity magnetic and bathymetric measurements from the central and northern parts of the red sea have been examined to discriminate between the earlier proposed hypotheses and to formulate an evolutionary model for the red sea the results show that oceanic crust extends for about 80 km at 19 n on each side of the spreading axis symmetrical seafloor spreading magnetic anomalies are recognized out to anomaly 3a 6 m y b p between 18 30 and 19 45 n anomaly 2a between 19 45 and 20 n and near 21 n anomaly 2 near 22 n and only anomaly 1 near 23 30 n although anomaly 3a is the oldest recognizable magnetic anomaly the extent and configuration of the oceanic basement as determined from seismic measurements indicate that spreading has started and continued without a hiatus since 10 12 m y b p prior to the initiation of oceanic accretion an extension of 40 55 km i e 50 70 has taken place in the proterozoic continental lithosphere whereas a separation of 70 160 km has occurred during the seafloor spreading phase several lines of evidence indicate that the shelves and the marginal parts of the main trough are of continental nature

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

the timing of uplift volcanism and rifting peripheral to the red sea a case for passive rifting prior to the formation of the red sea the northeastern afro arabian continent had low relief and was largely below sea level from the late cretaceous to the early oligocene the events leading to the formation of the red sea followed the sequence 1 alkaline volcanism and rifting beginning about 30 32 ma affecting a narrow linear zone in the continent 2 rotational block faulting and detachment faulting well underway by 25 ma 3 gabbro and diorite magmatism andesite to rhyolite volcanism and fine grained nonmarine sedimentation in the rift between 20 and 25 ma 4 fine grained marine sedimentation in the rift as the early shelves started to subside in the middle miocene and 5 uplift of the adjacent continents about 3 km and subsidence of the shelves about 4 km between 13 8 and 5 ma the youth of the uplift is suggested by 44 fission track dates on apatites from rocks of the proterozoic arabian shield that range in age from 13 8 to 568 ma the youngest of these ages coupled with the present high relief along the arabian escarpment and published heat flow measurements indicate that 2 5 4 km uplift has occurred in the last 13 8 m y the sequence volcanism rifting followed by uplift leads to our adoption of a passive mantle model for rift origin models that require uplift to create the rift are rejected because of the late uplift we advocate a model of lithospheric extension caused by two dimensional plate stress over those requiring tractional drag at the base of the lithosphere caused by vigorous flow in the asthenosphere it is acknowledged that traction models could explain the observed data but they imply a rigid static lithosphere and seem to require a link between the direction of flow in the asthenosphere and plate motions neither requirement is necessary in the extension model the rift starts with mechanical extension in a narrow zone of lithosphere between 25 32 ma in our model the thinned lithosphere is replaced by upwelling asthenosphere and by rocks from the adjacent deep continental lithosphere which flow into the rift ductile flow of the deep continental lithosphere is accelerated by partial melting as rocks flow upward toward the rift axis once partially melted rocks formerly part of the continental lithosphere join the upwelling asthenosphere resulting in a rapid erosion of the lithospheric mantle beneath the continent near the rift edge the resulting density decrease explains the uplift we think that the red sea began as a consequence of changing plate geometries resulting from the collision of india and eurasia after the collision the segment of the owens fracture zone north of the carlsberg ridge became locked forcing the northeast corner of afro arabia to rotate with the indian plate away from the rest of africa

a microseismicity survey of axial seamount juan de fuca ridge an array of ocean bottom seismometers and hydrophones were deployed within the caldera of axial seamount located at the intersection of the cobb eickelberg seamount chain and the juan de fuca ridge recent manned submersible dives have discovered the presence of two distinct hydrothermal vent fields the objective of the microseismicity survey was to determine baseline information necessary for designing an optimal experiment to monitor seismicity associated with submarine volcanic and hydrothermal systems with an instrument deployment period of eight days over 1500 events were recorded and examined most events occurred within three periods of swarm activity the location of the swarm events is believed to be within the low temperature vent field in the northwest portion of axial s caldera these impulsive high frequency events are interpreted to be tectonic in origin no harmonic tremor like events were observed on any of the instruments indicating a lack of magmatic movement only nine local events were recorded on more than one instrument and none of these could be located unambiguously monochromatic 20 hz signals were also recorded and are interpreted to originate from whales observed in the immediate vicinity

on the fate of sulfate during hydrothermal circulation at mid ocean ridges reliable sulfate data are reported for seafloor hydrothermal vents at the galapagos rift at 86 w and the east pacific rise at 21 n making use of sulfate 3he systematics a loss of sulfate comparable to the river input is estimated it is argued that the dominant chemical mechanism is reduction by iron a conclusion based on strontium systematics and indicators of the water rock ratios controls on the fate of sulfate in laboratory experiments are examined it is proposed that the mg ca ratio exerts a significant influence on the rate of sulfate reduction mixing of the hydrothermal endmember with seawater is considered with slow mixing alkaline earth sulfates should precipitate such precipitation accounts for the greater sulfate heat ratio in the galapagos fluids which are diluted extensively subsurface the particular sulfate mineral formed is not known because of questions concerning the control of alkalinity

seismic study of the crust of the northern red sea and gulf of suez we report the results of fifteen expanding spread profiles esp s and a seismic wide angle reflection refraction une performed during march april 1986 in the gulf of suez and the egyptian part of the northern red sea area north of 25 n four 16 4 air guns were used as a sound source on board r v le suroit and a 96 channel 2 4 km long streamer was towed by a supply vessel the whity tide most of the profiles show good crustal reflection and refraction arrivals and often good moho arrivals obtained for a distance of 80 to 100 km we present the results of x t and p analysis obtained by a velocity inversion performed in the p plane and by ray tracing modeling of both the p and the x t sections the velocity models are computed for planar and linear gradient velocity layers the northernmost part of the red sea appears to be characterized by a continental type crust extremely thinned 4 lying at a mean depth of 7 8 km whereas the moho is at a mean 14 15 km depth the southern part shows a seismic velocity structure of an oceanic type except in the 40 km closest to the coastline in both parts seismic waves progressively get more attenuated with distance from the shore to the axial zone which is presently tectonically active the difference between the northern continental and southern oceanic zone is related to the termination of the levant fault the northern continental area appears to represent the termination of the levant fault as a zone of distributed deformation

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

the east pacific rise near 21 n 13 n and 20 s inferences for along strike variability of axial processes of the mid ocean ridge abstract data obtained along various segments of the mid ocean ridge mor are used to construct an idealized model for crustal accretion the model seeks to predict the topographic volcanic tectonic and hydrothermal characteristics of any given spreading segment of the mor as a function of distance away from the bounding transform faults this model is based on a series of detailed mapping efforts carried out on segments of the mor having a broad range of spreading rates between 2 and 16 cm yr this paper includes the results of the french searise program carried out during the summer of 1980 aboard the n o jean charcot and two american cruises conducted in 1981 aboard the r v melville

rapid subsidence of small oceanic basins a study of the marsili basin central mediterranean many small backarc basins show subsidence rates significantly greater than those in major ocean basins of similar ages in this study we test by numerical modeling the hypothesis that small backarc basins are formed by the same fundamental processes as major oceanic basins i e by accretion spreading and cooling and that the rate of subsidence is modified and accelerated by lateral heat loss from the hot subbasinal asthenosphere to the surrounding cooler continental lithosphere bathymetric seismic and drilling data in the marsili basin a small and young backarc basin in the central mediterranean are used to constrain the models the results of this analysis confirm the above hypothesis they further suggest that for actively spreading backarc basins the most rapid subsidence occurs in a zone near the margin of the basin during the earliest stage of basin spreading whereas after spreading terminates the location for rapid subsidence migrates towards the basin center

potential field modeling of collapse prone submarine volcanoes in the southern tyrrhenian sea italy 1 hydrothermal alteration may weaken volcanic rocks causing the gravitational instability of portions of active volcanoes with potentially hazardous collapses here we show high resolution multibeam magnetic and gravity surveys of the marsili seamount the largest active volcano of europe located in the southern tyrrhenian back arc basin these surveys reveal zones with exceptionally low densities and with vanishing magnetizations due probably to the comminution of basalts during hyaloclastic submarine eruptions and to their post eruptive hydrothermal alteration the location of these regions correlates with morphological data showing the occurrence of past collapses similar evidence has been obtained from pre existing data at vavilov seamount another older volcanic system in the tyrrhenian back arc basin here a large volume of at least 50 km3 may have collapsed in a single event from its 40 km long western flank given the similarities between these volcanoes a large collapse event may also be expected at marsili

hydrothermal iron rich deposits from the teahitia mehitia and macdonald hot spot areas southwest pacific abstract during cruise so 47 of r v sonne iron oxyhydroxide crusts of restricted extent were recovered from teahitia and moua pihaa seamounts in the teahitia mehetia hot spot area and from macdonald seamount in the macdonald hot spot area the crusts show complex layering and incorporate volcanic ash layers they consist of goethite feldspar and pyroxene and display low contents of transition elements apart from fe it is believed that the crusts were deposited from relatively low temperature hydrothermal fluids these low temperatures preclude the formation of sulphide deposits the type of hydrothermal mineral recovered may be a function of the thermal regime of the system and the elevation which controls the pressure of the venting fluids the shallower depths of these seamounts compared to the axes of mid ocean ridges may therefore be responsible for the style of mineralization initially magmatic vapours migrating upwards through the hot spot volcano are thought to be absorbed into seawater at comparatively shallow levels to deposit sulphide minerals within the volcanic pile the fe oxyhydroxide deposits are the residual low temperature deposits formed when the spent hydrothermal fluid reached the crest of the seamount these deposits therefore differ fundamentally from those found at the crest of mid ocean ridges where sulphide mounds are deposited directly upon interaction of the boiling hydrothermal fluid with seawater the discovery of hydrothermal iron oxyhydroxide crusts in these two hot spot areas is only the second such finding after loihi seamount southeast of hawaii

magnetic evidence for the nature of the crust beneath the southern red sea magnetic data over the southern red sea are used to examine various models for the evolution of the region the linear nature of the magnetic anomalies over the coastal margins and adjacent portions of the main trough together with their decrease in amplitude with elevation is shown to be compatible with deep thin sources that are strongly magnetized the general lack of correspondence between gravity anomalies and deskewed magnetic anomalies indicates that the anomalies are due to intrabasement bodies rather than basement relief associated with density contrasts across axis correlations of individual anomalies indicate a symmetrical disposition of magnetic sources about a line close to the present deep water axis the regular symmetrical pattern of linear anomalies strongly suggests a seafloor spreading origin the combined geologic and geophysical evidence therefore supports a model in which the margins are underlain by material more typically oceanic in character than continental the magnetic evidence further implies that this oceanic crust formed during an earlier phase of spreading in the red sea although the age of this crust is unknown anomaly correlations suggest spreading may have taken place during the late oligocene early miocene

geological setting and mineralogical and geochemical investigations on sulfide deposits near 13 n on the east pacific rise geological mineralogical and geochemical investigations of hydrothermal deposits on one of the most studied segments of the east pacific rise near 13 n show that sulfides occur in four main tectonic settings these are 1 in the middle part of the central graben where active zones are located 2 at the top of the central graben where a second line of deposits is observed 3 on the marginal high which has sulfides both on its western tectonic side and at its top where a large deposit is located and 4 on the flank of the southeastern seamount which is the site of the most important deposit of the area the nature of the distribution of the samples through the area studied shows that hydrothermal activity has existed for more than 20 000 years mineral assemblages are dependent on the sample location the sulfide samples exhibit a wide range of mineralogical assemblages varying from high temperature hydrothermal cu fe rich chimneys black smokers which are characteristic structures of young edifices to mature cu fe rich massive sulfides similar to those found in the core of massive sulfide lenses mined on shore with respect to these observations a classification into nine types of sulfide assemblages is proposed hydrothermal activity at the axis is now increasing in intensity increase in temperature decrease in f s2 and f o2 the relative paucity of zn rich massive sulfides on the off axis structures is likely to be a consequence of the depletion of zn in the hydrothermal fluids r mode factor analysis applied to a population of nineteen representative bulk chemical analyses shows fractionation of the reduced sulfur fluid during the discharge into three main groups cu fe co se ca sr zn cd ag as pb and si ba displaying the principal types of sulfide samples collected in the 13 n area microprobe investigations conducted on sulfide minerals define the distribution of trace metal elements such as ag co se cd and as during the process of hydrothermal maturation of the deposits

processes associated with ocean crustal formation the juan de fuca ridge recent geophysical geological and geochemical studies on the juan de fuca ridge have been gathered into a special collection of manuscripts that demonstrate the interdisciplinary nature of the crustal formation and evolutionary processes on the ridge axis these new studies include long term sea floor and water column measurements that indicate real time changes due to volcanic activity and related hydrothermal flux sea floor measurements of seismic activity detecting swarms of earthquakes near the axial zone and submersible observations of a 2 km along strike spacing of large scale hydrothermal fields detailed sampling of axial basalts show that in spite of abundant evidence of excess volcanic activity the juan de fuca ridge is not a traditional hot spot on a larger scale geophysical surveys of the ridge flanks show that the upper crust can vary in thickness by a factor of two that crustal porosity systematically decreases with age and the topographic forcing is the dominant control of off axis hydrothermal circulation finally the juan de fuca ridge has long been a testing ground for innovative survey techniques and this issue includes reports on the development of acoustic backscatter as a quantitative measure of basement roughness and as an indicator of the presence of lithified sediments overlying zones of fluid upwelling

heat flow and lithospheric thinning in the ligurian basin n w mediterranean we present preliminary results of 78 new heat flow measurements collected on a transect across the central ligurian basin south france to corsica margins the basin wide flux has an average of 79 21 mwm 2 with significant local and regional variability correction for the deposition of up to 5 km of post rift sediment gives a mean value of 102 mwm 2 which is similar to values for normal oceanic crust of the same age 21 ma the total tectonic subsidence tts calculated for the region with probable oceanic crustal structure is comparable to or somewhat greater than 21 ma crust of the major ocean basins the heat flow data overall are compatible with the observed tts in showing substantial lithospheric thinning but do not tightly constrain thermal models of continental extension and rifting

subsidence of the shikoku back arc basin abstract sources of basaltic tephras found in miocene cores at dsdp sites 443 and 444 in the eastern side of shikoku basin are likely to be the kinan seamounts which were emergent during middle to early late miocene abnormally high vesicularity of dredged and dsdp pillow basalts together with existence of a thin layer of limestone or chalk overlying the basalts seem to indicate that the axial zone of shikoku basin was much shallower than the present depth immediately after solidification of the basement and that it subsided quickly shallow water depths of the back arc basins are characteristic of the present lau basin and okinawa trough whereas the mariana trough possesses water depths about 1000 m deeper than normal oceans of the same age most of inactive back arc basins have depth age relations consistent with that of mariana trough very shallow water depths of the axial zone of back arc basins similar to those of the shikoku basin axis during their opening period appear to be due to non isostatic uplift caused by excess mass and to isostatic elevation by buoyancy of hot partially molten materials accumulated above the subducting slab

effects of finite rifting times on the development of sedimentary basins abstract most thermo mechanical models for the development of sedimentary basins have assumed that the rifting responsible for the formation of the basin occurred instantaneously and have examined the post rift development of the basin this assumption greatly simplifies the mathematical treatment but is not in accord with what is found in nature where 10 to 50 m y rifting events commonly accompany the formation of sedimentary basins and continental margins the effects of a finite rifting time on the development of sedimentary basins are examined using an analytic technique which allows an arbitrary rifting history in both time and space and which considers the effects of both vertical and horizontal heat transfer this technique allows the thermal structure of the lithosphere to be calculated throughout the rifting event and thus permits the subsidence history and surface heat flow of the developing basin to be traced the effect of a finite duration extension event is that heat is lost during rifting increasing the syn rift subsidence at the expense of the post rift lateral heat flow which was not included in previous studies of the effect of finite rifting times has a significant effect on the subsidence history distribution of sediments and thermal history in particular the post rift subsidence is decreased by more than 25 for a 20 m y rifting event and by more than 10 15 for a rifting event as short as 10 m y this will significantly decrease the subsidence rates in the post rift stage and implies that inferences concerning the structure development and thermal history of the basin derived from using curves to interpret backstripped subsidence can be greatly in error variations in syn rift sediment accumulation and lithospheric thermal structure at the end of rifting resulting from different rifting histories can interact with other factors such as the flexural response of the lithosphere to sediment loading to affect the final width of the basin the total amount of sediments that accumulate and the basin stratigraphy

crustal helium in deep pacific waters we have analyzed deep helium isotope data from world ocean circulation experiment section p17 along 135 w in the pacific ocean combining these data with neon data we have derived the isotopic ratio rt of the nonatmospheric part of helium terrigenic helium the calculated rt values are significantly lower than helium isotope ratios from mid ocean ridge mor sources rmor 8ra ra 3he 4he air which are generally presumed to dominate the isotope characteristics of helium of nonatmospheric origin for the depths most influenced by the southern east pacific rise epr helium plume the isotopic ratio of terrigenic helium is 7 4 ra however both the northern epr plume and the juan de fuca ridge jdfr plume show significantly lower rt values of 6 8 ra we find minimum rt values of 5 6 ra at about 20 n to 30 n from 2000 m depth to the ocean floor we conclude that in addition to the epr and jdfr helium sources having mor isotope characteristics there is a flux of radiogenic 4he out of deep sea sediments and the oceanic crust which causes a significant deviation of rt toward values lower than rmor using our calculated terrigenic 4he concentrations and isotope ratios rt and the helium isotope ratios of the two end members mor helium and crustal helium we derive concentrations and ocean bottom fluxes of crustal 4he 4hecrust maximum concentrations of 4hecrust up to 30 pmol kg 1 are found at about 20 n to 30 n between 2000 m and 4000 m along the p17 section the mean crustal 4he flux is obtained as 1 0 0 4 109 atoms m 2earth s 1

a detailed study of the cobb offset of the juan de fuca ridge evolution of a propagating rift the cobb offset on the northern portion of the juan de fuca ridge has been identified as the tip of a northward propagating rift hey and wilson 1982 map compilations of magnetic and seismic data from four new cruises define the present locus of spreading and volcanism on the two ridge segments abutting the offset and permit detailed modeling of the recent evolution within this transform zone the axis of recent spreading on the southern ridge segment bends from the normal ridge trend n20 e to a n s trend north of 47 15 n the spreading axis on the northern ridge segment generally defines a n20 e trend except at the southern terminus where the spreading center is offset slightly to the east the two spreading centers overlap by about 33 km in the offset vicinity and there is evidence of recent volcanism on both segments present ridge axis morphology exhibits a transitional sequence from a symmetrical axial high along the more normal portions of each ridge segment to a grabenlike depression as the tip is approached the magnetic anomaly patterns observed in the cobb offset vicinity are not consistent with the patterns predicted by models of continuous northward propagation the magnetic anomaly patterns of the brunhes epoch require an event of rapid northward propagation about 0 7 m y b p followed by a more gradual southward propagation in the middle brunhes epoch most recently the spreading center on the southern ridge has extended northward to its present configuration prior to the brunhes epoch modeling of the magnetic anomaly patterns does not indicate a unique solution however net propagation has been northward we present alternative models for the period beginning 1 7 m y b p in the first model the cobb offset has evolved by a series of northward and southward events of propagation with net advance to the north in the second model stable asymmetric spreading from overlapping ridge segments has evolved into a transform fault offset by gradual decay of the northern ridge both models indicate that propagation has been discontinuous and relatively rapid compared to spreading rate and that realignment of ridge segments near the propagator tip has been an important component

diversity and distribution of subseafloor thermococcales populations in diffuse hydrothermal vents at an active deep sea volcano in the northeast pacific ocean received 30 august 2005 revised 28 july 2006 accepted 5 september 2006 published 20 december 2006 1 the presence diversity and distribution of a key group of subseafloor archaea the thermococcales was examined in multiple diffuse flow hydrothermal vents at axial seamount an active deep sea volcano located in the northeast pacific ocean a polymerase chain reaction pcr approach was used to determine if this group of subseafloor indicator organisms showed any phylogenetic distribution that may indicate distinct subseafloor communities at vents with different physical and chemical characteristics targeted primers for the thermococcales 16s rrna small subunit ribosomal rna gene and intergenic transcribed spacer its region were designed and applied to organisms filtered directly from a variety of diffuse flow vents thermococcales were amplified from 9 of 11 samples examined and it was determined that the its region is a better phylogenetic marker than the 16s rrna in defining consistent groups of closely related sequences results show a relationship between environmental clone distribution and source vent chemistry the most highly diluted vents with elevated iron and alkalinity contained a distinct group of thermococcales as defined by the its region suggesting separate subseafloor thermococcales populations at diffuse vents within the axial caldera

geochemistry of hydrothermal fluids from axial seamount hydrothermal emissions study vent field juan de fuca ridge subseafloor boiling and subsequent fluid rock interaction hydrothermal fluids collected from the ashes vent field in 1986 1987 and 1988 exhibit a very wide range of chemical composition over a small area 60 m in diameter compositions range from a 300 c gas enriched 285 mmol kg co2 low chlorinity 33 of seawater fluid to a 328 c relatively gas depleted 50 mmol kg co2 high chlorinity 116 of seawater fluid the entire range of measured compositions at ashes is best explained by a single hydrothermal fluid undergoing phase separation while rising through the ocean crust followed by partial segregation of the vapor and brine phases other mechanisms proposed to produce chlorinity variations in hydrothermal fluids precipitation dissolution of a chloride bearing mineral or crustal hydration cannot produce the covariation of chlorinity and gas content observed at ashes there is good agreement of the measured fluid compositions with compositions generated by a simple model of phase separation in which gases are partitioned according to henry s law and all salt remains in the liquid phase significant enrichments in silica lithium and boron in the low chlorinity fluids over levels predicted by the model are attributed to fluid rock interaction in the upflow zone depletions in iron and calcium suggest that these elements have been removed by iron sulfide and anhydrite precipitation at some time in the history of the low chlorinity fluids the distribution of low and high chlorinity venting is consistent with mechanisms of phase segregation based on differential buoyancy or relative permeability the relatively shallow depth of the seafloor 1540 m and the observed chemistry of ashes fluids are consistent with phase separation in the sub critical or near critical region

odp leg 107 in the tyrrhenian sea insights into passive margin and back arc basin evolution leg 107 of the ocean drilling program drilled a west northwest east southeast transect of seven sites across the tyrrhenian sea the youngest of the sub basins of the mediterranean sea sites 654 653 652 and 656 document the rifting and subsidence of the sardinia passive continental margin on the upper margin site 654 we cored a classic transgressive sequence subaerial conglomerates overlain by oyster bearing sands overlain by marine marl comparison between the recovered lithologies and seismic reflection profiles suggests that the synrift sediments on the upper margin are tortonian late miocene to messinian latest miocene in age whereas synrift sediments on the lower margin are messinian to pliocene in age during the messinian desiccation of the mediterranean sites 654 and 653 now on the upper sardinian margin apparently occupied a basinal setting where they received nannoplankton bearing clays interbedded with laminated gypsum sites 656 and 652 now on the lower sardinia margin were apparently higher standing during the desiccation event their messinian facies are subaerial and lacustrine respectively we infer from these lines of evidence that tilting and subsidence occurred more than a million years earlier on the upper margin than on the lower margin such diachroneity can be interpreted in terms of migration of the zone of maximum extension above a rolling back subduction zone or in terms of extension of continental crust by shear along a deep detachment fault sites 655 651 and 650 were drilled into two small basalt floored basins of the central and eastern tyrrhenian emplacement of basaltic crust in the central tyrrhenian vavilov basin apparently began more than a million years before emplacement of basaltic crust in the eastern tyrrhenian marsili basin this observation is compatible with previous suggestions that the tyrrhenian has grown southeastward in response to rollback of the down going slab that currently dips northwestward under the toe of italy at the easternmost site high vesicularity of the basalt and benthic foram assemblages in the oldest sediments imply that the basalt erupted in water shallower than 2 500 m it has apparently subsequently subsided to its present depth of 4 100 m below sea level nearly three times as fast as normal subsidence of crust formed at a mid ocean ridge

thermal and mechanical evolution of u s atlantic continental margin abstract the postrifting tectonic subsidence of the u s atlantic continental margin appears to be due to thermal contraction following continental breakup this subsidence is greatly increased by the effect of sediment loading models for the response of the lithosphere to loads show that the rigidity of the lithosphere is a strong function of temperature hence the flexural rigidity at the u s margin should vary in both space and time the thermal structure and subsidence of the margin has been modelled using a two dimensional extension model where the amount of extension has been allowed to vary across the margin and lateral conduction of heat has been included the equivalent elastic thickness of the lithosphere has been approximated as the depth to an isotherm thermoelastic effects of the cooling lithosphere as well as the flexural loading of the sediments have been included the lateral flow of heat and flexure have contrasting effects the lateral heat flow tends to cause uplift of the coastal plain region landward of the hinge zone and to increase the subsidence rate in the regions of the transitional crust in addition the effects of lateral heat flow modify the thermal history of the sediments and heat flow flexure on the other hand causes subsidence of the coastal plain and decreases the sediment accumulation beneath the transition zone the stratigraphy of the u s margin is seen to be the result of the complex interaction of these effects as their contributions vary through the history of the margin end of article last page 1671

calc alkaline magmatism and rifting of the deep water volcano of marsili aeolian back arc tyrrhenian sea abstract geochemistry and mineral chemistry data indicate that new lava samples recovered from a large depth interval of marsili volcano southern tyrrhenian sea are calc alkaline the deep to intermediate portions of the volcano are made up by calc alkaline basalts whereas the summit consists of high potassium calc alkaline andesites the contents of mgo cao total iron cr ni and v decrease with increasing silica while k 2 o rb ba th nb and lree exhibit the opposite behavior the andesite lavas of the top at waterdepth shallower than about 800 m are younger than 0 2 ma composition of the basalts varies with the waterdepth overall the deepest products of marsili are geochemically more mafic than those erupted in the higher physiographic positions geochemical features suggest the existence of a mantle source that was modified by metasomatic acquirement of subduction derived incompatible elements the seamount lies on oceanic crust which formed above subducting lithosphere not earlier than about 2 ma ago it is currently undergoing extension along linear faults the major distensional fractures trend n s to n15 e paralleling the physiographic elongation of the volcano at the summit later faults have an oblique ne sw orientation which is associated with the eruption of high potassium andesites the transition from basalts to high potassium andesites is discussed as a possible response to the extension and rapid subsidence that dominate the volcano and surrounding region

volcanic features of the north rockall trough application of visualisation techniques on 3d seismic reflection data understanding the volcanic processes operating during continental break up is hampered by the subsequent burial of the majority of the volcanic pile beneath thick sedimentary sequences currently located in bathymetrically deep offshore regions although portions of these volcanic systems are currently exposed on land a full understanding of the volcanic structure the eruptive styles and their evolution is not possible as these localities have been partially eroded furthermore as the onshore exposure represents a volumetrically minor part of the entire system the documented eruptive styles may not be representative the increasing availability of 3d seismic reflection data has the potential to significantly enhance our understanding of break up related volcanism as it allows direct access to detailed information from the buried volcanic succession however conventional seismic interpretation methodologies cannot determine lava flow morphologies and as a result eruptive styles and their evolution are still largely based on extrapolation from the accessible onshore outcrop data new 3d seismic volume visualisation techniques allow the buried basalt morphologies to be examined for the first time in a manner similar to outcrop aerial photographic or satellite based observations applying this new approach for 3d seismic data to the north rockall trough u k atlantic margin demonstrates that a range of volcanic features indicative of eruptive style can be determined the data reveal a complex terrain containing lava flows originating from discrete volcanic centres contemporaneous normal faults linear fissures a few kilometres long radial fissures and inflation ridges lava flow morphologies that are indicative of tube fed inflated sheetflows intracanyon flows and elongate subaerial flows that enter water downslope to produce a large hyaloclastite delta are observed

late oligocene to recent episodes of magmatism in and around the tyrrhenian sea implications for the processes of opening in a young inter arc basin of intra orogenic mediterranean type abstract five episodes of magmatic activity as deduced from geochronological data affected the tyrrhenian and peri tyr rhenian areas from the late oligocene 32 ma to recent times the episodes of w e migration of magmatism from the islands of sardinia and corsica to the eolian islands and the roman comagmatic region of central italy are related to the anti clockwise drift movements of the apenninic chain and clockwise drifting of the sicilian part of the maghrebian chain magmatic activity in the tyrrhenian sea initiated in late miocene times 8 5 6 5 ma in concomitance with a significant change of the geodynamic regime from compressional to tensional the tyrrhenian bathyal plain was affected by two episodes of spreading the first episode occurred around 7 ma and the second from about 1 9 to 1 3 ma these spreading events apparently developed during times of magmatic quiescence in the peri tyrrhenian areas two subsequent episodes of volcanism of the seamount type occurred in the bathyal plain one from 5 to 1 8 and the other from 1 3 to recent they are contemporaneously with extensive manifestations in the peri tyrrhenian areas the different types of magmatism of the late miocene back arc spreading in the central bathyal plain and acidic manifestations of intra crustal anatectic origin in the northern continental platform and slope i e islands of capraia elba and montecristo and seamount vercelli are possibly related to movements of upwelling of the astenosphere which in the southern tyrrhenian sea developed with a much stronger intensity timing of the volcanism in relationship with the overall structural evolution through time in the basin and neighbouring lands is considered

tectonic focusing of voluminous basaltic eruptions in magma deficient backarc rifts highlights shield type volcanoes in young backarc rifts may be linked to arc segmentation arc transverse faults act as magma conduits for voluminous basaltic eruptions the nifonea volcano hosts the first large lava lake described in a submarine backarc effusive eruptions produced short live hydrothermal venting and an event plume the coriolis troughs of the new hebrides subduction zone are among the youngest backarc rifts in the world they reach depths of 3 km despite their small size 100 km in length and only 25 45 km wide and their proximity to the arc front 50 km the narrow deep graben morphology is characteristic of magma deficient arc rifts in the early stages of backarc extension where the rate of extension and subsidence exceeds the magmatic input unexpectedly the youngest graben the vate trough contains a centrally located 1000 m tall and 14 km wide shield volcano with a large 5 8 km breached summit caldera the nifonea axial volcano has a volume of 126 km3 reflecting unusually high extrusion rates given its young age 3 ma and the summit caldera hosts the remnants of a large lava lake the first described from a submarine backarc setting extensive diffuse hydrothermal venting and several clusters of black smoker chimneys with the highest recorded fluid temperatures 368 c in the sw pacific occur on the youngest lava flows comparison with similar axial volcanoes on the mid ocean ridges suggests that the 46 106 m3 of sheet flows in the caldera could have been erupted in 30 hours the focusing of voluminous basaltic eruptions into an otherwise magma deficient backarc has been linked to strong left lateral transtension caused by clockwise rotation and segmentation of the southern portion of the arc after collision with d entrecasteaux ridge this study shows that the upper plate stresses can result in dramatic variability in magma supply and hydrothermal activity at the earliest stages of arc rifting and could explain the wide range of melt compositions volcanic styles and mineral deposit types found in nascent backarc rifts

thermal evolution of a rift basin the tyrrhenian sea existing geological and geophysical data support the view that the opening of the tyrrhenian sea was by rifting of a formerly continuous continental lithosphere which began in upper miocene on the upper sardinian margin and migrated southeastward to the calabria margin heat flow across the tyrrhenian sea shows a pronounced asymmetry from about 50 70 mw m 2 over the sardinian margin to more than 150 mw m 2 over its southeastern margin off southern italy and calabria in this study we use the newly acquired data from ocean drilling program leg 107 heat flow seismic velocity and bathymetric data to constrain a group of models that supposes the tyrrhenian sea opened by detachment of a formerly continuous lithosphere along a southeastern facing low angle normal fault specific models tested include 1 the wernicke model in which thinning and extension of the continental lithosphere are supposed to be accomplished by simple shear along a large scale gently dipping detachment fault zone which cut through the entire lithosphere and 2 the delamination model in which the low angle detachment fault is supposed to cut only the upper and middle crust but to merge subhorizontally into the lower crust below which concurrent pure shear may occur a versatile two dimensional finite element procedure is used to evaluate the tectonic and thermal evolution of these models and the results are compared with observations to test the validity of the models delamination models with large pure shear components in the lower lithosphere and the wernicke model predict heat flow within 1 of the measurements the wernicke model however predicts a basin configuration much too deep in comparison with bathymetry and a subbasinal lithospheric thickness substantially different from rayleigh wave dispersion results the delamination model with a large component of pure shear in the subbasinal lower lithosphere on the other hand predicts satisfactorily both the bathymetry and the lithospheric thickness beneath the basin

architecture and neogene to recent evolution of the western calabrian continental margin an upper plate perspective to the ionian subduction system central mediterranean the western calabria continental margin forms the transition between the pliocene to recent marsili spreading center and continental calabria all parts of the upper plate of the ionian subduction zone integrating high resolution and crustal seismic images constrained by gravity modeling we provide a detailed reconstruction of the architecture of the margin and develop a new scheme for its miocene to present evolution this time span encompasses the continent continent collision between africa and eurasia subsequent orogenic collapse and rifting apart between the two continental masses and the pliocene to recent emplacement of oceanic crust in the vavilov and marsili basins the crust of the margin thins from the calabria coast 25 km to the marsili continent ocean transition 12 km on the whole upper and lower crusts thin proportionally with pure shear geometry the continental margin is covered by an oligocene to present sedimentary succession reaching a maximum thickness of 6 0 km in the paola basin during the miocene the continental margin experienced regional shortening accommodated by a large number of mainly west vergent thrusts possibly associated with the late stages of the kabilo calabrian chain shortening continued through pliocene to recent but was accommodated by a limited number of west vergent thrust faults located in the western part of the margin and by a few tens of kilometers wide syncline located in the eastern part of the profile the accommodation space created in the syncline core hosted a 4 5 km thick plio quaternary sedimentary succession the paola basin no significant extensional fault is observed along the profile miocene to recent subsidence was controlled by 1 a short wavelength component related to shortening and responsible for the formation of the paola basin syncline and possibly contributing to the uplift of onshore calabria and 2 a long wavelength component responsible for the regional subsidence and oceanward tilting of the calabria margin short wavelength subsidence ended in the late pliocene but long wavelength downward movements persisted and even accelerated during late pliocene to quaternary times when the present day bathymetry was achieved both horizontal deformations and vertical movements are difficult to explain in the context of a normal back arc basin without taking into consideration patterns of secondary mantle flow generated by the retreat of the subducting slab