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Sample records for active spreading ridge

  1. Hydrothermal Activity on ultraslow Spreading Ridge: new hydrothermal fields found on the Southwest Indian ridge

    NASA Astrophysics Data System (ADS)

    Tao, C.; Li, H.; Deng, X.; Lei, J.; Wang, Y.; Zhang, K.; Zhou, J.; Liu, W.

    2014-12-01

    Ultraslow spreading ridge makes up about 25% of global mid-ocean ridge length. Previous studies believed that hydrothermal activity is not widespread on the ultraslow spreading ridge owing to lower magma supply. Southwest Indian ridge (SWIR) with the spreading rate between 1.2cm/a to 1.4cm/a, represents the ultraslow spreading ridge. In 2007, Chinese Cruise (CC) 19th discovered the Dragon Flag deposit (DFD) on the SWIR, which is the first active hydrothermal field found on the ultraslow spreading ridge. In recent years, over 10 hydrothermal fields have been found on the SWIR between Indomed and Gallieni transform faults by the Chinese team. Tao et al. (2012) implied that the segment sections with excess heat from enhanced magmatism and suitable crustal permeability along slow and ultraslow ridges might be the most promising areas for searching for hydrothermal activities. In 2014, CC 30thdiscovered five hydrothermal fields and several hydrothermal anomalies on the SWIR. Dragon Horn Area (DHA). The DHA is located on the southern of segment 27 SWIR, with an area of about 400 km2. The geophysical studies indicated that the DHA belongs to the oceanic core complex (OCC), which is widespread on the slow spreading ridges (Zhao et al., 2013). The rocks, such as gabbro, serpentinized peridotite, and consolidated carbonate were collected in the DHA, which provide the direct evidence with the existence of the OCC. However, all rock samples gathered by three TV-grab stations are basalts on the top of the OCC. A hydrothermal anomaly area, centered at 49.66°E,37.80° S with a range of several kms, is detected in the DHA. It is probably comprised of several hydrothermal fields and controlled by a NW fault. New discovery of hydrothermal fields. From January to April 2014, five hydrothermal fields were discovered on the SWIR between 48°E to 50°E during the leg 2&3 of the CC 30th, which are the Su Causeway field (48.6°E, 38.1°S), Bai Causeway field (48.8°E, 37.9 °S), Dragon

  2. Asymmetric active seismicity along the ultra-slow spreading Gakkel Ridge

    NASA Astrophysics Data System (ADS)

    Hopper, John R.; Voss, Peter H.; Lavier, Luc L.

    2015-04-01

    Ultra-slow spreading ridges are frequently characterised by spreading segments that are largely magma starved. Spreading along such segments does not occur by crustal creation/accretion processes such as intrusions, diking and volcanism, but rather by mechanical extension of the lithosphere, exposing the mantle to seafloor where it interacts with seawater to form serpentinite. Such exhumation is thought to occur along detachment faults that form concave down surfaces and produce an extensional geometry that is highly asymmetric. A consequence of all models that have been developed to simulate this type of extension is that stress and strain is focused primarily on the footwall block of the spreading system. This would predict that at any given time, only one side of the system should show active seismicity. In 2001, the Gakkel Ridge was extensively sampled by dredging during the AMORE cruise. These samples showed that the ridge is divided into distinct segments that today are either magmatically robust (only basalts recovered) or magmatically starved (dominantly serpentinised peridotite and gabbros recovered). We extracted earthquake data along the Gakkel Ridge from the global catalogs to investigate if these distinct segments exhibit any differences in active seismicity. We show that the western volcanic zone shows symmetric active seismicity, with earthquakes occurring on both sides of the ridge axis along a relatively restricted region. In contrast, the sparsely magmatic zone shows active seismicity dominantly along along the southern half of the ridge, with comparatively little seismicity to the north. These results are consistent with the proposed models for the formation of amagmatic spreading centers.

  3. Hydrothermal activity at slow-spreading ridges: variability and importance of magmatic controls

    NASA Astrophysics Data System (ADS)

    Escartin, Javier

    2016-04-01

    Hydrothermal activity along mid-ocean ridge axes is ubiquitous, associated with mass, chemical, and heat exchanges between the deep lithosphere and the overlying envelopes, and sustaining chemiosynthetic ecosystems at the seafloor. Compared with hydrothermal fields at fast-spreading ridges, those at slow spreading ones show a large variability as their location and nature is controlled or influenced by several parameters that are inter-related: a) tectonic setting, ranging from 'volcanic systems' (along the rift valley floor, volcanic ridges, seamounts), to 'tectonic' ones (rift-bounding faults, oceanic detachment faults); b) the nature of the host rock, owing to compositional heterogeneity of slow-spreading lithosphere (basalt, gabbro, peridotite); c) the type of heat source (magmatic bodies at depth, hot lithosphere, serpentinization reactions); d) and the associated temperature of outflow fluids (high- vs.- low temperature venting and their relative proportion). A systematic review of the distribution and characteristics of hydrothermal fields along the slow-spreading Mid-Atlantic Ridge suggests that long-lived hydrothermal activity is concentrated either at oceanic detachment faults, or along volcanic segments with evidence of robust magma supply to the axis. A detailed study of the magmatically robust Lucky Strike segment suggests that all present and past hydrothermal activity is found at the center of the segment. The association of these fields to central volcanos, and the absence of indicators of hydrothermal activity along the remaining of the ridge segment, suggests that long-lived hydrothermal activity in these volcanic systems is maintained by the enhanced melt supply and the associated magma chamber(s) required to build these volcanic edifices. In this setting, hydrothermal outflow zones at the seafloor are systematically controlled by faults, indicating that hydrothermal fluids in the shallow crust exploit permeable fault zones to circulate. While

  4. Kinematics of Mid-Ocean Ridge Relative Motions in the Indo-Atlantic Frame of Reference: Passive and Active Spreading Ridges

    NASA Astrophysics Data System (ADS)

    Rowan, C. J.; Rowley, D. B.; Forte, A. M.

    2011-12-01

    ridge parallel (N-S) motion in the past 30 Ma. Between 80 and 30 Ma, the midpoint of the Nazca-Pacific segment of the EPR moved ~2500 km parallel with the trend of the ridge. The absence of significant longitudinal motion of the EPR extends northward to the now subducted portion beneath eastern North America. We interpret this as an actively spreading ridge segment linked directly to and driven by active mantle upwelling associated with the mantle convective system.

  5. Evidence of recent volcanic activity on the ultraslow-spreading Gakkel ridge.

    PubMed

    Edwards, M H; Kurras, G J; Tolstoy, M; Bohnenstiehl, D R; Coakley, B J; Cochran, J R

    2001-02-15

    Seafloor spreading is accommodated by volcanic and tectonic processes along the global mid-ocean ridge system. As spreading rate decreases the influence of volcanism also decreases, and it is unknown whether significant volcanism occurs at all at ultraslow spreading rates (<1.5 cm yr(-1)). Here we present three-dimensional sonar maps of the Gakkel ridge, Earth's slowest-spreading mid-ocean ridge, located in the Arctic basin under the Arctic Ocean ice canopy. We acquired this data using hull-mounted sonars attached to a nuclear-powered submarine, the USS Hawkbill. Sidescan data for the ultraslow-spreading (approximately 1.0 cm yr(-1)) eastern Gakkel ridge depict two young volcanoes covering approximately 720 km2 of an otherwise heavily sedimented axial valley. The western volcano coincides with the average location of epicentres for more than 250 teleseismic events detected in 1999, suggesting that an axial eruption was imaged shortly after its occurrence. These findings demonstrate that eruptions along the ultraslow-spreading Gakkel ridge are focused at discrete locations and appear to be more voluminous and occur more frequently than was previously thought. PMID:11236991

  6. Seismicity and active accretion processes at the ultraslow-spreading Southwest and intermediate-spreading Southeast Indian ridges from hydroacoustic data

    NASA Astrophysics Data System (ADS)

    Tsang-Hin-Sun, Eve; Royer, Jean-Yves; Perrot, Julie

    2016-08-01

    Volcanic and tectonic events are the main processes involved in the generation of the oceanic crust and responsible for the seismicity associated with seafloor spreading. To monitor this activity, usually not or poorly detected by land-based seismological stations, we deployed from February 2012 to February 2013 a network of autonomous hydrophones to compare the behaviour of the ultraslow-spreading Southwest Indian ridge (SWIR) with that of the intermediate-spreading Southeast Indian ridge (SEIR). The rate of seismicity is similar for both ridges, suggesting that there is no systematic relationship between seismicity and spreading rates. The along-axis distribution of the seismic events, however, does differ, reflecting the rate dependence of accretion modes. Earthquakes are sparse and regularly spaced and scattered along the SWIR, reflecting prevailing tectonic processes. By contrast, along the SEIR, events are irregularly distributed and focus at ridge-segment ends and transforms faults, reflecting the ridge segmentation; only two swarms occurred at a segment centre and are probably caused by a magmatic event. This seismicity distribution thus looks controlled by segment-scale crustal heterogeneities along the SEIR and by regional-scale contrasting accretion processes along the SWIR, probably driven by different lithospheric and asthenospheric dynamics on either side of the Melville fracture zone. The comparison of hydroacoustic and teleseismic catalogues shows that, along these spreading ridges, the background seismicity observed in 1 yr by a hydroacoustic network is representative of the seismicity observed over two decades by land-based networks.

  7. Active spreading processes at ultraslow-spreading ridges: Relocalization and analysis of the 1999 earthquake swarm at Gakkel Ridge, Arctic Ocean

    NASA Astrophysics Data System (ADS)

    Korger, Edith; Schlindwein, Vera; Riedel, Carsten

    2010-05-01

    Gakkel Ridge is situated within the Arctic Ocean and spans from eastern Greenland to the continental Siberian shelf. It is termed an ultraslow spreading ridge (full spreading rate 6-14 mm/yr). At ultraslow spreading ridges, heat loss by conductive cooling is thought to decrease magma supply and focus magmatism in widely spaced discrete volcanic centres. In 1999 lasting over 7 months, the largest ever earthquake swarm at a mid-ocean ridge was teleseismically registered worldwide. It originated from around 85°E which was postulated to be a volcanic complex and featured 252 recorded events, including 11 events with a magnitude mb > 5.0. The unprecedented duration, magnitude and number of events ran counter to expectations at this location. Preliminary analysis with unreviewed datasets showed changing source-mechanisms from extensional to events with a greater non-double-couple component at around March, 4th and an abrupt decrease in the number of events on the 15th of April. Also the epicentres of the larger events seemed to migrate with advancing time to the NW, farther away from the volcanic centre. As these analyses were based on preliminary datasets it seemed indicated to relocalize the earthquakes when the reviewed bulletin of the International Seismological Centre was released. This relocalization was done with the earthquake localization programme NonLinLoc using a probabilistic approach and grid-search. The calculation of travel times used as velocity model both AK135 and IASP91 combined with a regional velocity model for recording stations with epicentral distance < 30°. We tested extensively the influence of localization algorithm, velocity model, station coverage and weighting on the localization result. The new locations of the epicentres show a clustering of events within the central rift valley and the southern rift flank. The dataset was further reviewed with regard to the quality of the localization and reduced to 93 well located events whose

  8. Seismicity and active accretion processes at the ultraslow-spreading Southwest and intermediate-spreading Southeast Indian ridges from hydroacoustic data

    NASA Astrophysics Data System (ADS)

    Tsang-Hin-Sun, Eve; Royer, Jean-Yves; Perrot, Julie

    2016-05-01

    Volcanic and tectonic events are the main processes involved in the generation of the oceanic crust and responsible for the seismicity associated with seafloor spreading. To monitor this activity, usually not or poorly detected by land-based seismological stations, we deployed from February 2012 to February 2013 a network of autonomous hydrophones to compare the behaviour of the ultraslow-spreading Southwest (SWIR) with that of the intermediate-spreading Southeast Indian ridges (SEIR). The rate of seismicity is similar for both ridges, suggesting that there is no systematic relationship between seismicity and spreading rates. The along-axis distribution of the seismic events, however, does differ, reflecting the rate-dependence of accretion modes. Earthquakes are sparse and regularly spaced and scattered along the SWIR, reflecting prevailing tectonic processes. By contrast, along the SEIR, events are irregularly distributed and focus at ridge-segment ends and transforms faults, reflecting the ridge segmentation; only two swarms occurred at a segment centre and are probably caused by a magmatic event. This seismicity distribution thus looks controlled by segment-scale crustal heterogeneities along the SEIR and by regional-scale contrasting accretion processes along the SWIR, probably driven by different lithospheric and asthenospheric dynamics on either side of the Melville FZ. The comparison of hydroacoustic and teleseismic catalogues shows that, along these spreading ridges, the background seismicity observed in one year by a hydroacoustic network is representative of the seismicity observed over two decades by land-based networks.

  9. Active spreading processes at ultraslow mid-ocean ridges: The 1999-2001 seismo-volcanic episode at 85°E Gakkel ridge, Arctic Ocean

    NASA Astrophysics Data System (ADS)

    Schlindwein, Vera; Riedel, Carsten; Korger, Edith; Läderach, Christine

    2010-05-01

    The rate of magma and crustal production at mid-ocean ridges is thought to decrease with decreasing spreading rate. At ultraslow spreading rates below 10-20 mm/y full rate, heat loss by conduction greatly reduces melt production with less melt produced at increasingly greater depths. Gakkel Ridge, the actively spreading mid-ocean ridge in the Arctic Ocean, opens at rates of 14 mm/y in the west decreasing to less than 6 mm/y at its eastern termination and demonstrates that magma production is not only a function of spreading rate. Whereas amagmatic spreading takes place at rates of about 12-10 mm/y, focussed melt production occurs at even lower spreading rates in long-lived discrete volcanic centres. One such centre is the 85°E volcanic complex at eastern Gakkel ridge where in 1999 a teleseismically recorded earthquake swarm consisting of more than 250 earthquakes over 9 months signalled the onset of an active spreading episode. The earthquake swarm is believed to be associated with volcanic activity although no concurrent lava effusion was found. We analysed the teleseismic earthquake swarm together with visual observation and microseismic data recorded at this site in 2001 and 2007 and noted the following characteristics which may be indicative for volcanic spreading events at the still poorly explored ultraslow spreading ridges: - unusual duration: The 1999 earthquake swarm lasted over 9 months rather than a few weeks as observed on faster spreading ridges. In addition, in 2001 seismoacoustic sounds which we interpret as gas discharge in Strombolian eruptions and a giant event plume maintained over more than one year indicate waxing and waning volcanic activity since 1999. - unusual strength: The earthquake swarm was detected at teleseismic distances of more than 1000 km and included 11 events with a magnitude >5. No other confirmed mid-ocean ridge eruption released a comparable seismic moment. Rather than focussing in a narrow area or showing pronounced

  10. Active spreading processes at ultraslow mid-ocean ridges: Unusual seismicity at the amagmatic Lena Trough, Arctic Ocean

    NASA Astrophysics Data System (ADS)

    Läderach, Christine; Schlindwein, Vera; Riedel, Carsten

    2010-05-01

    Lena Trough is the southern continuation of the ultraslow-spreading Gakkel Ridge and with its position in the Fram Strait between Greenland and Spitsbergen it is the only deep-sea gateway to the Arctic Ocean. DFG funded Emmy Noether group 'Mid-Ocean Volcanoes and Earthquakes' located at Alfred Wegener Institute for Polar and Marine Research is focusing on the seismicity of ultraslow spreading ridges and is especially interested in Lena Trough as an ultraslow spreading ridge in a developing stage. The southern Lena Trough shows similarities to the northern Red Sea spreading centre which is in the early stage of development from continental to oceanic rift. Cochran postulated in 2003 that the continental crust within the water-covered Red Sea is less than 10 km thick and that the northern part of the Red Sea rift spreads ultraslow as well. At Lena Trough an actively spreading mid-ocean ridge with a narrow rift valley has already developed but continental crust lies within a short distance. Lena Trough is extending from 83°N/5°W to 80.3°N/2°W where it passes into the transform fault of the Spitsbergen Fracture Zone. The geometry of Lena Trough and certain asymmetric structures in the rift valley indicate oblique spreading and mostly tectonic and amagmatic rifting. There are several topographic highs west of the ridge axis which could be bounded by deep faults with normal faulting or detachment character exposing mantle material at the surface. Seismicity at the Lena Trough shows apparently the same asymmetric character with epicenters of teleseismically recorded earthquakes concentrating predominantly west of the ridge axis. The most frequent focal mechanism of the earthquakes within the rift valley is normal faulting, whereas strike-slip faults occur in the Spitsbergen Fracture Zone. We relocalized teleseismic earthquakes recorded from May 1973 to April 2009 in the region using a refined localization algorithm and could confirm systematic asymmetry in the

  11. Reactive spreading: Adsorption, ridging and compound formation

    SciTech Connect

    Saiz, E.; Cannon, R.M.; Tomsia, A.P.

    2000-09-11

    Reactive spreading, in which a chemically active element is added to promote wetting of noble metals on nonmetallic materials, is evaluated. Theories for the energetics and kinetics of the necessary steps involved in spreading are outlined and compared to the steps in compound formation that typically accompany reactive wetting. These include: fluid flow, active metal adsorption, including nonequilibrium effects, and triple line ridging. All of these can be faster than compound nucleation under certain conditions. Analysis and assessment of recently reported experiments on metal/ceramic systems lead to a focus on those conditions under which spreading proceeds ahead of the actual formation of a new phase at the interface. This scenario may be more typical than believed, and perhaps the most effective situation leading to enhanced spreading. A rationale for the pervasive variability and hysteresis observed during high temperature wetting also emerges.

  12. Hydrothermal activity along the slow-spreading Lucky Strike ridge segment (Mid-Atlantic Ridge): Distribution, heatflux, and geological controls

    NASA Astrophysics Data System (ADS)

    Escartin, J.; Barreyre, T.; Cannat, M.; Garcia, R.; Gracias, N.; Deschamps, A.; Salocchi, A.; Sarradin, P. M.; Ballu, V.

    2015-12-01

    We have reviewed available visual information from the seafloor, and recently acquired microbathymetry for several traverses across the Lucky Strike segment to evaluate the distribution of hydrothermal activity. The Lucky Strike segment hosts three active hydrothermal fields: Capelinhos, Ewan, and the known Main Lucky Strike Hydrothermal Field (MLSHF). Capelinhos is located 1.3 km E of the axis and the MLSHF, and consists of a ~20 m sulfide mound with black smoker vents. Ewan is located ~1.8 km south from the MLSHF along the axial graben, and displays only diffuse flow along and around scarps of collapse structures associated with fault scarps. At the MLSHF we have identified an inactive site, thus broadening the extent of this field. Heat flux estimates from these new sites are relatively low and correspond to ~10% of the heat flux estimated for the Main field, with an integrated heatflux of 200-1200 MW. Overall, most of the flux (up to 80-90%) is associated with diffuse outflow, with the Ewan site showing solely diffuse flow and Capelinhos mostly focused flow. Microbathymetry also reveals a large, off-axis (~2.4 km) hydrothermal field, similar to the TAG mound in size, on the flanks of a rifted volcano. The association of these fields to a central volcano, and the absence of indicators of hydrothermal activity along the ridge segment, suggest that sustained hydrothermal activity is maintained by the enhanced melt supply and the associated magma chamber(s) required to build central volcanoes. Hydrothermal outflow zones at the seafloor are systematically controlled by faults, indicating that hydrothermal circulation in the shallow crust exploits permeable fault zones. Central volcanoes are thus associated with long-lived hydrothermal activity, and these sites may play a major role in the distribution and biogeography of vent communities.

  13. Characteristics of Hydrothermal Mineralization in Ultraslow Spreading Ridges

    NASA Astrophysics Data System (ADS)

    Zhou, H.; Yang, Q.; Ji, F.; Dick, H. J.

    2014-12-01

    Hydrothermal activity is a major component of the processes that shape the composition and structure of the ocean crust, providing a major pathway for the exchange of heat and elements between the Earth's crust and oceans, and a locus for intense biological activity on the seafloor and underlying crust. In other hand, the structure and composition of hydrothermal systems are the result of complex interactions between heat sources, fluids, wall rocks, tectonic controls and even biological processes. Ultraslow spreading ridges, including the Southwest Indian Ridge, the Gakkel Ridge, are most remarkable end member in plate-boundary structures (Dick et al., 2003), featured with extensive tectonic amagmatic spreading and frequent exposure of peridotite and gabbro. With intensive surveys in last decades, it is suggested that ultraslow ridges are several times more effective than faster-spreading ridges in sustaining hydrothermal activities. This increased efficiency could attributed to deep mining of heat and even exothermic serpentinisation (Baker et al., 2004). Distinct from in faster spreading ridges, one characteristics of hydrothermal mineralization on seafloor in ultraslow spreading ridges, including the active Dragon Flag hydrothermal field at 49.6 degree of the Southwest Indian Ridge, is abundant and pervasive distribution of lower temperature precipitated minerals ( such as Fe-silica or silica, Mn (Fe) oxides, sepiolite, pyrite, marcasite etc. ) in hydrothermal fields. Structures formed by lower temperature activities in active and dead hydrothermal fields are also obviously. High temperature precipitated minerals such as chalcopyrite etc. are rare or very limited in hydrothermal chimneys. Distribution of diverse low temperature hydrothermal activities is consistence with the deep heating mechanisms and hydrothermal circulations in the complex background of ultraslow spreading tectonics. Meanwhile, deeper and larger mineralization at certain locations along the

  14. Hydrothermal activity along the slow-spreading Lucky Strike ridge segment (Mid-Atlantic Ridge): Distribution, heatflux, and geological controls

    NASA Astrophysics Data System (ADS)

    Escartin, J.; Barreyre, T.; Cannat, M.; Garcia, R.; Gracias, N.; Deschamps, A.; Salocchi, A.; Sarradin, P.-M.; Ballu, V.

    2015-12-01

    We have reviewed available visual information from the seafloor, and recently acquired microbathymetry for several traverses across the Lucky Strike segment, to evaluate the distribution of hydrothermal activity. We have identified a new on-axis site with diffuse flow, Ewan, and an active vent structure ∼1.2 km from the axis, Capelinhos. These sites are minor relative to the Main field, and our total heatflux estimate for all active sites (200-1200 MW) is only slightly higher than previously published estimates. We also identify fossil sites W of the main Lucky Strike field. A circular feature ∼200 m in diameter located on the flanks of a rifted off-axis central volcano is likely a large and inactive hydrothermal edifice, named Grunnus. We find no indicator of focused hydrothermal activity elsewhere along the segment, suggesting that the enhanced melt supply and the associated melt lenses, required to form central volcanoes, also sustain hydrothermal circulation to form and maintain large and long-lived hydrothermal fields. Hydrothermal discharge to the seafloor occurs along fault traces, suggesting focusing of hydrothermal circulation in the shallow crust along permeable fault zones.

  15. Where are the undiscovered hydrothermal vents on oceanic spreading ridges?

    NASA Astrophysics Data System (ADS)

    Beaulieu, Stace E.; Baker, Edward T.; German, Christopher R.

    2015-11-01

    In nearly four decades since the discovery of deep-sea vents, one-third of the length of global oceanic spreading ridges has been surveyed for hydrothermal activity. Active submarine vent fields are now known along the boundaries of 46 out of 52 recognized tectonic plates. Hydrothermal survey efforts over the most recent decade were sparked by national and commercial interests in the mineral resource potential of seafloor hydrothermal deposits, as well as by academic research. Here we incorporate recent data for back-arc spreading centers and ultraslow- and slow-spreading mid-ocean ridges (MORs) to revise a linear equation relating the frequency of vent fields along oceanic spreading ridges to spreading rate. We apply this equation globally to predict a total number of vent fields on spreading ridges, which suggests that ~900 vent fields remain to be discovered. Almost half of these undiscovered vent fields (comparable to the total of all vent fields discovered during 35 years of research) are likely to occur at MORs with full spreading rates less than 60 mm/yr. We then apply the equation regionally to predict where these hydrothermal vents may be discovered with respect to plate boundaries and national jurisdiction, with the majority expected to occur outside of states' exclusive economic zones. We hope that these predictions will prove useful to the community in the future, in helping to shape continuing ridge-crest exploration.

  16. Chemical signatures from hydrothermal venting on slow spreading ridges

    NASA Astrophysics Data System (ADS)

    Edmonds, Henrietta N.

    At least 24 sites of active venting have been confirmed on slow and ultraslow spreading ridges, with dozens more indicated on the basis of hydrothermal plume distributions and/or dredge recovery of massive sulfides. Fluid chemistry data have been published for 13 sites: 8 on the northern Mid-Atlantic Ridge, 3 on the southern Mid-Atlantic Ridge, and 2 on the Central Indian Ridge. Three of these 13 sites (Rainbow, Logatchev, and Lost City) are known to be hosted in ultramafic terrain, and their fluid chemistries reflect the influence of serpentinization reactions, including elevated hydrogen and methane, and low silica concentrations. This brief review presents the published fluid chemistry for all 13 sites, including time series where available, and demonstrates the diversity of chemical compositions engendered by the myriad settings (near and off axis, young volcanic to ultramafic terrain, and depths up to 4100 m) of hydrothermal systems on slow and ultraslow spreading ridges.

  17. Hydrothermal activity in Tertiary Icelandic crust: Implication for cooling processes along slow-spreading mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Pałgan, D.; Devey, C. W.; Yeo, I. A.

    2015-12-01

    Known hydrothermal activity along the Mid-Atlantic Ridge is mostly high-temperature venting, controlled by volcano-tectonic processes confined to ridge axes and neotectonic zones ~15km wide on each side of the axis (e.g. TAG or Snake Pit). However, extensive exploration and discoveries of new hydrothermal fields in off-axis regions (e.g. Lost City, MAR) show that hydrothermalism may, in some areas, be dominated by off-axis venting. Little is known about nature of such systems, including whether low-temperature "diffuse" venting dominates rather than high-temperature black-smokers. This is particularly interesting since such systems may transport up to 90% of the hydrothermal heat to the oceans. In this study we use Icelandic hot springs as onshore analogues for off-shore hydrothermal activity along the MAR to better understand volcano-tectonic controls on their occurrence, along with processes supporting fluid circulation. Iceland is a unique laboratory to study how new oceanic crust cools and suggests that old crust may not be as inactive as previously thought. Our results show that Tertiary (>3.3 Myr) crust of Iceland (Westfjords) has widespread low-temperature hydrothermal activity. Lack of tectonism (indicated by lack of seismicity), along with field research suggest that faults in Westfjords are no longer active and that once sealed, can no longer support hydrothermal circulation, i.e. none of the hot springs in the area occur along faults. Instead, dyke margins provide open and permeable fluid migration pathways. Furthermore, we suggest that the Reykjanes Ridge (south of Iceland) may be similar to Westfjords with hydrothermalism dominated by off-axis venting. Using bathymetric data we infer dyke positions and suggest potential sites for future exploration located away from neotectonic zone. We also emphasise the importance of biological observations in seeking for low-temperature hydrothermal activity, since chemical or optical methods are not sufficient.

  18. An ultraslow-spreading class of ocean ridge.

    PubMed

    Dick, Henry J B; Lin, Jian; Schouten, Hans

    2003-11-27

    New investigations of the Southwest Indian and Arctic ridges reveal an ultraslow-spreading class of ocean ridge that is characterized by intermittent volcanism and a lack of transform faults. We find that the mantle beneath such ridges is emplaced continuously to the seafloor over large regions. The differences between ultraslow- and slow-spreading ridges are as great as those between slow- and fast-spreading ridges. The ultraslow-spreading ridges usually form at full spreading rates less than about 12 mm yr(-1), though their characteristics are commonly found at rates up to approximately 20 mm yr(-1). The ultraslow-spreading ridges consist of linked magmatic and amagmatic accretionary ridge segments. The amagmatic segments are a previously unrecognized class of accretionary plate boundary structure and can assume any orientation, with angles relative to the spreading direction ranging from orthogonal to acute. These amagmatic segments sometimes coexist with magmatic ridge segments for millions of years to form stable plate boundaries, or may displace or be displaced by transforms and magmatic ridge segments as spreading rate, mantle thermal structure and ridge geometry change. PMID:14647373

  19. The Influence of Ridge Geometry at Ultraslow Spreading Rates

    NASA Astrophysics Data System (ADS)

    Dick, H. J.; Standish, J.

    2004-12-01

    Ridges spreading at ultraslow rate less than 20 mm/yr have been identified as a unique class of ocean ridge as different from slow spreading as slow spreading are from fast 1. Ridge characteristics, such as the presence or absence of amagmatic accretionary segments, transform faults, axial valleys or axial rises, however, are not a simple function of spreading rate, and it is therefore difficult to define precisely ridge classes simply on this criterion. Ridge morphology, tectonics, and geochemistry are also largely a function of mantle thermal structure, upwelling rate, fertility, and ridge geometry. However, examination of ridge crustal structure with spreading rate clearly shows a sharp break, with seismic measurements of crustal thickness indicating highly variable, generally thin crust associated with spreading rates below 20 mm/yr. In contrast, crust formed at spreading rates greater than 20 mm/yr is generally thicker and less variable thickness, averaging between 6 and 7 km, without a clear relationship to spreading rate. The generally accepted explanation is the influence of conductive heat loss and the formation of a thick axial lithosphere due to slow mantle upwelling rates, thereby limiting melt production at ultraslow spreading rates 2. Comparatively, the influence of conductive heat loss at spreading rates greater than 20 mm/yr is likely negligible except near major large offset transforms. The latter effect is predicted by modeling to increase sharply with decreasing spreading rate below 20 mm/yr. Thus perturbations in ridge geometry that would otherwise have a negligible effect, can dramatically influence melt production and ridge tectonics at ultraslow spreading rates. Investigation of the SW Indian Ridge and along the Gakkel Ridge, for example, shows that where the effective spreading rate for mantle upwelling, which ridge obliquity, falls below ~12 mm/yr, long amagmatic accretionary ridge segments form and replace both magmatic accretionary ridge

  20. The Galapagos Spreading Center. Galapagos Rifts Expedition--Grades 9-12. Mid-Ocean Ridges.

    ERIC Educational Resources Information Center

    National Oceanic and Atmospheric Administration (DOC), Rockville, MD.

    This activity introduces students to the basic concept of seafloor spreading, the processes involved in creating new seafloor at a mid- ocean ridge, the Galapagos Spreading Center system, and the different types of plate motion associated with ridge segments and transform faults. The activity provides learning objectives, a list of needed…

  1. Ultraslow spreading processes along the Arctic mid-ocean ridge system

    NASA Astrophysics Data System (ADS)

    Schlindwein, Vera

    2013-04-01

    Generation of new seafloor in the Arctic Ocean occurs along the more than 2800 km long Arctic Ridge System from the Knipovich Ridge in the south to Gakkel ridge in the northeast. The plates separate at velocities of only 6-15 mm/y making the Arctic Ridge System the most prominent representative of an ultraslow spreading mid-ocean ridge. The engine of crustal production splutters at very low spreading rates such that ultraslow spreading ridges show a unique morphology: Isolated volcanoes, capable of vigorous eruptions, pierce the seafloor at distances of several hundred kilometres; in between there are long stretches without volcanism. My work group studies at global, regional and local scale the spreading processes of the Arctic ridge system, using earthquake records of ocean bottom seismometers, seismometers on drifting ice floes and of the global seismic network. We discovered that, contrary to faster spreading ridges, amagmatic portions of the Arctic ridge system are characterised by decreased seismicity rates with few and relatively weak earthquakes, whereas magmatically robust segments display more frequent seismic events. The maximum depth of earthquake hypocentres varies markedly along axis reaching maxima of 22 km depth below sea floor. Volcanic centres are characterized by vigorous earthquake swarm activity including large earthquake swarms that are recorded teleseismically. These earthquake swarms appear to be connected to episodes of active spreading as demonstrated at the 85°E volcanic complex at eastern Gakkel ridge which experienced an unusual spreading event between 1999 and 2001. The varying patterns of seismicity along the ridge axis correlate well with the pronounced differences in ridge morphology and petrology and its magnetic and gravimetric signatures. Our results support current theories of magma production at ultraslow spreading ridges which postulate a lateral melt flow towards isolated volcanic centres.

  2. An analysis of ridge axis gravity roughness and spreading rate

    NASA Technical Reports Server (NTRS)

    Small, Christopher; Sandwell, David T.

    1992-01-01

    Fast and slow spreading ridges have radically different morphologic and gravimetric characteristics. In this study, altimeter measurements from the Geosat Exact Repeat Mission are used to investigate spreading rate dependence of the ridge axis gravity field. Gravity roughness provides an estimate of the amplitude of the gravity anomaly and is robust to small errors in the location of the ridge axis. Gravity roughness as a weighted root mean square of the vertical deflection at 438 ridge crossings on the mid-ocean ridge system is computed. Ridge axis gravity anomalies show a decrease in amplitude with increasing spreading rate up to an intermediate rate of about 60-80 mm/yr and almost no change at higher rates; overall the roughness decreases by a factor of 10 between the lowest and highest rates. In addition to the amplitude decrease, the range of roughness values observed at a given spreading rate shows a similar order of magnitude decrease with transition between 60 and 80 mm/yr. The transition of ridge axis gravity is most apparent at three relatively unexplored locations on the Southeast Indian Ridge and the Pacific-Antarctic Rise; on these intermediate rate ridges the transition occurs abruptly across transform faults.

  3. Ultraslow, slow, or fast spreading ridges: an interplay between plate tectonics and mantle convection

    NASA Astrophysics Data System (ADS)

    Husson, Laurent; Yamato, Philippe; Bézos, Antoine

    2015-04-01

    Oceanic spreading rates are highly variable. These variations are known to correlate to a variety of surface observables, like magmatic production, heat flow or bathymetry, which lead to classify ridges into fast and slow spreading ridges, but also as the more peculiar ultraslow spreading regime. Here we explore the dynamic relationships between spreading ridges, plate tectonics and mantle flow. For this, we first focus on the thermal signature at deeper levels that we infer from the global S-wave seismic tomography model of Debayle and Ricard (2012). We show that the thermal structure of ridges gradually departs from the half-space cooling model for slow, and above all ultraslow spreading ridges. We also infer that the sub- lithospheric mantle temperature decreases by more than 180K from fast spreading to ultraslow spreading regimes. Both observations indicate that the mantle convection pattern is increasingly altered underneath slow and ultraslow spreading ridges. We suggest that this is due to far-field tectonics on the other ends of lithospheric plates. Not only it modulates the spreading rates but it also alters the convection regime: collisions at active plate boundaries obstruct plate motion and decrease their velocities. We then test this hypothesis using a thermo-mechanical model that represents a convection cell carrying a positively buoyant continental lithosphere on top. The continent gradually drifts away from the spreading ridge, from which the oceanic lithosphere grows and cools while the continent eventually collides at the opposite side. In turn, this event drastically modifies the upper kinematic condition for the convecting mantle that evolves from a mobile lid regime to an almost stagnant lid regime. Implications on spreading ridges are prominent: heat advection is slower than thermal diffusion, which causes the oceanic lithosphere to thicken faster; the oceanic plates get compressed and destabilized by a growing number of small scale transient

  4. Controls on melting at spreading ridges from correlated abyssal peridotite - mid-ocean ridge basalt compositions

    NASA Astrophysics Data System (ADS)

    Regelous, Marcel; Weinzierl, Christoph G.; Haase, Karsten M.

    2016-09-01

    Variations in the volume and major element composition of basalt erupted along the global mid-ocean ridge system have been attributed to differences in mantle potential temperature, mantle composition, or plate spreading rate and lithosphere thickness. Abyssal peridotites, the residues of mantle melting beneath mid-ocean ridges, provide additional information on the melting process, which could be used to test these hypotheses. We compiled a global database of abyssal peridotite compositions averaged over the same ridge segments defined by Gale et al. (2013). In addition, we calculated the distance of each ridge segment to the nearest hotspots. We show that Cr# in spinel in abyssal peridotites is negatively correlated with Na90 in basalts from the same ridge segments on a global scale. Ridge segments that erupt basalts apparently produced by larger degrees of mantle melting are thus underlain by peridotites from which large amounts of melt have been extracted. We find that near-ridge hotspots have a more widespread influence on mid-ocean ridge basalt (MORB) composition and ridge depth than previously thought. However, when these hotspot-influenced ridge segments are excluded, the remaining segments show clear relationships between MORB composition, peridotite composition, and ridge depth with spreading rate. Very slow-spreading ridges (<20 mm/yr) are deeper, erupt basalts with higher Na90, Al90, K90/Ti90, and lower Fe90, Ca90/Al90, and expose peridotites with lower Cr# than intermediate and fast-spreading ridges. We show that away from hotspots, the spreading-rate dependence of the maximum degree of mantle melting inferred from Cr# in peridotites (FM) and the bulk degree of melting inferred from Na90 in basalts (FB) from the same ridge segments is unlikely to be due to variations in mantle composition. Nor can the effects of dynamic mantle upwelling or incomplete melt extraction at low spreading rates satisfactorily explain the observed compositions of abyssal

  5. Neogene magmatism northeast of the Aegir and Kolbeinsey ridges, NE Atlantic: Spreading ridge-mantle plume interaction?

    NASA Astrophysics Data System (ADS)

    Breivik, AsbjøRn Johan; Faleide, Jan Inge; Mjelde, Rolf

    2008-02-01

    According to mantle plume theory the Earth's interior cools partly by localized large vertical mass transport, causing extensive decompression melting. The Iceland melt anomaly is regarded as a typical example of a mantle plume. However, there are centers of Miocene to recent magmatism in the Norwegian-Greenland Sea not easily explained by the plume theory. Here we present new data to document diffuse late Miocene magmatic underplating of older oceanic crust located mostly north of the Aegir Ridge, an extinct seafloor spreading axis in the Norway Basin. There is also a region with similar magmatism northeast of the presently spreading Kolbeinsey Ridge north of Iceland. Intraplate magmatism in these locations is not easily explained by local plume models, edge-driven convection, or by asthenosphere flow-lithosphere thickness interaction. On the basis of correlation between the magmatism and the active or extinct spreading ridges, we propose the mid-ocean ridge basalt-capture model, in which this magmatism can be understood through plume-spreading ridge interaction: The asthenosphere flow out from Iceland captures deeper, low-degree partially molten asthenospheric regions from underneath the spreading ridges and carry these across the terminating fracture zones, to subsequently underplate oceanic crust or to intrude and build seamounts. This model is similar to lithospheric cracking models for intraplate magmatism in requiring that low-degree partial melt can be retained in the asthenosphere over time but differ in that the magma is extracted by internal magma movement processes and not by external tectonic forces.

  6. Teleseismic earthquake swarms at ultraslow spreading ridges: indicator for dyke intrusions?

    NASA Astrophysics Data System (ADS)

    Schlindwein, V.

    2012-07-01

    Earthquakes at mid-ocean ridges reflect the active magmatic and tectonic processes that form new oceanic crust. Studies of large earthquakes observed on land and smaller earthquakes observed locally or regionally by ocean bottom seismometers or autonomous underwater hydrophones have greatly contributed to our understanding of the structure and active spreading processes at the mid-ocean ridges of the Atlantic and Pacific Ocean opening with velocities in excess of 25 mm yr-1. At spreading rates below 20 mm yr-1 full rate, the appearance and the accretion processes of mid-ocean ridges undergo fundamental changes as the melt supply is drastically reduced. The active spreading processes at these so-called ultraslow spreading ridges are still poorly known, as the main representatives, the Arctic Ridge System and the Southwest Indian Ridge, are poorly accessible and neither autonomous underwater hydrophone nor ocean bottom seismometer records of local seismicity are available. In an attempt to compare on a large scale the accretion style of ultraslow spreading ridge sections, I analyse the teleseismically recorded seismicity in 11 sections of the Arctic Ridge System and the Southwest Indian Ridge spanning altogether 7200 km. Epicentres located within 30-35 km of the rift axis were extracted from the catalogue of the International Seismological Centre for a time period of 35 yr. On the basis of a single-link cluster analysis, I identified 27 swarms with eight or more events. These swarms occur almost exclusively at centres of focussed magmatism suggesting that the swarms are probably initiated by magmatism. Normal faults along several tens of kilometres surrounding the volcanic centres react in large earthquakes (M > 5) to dyke emplacement. The routine generation of large earthquakes in the cold, brittle lithosphere of ultraslow spreading ridges makes the teleseismic record a valuable means to study ultraslow accretion processes and to provide a global framework for the

  7. Experimental study of structure-forming deformations in obliquely spreading ultra-slow ridges

    NASA Astrophysics Data System (ADS)

    Dubinin, Evgeniy; Kokhan, Andrey; Grokholsky, Andrey

    2013-04-01

    This paper is dedicated to obliquely spreading ultra-slow ridges of North Atlantic and Arctic. The study covers four ridges: Reikjanes, Kolbeynsey, Mohns and Knipovich They are rather young (spreading initiated 58-60 Myr ago) and angles between their trends and spreading direction are from 33 to 85°. All the ridges have peculiarities in structure patterns, kinematics, and morphology and develop in specific geodynamical environments. Kolbeynsey and Reikjanes ridges are developing under influence of Iceland hotspot. Knipovich ridge is developing in ancient slip zone along the heavily sedimented Spitzbergen margin. Spreading at Mohns ridge occurs in conditions of thick lithosphere and extremely narrow heating zone. In order to study geodynamical features of structure-forming on these ridges we apply experimental modeling The model material used in modeling is a colloidal system composed of mineral oils, solid hydrocarbon and surface-active substances. It has elastic-viscous-plastic properties, under temperature and strain rate, it is capable of failure like a brittle body. Reikjanes (ridge obliquity 60-65°) and Kolbeynsey (80-85°) ridges show changes of morphology with increasing distance from Iceland mantle plume. In proximity with Iceland they are characterized by axial rise with long s-shaped axial volcanic ridges (AVRs) offset by small discontinuities. Far from Iceland the AVRs are short and offset by large non-transform offsets which are situated in axial valley. In conditions of all these features are explained by influence of mantle flow from the Iceland mantle plum initiating the increasing of mantle temperature. It results in decreasing of lithospheric brittle layer with approaching to Iceland. In experimental sets reproducing conditions of proximate to Iceland part of the ridge were reproduced in sets with the widest weak zone and the smallest crustal thickness and vice versa. In sets reproducing conditions of proximate to Iceland received long and non

  8. Evidence for accumulated melt beneath the slow-spreading Mid-Atlantic Ridge

    NASA Astrophysics Data System (ADS)

    Sinha, M. C.; Navin, D. A.; MacGregor, L. M.; Constable, S.; Peirce, C.; White, A.; Heinson, G.; Inglis, M. A.

    The analysis of data from a multi-component geophysical experiment conducted on a segment of the slow-spreading (20 mm yr-1) Mid-Atlantic Ridge shows compelling evidence for a significant crustal magma body beneath the ridge axis. The role played by a crustal magma chamber beneath the axis in determining both the chemical and physical architecture of the newly formed crust is fundamental to our understanding of the accretion of oceanic lithosphere at spreading ridges, and over the last decade subsurface geophysical techniques have successfully imaged such magma chambers beneath a number of intermediate and fast spreading (60-140 mm yr-1 full rate) ridges. However, many similar geophysical studies of slow-spreading ridges have, to date, found little or no evidence for such a magma chamber beneath them. The experiment described here was carefully targeted on a magmatically active, axial volcanic ridge (AVR) segment of the Reykjanes Ridge, centred on 57 degrees 43 minutes North. It consisted of four major components: wide-angle seismic profiles using ocean bottom seismometers; seismic reflection profiles; controlled source electromagnetic sounding; and magneto-telluric sounding. Interpretation and modelling of the first three of these datasets shows that an anomalous body lies at a depth of between 2 and 3 km below the seafloor beneath the axis of the AVR. This body is characterized by anomalously low seismic P-wave velocity and electrical resistivity, and is associated with a seismic reflector. The geometry and extent of this melt body shows a number of similarities with the axial magma chambers observed beneath ridges spreading at much higher spreading rates. Magneto-telluric soundings confirm the existence of very low electrical resistivities in the crust beneath the AVR and also indicate a deeper zone of low resistivity within the upper mantle beneath the ridge.

  9. Magmatic and amagmatic seafloor generation at the ultraslow-spreading Gakkel ridge, Arctic Ocean.

    PubMed

    Michael, P J; Langmuir, C H; Dick, H J B; Snow, J E; Goldstein, S L; Graham, D W; Lehnert, K; Kurras, G; Jokat, W; Mühe, R; Edmonds, H N

    2003-06-26

    A high-resolution mapping and sampling study of the Gakkel ridge was accomplished during an international ice-breaker expedition to the high Arctic and North Pole in summer 2001. For this slowest-spreading endmember of the global mid-ocean-ridge system, predictions were that magmatism should progressively diminish as the spreading rate decreases along the ridge, and that hydrothermal activity should be rare. Instead, it was found that magmatic variations are irregular, and that hydrothermal activity is abundant. A 300-kilometre-long central amagmatic zone, where mantle peridotites are emplaced directly in the ridge axis, lies between abundant, continuous volcanism in the west, and large, widely spaced volcanic centres in the east. These observations demonstrate that the extent of mantle melting is not a simple function of spreading rate: mantle temperatures at depth or mantle chemistry (or both) must vary significantly along-axis. Highly punctuated volcanism in the absence of ridge offsets suggests that first-order ridge segmentation is controlled by mantle processes of melting and melt segregation. The strong focusing of magmatic activity coupled with faulting may account for the unexpectedly high levels of hydrothermal activity observed. PMID:12827193

  10. Hidden tectonics at slow-spreading ridges: distinguishing magmatic from tectonic spreading

    NASA Astrophysics Data System (ADS)

    MacLeod, C. J.; Searle, R. C.; Mallows, C.; Young, E. C.

    2011-12-01

    In the fifteen years since the discovery of oceanic core complexes (OCCs) at slow-spreading mid-ocean ridges our understanding of the processes of seafloor spreading has changed fundamentally. Following the 2010 Chapman Conference on Detachments in Oceanic Lithosphere there has been a general convergence of view that OCCs - the flat-topped domal massifs with spreading-direction-parallel corrugations found at intervals along slow-spreading ridges - represent the exposed, inactive portions of long-lived extensional detachment structures that exhume mantle rocks in their footwalls. Detachments appear to initiate and slip at steep angles before rolling over as a flexural response to unloading. It is recognised that detachment fault initiation, i.e. maintenance of slip on a single median valley fault, is favoured when the proportion of plate separation accommodated by magmatic accretion in the axial valley is about a third to a half of the total. Fault weakening, typically by formation of phyllosilicates such as talc as a result of deep penetration of fluids along the fault, appears also to be an essential pre-requisite for detachment fault formation. Considerably less well understood are the mechanisms of melt emplacement into the lithosphere and the nature of the interactions between tectonism and magmatism. In a recent paper on the 13°N region of the Mid-Atlantic Ridge (MAR; MacLeod et al. 2009 EPSL v.287, p.333-344) we showed that volcanism is suppressed when OCC detachment faults are active, but that renewed volcanism propagating laterally along strike from adjacent, magmatically robust segments intrudes into their footwalls and may eventually terminate them. If melt supply is insufficient to overwhelm the detachment it may instead be captured in the footwall of an OCC, decoupling the mantle melt flux from that contributing to magmatic accretion in the hanging wall and instead promoting asymmetric accretion. This model implicitly views oceanic detachments as

  11. New data about small-magnitude earthquakes of the ultraslow-spreading Gakkel Ridge, Arctic Ocean

    NASA Astrophysics Data System (ADS)

    Morozov, Alexey N.; Vaganova, Natalya V.; Ivanova, Ekaterina V.; Konechnaya, Yana V.; Fedorenko, Irina V.; Mikhaylova, Yana A.

    2016-01-01

    At the present time there is available detailed bathymetry, gravimetric, magnetometer, petrological, and seismic (mb > 4) data for the Gakkel Ridge. However, so far not enough information has been obtained on the distribution of small-magnitude earthquakes (or microearthquakes) within the ridge area due to the absence of a suitable observation system. With the ZFI seismic station (80.8° N, 47.7° E), operating since 2011 at the Frantz Josef Land Archipelago, we can now register small-magnitude earthquakes down to 1.5 ML within the Gakkel Ridge area. This article elaborates on the results and analysis of the ZFI station seismic monitoring obtained for the period from December 2011 to January 2015. In order to improve the accuracy of the earthquakes epicenter locations, velocity models and regional seismic phase travel-times for spreading ridges in areas within the Euro-Arctic Region have been calculated. The Gakkel Ridge is seismically active, regardless of having the lowest spreading velocity among global mid-ocean ridges. Quiet periods alternate with periods of higher seismic activity. Earthquakes epicenters are unevenly spread across the area. Most of the epicenters are assigned to the Sparsely Magmatic Zone, more specifically, to the area between 1.5° E and 19.0° E. We hypothesize that assignment of most earthquakes to the SMZ segment can be explained by the amagmatic character of the spreading of this segment. The structuring of this part of the ridge is characterized by the prevalence of tectonic processes, not magmatic or metamorphic ones.

  12. Slow-Spreading Oceanic Crust Formed By Steady-State Axial Volcanic Ridges

    NASA Astrophysics Data System (ADS)

    Murton, B. J.; Schroth, N.; LeBas, T.; Van Calsteren, P. W.; Yeo, I. A.; Achenbach, K. L.; Searle, R. C.

    2012-12-01

    Oceanic crust originates at mid-ocean spreading ridges (MORs), covers almost three quarters of the earth's surface and dominates the global magmatic flux. Axial volcanic ridges (AVRs) are almost ubiquitous features of orthogonal slow-spreading ridges, which account for three quarters of the global mid-ocean spreading ridge system today. Typically 3-6 km wide, 200-500 m high and 10-20 km long, AVRs are the loci of recent volcanic activity and form the most prominent topography rising above the otherwise flat-lying Median Valley floor. Previous studies indicate that AVRs, and their related crustal magma reservoirs are episodic, on a time scale of 150-300 ka. Yet their near ubiquitous occurrence at slow-spreading ridge segments provides us with a paradox: if AVRs have a life cycle of formation and degradation, does their near ubiquitous presence at slow spreading ridges imply their life-cycles are synchronised? In this contribution, we report the findings from a high-resolution study of a well-developed axial volcanic ridge (AVR) at 45°N on the Mid-Atlantic Ridge (MAR). Here, the MAR is typical of most slow-spreading ridges: it spreads generally symmetrically and orthogonally, at a full rate of 23.6 mm per year, has second and third-order segmentation, and contains a typical AVR. Using a combination of detailed micro-bathymetry, sidescan sonar, visual surveying and petrology, we suggest that the AVR is the product of quasi-steady state volcanotectonic processes. Small volume lava flows, originating at or near the crest and with short run-out lengths, form ~60 m high hummocky pillow-lava mounds that dominate the construction of the AVR. The lavas are the product of moderate degrees of mantle melting that are typical for normal mid-ocean ridge basalt. Synchronous with these eruptions the flanks of the AVR subside forming a structural horst. Subsidence is partially accommodated by a series of outward-facing volcanic growth faults that step-down and away from the AVR

  13. Subaerial Seafloor Spreading in Iceland: Segment-Scale Processes and Analogs for Fast-Spreading Mid-Ocean Ridge Spreading Centers

    NASA Astrophysics Data System (ADS)

    Karson, Jeffrey; Varga, Robert; Siler, Drew; Horst, Andrew

    2010-05-01

    The nature of oceanic crust and spreading center processes are derived from direct observations of surface features and geophysics at active spreading centers as well as from deep crustal drilling, tectonic windows into the upper oceanic crust, and ophiolites. Integrating active spreading processes with deeply eroded crustal structures in Iceland provides an additional perspective on subsurface processes that are likely to be important at mid-ocean ridge spreading centers. Spreading in Iceland strongly resembles second-order segment-scale processes of the fast-spreading centers. Along axis, major processes including subsidence, magmatic construction, and hydrothermal activity vary systematically over tens of kilometers from segment centers to ends. Near spreading segment centers ("central volcanoes") subsidence and crustal thickening are greatest. The intrusion of high-level sill and cone sheet complexes and small gabbroic plutons contribute substantially to upper crustal thickening. Both magma supply and tectonic movements have a very strong vertical component. In contrast, near segment ends (fissure swarms in active spreading areas) subsidence is limited, most thickening occurs in the lava units and lateral dike injection is likely to dominate. In both Iceland and fast-spread crust, where the magma supply is relatively high, subaxial subsidence is the key process that controls the construction and modification of the crust during spreading. Seafloor studies on fast-spreading ridge show lava flows fed by dike intrusion events focused along a narrow (<1 km) axial region with very limited relief. However, subsurface structures reveal that axial lavas must subside hundreds of meters immediately beneath the axis as the overlying lava pile thickens. Similar relationships occur in Iceland but over a wider region of active magmatism (neovolcanic zone tens of kilometers wide) and building a much thicker upper crust (~5 km). For both cases, in order for the lava units to

  14. Ridge-parallel-shearing and localized vertical melt migration during spreading: A phenomenon underestimated so far?

    NASA Astrophysics Data System (ADS)

    Herwegh, Marco; Mercolli, Ivan; Linckens, Jolien; Muntener, Othmar

    2016-04-01

    At spreading ridges, melt generation in the asthenospheric mantle followed by melt extraction, ascend, reaction and crystallization is an important mechanism for the generation of new oceanic crust. In contrast to original concepts, assuming a continuous and steady melt supply, recent studies indicate that melt ascend preferentially occurs in spatially restricted areas that both vary in space and time heavily segmenting the ridges. Based on field investigations in the Semail ophiolite (Oman) we found a close interplay between ductile strike-slip shearing along a vertical ridge-parallel shear zone in mantle harzburgite and melt ascend in the center of the shear zone. Here, melt rise was episodic and started at stages of initial high temperature deformation (>1100°C) but persisted during spreading induced retrograde cooling to deformation temperatures of about 800°C. The locations of ductile deformation and melt ascend (dikes) are very localized and coincide. Moreover, old dikes are spread over wider domains in the shear zones, while young dikes are concentrated in the immediate shear zone center, i.e. in the parts of youngest ductile deformation. These observations suggest that stress concentration along already crystallized dikes, continuous ductile strain localization and lubrication by younger melts all contributed to shear zone focusing with progressive cooling. Hence initial mechanical anisotropies are repeatedly overprinted resulting in a complex 3D shear zone pattern with temporal and spatial hydraulic variations in melt activity. Extrapolated to the scale of the entire Oman ophiolite we suggest that the ductile shear zones separate major mantle diapirs. They therefore seem to represent important zones of heterogeneous thermo-mechanical strain accommodation between the cooling diapirs and might be more common along spreading ridges than recognized so far contributing to ridge-segmentation.

  15. Subduction of the South-Chile active spreading ridge: a 17 Ma to 3 Ma magmatic record in central Patagonia (western edge of Meseta del Lago Buenos Aires, Argentina)

    NASA Astrophysics Data System (ADS)

    Boutonnet, Emmanuelle; Arnaud, Nicolas; Guivel, Christèle; Lagabrielle, Yves; Scalabrino, Bruno; Espinoza, Felipe

    2010-05-01

    The Chile Triple Junction is a natural laboratory to study the interactions between magmatism and tectonics during the subduction of an active spreading ridge beneath a continent. The MLBA plateau (Meseta del Lago Buenos Aires) is one of the Neogene alkali basaltic plateaus located in the back-arc region of the Andean Cordillera at the latitude of the current Chile Triple Junction. The genesis of MLBA can be related with successive opening of slabs windows beneath Patagonia: within the subducting Nazca Plate itself and between the Nazca and Antarctic plates. Detailed 40Ar/39Ar dating and geochemical analysis of bimodal magmatism from the western flank of the MLBA show major changes in the back-arc magmatism which occurred between 14.5 Ma and 12.5 Ma with the transition from calc-alkaline lavas (Cerro Plomo) to alkaline lavas (MLBA) in relation with slab window opening. In a second step, at 4- 3 Ma, alkaline felsic intrusions were emplaced in the western flank of the MLBA coevally with the MLBA basalts with which they are genetically related. These late OIB-like alkaline to transitional basalts were generated by partial melting of the subslab asthenosphere of the subducting Nazca plate during the opening of the South Chile spreading ridge-related slab window. These basalts differentiated with small amounts of assimilation in shallow magma chambers emplaced along transtensional to extensional zones. The close association of bimodal magmatism with extensional tectonic features in the western MLBA is a strong support to the model of Patagonian collapse event proposed to have taken place between 5 and 3 Ma as a consequence of the presence of the asthenospheric window (SCR-1 segment of South Chile Ridge) below the MLBA area.

  16. The formation and linking of mid-segment detachment faults at the slow-spreading Mid-Atlantic Ridge

    NASA Astrophysics Data System (ADS)

    Schouten, H.; Smith, D. K.; Dick, H. J.; Escartin, J.

    2011-12-01

    The Mid-Atlantic Ridge axis at 16.5N has a remarkably high rate of teleseismic and hydrophone-recorded seismicity, and we have identified it as a region of active detachment faulting. Limited multibeam bathymetry data on the west side of the median valley show two parallel, linear ridges: 50-km-long West Ridge at 15 km west of the volcanic axis, and 10-km-long East Ridge at only 6 km from the axis. The ridges are interpreted to be the tops of rotated detachment fault scarps (breakaways), indicating significant fault rotation (> 25 degrees). A striated surface, characteristic of a core complex, is associated with West Ridge. This region stands out because it presents a dramatic demonstration of a new detachment fault forming nearer to the axis (East Ridge) and interrupting the overall development of what we interpret to be a longer, older and still active detachment fault that has its breakaway at the older West Ridge. We hypothesize that the section of the West Ridge detachment behind the East Ridge detachment was deactivated when East Ridge formed and furthermore, that the East Ridge detachment has linked into the West Ridge detachment to form a single detachment fault. This area represents an opportunity to address the initiation and cessation of mid-segment detachment faulting as well as how the faults link along the axis. Sampling of the detachment footwall will allow us to relate the subcrustal architecture of the segment to the local magmatic budget, and how this influences the initiation and geometry of the faulting. A broad, well-developed neovolcanic zone at the adjacent spreading axis suggests abundant volcanism. The greater depth of the local off-axis morphology, though, indicates that East Ridge may have formed in a relatively amagmatic corridor. Massifs at the western limit of the multibeam bathymetry data suggest asymmetric spreading through detachment faulting has dominated this region for at least the last several million years and perhaps much

  17. Upper plate responses to active spreading ridge/transform subduction: The tectonics, basin evolution, and seismicity of the Taita area, Chile Triple Junction

    SciTech Connect

    Flint, S.; Prior, D. ); Styles, P.; Murdie, R. ); Agar, S.; Turner, P. )

    1993-02-01

    Integrated field geophysical, structural and stratigraphic studies are attempting to elucidate the mechanisms and consequences of the Late Miocene-present day subduction of the Chile Ridge triple junction system. Preliminary data indicate a shallow plane of seismicity at about 15 km to 20 km depth below the Taitao peninsula. The depths correspond to the predicted depth range of subducted upper ocean crust. The calculated Bouguer anomaly map cannot be explained by the upper plate geology, suggesting that gravity is influenced by heterogeneities in the subducting oceanic plate. Seismic data imply that a subducted transform system underlying the inner Taitao Peninsula is still an active structure. A series of Middle-Late Tertiary sedimentary basins lie inboard of the triple junction. Within the Cosmelli basin, abrupt marine to continental facies transitions give clear evidence of base level changes. The amount of basinward shift of facies across sequence boundaries gets progressively greater up stratigraphy, indicating progressively greater base level changes. The lower part of the basin fill is folded and then thrusted eastward as a series of imbricates, while the overlying, greater thickness of fluvial sediments are only gently tilted westwards. We provisionally interpret this geometry to indicate that the early basin fill was deforming due to contractional tectonics while the later basin fill was being deposited. This complex basin history may reflect initiation and development of triple junction subduction.

  18. Experimental study of structure-forming deformations in ultra-slow spreading Arctic and Polar Atlantic ridges

    NASA Astrophysics Data System (ADS)

    Dubinin, E. P.; Grokholsky, A. L.; Kokhan, A. V.

    2010-05-01

    The system of regional spreading ridges includes Reikjanes, Kolbeinsey, Mohns, Knipovich and Gakkel ridges. They are rather young (spreading initiated 58-60 Myr ago) and ultra-slow (spreading velocity < 20 mm/year). But all of them have peculiarities in structure patterns, kinematics, and morphology. In order to study geodynamical features of structure-forming on these ridges we apply experimental modeling. This study covers three of the ridges mentioned above: Reikjanes, Knipovich and Gakkel. The specified experimental complex and model material were used in modeling sets. The model material used in modeling is a colloidal system composed of mineral oils, solid hydrocarbon and surface-active substances. It has elastic-viscous-plastic properties, under temperature and strain rate, it is capable of failure like a brittle body. All experiments were held according to similarity conditions. Reikjanes ridge is situated south-west of Iceland. It shows changes of morphology from north to south. The northern part of it is characterized by axial rise, the southern part - by axial valley. The main feature of axial morphology is presence of s-shaped axial volcanic ridges (AVRs). The angle between ridge trend and plate divergence trend is nearly 60°. All these features are explained by influence of mantle flow from the Iceland mantle plum initiating the increasing of mantle temperature. It results in decreasing of litospheric brittle layer with approaching to Iceland. The experimental setting was following. The weak zone was emplaced obliquely, crustal thickness and width of weak zone varied in sets. Conditions of northern part of the ridge were reproduced in sets with the widest weak zone and the smallest crustal thickness and vice versa. In sets reproducing conditions of northern province we received long and non-discontinued AVRs, on the other hand we received short and displaced AVRs in south-like conditions. Knipovich ridge stretches along Spitsbergen continental margin

  19. Increased Spreading Activation in Depression

    ERIC Educational Resources Information Center

    Foster, Paul S.; Yung, Raegan C.; Branch, Kaylei K.; Stringer, Kristi; Ferguson, Brad J.; Sullivan, William; Drago, Valeria

    2011-01-01

    The dopaminergic system is implicated in depressive disorders and research has also shown that dopamine constricts lexical/semantic networks by reducing spreading activation. Hence, depression, which is linked to reductions of dopamine, may be associated with increased spreading activation. However, research has generally found no effects of…

  20. Explosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean.

    PubMed

    Sohn, Robert A; Willis, Claire; Humphris, Susan; Shank, Timothy M; Singh, Hanumant; Edmonds, Henrietta N; Kunz, Clayton; Hedman, Ulf; Helmke, Elisabeth; Jakuba, Michael; Liljebladh, Bengt; Linder, Julia; Murphy, Christopher; Nakamura, Ko-Ichi; Sato, Taichi; Schlindwein, Vera; Stranne, Christian; Tausenfreund, Maria; Upchurch, Lucia; Winsor, Peter; Jakobsson, Martin; Soule, Adam

    2008-06-26

    Roughly 60% of the Earth's outer surface is composed of oceanic crust formed by volcanic processes at mid-ocean ridges. Although only a small fraction of this vast volcanic terrain has been visually surveyed or sampled, the available evidence suggests that explosive eruptions are rare on mid-ocean ridges, particularly at depths below the critical point for seawater (3,000 m). A pyroclastic deposit has never been observed on the sea floor below 3,000 m, presumably because the volatile content of mid-ocean-ridge basalts is generally too low to produce the gas fractions required for fragmenting a magma at such high hydrostatic pressure. We employed new deep submergence technologies during an International Polar Year expedition to the Gakkel ridge in the Arctic Basin at 85 degrees E, to acquire photographic and video images of 'zero-age' volcanic terrain on this remote, ice-covered ridge. Here we present images revealing that the axial valley at 4,000 m water depth is blanketed with unconsolidated pyroclastic deposits, including bubble wall fragments (limu o Pele), covering a large (>10 km(2)) area. At least 13.5 wt% CO(2) is necessary to fragment magma at these depths, which is about tenfold the highest values previously measured in a mid-ocean-ridge basalt. These observations raise important questions about the accumulation and discharge of magmatic volatiles at ultraslow spreading rates on the Gakkel ridge and demonstrate that large-scale pyroclastic activity is possible along even the deepest portions of the global mid-ocean ridge volcanic system. PMID:18580949

  1. The plutonic foundation of a slow-spreading ridge

    NASA Astrophysics Data System (ADS)

    Dick, Henry J. B.; Robinson, Paul T.; Meyer, Peter S.

    Hole 735B drilled 500 m of gabbroic layer 3 at the SW Indian Ridge. The section consists of small intrusions with no evidence for a large, steady-state magma chamber. The complex stratigraphy represents multiple phases of magmatism, alteration, and ongoing deformation in the zone of lithospheric necking and crustal accretion beneath a slow-spreading ocean ridge. Magma evolution was by fractional crystallization of intercumulus melt in crystal mush, by melt-rock reaction and wall rock assimilation as batches of melt migrated upward through the crust. The major form of igneous layering, undeformed olivine gabbro cut by numerous layers of sheared ferrogabbro, formed by synkinematic differentiation in deforming, partially molten gabbro. This process drove late intercumulus melt into shear zones, where reaction, crystallization, oxide precipitation and melt trapping transformed the rock to ferrogabbro. The section underwent extensive brittle-ductile deformation, with shear zones forming while the section was still partially molten, under anhydrous granulite conditions, and at higher water-rock ratios in the amphibolite facies. The shear zone is extensively controlled both magmatic and subsolidus fluid flow and alteration. Alteration and circulation of seawater was tectonically enhanced, with extension and lithospheric necking superimposed on the dilational thermal stress available for cracking, resulting in high permeabilities. Alteration decreased abruptly in mid-amphibolite facies with the end of brittle-ductile deformation as the section was transferred into the rift valley wall and the zone of block uplift. Alteration conditions then closely resembled those of the statically cooled Skaergaard intrusion, with diopside replacing amphibole as the principal mafic hydrothermal vein mineral. This is attributed to low permeability and more reacted fluid with cracking driven only by thermal dilation during static cooling. Late trondhjemite veins formed by fractional

  2. Miocene to Late Quaternary Patagonian basalts (46 47°S): Geochronometric and geochemical evidence for slab tearing due to active spreading ridge subduction

    NASA Astrophysics Data System (ADS)

    Guivel, Christèle; Morata, Diego; Pelleter, Ewan; Espinoza, Felipe; Maury, René C.; Lagabrielle, Yves; Polvé, Mireille; Bellon, Hervé; Cotten, Joseph; Benoit, Mathieu; Suárez, Manuel; de la Cruz, Rita

    2006-01-01

    Miocene to Quaternary large basaltic plateaus occur in the back-arc domain of the Andean chain in Patagonia. They are thought to result from the ascent of subslab asthenospheric magmas through slab windows generated from subducted segments of the South Chile Ridge (SCR). We have investigated three volcanic centres from the Lago General Carrera-Buenos Aires area (46-47°S) located above the inferred position of the slab window corresponding to a segment subducted 6 Ma ago. (1) The Quaternary Río Murta transitional basalts display major, trace elements, and Sr and Nd isotopic features similar to those of oceanic basalts from the SCR and from the Chile Triple Junction near Taitao Peninsula (e.g., ( 87Sr/ 86Sr) o = 0.70396-0.70346 and ɛNd = + 5.5 - + 3.0). We consider them as derived from the melting of a Chile Ridge asthenospheric mantle source containing a weak subduction component. (2) The Plio-Quaternary (< 3.3 Ma) post-plateau basanites from Meseta del Lago Buenos Aires (MLBA), Argentina, likely derive from small degrees of melting of OIB-type mantle sources involving the subslab asthenosphere and the enriched subcontinental lithospheric mantle. (3) The main plateau basaltic volcanism in this region is represented by the 12.4-3.3-Ma-old MLBA basalts and the 8.2-4.4-Ma-old basalts from Meseta Chile Chico (MCC), Chile. Two groups can be distinguished among these main plateau basalts. The first group includes alkali basalts and trachybasalts displaying typical OIB signatures and thought to derive from predominantly asthenospheric mantle sources similar to those of the post-plateau MLBA basalts, but through slightly larger degrees of melting. The second one, although still dominantly alkalic, displays incompatible element signatures intermediate between those of OIB and arc magmas (e.g., La/Nb > 1 and TiO 2 < 2 wt.%). These intermediate basalts differ from their strictly alkalic equivalents by having lower High Field Strength Element (HFSE) and higher ɛNd (up to

  3. Magmatism, Hydrothermal Cooling and Asymmetric Accretion at Slow-spreading Ridges

    NASA Astrophysics Data System (ADS)

    Bai, H.; Montesi, L.

    2014-12-01

    Asymmetric spreading is common at slow-spreading mid-ocean ridges when an active detachment fault accommodates a portion of the total plate separation. Basalts erupted along asymmetric segments have lower Ca, higher Fe, Na, K than the ones collected from symmetric segments, indicating higher pressures of fractionation and lower extents of partial melting of the mantle [Langmuir et al., AGU, 2013]. Seismic evidence also shows a thicker and colder axial lithosphere at asymmetric sections of the ridge [Escartín et al., 2008]. This phenomenon is most obvious when the asymmetric spreading centers are also oblique to its opening direction. The reduced melt supply beneath asymmetric spreading segments may be attributed to distorted mantle upwelling, enhanced hydrothermal cooling, and enriched compositional heterogeneities in the upper mantle. We construct two-dimensional thermo-mechanical models of symmetric and asymmetric spreading centers, and test the effects of asymmetric accretion and hydrothermal circulation on mantle melting. A temperature-dependent mantle viscosity is used. The hydrothermal circulation is implemented as an enhanced thermal conductivity limited by cutoff depth and temperature. The effect of oblique spreading is incorporated in the model as reduced effective spreading rate. Mantle flow and thermal structure are solved in the commercial finite element software COMSOL Multiphysics®. Melt production and flux are estimated in Matlab® using a nonlinear melting function [Katz et al., 2003]. We show that the asymmetric accretion alone does not affect the extent of melting or reduce the melt flux significantly. Hydrothermal cooling can plays an important role in deepening the melting depth and lowering the melt extent. Therefore, the difference in the extent of melting between asymmetric and symmetric spreading models can be explained by an enhanced hydrothermal circulation at asymmetric segments. This correlation is supported by the observation made at

  4. Hydrological response to a seafloor spreading episode on the Juan de Fuca ridge.

    PubMed

    Davis, Earl; Becker, Keir; Dziak, Robert; Cassidy, John; Wang, Kelin; Lilley, Marvin

    2004-07-15

    Seafloor hydrothermal systems are known to respond to seismic and magmatic activity along mid-ocean ridges, often resulting in locally positive changes in hydrothermal discharge rate, temperature and microbial activity, and shifts in composition occurring at the time of earthquake swarms and axial crustal dike injections. Corresponding regional effects have also been observed. Here we present observations of a hydrological response to seafloor spreading activity, which resulted in a negative formation-fluid pressure transient during and after an earthquake swarm in the sediment-sealed igneous crust of the Middle Valley rift of the northernmost Juan de Fuca ridge. The observations were made with a borehole seal and hydrologic observatory originally established in 1991 to study the steady-state pressure and temperature conditions in this hydrothermally active area. The magnitude of the co-seismic response is consistent with the elastic strain that would be expected from the associated earthquakes, but the prolonged negative pressure transient after the swarm is surprising and suggests net co-seismic dilatation of the upper, permeable igneous crust. The rift valley was visited four weeks after the onset of the seismic activity, but no signature of increased hydrothermal activity was detected in the water column. It appears that water, not magma, filled the void left by this spreading episode. PMID:15254534

  5. The SHEBA Ridge : a Particular Spreading Center or an End-member of the Slow Spreading Processes ?

    NASA Astrophysics Data System (ADS)

    GENTE, P.; LEROY, S.; BLAIS, A.; d'ACREMONT, E.; PATRIAT, P.; FLEURY, J.; MAIA, M.; PERROT, J.; FOURNIER, M.

    2001-12-01

    We analyze multibeam bathymetry, acoustic imagery, magnetic and gravity data collected during the Encens-Sheba cruise of the NO Marion Dufresne. The survey covered the axis and the flanks up to the continental margins of the Sheba Ridge between 52oE and 54o30'E, at the oriental extremity of the Aden gulf. The full spreading rate in this young oceanic basin is about 2 cmy since the continental rifting. Three second-order segments, one presenting an anomalously shallow axis, characterize this part of the Sheba ridge. The new bathymetry data reveal a particular fabric on the flanks and at the axis for the long (120 km) and shallow spreading center. The flanks, like the ridge axis, are marked by large, more or less circular, volcanic domes. They are built by a few large volcanoes (5-10 km diameter) and by several smaller (1-2 km diameter) edifices. Many of these volcanoes present a well-developed caldera. These volcanic constructions are well developed in the southern part of the axis. Close to the axis, the higher reliefs culminate at a depth of 1000 m. Tectonic scarps limit a deep axial valley at the extremities of this long segment. The deformation, diffuse at the ends, becomes more focused toward the center of this segment and is arranged in an hourglass pattern. A negative mantle Bouguer anomaly elongated in the spreading direction marks this segment. The differences in MBA (~70 mgals) and in depth (more than 2 km) between the center and the ends of this segment are the largest, highest of the slow spreading ridges. Acoustic imagery, axial magnetic and mantle Bouguer anomalies generally permit to precise the location of the spreading axis. In this segment, if the axial area is clearly defined, the neovolcanic zone is more difficult to localize. This suggests a diffuse volcanism at the center of the segment at the origin of the numerous small volcanoes. The other segments of the Sheba ridge present a more typical slow spreading axial valley. The discontinuities

  6. Temporal and spatial cyclicity of accretion at slow-spreading ridges-evidence from the Reykjanes Ridge

    NASA Astrophysics Data System (ADS)

    Peirce, Christine; Gardiner, Alex; Sinha, Martin

    2005-10-01

    A unifying model of oceanic crustal development at slow spreading rates is presented in which accretion follows a cyclic pattern of magmatic construction and tectonic destruction, controlled by along-axis variation in melt supply and coupled to along-axis variation in spreading rate and across-axis asymmetry in spreading. This study focuses on the Reykjanes Ridge, Mid-Atlantic Ridge south of Iceland, which is divided along its entire length into numerous axial volcanic ridges (AVR). Five adjacent AVRs have been analysed, located between 57°30'N and 58°30'N and south of any strong Iceland hotspot influence. The seabed morphology of each AVR is investigated using sidescan sonar data to determine relative age and eruptive history. Along-axis gravity profiles for each AVR are modelled relative to a seismically derived crustal reference model, to reveal the underlying crustal thickness and density structure. Correlating these models with seabed features, crustal structure, ridge segment morphology and relative ages, a model of cyclic ridge segmentation is developed in which accretion results in adjacent AVRs with a range of crustal features which, when viewed collectively, reveal that second-order segments on the Reykjanes Ridge have an along-axis length of ~70 km and comprise several adjacent AVRs which, in turn, reflect the pattern of third-order segmentation. Tectono-magmatic accretion is shown to operate on the scale of individual AVRs, as well as on the scale of the second-order segment as a whole.

  7. Gravity tectonics of topographic ridges: Halokinesis and gravitational spreading in the western Ogaden, Ethiopia

    NASA Astrophysics Data System (ADS)

    Mège, Daniel; Le Deit, Laetitia; Rango, Tewodros; Korme, Tesfaye

    2013-07-01

    The Cenozoic history of the western Ogaden region of Ethiopia, between the Ethiopian rift and the South Afar margin, is marked by uplift and incision of the Ogaden plateau down to the Gorrahei Formation, an upper Cretaceous evaporite formation. Debuttressing of this and the overlying sedimentary formations resulted in widespread and spectacular gravitational spreading landforms over a minimum surface area of 15,000 km2, most of which remains unstudied. After clearing up some misconceptions about the surface geology of the study area, the Kebenawa Ridge in the Audo Range, observations are reported that point to a tectonic style controlled by halokinesis and subsequently, gravitational spreading. The role of diapirism and karstification in the observed halokinesis is discussed, as well as the influence of halokinesis on gravitational spreading. Spreading is in part akin to sackung, in that ridge deformation features include a crestal graben and basal ridge topography extrusion, and deformation was triggered by lateral ridge debuttressing. Ridge spreading also presents analogy with gravitational spreading of the Canyonlands grabens in the Needles District, Canyonlands National Park, Utah. The scale and the mechanisms are found to be basically similar, but two differences are noted. First, incision by the drainage network in response to plateau uplift in Ethiopia has debuttressed the topography along two parallel rivers, instead of a single river (the Colorado River) in Utah. Secondly, incision proceeded to the base of the evaporite layer in the Ogaden, whereas incision has not exceeded the top of the evaporite layer in Utah. These differences may have influenced the details of the spreading mechanisms in ways that remain to be investigated. Overall, in Ethiopia, association of halokinesis and a transitional mode of gravitational spreading at the interface between narrow ridge spreading (sackung) and plateau spreading (Canyonlands-type), illustrates a fascinating and

  8. Hydrothermal activity at the Arctic mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Pedersen, Rolf B.; Thorseth, Ingunn H.; Nygård, Tor Eivind; Lilley, Marvin D.; Kelley, Deborah S.

    Over the last 10 years, hydrothermal activity has been shown to be abundant at the ultraslow spreading Arctic Mid-Ocean Ridges (AMOR). Approximately 20 active and extinct vent sites have been located either at the seafloor, as seawater anomalies, or by dredge sampling hydrothermal deposits. Decreasing spreading rates and decreasing influence of the Icelandic hot spot toward the north along the AMOR result in a north-south change from a shallow and magmatically robust to a deep and magmatically starved ridge system. This contrast gives rise to large variability in the ridge geology and in the nature of the associated hydrothermal systems. The known vent sites at the southern part of the ridge system are either low-temperature or white smoker fields. At the deep, northern parts of the ridge system, a large black smoker field has been located, and seawater anomalies and sulfide deposits suggest that black smoker-type venting is common. Several of these fields may be peridotite-hosted. The hydrothermal activity at parts of the AMOR exceeds by a factor of 2 to 3 what would be expected by extrapolating from observations on faster spreading ridges. Higher fracture/fault area relative to the magma volume extracted seems a likely explanation for this. Many of the vent fields at the AMOR are associated with axial volcanic ridges. Strong focusing of magma toward these ridges, deep rifting of the ridges, and subsequent formation of long-lived detachment faults that are rooted below the ridges may be the major geodynamic mechanisms causing the unexpectedly high hydrothermal activity.

  9. Hydrothermal Activity Along Multiple Ridge Segments of the Northern Central Indian Ridge, 8°-17°S

    NASA Astrophysics Data System (ADS)

    Son, J.; Kim, J.; Pak, S.; Son, S.; Moon, J.; Baker, E. T.

    2012-12-01

    We report the first systematic hydrothermal plume surveys conducted on the northern Central Indian Ridge (CIR, 8°-17°S), a slow spreading ridge with rates between ~35 and 40 mm/yr, during the CIR research program of KORDI between 2009 and 2011. Using a combined CTD/Miniature Autonomous Plume Recorder (MAPR) package we occupied 208 vertical casts and 82 tows along seven segments of the CIR totaling ~700 km of ridge length to estimate the frequency of hydrothermal activity on this slow-spreading ridge. Evidence for hydrothermal activity was found on each of the seven segments, with most plumes found between 3000 and 3500 m. Using only stations within the rift valley, the estimated value of plume incidence (ph=0.19) coincides with the global trend between the spatial density of hydrothermal plumes and full-spreading rate (an indicator of magmatic budget). However, there are also indications of possible discharge from hydrothermal activity or serpentinization from the ridge flanks (possible ocean core complexes), as has been observed along the Mid-Atlantic Ridge. For example, some sites show methane anomalies unaccompanied by any optical anomaly. Our preliminary results support the increasing role of tectonic control on hydrothermal activity as spreading rates decrease. Further examination of the plume signals, combined with chemical composition of sampled water and geological data, will provide valuable insights into hydrothermal activity on slow spreading ridges.

  10. The influence of isotropic and anisotropic crustal permeability on hydrothermal flow at fast spreading ridges

    NASA Astrophysics Data System (ADS)

    Hasenclever, Jörg; Rüpke, Lars; Theissen-Krah, Sonja; Morgan, Jason

    2016-04-01

    We use 3-D numerical models of hydrothermal fluid flow to assess the magnitude and spatial distribution of hydrothermal mass and energy fluxes within the upper and lower oceanic crust. A better understanding of the hydrothermal flow pattern (e.g. predominantly on-axis above the axial melt lens vs. predominantly off-axis and ridge-perpendicular over the entire crustal thickness) is essential for quantifying the volume of oceanic crust exposed to high-temperature fluid flow and the associated leaching and redistribution of economically interesting metals. The initial setup of all 3-D models is based on our previous 2-D studies (Theissen-Krah et al., 2011), in which we have coupled numerical models for crustal accretion and hydrothermal fluid flow. One result of these 2-D calculations is a crustal permeability field that leads to a thermal structure in the crust that matches seismic tomography data at the East Pacific Rise. Our reference 3-D model for hydrothermal flow at fast-spreading ridges predicts the existence of a hybrid hydrothermal system (Hasenclever et al., 2014) with two interacting flow components that are controlled by different physical mechanisms. Shallow on-axis flow structures develop owing to the thermodynamic properties of water, whereas deeper off-axis flow is strongly shaped by crustal permeability, particularly the brittle-ductile transition. About ˜60% of the discharging fluid mass is replenished on-axis by warm (up to 300oC) recharge flow surrounding the hot thermal plumes. The remaining ˜40%, however, occurs as colder and broader recharge up to several kilometres away from the ridge axis that feeds hot (500-700oC) deep off-axis flow in the lower crust towards the ridge. Both flow components merge above the melt lens to feed ridge-centred vent sites. In a suite of 3-D model calculations we vary the isotropic crustal permeability to quantify its influence on on-axis vs. off-axis hydrothermal fluxes as well as on along-axis hydrothermal

  11. Venus trough-and-ridge tessera - Analog to earth oceanic crust formed at spreading centers?

    NASA Technical Reports Server (NTRS)

    Head, James W.

    1990-01-01

    The similarity between the morphologies of Venus trough-and-ridge tessera and the earth's ocean floor is discussed. The hypothesis that tessera texture might be related to a crustal fabric produced at spreading centers is examined. It is suggested that the proccesses that produce the ocean floor fabric on earth are good candidates for the origin and production of the trough-and-ridge tessera. To support this hypothesis, the characteristics of the trough-and-ridge terrain in Laima Tessera are described and compared to the seafloor at spreading centers.

  12. Characteristics of Seamounts in the South China Sea: Implications on Mid-Ocean Ridge Magmatism during Cessation of Seafloor Spreading

    NASA Astrophysics Data System (ADS)

    Zhang, X.; Lin, J.

    2014-12-01

    We have analyzed bathymetric, seismic, and geophysical data of the South China Sea (SCS) to investigate the characteristics of seamounts and their implications on the magmatic evolution of a mid-ocean ridge from active seafloor spreading to post-spreading volcanism. Among the two dozens of seamounts that can be identified with ease on multi-beam bathymetry data, about half of them are located along the fossil spreading ridge while the remaining located off axis. The distribution of off-axis seamounts also shows strong asymmetry about the fossil spreading ridge with a much greater population of seamounts concentrated in the northern basin. The shape of individual seamounts is approximated as elliptical cones to yield best-fitting models: the population of seamounts in the SCS has an average major-axis length of 21 km (standard deviation σ = 8.3 km) and an average minor-axis length of 14 km (σ = 5.3 km); The above-seafloor height and volume of the seamounts have average value of 2.9 km (σ = 1.1 km) and 1.21*1012 m3 (σ = 0.99*1012 m3), respectively. However, limited seismic reflection data show that when the parts of the seamounts buried in the sediment are considered, the resulting estimations of the seamount dimension and volume would increase significantly. Overall, seamounts located on the fossil ridge in the East Subbasin have larger dimension and volume than those in the Southwest Subbasin, except for a seamount neat the Zhongnan Fault between the two subbasins. Adjacent seamounts on the fossil ridge have an average separation distance of 53 km (σ = 9.4 km). We are currently investing lithospheric and mantle melting mechanisms that might control the observed characteristics of the spatial distribution and dimension of seamounts in the SCS. Keywords: the South China Sea, fossil spreading ridge, seamounts

  13. Mechanics of gravitational spreading of steep-sided ridges («sackung»)

    USGS Publications Warehouse

    Savage, W.Z.; Varnes, D.J.

    1987-01-01

    Large-scale gravitational spreading of steep-sided ridges characterized by linear fissures, trenches, and uphill-facing scarps high on the sides and tops of ridges are known worldwide. Such spreading, termed sackung, is commonly attributed to pervasive plastic deformation of a rock mass, and is here analyzed as such. Beginning with a previously developed exact elastic solution for gravity-induced stresses in a symmetric ridge, stresses calculated from the exact solution are used in the Coulomb failure criterion to determine the extent of ridge failure under self-weight. Finally, when the regions of failure are established, a plastic flow solution is applied to predict the location of and sense of movement on upward-facing scarps near ridge crests and other features common in sackung. ?? 1987 International Assocaition of Engineering Geology.

  14. Seismological imaging of ridge-arc interaction beneath the Eastern Lau Spreading Center from OBS ambient noise tomography

    NASA Astrophysics Data System (ADS)

    Zha, Yang; Webb, Spahr C.; Wei, S. Shawn; Wiens, Douglas A.; Blackman, Donna K.; Menke, William; Dunn, Robert A.; Conder, James A.

    2014-12-01

    The Lau Basin displays large along-strike variations in ridge characters with the changing proximity of the adjacent subduction zone. The mechanism governing these changes is not well understood but one hypotheses relates them to interaction between the arc and back-arc magmatic systems. We present a 3D seismic velocity model of the shallow mantle beneath the Eastern Lau back-arc Spreading Center (ELSC) and the adjacent Tofua volcanic arc obtained from ambient noise tomography of ocean bottom seismograph data. Our seismic images reveal an asymmetric upper mantle low velocity zone (LVZ) beneath the ELSC. Two major trends are present as the ridge-to-arc distance increases: (1) the LVZ becomes increasingly offset from the ridge to the north, where crust is thinner and the ridge less magmatically active; (2) the LVZ becomes increasingly connected to a sub-arc low velocity zone to the south. The separation of the ridge and arc low velocity zones is spatially coincident with the abrupt transition in crustal composition and ridge morphology. Our results present the first mantle imaging confirmation of a direct connection between crustal properties and uppermost mantle processes at ELSC, and support the prediction that as ELSC migrates away from the arc, a changing mantle wedge flow pattern leads to the separation of the arc and ridge melting regions. Slab-derived water is cutoff from the ridge, resulting in abrupt changes in crustal lava composition and crustal porosity. The larger offset between mantle melt supply and the ridge along the northern ELSC may reduce melt extraction efficiency along the ridge, further decreasing the melt budget and leading to the observed flat and faulted ridge morphology, thinner crust and the lack of an axial melt lens.

  15. Ultraslow, slow, or fast spreading ridges: Arm wrestling between mantle convection and far-field tectonics

    NASA Astrophysics Data System (ADS)

    Husson, Laurent; Yamato, Philippe; Bézos, Antoine

    2015-11-01

    Oceanic spreading rates are highly variable, and these variations are known to correlate to a variety of surface observables, like magmatic production, heat flow or bathymetry. This correlation lead to classify ridges into fast and slow spreading ridges, but also into the more peculiar ultraslow spreading regime. Here we explore the dynamic relationships between spreading ridges, plate tectonics and mantle flow. We first focus on the thermal signature of the mantle, that we infer from the global S-wave seismic tomography model of Debayle and Ricard (2012). We show that the thermal structure of ridges gradually departs from the half-space cooling model for slow, and above all ultraslow spreading ridges. We also infer that the sublithospheric mantle temperature decreases by more than 150 °C from fast to ultraslow spreading regimes. Both observations overall indicate that the mantle convection pattern is increasingly chaotic underneath slow and ultraslow spreading ridges. We suggest that this is due to far-field tectonics at the other ends of lithospheric plates: not only it modulates the spreading rates but it also alters the convection regime by obstructing the circulation of plates, which in turn modifies the surface kinematic conditions for the convecting mantle. We test this hypothesis using a thermo-mechanical model that represents a convection cell carrying a continental lithosphere atop. The continent gradually drifts away from the spreading ridge, from which the oceanic lithosphere grows and cools while the continent eventually collides at the opposite side. In turn, this event drastically modifies the upper kinematic condition for the convecting mantle that evolves from a mobile lid regime to an almost stagnant lid regime. Implications on spreading ridges are prominent: heat advection decreases with respect to thermal conduction, which causes the oceanic lithosphere to thicken faster; the oceanic plates get compressed and destabilized by a growing number of

  16. Ultraslow, slow, or fast spreading ridges: Arm wrestling between mantle convection and far-field tectonics

    NASA Astrophysics Data System (ADS)

    Husson, Laurent; Yamato, Philippe; Bezos, Antoine

    2016-04-01

    Oceanic spreading rates are highly variable, and these variations are known to correlate to a variety of surface observables, like magmatic production, heat flow or bathymetry. This correlation lead to classify ridges into fast and slow spreading ridges, but also into the more peculiar ultraslow spreading regime. Here we explore the dynamic relationships between spreading ridges, plate tectonics and mantle flow. We first focus on the thermal signature of the mantle, that we infer from the global S-wave seismic tomography model of Debayle and Ricard (2012). We show that the thermal structure of ridges gradually departs from the half-space cooling model for slow, and above all ultraslow spreading ridges. We also infer that the sublithospheric mantle temperature decreases by more than 150 degrees C from fast to ultraslow spreading regimes. Both observations overall indicate that the mantle convection pattern is increasingly chaotic underneath slow and ultraslow spreading ridges. We suggest that this is due to far-field tectonics at the other ends of lithospheric plates: not only it modulates the spreading rates but it also alters the convection regime by obstructing the circulation of plates, which in turn modifies the surface kinematic conditions for the convecting mantle. We test this hypothesis using a thermo-mechanical model that represents a convection cell carrying a continental lithosphere atop. The continent gradually drifts away from the spreading ridge, from which the oceanic lithosphere grows and cools while the continent eventually collides at the opposite side. In turn, this event drastically modifies the upper kinematic condition for the convecting mantle that evolves from a mobile lid regime to an almost stagnant lid regime. Implications on spreading ridges are prominent: heat advection decreases with respect to thermal conduction, which causes the oceanic lithosphere to thicken faster; the oceanic plates get compressed and destabilized by a growing

  17. "Near-bottom sonar mapping along the Eastern Lau Spreading Center and Valu Fa Ridge"

    NASA Astrophysics Data System (ADS)

    Sleeper, J. D.; Martinez, F.; Edwards, M.; Dunn, R.; Baker, E. T.

    2009-12-01

    volcaniclastic sediments. Faulting is relatively limited and fault throws are significantly smaller than those at the ELSC. The northern ELSC axis forms a deep graben bounded by larger, more continuous abyssal hill faults than those at the VFR or other MOR's with similar spreading rates. An objective of the survey was to determine the off-axis extent of hydrothermal activity, the associated geologic and volcanic features and whether these may be related to the lateral extent of the seismic low velocity zone. Hydrothermal activity, as indicated primarily by particle plumes and oxidation-reduction potential anomalies appears largely restricted to within ~1 km of the ridge axis. Preliminary results from the L-SCAN seismic tomography experiment, however, suggest a narrower than expected low velocity zone underlying the axis, possibly accounting for the narrow lateral extent of hydrothermal activity.

  18. The East Pacific Rise: An Active Not Passive Spreading System

    NASA Astrophysics Data System (ADS)

    Rowley, D. B.; Rowan, C. J.; Forte, A. M.; Moucha, R.; Grand, S. P.; Simmons, N. A.

    2011-12-01

    Traditional plate tectonic interpretations of mid-oceanic ridges regard spreading as an entirely passive phenomenon. From this one would assume that the oceanic ridges will move over the mantle in response to the geodynamics of the diverging plates, and do not remain fixed spatially over any protracted period of time. An analysis of the kinematics of ridge motions in the Indo-Atlantic hotspot frame of reference since 83 Ma generally supports this view, with the notable exception of the East Pacific Rise (EPR). The Pacific-Nazca/Farallon segment of the EPR north of Easter Island (27°S) is oriented essentially N-S, and has produced more than 9500km of E-W spreading in the past 80 Ma, making it the dominant ridge in the world's plate system over this interval of time. Yet despite the large amount of E-W divergence, the spreading center has maintained its longitudinal position to within <±250 km of the current ridge axis. Global mantle convective flow modeling indicates that the EPR, unlike any other extensive segment of the mid-oceanic ridge system, is underlain by an active upwelling system extending from the core-mantle boundary to the surface. We suggest that the lack of E-W motion of the EPR apparent from the kinematics is a consequence of these mantle dynamics; this ridge is thus not behaving as a passive plate boundary, but is actively and directly linked to, and controlled by, whole mantle upwelling. This observation overturns the notion that ridges are always entirely passive features of the plate system. Subduction of the northern EPR beneath western North America has thus resulted in the overriding of an active upwelling system that has contributed significantly to the evolution of Basin and Range kinematics and superimposed dynamics, including significant contributions from dynamic topography.

  19. Asymmetric seafloor spreading and short ridge jumps in the Australian-Antarctic discordance

    NASA Astrophysics Data System (ADS)

    Marks, Karen M.; Stock, Joann M.

    1995-08-01

    The crenulated geometry of the Southeast Indian ridge within the Australian-Antarctic discordance is formed by numerous spreading ridge segments that are offset, alternately to the north and south, by transform faults. Suggested causes for these offsets, which largely developed since ~ 20 Ma, include asymmetric seafloor spreading, ridge jumps, and propagating rifts that have transferred seafloor from one flank of the spreading ridge to the other. Each of these processes has operated at different times in different locations of the discordance; here we document an instance where a small (~ 20 km), young (< 0.2 Ma), southward ridge jump has contributed to the observed asymmetry. When aeromagnetic anomalies from the Project Investigator-1 survey are superposed on gravity anomalies computed from Geosat GM and ERM data, we find that in segment B4 of the discordance (between 125° and 126° E), the roughly east-west-trending gravity low, correlated with the axial valley, is 20 25 km south of the ridge axis position inferred from the center of magnetic anomaly 1. Elsewhere in the discordance, the inferred locations of the ridge axis from magnetics and gravity are in excellent agreement. Ship track data confirm these observations: portions of Moana Wave track crossing the ridge in B4 show that a topographic valley correlated with the gravity anomaly low lies south of the center of magnetic anomaly 1; while other ship track data that cross the spreading ridge in segments B3 and B5 demonstrate good agreement between the axial valley, the gravity anomaly low, and the central magnetic anomaly. Based on these observations, we speculate that the ridge axis in B4 has recently jumped to the south, from a ridge location closer to the center of the young normally magnetized crust, to that of the gravity anomaly low. The position of the gravity low essentially at the edge of normally magnetized crust requires a very recent (< 0.2 Ma) arrival of the ridge in this new location. Because

  20. Paradise Lost: Uncertainties in melting and melt extraction processes beneath oceanic spreading ridges

    NASA Astrophysics Data System (ADS)

    Kelemen, P. B.

    2014-12-01

    In many ways, decompression melting and focused melt transport beneath oceanic spreading ridges is the best understood igneous process on Earth. However, there are remaining - increasing - uncertainties in interpreting residual mantle peridotites. Indicators of degree of melting in residual peridotite are questionable. Yb concentration and spinel Cr# are affected by (a) small scale variations in reactive melt transport, (b) variable extents of melt extraction, and (c) "impregnation", i.e. partial crystallization of cooling melt in pore space. Roughly 75% of abyssal peridotites have undergone major element refertilization. Many may have undergone several melting events. The following three statements are inconsistent: (1) Peridotite melt productivity beyond cpx exhaustion is > 0.1%/GPa. (2) Crustal thickness is independent of spreading rate at rates > 2 cm/yr full rate (excluding ultra-slow spreading ridges). (3) Thermal models predict, and observations confirm, thick thermal boundary layers beneath slow spreading ridges. If (a) melt productivity is << 0.1%/GPa beyond cpx-out, and (b) cpx-out occurs > 15 km below the seafloor beneath most ridges, then the independence of crustal thickness with spreading rate can be understood. Most sampled peridotites from ridges melted beyond cpx-out. Cpx in these rocks formed via impregnation and/or exsolution during cooling. Most peridotites beneath ridges may undergo cpx exhaustion during decompression melting. This would entail an upward modification of potential temperature estimates. Alternatively, perhaps oceanic crustal thickness does vary with spreading rate but this is masked by complicated tectonics and serpentinization at slow-spreading ridges. Dissolution channels (dunites) are predicted to coalesce downstream, but numerical models of these have not shown why > 95% of oceanic crust forms in a zone < 5 km wide. There may be permeability barriers guiding deeper melt toward the ridge, but field studies have not identified

  1. Gravitational spreading of steep-sided ridges ("sackung") in Western United States

    USGS Publications Warehouse

    Radbruch-Hall, D. H.; Varnes, D.J.; Savage, W.Z.

    1976-01-01

    Large-scale gravitational spreading and movement along fractures of steep-sided ridges in the mountainous areas of the western United States are characterized by linear fissures, trenches, and uphill-facing scarps on tops and sides of ridges. Spreading appears to take place by movement along disconnected planes and/or by slow plastic deformation of a rock mass. In some places, valleyward squeezing out of weak shales overlain by rigid rocks causes extensional fracturing and outward movement of the rigid layers, as illustrated by extension of two laccoliths overlying Mancos Shale, one at Dolores Peak and another at Crested Butte in western Colorado. Gravitational forces acting on a ridge of more homogeneous material causes tensional spreading of the ridge parallel to its long axis, for example in fractured granitic rock north of Mt. Massive in central Colorado, where a survey course has been established to monitor the movement. Recognition and understanding of these large-scale gravitational features and the mechanism that causes them are pertinent to site selection and design of engineering structures in high mountains. If fractures extend to considerable depth and if movement is continuing, engineering structures in valleys or tunnels through the spreading ridges could be damaged. ?? 1976 International Association of Engineering Geology.

  2. Dating the growth of oceanic crust at a slow-spreading ridge.

    PubMed

    Schwartz, Joshua J; John, Barbara E; Cheadle, Michael J; Miranda, Elena A; Grimes, Craig B; Wooden, Joseph L; Dick, Henry J B

    2005-10-28

    Nineteen uranium-lead zircon ages of lower crustal gabbros from Atlantis Bank, Southwest Indian Ridge, constrain the growth and construction of oceanic crust at this slow-spreading midocean ridge. Approximately 75% of the gabbros accreted within error of the predicted seafloor magnetic age, whereas approximately 25% are significantly older. These anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or, more likely, crystallization of older gabbros at depths of approximately 5 to 18 kilometers below the base of crust in the cold, axial lithosphere, which were uplifted and intruded by shallow-level magmas during the creation of Atlantis Bank. PMID:16254183

  3. Dating the growth of oceanic crust at a slow-spreading ridge

    USGS Publications Warehouse

    Schwartz, J.J.; John, Barbara E.; Cheadle, Michael J.; Miranda, E.A.; Grimes, Craig B.; Wooden, J.L.; Dick, H.J.B.

    2005-01-01

    Nineteen uranium-lead zircon ages of lower crustal gabbros from Atlantis Bank, Southwest Indian Ridge, constrain the growth and construction of oceanic crust at this slow-spreading midocean ridge. Approximately 75% of the gabbros accreted within error of the predicted seafloor magnetic age, whereas ???25% are significantly older. These anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or, more likely, crystallization of older gabbros at depths of ???5 to 18 kilometers below the base of crust in the cold, axial lithosphere, which were uplifted and intruded by shallow-level magmas during the creation of Atlantis Bank.

  4. The RAMESSES experiment-II. Evidence for accumulated melt beneath a slow spreading ridge from wide-angle refraction and multichannel reflection seismic profiles

    NASA Astrophysics Data System (ADS)

    Navin, D. A.; Peirce, C.; Sinha, M. C.

    1998-12-01

    The RAMESSES study (Reykjanes Axial Melt Experiment: Structural Synthesis from Electromagnetics and Seismics) targeted an apparently magmatically active axial volcanic ridge (AVR), centred on 57°45'N at the Reykjanes Ridge, with the aim of investigating the processes of crustal accretion at a slow spreading mid-ocean ridge. As part of this multicomponent experiment, airgun and explosive wide-angle seismic data were recorded by 10 digital ocean-bottom seismometers (OBSs) along profiles oriented both across- and along-axis. Coincident normal-incidence seismic, bathymetry and underway gravity and magnetic data were also collected. Forward modelling of the seismic and gravity data has revealed layer thicknesses, velocities and densities similar to those observed elsewhere within the oceanic crust near mid-ocean ridges. At 57°45'N, the Reykjanes Ridge has a crustal thickness of approximately 7.5 km on-axis. However, the crust is modelled to decrease in thickness slightly off-axis (i.e. with age), which implies that full crustal thickness is achieved on-axis and that it is subsequently thinned, most likely, by off-axis extension. Modelling also indicates that the AVR is underlain by a thin (~100 m), narrow (~4 km) melt lens some 2.5 km beneath the seafloor, which overlies a broader zone of partial melt approximately 8 km in width. Thus the results of this study provide the first clear evidence for a crustal magma chamber beneath any slow spreading ridge. The size and depth of this magma chamber (the melt lens and underlying zone of partial melt) are similar to those observed beneath fast and intermediate spreading ridges, which implies that the processes of crustal accretion are similar at all spreading rates. Hence the lack of previous observations of magma chambers beneath slow spreading ridges is probably temporally related to the periods of magmatic activity being considerably shorter and more widely spaced in time than at fast and intermediate spreading ridges.

  5. No spreading across the southern Juan de Fuca ridge axial cleft during 1994-1996

    USGS Publications Warehouse

    Chadwell, C.D.; Hildebrand, J.A.; Spiess, Fred N.; Morton, J.L.; Normark, W.R.; Reiss, C.A.

    1999-01-01

    Direct-path acoustic measurements between seafloor transponders observed no significant extension (-10 ?? 14 mm/yr) from August 1994 to September 1996 at the southern Juan de Fuca Ridge (44??40' N and 130??20' W). The acoustic path for the measurement is a 691-m baseline straddling the axial cleft, which bounds the Pacific and Juan de Fuca plates. Given an expected full-spreading rate of 56 mm/yr, these data suggest that extension across this plate boundary occurs episodically within the narrow (~1 km) region of the axial valley floor, and that active deformation is occurring between the axial cleft and the plate interior. A cleft-parallel 714-m baseline located 300 m to the west of the cleft on the Pacific plate monitored system performance and, as expected, observed no motion (+5??7 mm/yr) between the 1994 and 1996 surveys.Direct-path acoustic measurements between seafloor transponders observed no significant extension (-10 ?? 14 mm/yr) from August 1994 to September 1996 at the southern Juan de Fuca Ridge (44??40 minutes N and 130??20 minutes W). The acoustic path for the measurement is a 691-m baseline straddling the axial cleft, which bounds the Pacific and Juan de Fuca plates. Given an expected full-spreading rate of 56 mm/yr, these data suggest that extension across this plate boundary occurs episodically within the narrow (approx. 1 km) region of the axial valley floor, and that active deformation is occurring between the axial cleft and the plate interior. A cleft-parallel 714-m baseline located 300 m to the west of the cleft on the Pacific plate monitored system performance and, as expected, observed no motion (+5 ?? 7 mm/yr) between the 1994 and 1996 surveys.

  6. Acoustic detection of a seafloor spreading episode on the Juan de Fuca Ridge using military hydrophone arrays

    NASA Astrophysics Data System (ADS)

    Fox, Christopher G.; Radford, W. Eddie; Dziak, Robert P.; Lau, Tai-Kwan; Matsumoto, Haruyoshi; Schreiner, Anthony E.

    1995-01-01

    Until recently, no practical method has been available to continuously monitor seismicity of seafloor spreading centers. The availability of the U.S. Navy's SOund SUrveillance System (SOSUS) for environmental research has allowed the continuous monitoring of low-level seismicity of the Juan de Fuca Ridge in the northeast Pacific. On June 22, 1993, NOAA installed a prototype system at U.S Naval Facility Whidbey Island to allow real-time acoustic monitoring of the Juan de Fuca Ridge using SOSUS. On June 26, 2145 GMT, a burst of low-level seismic activity, with accompanying harmonic tremor, was observed and subsequently located near 46 deg 15 min N, 129 deg 53 min W, on the spreading axis of the Juan de Fuca Ridge. Over the following 2 days, the activity migrated to the NNE along the spreading axis with the final locus of activity near 46 deg 31.5 min N, 129 deg 35 min W. The nature of the seismicity was interpreted to represent a lateral dike injection with the possibility of eruption on the seafloor. Based on this interpretation, a response effort was initated by U.S. and Canadian research vessels, and both warm water plumes and fresh lavas were subsequently identified at the site.

  7. Deep-Towed Sidescan Sonar Studies of Amagmatic Spreading Centres: the Mid-Atlantic Ridge at 13N

    NASA Astrophysics Data System (ADS)

    Mallows, C.; Searle, R. C.; Party, J. S.

    2007-12-01

    In areas of sparse magmatism, plate separation is accommodated predominately by tectonic processes, which is often observed in the form of long-lived detachment faults exhuming lower crustal rocks and mantle peridotites to the seafloor. During research cruise JC007, a number of these oceanic core complexes (OCCs) recently identified by Smith et al. (2006) along the Mid-Atlantic Ridge (MAR) were imaged using the National Oceanography Centres Towed Ocean Bottom Instrument (TOBI). Sidescan sonar data were collected across two active OCCs at 1320N and 1330N and one inactive OCC at 1350N, including the intervening regions. We made extensive use of 3-D Fledermaus visualisations during our interpretations, and will include these in our presentation. The sidescan sonar data show distinct similarities between the two active OCCs. They both appear as large domal detachment surfaces being exhumed from beneath the ridge axis that are capped by a complexly deformed central massif and volcanic ridge at the breakaway (as suggested by Smith et al, 2006). The structures extend for c.20km off axis and c.10km along axis, and the detachment faults are characterised by large-scale spreading-parallel bathymetric corrugations and fine scale striations (interpreted as streams of basalt derived from tectonic erosional screes imaged at the terminations). The hanging walls above the zones of emergence of the detachment faults exhibit faulting that appears to trend from the ridge axis (N and S of the OCC) towards the spreading direction (at the OCC). The neo-volcanic zones along the ridge axis are apparently absent opposite active OCC formation, which is likely indicative of predominately tectonic spreading. In the regions between the active OCCs, sidescan sonar imagery shows a clear neo-volcanic zone at the ridge axis, indicating robust magmatic plate separation and accretion. The extinct OCC is characterised by heavy sediment cover over a much shallower detachment surface, presumably due to

  8. The formation of post-spreading volcanic ridges in the South China Sea

    NASA Astrophysics Data System (ADS)

    Zhao, Minghui; Sibuet, Jean-Claude; He, Enyuan; Tan, Pingchuan; Wang, Jian; Qiu, Xuelin

    2016-04-01

    In the South China Sea (SCS), the post-spreading magmatism (˜8-13 Ma) largely masks the previous spreading fabric. The resulting post-spreading seamounts are more numerous in the northern part than in the southern part of the East sub-basin. In the eastern part of the East sub-basin, the post-spreading volcanic ridge (PSVR) is approximately N055° oriented and follows the extinct spreading ridge (ESR). In the western part of the East sub-basin, the PSVR, called the Zhenbei-Huangyan seamounts chain, is E-W oriented and hides the ESR. Based on swath bathymetric and magnetic data, the ESR in the eastern part of the East basin is also N055° oriented and thus is oblique the E-W Zhenbei-Huangyan seamounts chain (Sibuet et al., 2016). We conducted a seismic refraction survey covering both the Zhenbei-Huangyan seamounts chain and the adjacent ESR, providing new constraints for understanding the relationship between the PSVR and the ESR. The detailed velocity structure shows that the Zhenbei-Huangyan seamounts chain was emplaced through a typical oceanic crust. The thicknesses of Zhenbei (14 km) and Huangyan seamounts (8 km) are larger than the ones of the normal oceanic crust. The correlation between crustal thicknesses and mean lower-crustal seismic velocities suggest that an asymmetric generation of seamounts in the East sub-basin where active upwelling mantle (Holbrook et al., 2001), the presence of a fertile mantle component (Korenaga et al., 2002), or buoyancy-driven decompression melting may happened (Castillo et al., 2010). Below the seamounts, the thickened lower crust is probably due to secondary magmatic intrusions and the large thickness of upper crust is possibly due to volcanic extrusions. The crustal thicknesses as well as the mean lower-crustal velocities of the Zhenbei and Huangyan seamounts are different, suggesting an independent origin for magmatic feeding. This research was granted by the Natural Science Foundation of China (91028002, 91428204

  9. The RAMESSES experiment-V. Crustal accretion at axial volcanic ridge segments-a gravity study at 57°45'N on the slow spreading Reykjanes Ridge

    NASA Astrophysics Data System (ADS)

    Peirce, Christine; Navin, Debbie A.

    2002-04-01

    Presented in this paper are the results of a two-stage analysis of gravity data acquired during a multidisciplinary geophysical survey of a magmatically active axial volcanic ridge (AVR) segment located at 57°45'N on the Reykjanes Ridge, part of the slow spreading Mid-Atlantic Ridge south of Iceland. Modelling of the free-air anomaly in 2-D shows that, across-axis, the observed anomaly results largely from density and layer thickness variation in the mid-lower crust. Although seismic control on crustal thickness along-axis is limited, modelling also suggests that both crustal density and thickness also vary towards AVR tips. Using the 2-D modelling results as crustal reference, the residual mantle Bouguer anomaly (RMBA) is calculated to assess whether magma-related density anomalies are present in the mantle and to investigate the structure of, and relationship between, adjacent AVRs along-axis. RMBA lows are associated with both an along-ridge trend encompassing a number of adjacent AVRs and with individual, more topographically robust AVRs. Modelling of the RMBA low associated with the 57°45'N AVR further suggests that along-axis density variation is confined to the central region of this AVR and that the anomaly can largely be accounted for by density variation within Layer 3 and a degree of crustal thinning towards AVR tips. The nature of the along-axis variation in crustal density further suggests that it may result from repeated phases of magma supply to the crustal system from the mantle. Within the resolution of the RMBA, modelling does not confirm or preclude the presence of a subcrustal density anomaly associated with retention of a small percentage of melt in the mantle. However, a melt-free model for at least the top 40 km of the mantle is preferred as this is consistent with the results of modelling a coincident magnetotelluric data set. The ridge-trend characteristics of the RMBA also suggest that magma delivery may take place along this trend, and

  10. Magmatic processes at slow spreading ridges: implications of the RAMESSES experiment at 57° 45'N on the Mid-Atlantic Ridge

    NASA Astrophysics Data System (ADS)

    Sinha, M. C.; Constable, S. C.; Peirce, C.; White, A.; Heinson, G.; MacGregor, L. M.; Navin, D. A.

    1998-12-01

    This paper is the first in a series of three (this issue) which present the results of the RAMESSES study (Reykjanes Axial Melt Experiment: Structural Synthesis from Electromagnetics and Seismics). RAMESSES was an integrated geophysical study which was carefully targeted on a magmatically active, axial volcanic ridge (AVR) segment of the Reykjanes Ridge, centred on 57°45'N. It consisted of three major components: wide-angle seismic profiles along and across the AVR, using ocean-bottom seismometers, together with coincident seismic reflection profiles; controlled-source electromagnetic sounding (CSEM); and magnetotelluric sounding (MT). Supplementary data sets included swath bathymetry, gravity and magnetics. Analyses of the major components of the experiment show clearly that the sub-axial geophysical structure is dominated by the presence and distribution of aqueous and magmatic fluids. The AVR is underlain by a significant crustal magma body, at a depth of 2.5 km below the sea surface. The magma body is characterized by low seismic velocities constrained by the wide-angle seismic data; a seismic reflection from its upper surface; and a region of anomalously low electrical resistivity constrained by the CSEM data. It includes a thin, ribbon-like melt lens at the top of the body and a much larger region containing at least 20 per cent melt in a largely crystalline mush zone, which flanks and underlies the melt lens. RAMESSES is the first experiment to provide convincing evidence of a significant magma body beneath a slow spreading ridge. The result provides strong support for a model of crustal accretion at slow spreading rates in which magma chambers similar to those at intermediate and fast spreading ridges play a key role in crustal accretion, but are short-lived rather than steady-state features. The magma body can exist for only a small proportion of a tectono-magmatic cycle, which controls crustal accretion, and has a period of at least 20000 years. These

  11. Shallow axial magma chamber at the slow-spreading Erta Ale Ridge

    NASA Astrophysics Data System (ADS)

    Pagli, Carolina; Wright, Tim J.; Ebinger, Cynthia J.; Yun, Sang-Ho; Cann, Johnson R.; Barnie, Talfan; Ayele, Atalay

    2012-04-01

    The existence of elongated, shallow magma chambers beneath the axes of fast-spreading mid-ocean ridges is well established. Yet, at slow-spreading ridges such shallow and elongated magma chambers are much less evident. Simple thermal models therefore predict that spreading velocity and magma supply may provide the main controls on magma-chamber depth and morphology. Here we use interferometric synthetic aperture radar data to investigate the dynamics of the magma chamber beneath the slow-spreading Erta Ale segment of the Ethiopian Rift. We show that an eruption from Alu-Dalafilla in November 2008 was sourced from a shallow, 1km deep, elongated magma chamber that is divided into two segments. The eruption was probably triggered by a small influx of magma into the northern segment. Both segments of the magma chamber fed the main eruption through a connecting dyke and both segments have been refilling rapidly since the eruption ended. Our results support the presence of independent sources of magma supply to segmented chambers located along the axes of spreading centres. However, the existence of a shallow, elongated axial chamber at Erta Ale indicates that spreading rate and magma supply may not be the only controls on magma-chamber characteristics.

  12. Volcanic accretion, tectonic extension and the second-order segmentation of slow and ultraslow-spreading mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Cannat, M.; Sauter, D.; Escartin, J.

    2011-12-01

    In this presentation we compare the segmentation and seafloor geology record of slow and ultraslow ridges with variable volcanic input. The easternmost Southwest Indian Ridge (SWIR), where long stretches of the axis lack volcanism is our volcanism-poor end-member, which we contrast with volcanically more active parts of the Mid-Atlantic Ridge (MAR). Keeping the differences of spreading rates in perspective, we use this comparison to analyze and discuss the respective roles of tectonic extension, which ultimately leads to the exhumation of deeply-derived rocks (ultramafics and gabbros), and of volcanic accretion, in shaping the geometry of the plate boundary. Second-order segments at slow and ultraslow ridges are typically 30 to 100 km-long, and separated by transform, or so-called "non-transform" discontinuities. Segment centers typically have a thicker crust, and in most cases have a thinner axial lithosphere, than segment ends. Although we do not resolve the controversy of whether these characteristics are produced by discrete melt and/or mantle diapirs in the subaxial asthenosphere (eg Lin et al. 1990), or by melt channeling toward regions of thinner axial lithosphere (eg Magde and Sparks, 1997), we show that melt supply and volcanism are needed to initiate second-order ridge segmentation. Axial valley bounding faults in our SWIR volcanism-poor end-member go un-segmented for up to 170 km along-axis.

  13. Distribution and Alignment of Small Volcanic Edifices at Slow-Spreading Mid-Ocean Ridges

    NASA Astrophysics Data System (ADS)

    Yeo, Isobel; Achenbach, Kay L.; Searle, Roger C.

    2010-05-01

    Around seventy percent of the Earth's surface is covered by oceanic crust produced along the 80,000km of mid-ocean ridges. However, despite its prevalence the processes by which ocean crust is built remain poorly understood, particularly at slow-spreading centres. This study uses sidescan sonar images from several volcanic centres along the Mid-Atlantic Ridge and Southwest Indian Ridge to statistically quantify the distribution and structural influence on volcanic edifice emplacement. Volcanism at slow-spreading mid-ocean ridges is concentrated on Axial Volcanic Ridges (AVRs), elongate topographic highs found within the axial valley. AVRs may reach a kilometre above the seafloor and tens of kilometres in length and are composite piles of volcanics, constructed almost entirely of 'hummocks', small volcanic edifices usually between fifty and several hundred metres across. In this study we analyse sidescan sonar data from three slow-spreading ridges, two on the Mid-Atlantic Ridge (at 45° N and 29° N) and one on the Southwest Indian Ridge (at 64° E). Nearest neighbour analyses are used to identify statistically significant degrees of cluster of hummocks at the three study areas and maps of areal volcano density are used to asses the distribution of high and low density areas. From these analyses hundreds of small clustered groups can be identified, possibly corresponding to individual sources of melt. Larger areas of lower density can also be identified and appear to be linked to the initiation of new faulting. This implies either a lower melt flux to these areas or a low flux of volcanic cone building material. The initiation of new faulting and the narrowing of the axial valley suggest the first option is more likely. This study records the presence of lateral volcanic 'spurs' running off either side of the AVR for the first time. Several methods are used to quantify the alignments of both these features and the hummocks on the main body of the AVR. We find that

  14. Local Seismicity of the slow-spreading Mid-Atlantic ridge: median valley earthquakes shallow towards segment ends

    NASA Astrophysics Data System (ADS)

    Tilmann, F.; Planert, L.; Flueh, E.; Reston, T.; Weinrebe, W.

    2003-04-01

    Slow spreading mid-ocean ridges are characterized by along-axis segmentation where crustal composition and structure varies significantly within a segment and across transform faults and other ridge axis discontinuities. In May 2000, the GERSHWIN experiment (Geophysical Experiments to investigate Ridge Segmentation HoW INside and outside corners forms) investigated the Mid-Atlantic Ridge (MAR) at 5oS during cruise M47/2 of RV Meteor. The work program included seismic refraction profiling, bathymetric mapping, dredging, and a passive seismological survey, the results of which we are reporting here. In the study area, two spreading segments of the MAR are separated by a 70 km offset transform fault. This segment of the ridge is unusual in that the inside corner high has been split by a change in location of active seafloor spreading. (Reston et al., 2002). Just south of the 5oS transform fault, a network of up to 15 ocean bottom stations (13 hydrophones and 2 seismometers), recorded micro-earthquake activity for a duration of altogether 10 days (because of instrument failures and early recovery instrument numbers vary throughout this period, though). Approximately, 150 earthquakes produced clear arrivals on three or more stations. Approximately half of these events have five or more picks and a azimuthal gap less than 300o, so can be considered well located; 49 events have good depth control. Earthquake activity is concentrated along a narrow zone along the median valley. A few events occur along the transform fault, and in diffuse regions within the Inside Corner High and the bounding massif near the centre of the segment. Event depths vary between 5 and 13 km below sea level (approx. 1-9 km below the seafloor), with most occurring at 7-9 km depth below seafloor. Earthquake depths within the median valley shallow towards the segment end, however, there is no significant seismicity within the immediate neighbourhood of the fracture zone or beneath the volcanic ridge

  15. Negative gravity anomaly over spreading rift valleys: Mid-Atlantic Ridge at 26°N

    NASA Astrophysics Data System (ADS)

    Bowin, Carl; Milligan, Julie

    1985-03-01

    A pronounced negative free-air gravity anomaly commonly occurs over the median valley of slow spreading ocean ridges. Previous results, using Wiener filtering and cross-spectral analysis techniques for the Mid-Atlantic Ridge, obtained estimates of the elastic plate thickness in the range of 7-13 km and the existence of a residual negative gravity anomaly over the median rift valley, suggesting that the rift valley has a response function different than the remainder of the spreading ridge. In this paper we have improved the derivation of the topography-gravity admittance function for spreading ocean crust by carefully avoiding several sources of spectral splattering when processing the data: (1) selecting data from a cruise that followed a flowline of central North Atlantic relative plate motion and hence is least corrupted by fracture zones; and (2) accounting for the difference in distance between the gravity meter and the regional variation in elevation as the ridge crest is traversed. Improvements of lesser importance include the use of cubic splines to interpolate to equally spaced data rather than linear interpolation, and correction of the free-air anomaly values for long-wavelength variations of the indirect effect. Comparison of the resulting admittance function to elastic flexure response functions suggests an elastic plate thickness of about 8 km. The improved admittance function, when convolved with the ridge topography, provides a predicted gravity profile that accounts very well for the negative anomaly over the rift valley. Therefore, the isostatic response function for the rift valley is similar to that for the topography away from the rift valley.

  16. Do ridge segments with asymmetric and symmetric spreading have distinctive geochemical signatures? (Invited)

    NASA Astrophysics Data System (ADS)

    Langmuir, C. H.; Hamelin, C.; Chen, Z.; Escartin, J.; Laubier, M.; Jagoutz, O. E.; Looney, J.

    2013-12-01

    The MARPEX expedition in summer 2012 intensively sampled 650km of the mid-Atlantic ridge between the Kane and Atlantis transform faults. Between Kane and Atlantis there are no additional transform faults, but 14 second order ridge segments of various types. Some segments (e.g. Broken Spur near 29'N) are symmetric with well-defined axial volcanic ridges and a strike perpendicular to the spreading direction. Some (e.g. TAG) are highly asymmetric with a well developed detachment fault on one side. Others (e.g. immediately south of Atlantis) have a detachment fault at one end, but become more symmetric in the middle of the segment. In other regions the apparent locus of spreading is not perpendicular to spreading direction, but rough terrain with no clearly defined neo-volcanic zone has an oblique orientation. In the south there is 70 km of oblique spreading with abyssal hills rotated 40 degrees relative to spreading direction. This region shows the simple plate tectonic paradigm of ridge segments and transform faults does not strictly apply to slow spreading ridges. The sampling campaign led to 180 new stations in this region, that have been combined with existing data from some 90 other stations to provide an unprecedented sample set from a long, normal section of slow-spreading ridge distant from hot spots. Sampling density is 2-3km along the entire length of ridge. All the samples are depleted MORB with no evidence of enriched basalts. The depletion is such that every sample has less Ba than the globally averaged N-MORB of Gale et al. (2013), and mean K2O at 8% MgO is only 0.1. These data show that small heterogeneities of enriched material are not ubiquitous. Within this depletion there is substantial diversity, with Zr/Y and La/Sm both varying by a factor of more than two, and a long wavelength signal of heterogeneity with maximum depletion in terms of both trace elements and isotopes occurring approximately mid-way between the two transform faults. The region

  17. Hydrothermal activity on near-arc sections of back-arc ridges: Results from the Mariana Trough and Lau Basin

    NASA Astrophysics Data System (ADS)

    Baker, Edward T.; Massoth, Gary J.; Nakamura, Ko-Ichi; Embley, Robert W.; de Ronde, Cornel E. J.; Arculus, Richard J.

    2005-09-01

    The spatial density of hydrothermal venting is strongly correlated with spreading rate on mid-ocean ridges (with the interesting exception of hot spot-affected ridges), evidently because spreading rate is a reliable proxy for the magma budget. This correlation remains untested on spreading ridges in back-arc basins, where the magma budget may be complicated by subduction-induced variations of the melt supply. To address this uncertainty, we conducted hydrothermal plume surveys along slow-spreading (40-60 mm/yr) and arc-proximal (10-60 km distant) sections of the southern Mariana Trough and the Valu Fa Ridge (Lau Basin). On both sections we found multiple plumes overlying ˜15-20% of the total length of each section, a coverage comparable to mid-ocean ridges spreading at similar rates. These conditions contrast with earlier reported results from the two nearest-arc segments of a faster spreading (60-70 mm/yr) back-arc ridge, the East Scotia Ridge, which approaches no closer than 100 km to its arc. There, hydrothermal venting is relatively scarce (˜5% plume coverage) and the ridge characteristics are distinctly slow-spreading: small central volcanic highs bookended by deep median valleys, and axial melt lenses restricted to the volcanic highs. Two factors may contribute to an unexpectedly low hydrothermal budget on these East Scotia Ridge segments: they may lie too far from the adjacent arc to benefit from near-arc sources of melt supply, and subduction-aided migration of mantle from the Bouvet hot spot may reduce hydrothermal circulation by local crustal warming and thickening, analogous to the Reykjanes Ridge. Thus the pattern among these three ridge sections appears to mirror the larger global pattern defined by mid-ocean ridges: a well-defined trend of spreading rate versus hydrothermal activity on most ridge sections, plus a subset of ridge sections where unusual melt delivery conditions diminish the expected hydrothermal activity.

  18. Seismic observation of an extremely magmatic accretion at the ultraslow spreading Southwest Indian Ridge

    NASA Astrophysics Data System (ADS)

    Li, Jiabiao; Jian, Hanchao; Chen, Yongshun John; Singh, Satish C.; Ruan, Aiguo; Qiu, Xuelin; Zhao, Minghui; Wang, Xianguang; Niu, Xiongwei; Ni, Jianyu; Zhang, Jiazheng

    2015-04-01

    The oceanic crust is formed by a combination of magmatic and tectonic processes at mid-ocean spreading centers. Under ultraslow spreading environment, however, observations of thin crust and mantle-derived peridotites on the seafloor suggest that a large portion of crust is formed mainly by tectonic processes, with little or absence of magmatism. Using three-dimensional seismic tomography at an ultraslow spreading Southwest Indian Ridge segment containing a central volcano at 50°28'E, here we report the presence of an extremely magmatic accretion of the oceanic crust. Our results reveal a low-velocity anomaly (-0.6 km/s) in the lower crust beneath the central volcano, suggesting the presence of partial melt, which is accompanied by an unusually thick crust (~9.5 km). We also observe a strong along-axis variation in crustal thickness from 9.5 to 4 km within 30-50 km distance, requiring a highly focused melt delivery from the mantle. We conclude that the extremely magmatic accretion is due to localized melt flow toward the central volcano, which was enhanced by the significant along-axis variation in lithosphere thickness at the ultraslow spreading Southwest Indian Ridge.

  19. RAMESSES II - Mapping an Axial Melt System Beneath a Slow-Spreading Ridge

    NASA Astrophysics Data System (ADS)

    Gill, C. J.; Peirce, C.; Sinha, M.

    2002-12-01

    RAMESSES II is the second phase of a multi-component geophysical investigation into an axial volcanic ridge (AVR) segment at 57° 45' N 32° 35' W close to the southern tip of the slow-spreading Reykjanes Ridge. Results from the original RAMESSES experiment showed the first unequivocal evidence for the presence of a crustal melt body beneath any slow-spreading ridge using a combination of wide-angle and 4-fold multi-channel seismics (Navin et al., Geophys. J. Int. (1998) 135, 746-772) plus electromagenetic data. RAMESSES II aimed to map and further constrain the properties of the magmatic system and crustal structure along-axis between adjacent AVRs which, in turn, will result in a better understanding of the accretionary process and melt supply mechanism from the mantle source. The RAMESSES II 2-D seismic reflection data set comprises 32 across-AVR profiles spaced by 1.5 km and 5 AVR-parallel profiles at 5.0 km intervals. All profiles were acquired with a 96-channel streamer and an 84.6 l (5172 in3) 12-airgun source array, with a shot instant of 15 s (37.5 m). Here we present an approach to, and results of, data processing, where the data is significantly contaminated by reflections and seismic energy scattered by the rough seafloor typical of slow-spreading ridges. We also present a simple approach to identifying scattered energy within stacked sections enabling real sub-surface reflection events to be distinguished.

  20. Crustal structure of the Boreas Basin formed at ultraslow spreading Knipovich Ridge - Northern North Atlantic

    NASA Astrophysics Data System (ADS)

    Hermann, T.; Jokat, W.

    2012-04-01

    The Boreas Basin is located in Norwegian Greenland Sea bordered by the Greenland Fracture Zone in the south and the Hovgard Ridge in the north, respectively. In the east it adjoins the ultraslow mid-ocean Knipovich Ridge. Previous seismic reflection studies in the Boreas Basin have shown that the basement topography has a roughness, which is typical for ultraslow spreading ridges. This observation supports assumptions that the basin was formed at ultraslow spreading rates during its entire geological history. However, the detailed crustal structure remained unresolved. In summer 2009 new seismic refraction data were acquired in the Boreas Basin during the expedition ARK-XXIV/3 with the research vessel Polarstern. The deep seismic sounding line has a length of 340 km. Forward modelling of the data of 18 ocean bottom seismometers deployed along the NW-SE trending profile reveal an unusual 3.2 km thin oceanic crust. The crustal model is further constrained by S-wave and 2D gravity modelling. The P-wave velocity model shows a layered oceanic crust without oceanic layer 3 and with velocities less than 6.3 km/s except beneath a nearly 2000 m high seamount. Beneath the seamount velocities of up to 6.7 km/s were observed. The mantle velocities range between 7.5 km/s in the uppermost mantle and 8.0 km/s in almost 15 km depth. A serpentinisation of approximately 13% in the uppermost mantle decreasing downwards can explain the low mantle velocities. In summary, the transect confirms earlier models that the entire Boreas Basin was formed at ultraslow spreading rates. Indications for this are the basement roughness and the overall thin oceanic crust. Both observations are typical for ultraslow spreading systems.

  1. Fault Characteristics and Strain Accommodation at the Ultra Slow Spreading South West Indian Ridge (SWIR).

    NASA Astrophysics Data System (ADS)

    Bralee, A. V.; Searle, R. C.

    2001-12-01

    Spreading rate is one of the fundamental physical processes controlling mid-ocean ridge structure. The ultra slow spreading (16mm/yr full rate) and expected low magma budget of the SWIR suggest that faulting characteristics may be particulary distinctive. Brittle failure extending through the crust into the upper mantle and extension along these faults forming low angle detachment surfaces along with normal faulting may produce fault patterns and parameters very different to those observed at faster spreading ridges. Data collected from the FUJI and INDOYO cruises of 1997 and 1998 have been used to map the faults at 64 deg. East, Segment 11 of the SWIR. TOBI sidescan sonar images and shipboard bathymetry display faults to a resolution of 100m. TOBI phase bathymetry supplements the datasets with a resolution of 20m. Fault parameters have been measured and strain calculations performed to make a direct comparison with 29 deg North of the slow spreading (25mm/yr) Mid-Atlantic Ridge (MAR). (JGR B, 104, 10,421. 1999) Total tectonic strain at the SWIR is shown to be 9% compared to 11.5% at the MAR. The strain measurements at the outside corner flank of each segment are more comparable with 6.67% and 6.8% at the SWIR and MAR respectively. Strain results for the inside corner and strain accommodation through outward facing faults at each ridge will also be presented. At the MAR tectonic strain remains constant along the segment axis suggesting that variations in fault development may be reflecting variations in the rheology of the lithosphere and not variations in tectonic strain or magmatic supply. The situation at the SWIR will be discussed. Gravity measurements are expected to show a Mantle Bouguer Anomaly over the FUJI Dome (inferred detachment surface) that will be modelled to determine relative crustal thickness. Sub-seafloor density measurements will discriminate crustal from mantle rocks and provide information about the origin of the FUJI Dome. Magnetic signatures

  2. Intraplate compressional deformation in West-Congo and the Congo basin: related to ridge-puch from the South Atlantic spreading ridge?

    NASA Astrophysics Data System (ADS)

    Delvaux, Damien; Everaerts, Michel; Kongota Isasi, Elvis; Ganza Bamulezi, Gloire

    2016-04-01

    After the break-up and separation of South America from Africa and the initiation of the South-Atlantic mid-oceanic ridge in the Albian, at about 120 Ma, ridge-push forces started to build-up in the oceanic lithosphere and were transmitted to the adjacent continental plates. This is particularly well expressed in the passive margin and continental interior of Central Africa. According to the relations of Wiens and Stein (1985) between ridge-push forces and basal drag in function of the lithospheric age of oceanic plates, the deviatoric stress reaches a compressional maximum between 50 and 100, Ma after the initiation of the spreading ridge, so broadly corresponding to the Paleocene in this case (~70-20 Ma). Earthquake focal mechanism data show that the West-Congo margin and a large part of the Congo basin are still currently under compressional stresses with an horizontal compression parallel to the direction of the active transform fracture zones. We studied the fracture network along the Congo River in Kinshasa and Brazzaville which affect Cambrian sandstones and probably also the late Cretaceous-Paleocene sediments. Their brittle tectonic evolution is compatible with the buildup of ridge-push forces related to the South-Atlantic opening. Further inland, low-angle reverse faults are found affecting Jurassic to Middle Cretaceous cores from the Samba borehole in the Congo basin and strike-slip movements are recorded as a second brittle phase in the Permian cores of the Dekese well, at the southern margin of the Congo basin. An analysis of the topography and river network of the Congo basin show the development of low-amplitude (50-100 m) long wavelengths (100-300 km) undulations that can be interpreted as lithospheric buckling in response to the compressional intraplate stress field generated by the Mid-Atlantic ridge-push. Wiens, D.A., Stein, S., 1985. Implications of oceanic intraplate seismicity for plate stresses, driving forces and theology. Tectonophysics

  3. Geochemical Characterization of Hydrothermal Plume Fluids From Peridotite- and Basalt- Dominated Regions of the Ultra-Slow Spreading Gakkel Ridge

    NASA Astrophysics Data System (ADS)

    Upchurch, L.; Edmonds, H. N.; Resing, J.; Nakamura, K.; Buck, N.; Liljebladh, B.; Stranne, C.; Tupper, G.; Winsor, P.

    2007-12-01

    Geochemical characterization of hydrothermal plumes initially located during the 2001 AMORE cruise to the Gakkel Ridge was undertaken as part of the 2007 Arctic Gakkel Vents Expedition (AGAVE). One peridotite- and one basalt-dominated area were targeted for this exploration to constrain the range of venting environments found on the Gakkel Ridge, the ultra-slow spreading endmember of the global mid-ocean ridge. CTD hydrocasts at the 7 E peridotite-hosted site relocated the plumes found initially on the AMORE cruise. The target plume was located between 2800 and 2950 meters and exhibited a localized signal in temperature and light scattering. While shipboard analysis of dissolved gases was unavailable at the 7 E site, samples were preserved for manganese and helium measurements. No Eh signal was found at the 7 E site. The 85 E basalt-hosted site has experienced recent volcanic activity and was more extensively studied relative to the 7 E site during the AGAVE cruise. CTD casts detected numerous temperature, light scattering, and Eh plumes at 85 E indicative of multiple hydrothermal sources. Three of the plumes sampled exhibited methane concentrations ranging from 20 nM to greater than 250 nM and hydrogen concentrations ranging from 10nM to 100nM. In situ Eh measurements recorded negative excursions of at least 25 mV in each plume. Associated manganese and particle chemistry samples collected at both sites will be analyzed in time for this meeting.

  4. Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge.

    PubMed

    Zhang, Chao; Koepke, Juergen; Kirchner, Clemens; Götze, Niko; Behrens, Harald

    2014-01-01

    Axial melt lenses sandwiched between the lower oceanic crust and the sheeted dike sequences at fast-spreading mid-ocean ridges are assumed to be the major magma source of oceanic crust accretion. According to the widely discussed "gabbro glacier" model, the formation of the lower oceanic crust requires efficient cooling of the axial melt lens, leading to partial crystallization and crystal-melt mush subsiding down to lower crust. These processes are believed to be controlled by periodical magma replenishment and hydrothermal circulation above the melt lens. Here we quantify the cooling rate above melt lens using chemical zoning of plagioclase from hornfelsic recrystallized sheeted dikes drilled from the East Pacific at the Integrated Ocean Drilling Program Hole 1256D. We estimate the cooling rate using a forward modelling approach based on CaAl-NaSi interdiffusion in plagioclase. The results show that cooling from the peak thermal overprint at 1000-1050°C to 600°C are yielded within about 10-30 years as a result of hydrothermal circulation above melt lens during magma starvation. The estimated rapid hydrothermal cooling explains how the effective heat extraction from melt lens is achieved at fast-spreading mid-ocean ridges. PMID:25209311

  5. Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge

    PubMed Central

    Zhang, Chao; Koepke, Juergen; Kirchner, Clemens; Götze, Niko; Behrens, Harald

    2014-01-01

    Axial melt lenses sandwiched between the lower oceanic crust and the sheeted dike sequences at fast-spreading mid-ocean ridges are assumed to be the major magma source of oceanic crust accretion. According to the widely discussed “gabbro glacier” model, the formation of the lower oceanic crust requires efficient cooling of the axial melt lens, leading to partial crystallization and crystal-melt mush subsiding down to lower crust. These processes are believed to be controlled by periodical magma replenishment and hydrothermal circulation above the melt lens. Here we quantify the cooling rate above melt lens using chemical zoning of plagioclase from hornfelsic recrystallized sheeted dikes drilled from the East Pacific at the Integrated Ocean Drilling Program Hole 1256D. We estimate the cooling rate using a forward modelling approach based on CaAl-NaSi interdiffusion in plagioclase. The results show that cooling from the peak thermal overprint at 1000–1050°C to 600°C are yielded within about 10–30 years as a result of hydrothermal circulation above melt lens during magma starvation. The estimated rapid hydrothermal cooling explains how the effective heat extraction from melt lens is achieved at fast-spreading mid-ocean ridges. PMID:25209311

  6. Three-dimensional seismic structure of the Dragon Flag oceanic core complex at the ultraslow spreading Southwest Indian Ridge (49°39'E)

    NASA Astrophysics Data System (ADS)

    Zhao, Minghui; Qiu, Xuelin; Li, Jiabiao; Sauter, Daniel; Ruan, Aiguo; Chen, John; Cannat, Mathilde; Singh, Satish; Zhang, Jiazheng; Wu, Zhenli; Niu, Xiongwei

    2013-10-01

    The Southwest Indian Ridge (SWIR) is an ultraslow spreading end-member of mid-ocean ridge system. We use air gun shooting data recorded by ocean bottom seismometers (OBS) and multibeam bathymetry to obtain a detailed three-dimensional (3-D) P wave tomographic model centered at 49°39'E near the active hydrothermal "Dragon Flag" vent. Results are presented in the form of a 3-D seismic traveltime inversion over the center and both ends of a ridge segment. We show that the crustal thickness, defined as the depth to the 7 km/s isovelocity contour, decreases systematically from the center (˜7.0-8.0 km) toward the segment ends (˜3.0-4.0 km). This variation is dominantly controlled by thickness changes in the lower crustal layer. We interpret this variation as due to focusing of the magmatic activity at the segment center. The across-axis velocity model documents a strong asymmetrical structure involving oceanic detachment faulting. A locally corrugated oceanic core complex (Dragon Flag OCC) on the southern ridge flank is characterized by high shallow crustal velocities and a strong vertical velocity gradient. We infer that this OCC may be predominantly made of gabbros. We suggest that detachment faulting is a prominent process of slow spreading oceanic crust accretion even in magmatically robust ridge sections. Hydrothermal activity at the Dragon Flag vents is located next to the detachment fault termination. We infer that the detachment fault system provides a pathway for hydrothermal convection.

  7. First Geochemical Evidences for Existence of Slow-Spreading Ridges in the Pacific Ocean

    NASA Astrophysics Data System (ADS)

    Krasnova, E. A.; Portnyagin, M.; Silantyev, S.; Werner, R.; Hoernle, K.

    2012-12-01

    Stalemate Fracture Zone (SFZ) is a 500 km long SE-NW trending transverse ridge between the northernmost Emperor Seamounts and the Aleutian Trench which originated by flexural uplift of Cretaceous (?) oceanic lithosphere along a transform fault at the Kula-Pacific plate boundary [1]. Sampling at the SFZ and the fossil Kula-Pacific Rift valley was carried out during the R/V SONNE cruise SO201 Leg 1b in July 2009. These rocks are thought to represent a complete section of oceanic lithosphere formed at the fossil Kula-Pacific Spreading Center. A broad spectrum of mantle peridotites ranging from spinel lherzolites to dunites were dredged at station DR37 at the northern bend of SFZ. Spinel in lherzolites has Mg#=0.65-0.68, NiO=0.26-0.34 wt%, Cr#=0.26-0.33, Fe3+#=0.021-0.030 and TiO2=0.04-0.09 wt%. Clinopyroxene has Mg#=91.7-92.4, Cr#=0.12-0.16, TiO2=0.06- 0.15 wt%, Na2O=0.19-0.41 wt%, NiO=0.06-0.09 wt% and is moderately depleted in HREE and extremely depleted in MREE and Zr (C1-normalized YbN=4.0- 5.6, [Sm/Yb]N=0.05-0.14, [Zr/Y]N=0.001-0.009) [2,3]. In terms of spinel and clinopyroxene Cr# and absolute concentrations of HREE, Ti and Na, these compositions are less depleted than those from the Hess Deep peridotites [4] formed at the fast spreading East-Pacific Rise. The SFZ peridotites are more similar to abyssal peridotites from slow-spreading ridges [e.g., 5]. Geochemical modeling suggests that the SFZ peridotites can be formed by 10-12% of near-fractional partial melting of depleted MORB mantle. We used the correlation between degree of partial mantle melting and full spreading rate [6] to estimate the spreading rate of 4-5 cm/year at the formation of the SFZ residual lherzolites (Fig.1). These results agree well with paleomagnetic data [1] suggesting asymmetric spreading at the ancient Kula-Pacific Center with the full rate of ~6 cm/year. Thus both geochemical and paleomagnetic data suggest the existence of slow-spreading ridges in the Pacific Basin during the Old

  8. New Frontiers in Arctic Exploration: Autonomous Location and Sampling of Hydrothermal Vents Under the Ice at Earth's Slowest Spreading Ridge (IPY Project 173)

    NASA Astrophysics Data System (ADS)

    Edmonds, H. N.; Reves-Sohn, R.; Singh, H.; Shank, T. M.; Humphris, S.; Seewald, J.; Akin, D.; Bach, W.; Nogi, Y.; Pedersen, R.

    2006-12-01

    As part of IPY project #173, we are planning an international expedition for 2007 to locate and study hydrothermal vents on the ultraslow-spreading Gakkel Ridge, at depths greater than 4000 m beneath the permanent ice cap. This effort necessitates the development of novel exploration technologies, because the Gakkel Ridge rift valley is inaccessible to traditional deep submergence tools. With funding from NASA, NSF, and the private sector we have developed two new autonomous underwater vehicles that will find and map hydrothermal plumes in the water column, trace the buoyant plume stem to the seafloor source, and then map, photograph, and collect samples from the vent sites. The Gakkel Ridge is a key target for hydrothermal exploration not only because of its spreading rate but also because its geographic and hydrographic isolation from other portions of the mid-ocean ridge system have important implications for novel endemic vent fauna. Our major scientific themes are the geological diversity and biogeography of hydrothermal vents on the Arctic mid-ocean ridge system. Our major technology theme is autonomous exploration and sample return with an explicit mandate to develop techniques and methods for eventual use in astrobiology missions to search for life under the ice covered oceans of Europa, a moon of Jupiter. In addition to the US-led Gakkel Ridge expedition, a Norway-led expedition will target sites in seasonally ice-free water over the Mohns Ridge. The results of these two expeditions will be combined to reveal systematic patterns regarding biogeography (through both community-level and genetic-level investigations) of vent-endemic fauna, to study the differences between basalt vs. peridotite hosted vent fields, and to improve our understanding of hydrothermal circulation at ultra- slow spreading plate boundaries where amagmatic extension and long-lived faulting predominate. The expeditions will provide educational and outreach activities through the award

  9. Evidence from gabbro of the Troodos ophiolite for lateral magma transport along a slow-spreading mid-ocean ridge.

    PubMed

    Abelson, M; Baer, G; Agnon, A

    2001-01-01

    The lateral flow of magma and ductile deformation of the lower crust along oceanic spreading axes has been thought to play a significant role in suppressing both mid-ocean ridge segmentation and variations in crustal thickness. Direct investigation of such flow patterns is hampered by the kilometres of water that cover the oceanic crust, but such studies can be made on ophiolites (fragments of oceanic crust accreted to a continent). In the Oman ophiolite, small-scale radial patterns of flow have been mapped along what is thought to be the relict of a fast-spreading mid-ocean ridge. Here we present evidence for broad-scale along-axis flow that has been frozen into the gabbro of the Troodos ophiolite in Cyprus (thought to be representative of a slow-spreading ridge axis). The gabbro suite of Troodos spans nearly 20 km of a segment of a fossil spreading axis, near a ridge-transform intersection. We mapped the pattern of magma flow by analysing the rocks' magnetic fabric at 20 sites widely distributed in the gabbro suite, and by examining the petrographic fabric at 9 sites. We infer an along-axis magma flow for much of the gabbro suite, which indicates that redistribution of melt occurred towards the segment edge in a large depth range of the oceanic crust. Our results support the magma plumbing structure that has been inferred indirectly from a seismic tomography experiment on the slow-spreading Mid-Atlantic Ridge. PMID:11343114

  10. Seismic anisotropy indicates ridge-parallel asthenospheric flow beneath the Eastern Lau Spreading Center

    NASA Astrophysics Data System (ADS)

    Menke, William; Zha, Yang; Webb, Spahr C.; Blackman, Donna K.

    2015-02-01

    Seismic anisotropy beneath the Eastern Lau Spreading Center (ELSC) is investigated using both Rayleigh waves and shear waves, using data from the 2009-2010 ELSC ocean bottom seismograph experiment. Phase velocities of Rayleigh waves determined by ambient noise cross correlation are inverted for azimuthally anisotropic phase velocity maps. Splitting of S waves from five intermediate and deep focus earthquakes was determined by waveform analysis. Taken together, Rayleigh wave and S wave data indicate that significant (~2%) anisotropy extends to at least 300 km depth. Both data sets indicate a fast direction aligned within a few degrees of the N10°E striking ELSC and somewhat oblique to the N25°E strike of the neighboring volcanic arc. We therefore describe the fast direction as spreading perpendicular, not convergence perpendicular and interpret it as due to ridge-parallel flow of the asthenosphere. However, the region arcward (east) of the ELSC has the stronger anisotropy, suggesting that the strongest flow gradients may occur in the region between the spreading center and the arc, in contrast to being centered beneath the ELSC. Fluids released from the underlying plate may produce anisotropic hydrous materials, but more importantly lower the viscosity, thus enhancing along-strike flow. Both could contribute to an along-strike fast direction signature. Seafloor spreading diminishes south of the seismic array, ceasing altogether south of latitude 25°S (500 km south of the array center), a region dominated by much slower tectonic extension, suggesting that asthenosphere is inflowing from the north to accommodate the increase in asthenospheric volume associated with the seafloor spreading.

  11. Effects of Fertile Mantle Compositional Variation and Spreading Rate Variation on the Working of Global Ocean Ridges

    NASA Astrophysics Data System (ADS)

    Niu, Y.; O'Hara, M. J.

    2014-12-01

    Mantle temperature variation, plate spreading rate variation and mantle compositional variation have been considered to be the three fundamental variables that govern the working of global ocean ridges [1]. An analysis demonstrates that mantle compositional variation exerts the primary control on ocean ridge processes; it determines (1) variation in both composition and mode of mantle mineralogy, (2) variation of mantle density, (3) variation of ridge axial depth, (4) source-inherited MORB compositional variation, (4) density-controlled variation in the amplitude of mantle upwelling, (5) apparent variation in the extent of melting, and (6) the correlated variation of MORB chemistry with ridge axial depth [2]. The above interpretations are reinforced by the updated MORB database [3]. The new database also confirms spreading rate control on the extent of melting as shown previously [4]. Mantle temperature variation could play a part, but its overstated role [3,5] results from a basic error (1) in treating ridge axial depth variation as evidence of mantle temperature variation by ignoring the intrinsic control of mantle composition, (2) in treating "mantle plume" influenced ridges (e.g., Iceland) as normal ridges of plate spreading origin, and (3) in treating low Vs at greater depths (> 300 km vs. < 200 km beneath ridges) beneath these "mantle plume" influenced ridges as evidence for hot ridge mantle. In order to understand the working of global ocean ridges, we must avoid plume-influenced ridges (e.g., in the vicinity of Iceland) and remove/average out data from such ridges. As a result, the correlations (e.g., between ridge axial depth, mantle low Vs anomaly, and some geochemical parameters) required for the interpretation of mantle temperature control all disappear. There is thus no evidence for large mantle temperature variation away from ridges influenced by "mantle plumes". References: [1] Niu et al., 2001, Earth Planet Sci. Lett., 186, 383-399; [2] Niu & O

  12. Spreading-rate dependence of melt extraction at mid-ocean ridges from mantle seismic refraction data.

    PubMed

    Lizarralde, Daniel; Gaherty, James B; Collins, John A; Hirth, Greg; Kim, Sangmyung D

    2004-12-01

    A variety of observations indicate that mid-ocean ridges produce less crust at spreading rates below 20 mm yr(-1) (refs 1-3), reflecting changes in fundamental ridge processes with decreasing spreading rate. The nature of these changes, however, remains uncertain, with end-member explanations being decreasing shallow melting or incomplete melt extraction, each due to the influence of a thicker thermal lid. Here we present results of a seismic refraction experiment designed to study mid-ocean ridge processes by imaging residual mantle structure. Our results reveal an abrupt lateral change in bulk mantle seismic properties associated with a change from slow to ultraslow palaeo-spreading rate. Changes in mantle velocity gradient, basement topography and crustal thickness all correlate with this spreading-rate change. These observations can be explained by variations in melt extraction at the ridge, with a gabbroic phase preferentially retained in the mantle at slower spreading rates. The estimated volume of retained melt balances the approximately 1.5-km difference in crustal thickness, suggesting that changes in spreading rate affect melt-extraction processes rather than total melting. PMID:15592410

  13. How the growth and freeboard of continents may relate to geometric and kinematic parameters of mid-ocean spreading ridges

    USGS Publications Warehouse

    Howell, D.G.

    1989-01-01

    If the volume of continents has been growing since 4 Ga then the area of the ocean basins must have been shrinking. Therefore, by inferring a constant continental freeboard, in addition to constant continental crustal thicknesses and seawater volume, it is possible to calculate the necessary combinations of increased ridge lengths and spreading rates required to displace the seawater in the larger oceans of the past in order to maintain the constant freeboard. A reasonable choice from the various possibilities is that at 4 Ga ago, the ridge length and spreading rates were ca. 2.5 times greater than the averages of these parameters during the past 200 Ma. By 2.5 Ga ago the ridge length and spreading rate decreased to about 1.8 times the recent average and by 1 Ga ago these features became reduced to approximately 1.4 times recent averages. ?? 1989.

  14. Detection and Response to a Seafloor Spreading Episode on the Central Gorda Ridge, April 2001

    NASA Astrophysics Data System (ADS)

    Fox, C. G.; Cowen, J. P.; Dziak, R. P.; Baker, E. T.; Embley, R. W.; Chadwick, W. W.; Lupton, J. E.; Resing, J. A.; Hammond, S. R.

    2001-12-01

    n April 3, 2001, volcanic seismicity was detected from the Jackson Segment of the Gorda Ridge (near 42o 9'N; 127o 3'W) by the NOAA/PMEL T-phase Real-Time Monitoring System, which monitors data collected from the U. S. Navy SOSUS in the North Pacific. The character of the hydroacoustic signals was nearly identical to that observed from three earlier events that occurred along northeast Pacific spreading centers and were later confirmed to have produced hydrothermal megaplumes and seafloor eruptions. A field response effort was mounted on R/V New Horizon and arrived on the site within 8 days of the initiation of the activity. Unlike earlier response efforts, and despite the fact that this event was well located and field CTD casts were of adequate density and coverage, no evidence for a hydrothermal plume was found. A particle signal that was observed arose from a bottom nepheloid layer of resuspended sediments of a non-hydrothermal origin. Also, pH and 3He profiles were similar to background levels, indicating no input of volcanic CO2 or helium into the water column. In addition, two short camera tows were collected on the axis of the segment near the center of the earthquake epicenters, but no evidence of new lavas or seafloor venting was found. Detailed post-event analyses of the hydroacoustic data indicate that most of the characteristics of earlier plume-producing, extrusive events were present in this episode, including a vigorous earthquake swarm (over 50 events per hour during the first day), lasting nearly ten days, with no initial main shock and a continuous background level of volcanic tremor. Detailed analyses of the t-wave rise times are also consistent with very shallow source locations. The primary difference in this event is that no significant horizontal migration of epicenters, characteristic of lateral dike injection, was recorded. The acoustic results indicate a vertical dike injection that although shallow, apparently did not penetrate the

  15. Degassing of carbon dioxide from basaltic magma at spreading centers: II. mid-oceanic ridge basalts

    NASA Astrophysics Data System (ADS)

    Gerlach, Terrence M.

    1989-11-01

    This study examines the hypothesis that a significant fraction of the CO 2 in basalt supplied to mid-oceanic spreading centers escapes by degassing from magma chambers of the oceanic crust. The approach employs mass balance calculations, stepped-heating data for carbon in sea-floor mid-oceanic ridge basalts (MORBs), and the CO 2 content determined in part I for transitional basalt supplied to the Erta'Ale spreading center in the Afar depression. It is also shown as part of the analysis that carbon data acquired for sea-floor MORBs by single-heating techniques give systematically high values that are difficult to reconcile with solubility data for CO 2 in MORB liquid and the depths of subridge magma chambers. The results confirm the hypothesis. A conservative estimate of the average loss of CO 2 during degassing from subridge magma chambers is between 30 and 65% of the CO 2 initially present in the magma. The conservative estimate for CO 2 degassing at depth is between two and seven times greater than the maximum amount of volcanic CO 2 degassing from MORB. CO 2 degassing from subridge magma chambers is sufficient to supply much (perhaps most) of the mantle carbon discharged from the global mid-oceanic spreading system to the oceans. CO 2 degassing from subridge magma chambers has several important implications. It may be more important than hydrothermal stripping in supplying mantle carbon to mid-oceanic hydrothermal vents. It is an effective mechanism for removing most of the heavier rare gases (Ar, Kr, Xe, and Rn) and much of the He and Ne in magma supplied to mid-oceanic spreading centers and for transporting them and possibly other volatiles from subridge magma chambers to the ocean-atmosphere system. It should also be an effective mechanism for fractionating carbon isotopes in MORB.

  16. Exploring the plutonic crust at a fast-spreading ridge:new drilling at Hess Deep

    SciTech Connect

    Gillis, Kathryn M.; Snow, Jonathan E.; Klaus, Adam; Guerin, Gilles; Abe, Natsue; Akizawa, Norikatsu; Ceuleneer, Georges; Cheadle, Michael J.; Adriao, Alden de Brito; Faak, Kathrin; Falloon, Trevor J.; Friedman, Sarah A.; Godard, Marguerite M.; Harigane, Yumiko; Horst, Andrew J.; Hoshide, Takashi; Ildefonse, Benoit; Jean, Marlon M.; John, Barbara E.; Koepke, Juergen H.; Machi, Sumiaki; Maeda, Jinichiro; Marks, Naomi E.; McCaig, Andrew M.; Meyer, Romain; Morris, Antony; Nozaka, Toshio; Python, Marie; Saha, Abhishek; Wintsch, Robert P.

    2013-02-28

    Integrated Ocean Drilling Program (IODP) Hess Deep Expedition 345 was designed to sample lower crustal primitive gabbroic rocks that formed at the fast-spreading East Pacific Rise (EPR) in order to test models of magmatic accretion and the intensity of hydrothermal cooling at depth. The Hess Deep Rift was selected to exploit tectonic exposures of young EPR plutonic crust, building upon results from ODP Leg 147 as well as more recent submersible, remotely operated vehicle, and near-bottom surveys. The primary goal was to acquire the observations required to test end-member crustal accretion models that were in large part based on relationships from ophiolites, in combination with mid-ocean ridge geophysical studies. This goal was achieved with the recovery of primitive layered olivine gabbros and troctolites with many unexpected mineralogical and textural relationships, such as the abundance of orthopyroxene and the preservation of delicate skeletal olivine textures.

  17. Numerical 3D models support two distinct hydrothermal circulation systems at fast spreading ridges

    NASA Astrophysics Data System (ADS)

    Hasenclever, Jörg; Theissen-Krah, Sonja; Rüpke, Lars

    2013-04-01

    We present 3D numerical calculations of hydrothermal fluid flow at fast spreading ridges. The setup of the 3D models is based our previous 2D studies, in which we have coupled numerical models for crustal accretion and hydrothermal fluid flow. One result of these calculations is a crustal permeability field that leads to a thermal structure in the crust that matches seismic tomography data of the East Pacific Rise (EPR). The 1000°C isotherm obtained from the 2D results is now used as the lower boundary of the 3D model domain, while the upper boundary is a smoothed bathymetry of the EPR. The same permeability field as in the 2D models is used, with the highest permeability at the ridge axis and a decrease with both depth and distance to the ridge. Permeability is also reduced linearly between 600 and 1000°C. Using a newly developed parallel finite element code written in Matlab that solves for thermal evolution, fluid pressure and Darcy flow, we simulate the flow patterns of hydrothermal circulation in a segment of 5000m along-axis, 10000m across-axis and up to 5000m depth. We observe two distinct hydrothermal circulation systems: An on-axis system forming a series of vents with a spacing ranging from 100 to 500m that is recharged by nearby (100-200m) downflows on both sides of the ridge axis. Simultaneously a second system with much broader extensions both laterally and vertically exists off-axis. It is recharged by fluids intruding between 1500m to 5000m off-axis and sampling both upper and lower crust. These fluids are channeled in the deepest and hottest regions with high permeability and migrate up-slope following the 600°C isotherm until reaching the edge of the melt lens. Depending on the width of the melt lens these off-axis fluids either merge with the on-axis hydrothermal system or form separate vents. We observe separate off-axis vent fields if the magma lens half-width exceeds 1000m and confluence of both systems for half-widths smaller than 500m. For

  18. Effusive and explosive volcanism on the ultraslow-spreading Gakkel Ridge, 85°E

    NASA Astrophysics Data System (ADS)

    Pontbriand, Claire W.; Soule, S. Adam; Sohn, Robert A.; Humphris, Susan E.; Kunz, Clayton; Singh, Hanumant; Nakamura, Ko-Ichi; Jakobsson, Martin; Shank, Timothy

    2012-10-01

    We use high-definition seafloor digital imagery and multibeam bathymetric data acquired during the 2007 Arctic Gakkel Vents Expedition (AGAVE) to evaluate the volcanic characteristics of the 85°E segment of the ultraslow spreading Gakkel Ridge (9 mm yr-1full rate). Our seafloor imagery reveals that the axial valley is covered by numerous, small-volume (order ˜1000 m3) lava flows displaying a range of ages and morphologies as well as unconsolidated volcaniclastic deposits with thicknesses up to 10 cm. The valley floor contains two prominent volcanic lineaments made up of axis-parallel ridges and small, cratered volcanic cones. The lava flows appear to have erupted from a number of distinct source vents within the ˜12-15 km-wide axial valley. Only a few of these flows are fresh enough to have potentially erupted during the 1999 seismic swarm at this site, and these are associated with the Oden and Loke volcanic cones. We model the widespread volcaniclastic deposits we observed on the seafloor as having been generated by the explosive discharge of CO2 that accumulated in (possibly deep) crustal melt reservoirs. The energy released during explosive discharge, combined with the buoyant rise of hot fluid, lofted fragmented clasts of rapidly cooling magma into the water column, and they subsequently settled onto the seafloor as fall deposits surrounding the source vent.

  19. Potassium-Argon Ages and Spreading Rates on the Mid-Atlantic Ridge at 45{degrees} North.

    PubMed

    Aumento, F; Wanless, R K; Stevens, R D

    1968-09-27

    Potassium-argon dates obtained from extrusives collected on a traverse across the Mid-Atlantic Ridge at 45 degrees N are consistent with the hypothesis of ocean-floor spreading. The dates suggest a spreading rate in the range of 2.6 to 3.2 centimeters per year near the axis of the ridge; the rate agrees with that computed from fission-track dating of basalt glasses. Additional data for a basalt collected 62 kilometers west of the axis gives a spreading rate of 0.8 centimeter per year, which is similar to the rate inferred from magnetic anomaly patterns in the area. Reasons for the difference in calculated spreading rates are discussed. PMID:17831344

  20. Crustal accretion at fast spreading ridges and implications for hydrothermal circulation

    NASA Astrophysics Data System (ADS)

    Theissen-Krah, S.; Rupke, L.; Hasenclever, J.

    2015-12-01

    Oceanic crust is continuously created at mid-ocean ridges, but the location of lower crust crystallization continues to be debated since the proposal of the gabbro glacier and many sills end-member models. Geophysical and geochemical studies find evidence for either of the models. The crust is cooled by a combination of heat diffusion and advection, and hydrothermal circulation is thought to play a key role in distinguishing between both models. We use our numerical model for joint modeling of crustal accretion and hydrothermal circulation1 to test different accretion and hydrothermal cooling scenarios. The results match the seismic and structural observations from the East Pacific Rise2 and the Oman Ophiolite3, with a shallow melt lens at the correct location overlaying a narrow volume of partially molten rocks. Our results show that no more than 25-50% of the lower crust crystallizes in situ and that deep circulation is likely to occur at fast and intermediate spreading ridges. The occurrence of deep hydrothermal cooling however does not rule out that a major portion of the lower crust is formed in the shallow melt lens; our simulations rather suggest that it is necessary independent of where in the lower crust crystallization takes place. 1 Theissen-Krah, S., Iyer, K., Rupke, L. H. & Morgan, J. P. Coupled mechanical and hydrothermal modeling of crustal accretion at intermediate to fast spreading ridges. Earth and Planetary Science Letters 311, 275-286, doi:10.1016/j.epsl.2011.09.018 (2011). 2 Dunn, R. A., Toomey, D. R. & Solomon, S. C. Three-dimensional seismic structure and physical properties of the crust and shallow mantle beneath the East Pacific Rise at 9 degrees 30'N. Journal of Geophysical Research-Solid Earth 105, 23537-23555 (2000). 3 Nicolas, A. & Boudier, F. Structural contribution from the Oman ophiolite to processes of crustal accretion at the East Pacific Rise. Terra Nova 27, 77-96, doi:10.1111/ter.12137 (2015).

  1. Analysis of local spread of equine influenza in the Park Ridge region of Queensland.

    PubMed

    Davis, J; Garner, M G; East, I J

    2009-03-01

    In 2007, an incursion of equine influenza (EI) occurred in Australia. Accurate maps of property boundaries were used to examine the pattern and mechanism of local spread of EI. This study focussed on a cluster of infected premises (IPs) at Park Ridge, a peri-urban suburb 26 km south of Brisbane, Queensland. The cluster recorded 437 IPs and 81% of these were not contiguous to a previously IP. The mean distance from each new IP to the closest previous IP was 0.85 +/- 1.50 km with a range of 0.01-12.94 km. Eighty-two percent of new IPs were within 1 km of a previous IP. The spatial mean for each week's new IPs showed a consistent trend of movement from east to west throughout the epizootic consistent with the predominant wind patterns. The findings were consistent with the conclusion that EI will routinely spread over 1-2 km via wind-borne aerosol. PMID:19200296

  2. Morphology and genesis of slow-spreading ridges-seabed scattering and seismic imaging within the oceanic crust

    NASA Astrophysics Data System (ADS)

    Peirce, Christine; Sinha, Martin; Topping, Simon; Gill, Christopher

    2007-01-01

    A grid of 32 across-axis and five axis-parallel multichannel seismic (MCS) reflection profiles were acquired at an axial volcanic ridge (AVR) segment at 57° 45'N, 32° 35'W on the slow-spreading Reykjanes Ridge, Mid-Atlantic Ridge, to determine the along-axis variation and geometry of the axial magmatic system and to investigate the relationship between magma chamber structure, the along-axis continuity and segmentation of melt supply to the crust, the development of faulting and the thickness of oceanic layer 2A. Seismic reflection profiles acquired at mid-ocean ridges are prone to being swamped by high amplitude seabed scattered noise which can either mask or be mistaken for intracrustal reflection events. In this paper, we present the results of two approaches to this problem which simulate seabed scatter and which can either be used to remove or simply predict events within processed MCS profiles. The 37 MCS profiles show clear intracrustal seismic events which are related to the structure of oceanic layer 2, to the axial magmatic system and to the faults which dismember each AVR as it ages through its tectono-magmatic life cycle and which form the median valley walls. The layer 2A event can be mapped around the entirety of the survey area between 0.1 and 0.5 s two-way traveltime below the seabed, being thickest at AVR centres, and thinning both off-axis and along-axis towards AVR tips. Both AVR-parallel and ridge-parallel trends are observed, with the pattern of on-axis layer 2A thickness variation preserved beneath relict AVRs which are rafted off-axis largely intact. Each active AVR is underlain by a mid-crustal melt lens reflection extending almost along its entire length. Similar reflection events are observed beneath the offset basins between adjacent AVRs. These are interpreted as new AVRs at the start of their life cycle, developing centrally within the median valley. The east-west spacings of relict AVRs and offset basins is ~5-7 km, corresponding to

  3. Hydrothermal Exploration of the Mid-Cayman Spreading Center: Isolated Evolution on Earth’s Deepest Mid-Ocean Ridge?

    NASA Astrophysics Data System (ADS)

    German, C. R.; Bowen, A.; Coleman, M. L.; Huber, J. A.; Seewald, J.; van Dover, C.; Whitcomb, L. L.; Yoerger, D.; Connelly, D.; Honig, D. L.; Jakuba, M.; Kinsey, J. C.; McDermott, J.; Nakamura, K.; Sands, C.; Smith, J.; Sylva, S.

    2009-12-01

    We report the first systematic exploration for and characterization of hydrothermal vents and vent ecosystems on the short (~110 km), deep (> 5000 m), ultra-slow-spreading (<20 mm yr-1) Mid-Cayman Rise in the Caribbean Sea. This work was carried out aboard the RV Cape Hatteras in October-November 2009 as part of the ChEss Project of the Census of Marine Life, funded through NASA’s ASTEP program and represents the first scientific field program funded to use WHOI’s new hybrid deep submergence vehicle, Nereus, first in AUV mode then in ROV mode. Prior to this work, evidence for hydrothermal venting had been found on every active spreading center investigated, including the comparably ultra-slow spreading ridges in the SW Indian Ocean and in the Arctic (Mohns, Knipovich & Gakkel Ridges). The organisms colonizing vents are renowned for their endemicity, their adaptations to the extreme chemical and physical conditions encountered and for differences in species level from one ocean basin to another. Consequently, the identification of any organisms colonizing vents of the Mid-Cayman Spreading Center offers a critical opportunity to build upon our understanding of the dispersion of vent species and the potential role of the rise of the Isthmus of Panama (dating from ~5 Ma) as a vicariant event leading to the evolutionary divergence of Atlantic and Pacific vent faunas. Further, the MCSC is so deep that any vents present may occur at depths greater than all previously known vent systems, extending the known limits to life on our planet in terms of pressure, temperature, and vent-fluid chemistry. Finally, hydrothermal circulation through ultramafic rocks can generate abiotic synthesis of organic matter: an analog for the prebiotic basis for the origin of life on early Earth and Mars. In future years of this 4-year study, therefore, we will also aim to assess the relative importance of abiotic organic synthesis versus recycling of bio-organic material and/or chemical

  4. Microbial community diversity in seafloor basalt from the Arctic spreading ridges.

    PubMed

    Lysnes, Kristine; Thorseth, Ingunn H; Steinsbu, Bjørn Olav; Øvreås, Lise; Torsvik, Terje; Pedersen, Rolf B

    2004-11-01

    Microbial communities inhabiting recent (< or =1 million years old; Ma) seafloor basalts from the Arctic spreading ridges were analyzed using traditional enrichment culturing methods in combination with culture-independent molecular phylogenetic techniques. Fragments of 16S rDNA were amplified from the basalt samples by polymerase chain reaction, and fingerprints of the bacterial and archaeal communities were generated using denaturing gradient gel electrophoresis. This analysis indicates a substantial degree of complexity in the samples studied, showing 20-40 dominating bands per profile for the bacterial assemblages. For the archaeal assemblages, a much lower number of bands (6-12) were detected. The phylogenetic affiliations of the predominant electrophoretic bands were inferred by performing a comparative 16S rRNA gene sequence analysis. Sequences obtained from basalts affiliated with eight main phylogenetic groups of Bacteria, but were limited to only one group of the Archaea. The most frequently retrieved bacterial sequences affiliated with the gamma-proteobacteria, alpha-proteobacteria, Chloroflexi, Firmicutes, and Actinobacteria. The archaeal sequences were restricted to the marine Group 1: Crenarchaeota. Our results indicate that the basalt harbors a distinctive microbial community, as the majority of the sequences differed from those retrieved from the surrounding seawater as well as from sequences previously reported from seawater and deep-sea sediments. Most of the sequences did not match precisely any sequences in the database, indicating that the indigenous Arctic ridge basalt microbial community is yet uncharacterized. Results from enrichment cultures showed that autolithotrophic methanogens and iron reducing bacteria were present in the seafloor basalts. We suggest that microbial catalyzed cycling of iron may be important in low-temperature alteration of ocean crust basalt. The phylogenetic and physiological diversity of the seafloor basalt

  5. Spread of epileptic activity in human brain

    NASA Astrophysics Data System (ADS)

    Milton, John

    1997-03-01

    For many patients with medically refractory epilepsy surgical resection of the site of seizure onset (epileptic focus) offers the best hope for cure. Determination of the nature of seizure propagation should lead to improved methods for locating the epileptic focus (and hence reduce patient morbidity) and possibly to new treatment modalities directed at blocking seizure spread. Theoretical studies of neural networks emphasize the role of traveling waves for the propagation of activity. However, the nature of seizure propagation in human brain remains poorly characterized. The spread of epileptic activity in patients undergoing presurgical evaluation for epilepsy surgery was measured by placing subdural grids of electrodes (interelectrode spacings of 3-10 mm) over the frontal and temporal lobes. The exact location of each electrode relative to the surface of the brain was determined using 3--D MRI imaging techniques. Thus it is possible to monitor the spread of epileptic activity in both space and time. The observations are discussed in light of models for seizure propagation.

  6. Fault structure and detailed evolution of a slow spreading ridge segment: the Mid-Atlantic Ridge at 29°N

    NASA Astrophysics Data System (ADS)

    Searle, R. C.; Cowie, P. A.; Mitchell, N. C.; Allerton, S.; MacLeod, C. J.; Escartin, J.; Russell, S. M.; Slootweg, P. A.; Tanaka, T.

    1998-01-01

    We present preliminary results of a detailed near-bottom study of the morphology and tectonics of the 29°N "Broken Spur" segment on the slow spreading Mid-Atlantic Ridge, using principally the TOBI deep-towed instrument. The survey covered two-thirds of the segment length, including all of its southern non-transform boundary, and extended off-axis of 40 km (3.3 Ma) on either side. We obtained nearly complete near-bottom sidescan sonar coverage and deep-towed three-component magnetic observations along 2-km-spaced E-W tracks. Sidescan data reveal new details of fault structure and evolution. Faults grow by along-axis linkage. In the inside corner, they also link in the axis-normal direction by curving to meet the next outer (older) fault; this leads to wider-spaced faults compared to segment centre or outside corner. Outward facing faults exist but are rare. The non-transform offset is characterised by faults that are highly oblique, not parallel, to the spreading direction, and show cross-cutting relations with ridge-parallel faults to the north, suggesting along-axis migration of the offset. Almost all volcanic activity occurs within 5 km of the axis. Most fault growth is complete within 15 km of the axis (1.2 Ma), though large scarps continue to be degraded by mass-wasting beyond there. Crustal magnetisation is strongly three-dimensional. The current neovolcanic zone is slightly oblique to earlier reversal boundaries, and its magnetisation rises to a maximum of 30 A m -1 near its southern tip. The central magnetisation high tapers southwards and is asymmetric, with a sharp western but gradual eastern boundary. We infer a highly asymmetric accretion of layer 2 near the segment end. Older magnetic anomalies are kinked and sometimes missing. We interpret these observations as evidence of a rapid, 18 km southward migration of the segment boundary during the past 1.8 Ma, and present a series of reconstructions illustrating this tectonic history.

  7. Fault structure and detailed evolution of a slow spreading ridge segment: the Mid-Atlantic Ridge at 29°N

    NASA Astrophysics Data System (ADS)

    Tanaka, T.; Slootweg, P. A.; Russell, S. M.; Escartin, J.; MacLeod, C. J.; Allerton, S.; Mitchell, N. C.; Cowie, P. A.; Searle, R. C.

    1998-01-01

    We present preliminary results of a detailed near-bottom study of the morphology and tectonics of the 29°N ``Broken Spur'' segment on the slow spreading Mid-Atlantic Ridge, using principally the TOBI deep-towed instrument. The survey covered two-thirds of the segment length, including all of its southern non-transform boundary, and extended off-axis of 40 km (3.3 Ma) on either side. We obtained nearly complete near-bottom sidescan sonar coverage and deep-towed three-component magnetic observations along 2-km-spaced E-W tracks. Sidescan data reveal new details of fault structure and evolution. Faults grow by along-axis linkage. In the inside corner, they also link in the axis-normal direction by curving to meet the next outer (older) fault; this leads to wider-spaced faults compared to segment centre or outside corner. Outward facing faults exist but are rare. The non-transform offset is characterised by faults that are highly oblique, not parallel, to the spreading direction, and show cross-cutting relations with ridge-parallel faults to the north, suggesting along-axis migration of the offset. Almost all volcanic activity occurs within 5 km of the axis. Most fault growth is complete within 15 km of the axis (1.2 Ma), though large scarps continue to be degraded by mass-wasting beyond there. Crustal magnetisation is strongly three-dimensional. The current neovolcanic zone is slightly oblique to earlier reversal boundaries, and its magnetisation rises to a maximum of 30 A m-1 near its southern tip. The central magnetisation high tapers southwards and is asymmetric, with a sharp western but gradual eastern boundary. We infer a highly asymmetric accretion of layer 2 near the segment end. Older magnetic anomalies are kinked and sometimes missing. We interpret these observations as evidence of a rapid, 18 km southward migration of the segment boundary during the past 1.8 Ma, and present a series of reconstructions illustrating this tectonic history.

  8. Controls of Plume Dispersal at the Slow Spreading Mid-Atlantic Ridge

    NASA Astrophysics Data System (ADS)

    Walter, M.; Mertens, C.; Koehler, J.; Sueltenfuss, J.; Rhein, M.; Keir, R. S.; Schmale, O.; Schneider v. Deimling, J.; German, C. R.; Yoerger, D. R.; Baker, E. T.

    2011-12-01

    The slow-spreading Mid-Atlantic Ridges hosts a multitude of different types of hydrothermal systems. Here, we compare the fluxes and the plume dispersal at three high temperature sites located in very diverse settings at comparable depths (~3000m): The recently discovered sites Turtle Pits, and Nibelungen on the southern MAR, and the Logatchev field in the North Atlantic. Plume mapping for these sites on cruises between 2004 and 2009 consisted of CTD Towyo-, Yoyo,- and station work, including velocity profiling, as well as water sampling for analysis of trace gases (CH4, H2, 3He/4He) and metals; temperature measurements and fluid sampling at the vent sites were carried out with an ROV. The aim of this work is to gain a better understanding of how the setting of a vent site affects the dispersal of the particle plume, and what means can be used to infer possible locations of vent sites based on the hydrographic properties and plume observations, using high resolution bathymetric mapping and hydrographic information. The ultramafic-hosted Nibelungen site (8°18'S) consists of a single active smoking crater, along with several extinct smokers, which is located off-axis south of a non-transform offset. The setting is characterized by rugged topography, favorable for the generation of internal tides, internal wave breaking, and vertical mixing. Elevated mixing with turbulent diffusivities Kρ up to 0.1 m2 s-1, 3 to 4 orders of magnitude higher than open ocean values, was observed close to the vent site. The mixing as well as the flow field exhibited a strong tidal cycle; the plume dispersal is thus dominated by the fast and intermittent vertical exchange and characterized by small scale spatial and temporal variability. The Turtle Pits vent fields (4°48'S) are located on a sill in a north-south orientated rift valley. The site consists of three (known) high temperature fields: Turtle Pits, Comfortless Cove, and Red Lion. The particle plume is confined to the rift

  9. A Review on Forearc Ophiolite Obduction, Adakite-Like Generation, and Slab Window Development at the Chile Triple Junction Area: Uniformitarian Framework for Spreading-Ridge Subduction

    NASA Astrophysics Data System (ADS)

    Bourgois, Jacques; Lagabrielle, Yves; Martin, Hervé; Dyment, Jérôme; Frutos, Jose; Cisternas, Maria Eugenia

    2016-05-01

    This paper aggregates the main basic data acquired along the Chile Triple Junction (CTJ) area (45°-48°S), where an active spreading center is presently subducting beneath the Andean continental margin. Updated sea-floor kinematics associated with a comprehensive review of geologic, geochemical, and geophysical data provide new constraints on the geodynamics of this puzzling area. We discuss: (1) the emplacement mode for the Pleistocene Taitao Ridge and the Pliocene Taitao Peninsula ophiolite bodies. (2) The occurrence of these ophiolitic complexes in association with five adakite-like plutonic and volcanic centers of similar ages at the same restricted locations. (3) The inferences from the co-occurrence of these sub-coeval rocks originating from the same subducting oceanic lithosphere evolving through drastically different temperature-pressure (P-T) path: low-grade greenschist facies overprint and amphibolite-eclogite transition, respectively. (4) The evidences that document ridge-jump events and associated microplate individualization during subduction of the SCR1 and SCR-1 segments: the Chonos and Cabo Elena microplates, respectively. The ridge-jump process associated with the occurrence of several closely spaced transform faults entering subduction is controlling slab fragmentation, ophiolite emplacement, and adakite-like production and location in the CTJ area. Kinematic inconsistencies in the development of the Patagonia slab window document an 11- km westward jump for the SCR-1 spreading segment at ~6.5-to-6.8 Ma. The SCR-1 spreading center is relocated beneath the North Patagonia Icefield (NPI). We argue that the deep-seated difference in the dynamically sustained origin of the high reliefs of the North and South Patagonia Icefield (NPI and SPI) is asthenospheric convection and slab melting, respectively. The Chile Triple Junction area provides the basic constraints to define the basic signatures for spreading-ridge subduction beneath an Andean

  10. Subalkaline andesite from Valu Fa Ridge, a back-arc spreading center in southern Lau Basin: petrogenesis, comparative chemistry, and tectonic implications

    USGS Publications Warehouse

    Vallier, T.L.; Jenner, G.A.; Frey, F.A.; Gill, J.B.; Davis, A.S.; Volpe, A.M.; Hawkins, J.W.; Morris, J.D.; Cawood, Peter A.; Morton, J.L.; Scholl, D. W.; Rautenschlein, M.; White, W.M.; Williams, Ross W.; Stevenson, A.J.; White, L.D.

    1991-01-01

    Tholeiitic andesite was dredged from two sites on Valu Fa Ridge (VFR), a back-arc spreading center in Lau Basin. Valu Fa Ridge, at least 200 km long, is located 40-50 km west of the active Tofua Volcanic Arc (TVA) axis and lies about 150 km above the subducted oceanic plate. One or more magma chambers, traced discontinuously for about 100 km along the ridge axis, lie 3-4 km beneath the ridge. The mostly aphyric and glassy lavas had high volatile contents, as shown by the abundance and large sizes of vesicles. An extensive fractionation history is inferred from the high SiO2 contents and FeO* MgO ratios. Chemical data show that the VFR lavas have both volcanic arc and back-arc basin affinities. The volcanic arc characteristics are: (1) relatively high abundances of most alkali and alkaline earth elements; (2) low abundances of high field strength elements Nb and Ta; (3) high U/Th ratios; (4) similar radiogenic isotope ratios in VFR and TVA lavas, in particular the enrichment of 87Sr 86Sr relative to 206Pb 204Pb; (5) high 238U 230Th, 230Th 232Th, and 226Ra 230Th activity ratios; and (6) high ratios of Rb/Cs, Ba/Nb, and Ba/La. Other chemical characteristics suggest that the VFR lavas are related to MORB-type back-arc basin lavas. For example, VFR lavas have (1) lower 87Sr 86Sr ratios and higher 143Nd 144Nd ratios than most lavas from the TVA, except samples from Ata Island, and are similar to many Lau Basin lavas; (2) lower Sr/REE, Rb/Zr, and Ba/Zr ratios than in arc lavas; and (3) higher Ti, Fe, and V, and higher Ti/V ratios than arc lavas generally and TVA lavas specifically. Most characteristics of VFR lavas can be explained by mixing depleted mantle with either small amounts of sediment and fluids from the subducting slab and/or an older fragment of volcanic arc lithosphere. The eruption of subalkaline andesite with some arc affinities along a back-arc spreading ridge is not unique. Collision of the Louisville and Tonga ridges probably activated back-arc extension

  11. Dynamics of the Axial Melt Lens/Dike transition at fast spreading ridges: assimilation and hydrous partial melting

    NASA Astrophysics Data System (ADS)

    France, L.; Ildefonse, B.; Koepke, J.

    2009-04-01

    Recent detailed field studies performed in the Oman ophiolite on the gabbro/sheeted dike transition, compared to corresponding rocks from the EPR drilled by IODP (Site 1256), constrain a general model for the dynamics of the axial melt lens (AML) present at fast spreading ridges (France et al., 2008). This model implies that the AML/dike transition is a dynamic interface migrating up- and downward, and that the isotropic gabbro horizon on top of the igneous section represents its fossilization. It is also proposed that upward migrations are associated to reheating of the base of the sheeted dike complex and to assimilation processes. Plagiogranitic lithologies are observed close to the truncated base of the dikes and are interpreted to represent frozen melts generated by partial melting of previously hydrothermalized sheeted dikes. Relicts of previously hydrothermalized lithologies are also observed in the fossil melt lens, and are associated to lithologies that have crystallized under high water activities, with clinopyroxene crystallizing before plagioclase, and An-rich plagioclase. To better understand our field data, we performed hydrous partial melting experiments at shallow pressures (0.1 GPa) under slightly oxidizing conditions (NNO oxygen buffer) and water saturated conditions on hydrothermalized sheeted dike sample from the Oman ophiolite. These experiments have been performed between 850°C and 1030°C; two additional experiments in the subsolidus regime were also conducted (750°C and 800°C). Clinopyroxenes formed during incongruent melting at low temperature (<910°C) have compositions that match those from the corresponding natural rocks (reheated base of the sheeted dike and relicts of assimilated lithologies). In particular, the characteristic low TiO2 and Al2O3 contents are reproduced. The experimental melts produced at low temperatures correspond to compositions of typical natural plagiogranites. In natural settings, these silicic liquids would be

  12. Linkage between crustal structure, mantle temperature and axial morphology: A study of the intermediate spreading Southeast Indian Ridge

    NASA Astrophysics Data System (ADS)

    Baran, Janet Maureen

    There is a systematic variation in axial morphology and axial depth along the Southeast Indian ridge (SEIR) with distance away from the Australian Antarctic Discordance, an area of cold uppermost mantle. Since spreading rate (72-76 mm/yr) and mantle geochemistry appear constant along this portion of the SEIR, the observed variations in axial morphology and axial depth are attributed to a gradient in mantle temperature. In this study, we report results from a multichannel seismic investigation of on-axis and off-axis crustal structure along this portion of SEIR. Axial highs have a shallow magma lens and a thin layer 2A along the ridge crest that doubles in thickness off-axis. Rifted axial highs have a deeper magma lens and thicker layer 2A on-axis that thickens by 50% off-axis. Beneath shallow axial valleys, no magma lens is imaged, and layer 2A is thick with no pattern of thickening off-axis. Near the ridge axis hydrothermal circulation has a large role in the observed seismic layer 2A velocity increase and thickening off-axis. Near-axis, in the vicinity of a magma lens, active hydrothermal circulation occurs, infilling pore spaces, as well as resulting downward cracking. Where there is no magma lens imaged, little active hydrothermal circulation occurs, thus resulting in a small velocity increase. With an observed magma lens, eruption of less viscous, far-traveling lobate laves can extend far off-axis, whereas in areas with no observed magma lens, lava may be emplaced through eruption of cooler more viscous lavas that form constructional pillows. The velocity of layer 2B increases through cooling and healing of fractures through infilling due to hydrothermal circulation. We conclude that a small change in melt production can cause a large change in axial morphology and shallow crustal structure. This implies a threshold type behavior where a small change in melt production results in a transition between two very different modes of crustal generation, resulting in

  13. High-resolution Remotely Operated Vehicle (ROV) mapping of a slow-spreading ridge: Mid-Atlantic Ridge 45°N

    NASA Astrophysics Data System (ADS)

    Yeo, I. A.; Searle, R. C.

    2013-06-01

    Axial volcanic ridges (AVRs) are found on most slow-spreading mid-ocean ridges and are thought to be the main locus of volcanism there. In this study we present high-resolution mapping of a typical, well-defined AVR on the Mid-Atlantic Ridge at 45°N. The AVR is characterized by "hummocky terrain," composed typically of hummocks with pillowed or elongate pillowed flanks with pillowed or lobate lava flow summits, often with small haystacks sitting on their highest points. The AVR is surrounded by several areas of "flat seafloor," composed of lobate and sheet lava flows. The spatial and morphological differences between these areas indicate different eruption processes operating on and off the AVR. Volcanic fissures are found all around and on the AVR, although those with the greatest horizontal displacement are found on the ridge crest and flat seafloor. Clusters of fissures may represent volcanic vents. Extremely detailed comparisons of sediment coverage and examination of contact relations around the AVR suggest that many of the areas of flat seafloor are of a similar age or younger than the hummocky terrain of the AVR. Additionally, all the lavas surveyed have similar degrees of sediment cover, suggesting that the AVR was either built or resurfaced in the same 50 ka time frame as the flat seafloor.

  14. Controls on segmentation and morphology along the back-arc Eastern Lau Spreading Center and Valu Fa Ridge

    NASA Astrophysics Data System (ADS)

    Sleeper, Jonathan D.; Martinez, Fernando

    2014-03-01

    Back-arc spreading centers increasingly depart from mid-ocean ridge (MOR) characteristics with proximity to the arc volcanic front. The close association of these departures with slab-derived materials in erupted lavas suggests that subduction-related chemical effects are their primary cause. The Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR) are type examples of this process. Together they constitute a first-order spreading center in the Lau back-arc basin that progressively converges on the Tofua arc volcanic front from north to south. Here we use ship multibeam and deep-towed side-scan sonar data to examine variations in axial morphology and volcanism at the second- and third-order segment scale along these ridges and develop a model for the processes that control them. Closest to the arc, VFR, and the southern segment of the ELSC shoal toward second-order segment ends, in contrast to MORs. Northward and beyond ~70 km from the arc, the axis becomes abruptly deeper and flatter and no longer shoals toward second-order segment ends. At VFR, along-axis topographic highs correlate with the location of arc volcanoes along slab flow lines. These correlations are weaker along the southernmost ELSC segment and absent along ELSC segments farther north. The observations show a modulation of back-arc segmentation with arc proximity that rapidly diminishes with distance. They support a model of the mantle wedge with a strongly hydrous domain within ~70 km of the arc within which the arc and ridge interact and a much less hydrous domain farther from the arc without evident arc-ridge interactions.

  15. Composition of hydrothermal fluids and mineralogy of associated chimney material on the East Scotia Ridge back-arc spreading centre

    NASA Astrophysics Data System (ADS)

    James, Rachael H.; Green, Darryl R. H.; Stock, Michael J.; Alker, Belinda J.; Banerjee, Neil R.; Cole, Catherine; German, Christopher R.; Huvenne, Veerle A. I.; Powell, Alexandra M.; Connelly, Douglas P.

    2014-08-01

    The East Scotia Ridge is an active back-arc spreading centre located to the west of the South Sandwich island arc in the Southern Ocean. Initial exploration of the ridge by deep-tow surveys provided the first evidence for hydrothermal activity in a back-arc setting outside of the western Pacific, and we returned in 2010 with a remotely operated vehicle to precisely locate and sample hydrothermal sites along ridge segments E2 and E9. Here we report the chemical and isotopic composition of high- and low-temperature vent fluids, and the mineralogy of associated high-temperature chimney material, for two sites at E2 (Dog’s Head and Sepia), and four sites at E9 (Black & White, Ivory Tower, Pagoda and Launch Pad). The chemistry of the fluids is highly variable between the ridge segments. Fluid temperatures were ∼350 °C at all vent sites except Black & White, which was significantly hotter (383 °C). End-member chloride concentrations in E2 fluids (532-536 mM) were close to background seawater (540 mM), whereas Cl in E9 fluids was much lower (98-220 mM) indicating that these fluids are affected by phase separation. Concentrations of the alkali elements (Na, Li, K and Cs) and the alkaline earth elements (Ca, Sr and Ba) co-vary with Cl, due to charge balance constraints. Similarly, concentrations of Mn and Zn are highest in the high Cl fluids but, by contrast, Fe/Cl ratios are higher in E9 fluids (3.8-8.1 × 10-3) than they are in E2 fluids (1.5-2.4 × 10-3) and fluids with lowest Cl have highest Cu. Although both ridge segments are magmatically inflated, there is no compelling evidence for input of magmatic gases to the vent fluids. Fluid δD values range from 0.2‰ to 1.5‰, pH values (3.02-3.42) are not especially low, and F concentrations (34.6-54.4 μM) are lower than bottom seawater (62.8 μM). The uppermost sections of conjugate chimney material from E2, and from Ivory Tower and Pagoda at E9, typically exhibit inner zones of massive chalcopyrite enclosed

  16. Fluid composition of the sediment-influenced Loki's Castle vent field at the ultra-slow spreading Arctic Mid-Ocean Ridge

    NASA Astrophysics Data System (ADS)

    Baumberger, Tamara; Früh-Green, Gretchen L.; Thorseth, Ingunn H.; Lilley, Marvin D.; Hamelin, Cédric; Bernasconi, Stefano M.; Okland, Ingeborg E.; Pedersen, Rolf B.

    2016-08-01

    The hydrothermal vent field Loki's Castle is located in the Mohns-Knipovich bend (73°N) of the ultraslow spreading Arctic Mid-Ocean Ridge (AMOR) close to the Bear Island sediment fan. The hydrothermal field is venting up to 320° C hot black smoker fluids near the summit of an axial volcanic ridge. Even though the active chimneys have grown on a basaltic ridge, geochemical fluid data show a strong sedimentary influence into the hydrothermal circulation at Loki's Castle. Compelling evidence for a sediment input is given by high alkalinity, high concentrations of NH4+, H2, CH4, C2+ hydrocarbons as well as low Mn and Fe contents. The low δ13C values of CO2 and CH4 and the thermogenic isotopic pattern of the C2+ hydrocarbons in the high-temperature vent fluids clearly point to thermal degradation of sedimentary organic matter and illustrate diminution of the natural carbon sequestration in sediments by hydrothermal circulation. Thus, carbon-release to the hydrosphere in Arctic regions is especially relevant in areas where the active Arctic Mid-Ocean Ridge system is in contact with the organic matter rich detrital sediment fans.

  17. Widespread active detachment faulting and core complex formation near 13 degrees N on the Mid-Atlantic Ridge.

    PubMed

    Smith, Deborah K; Cann, Johnson R; Escartín, Javier

    2006-07-27

    Oceanic core complexes are massifs in which lower-crustal and upper-mantle rocks are exposed at the sea floor. They form at mid-ocean ridges through slip on detachment faults rooted below the spreading axis. To date, most studies of core complexes have been based on isolated inactive massifs that have spread away from ridge axes. Here we present a survey of the Mid-Atlantic Ridge near 13 degrees N containing a segment in which a number of linked detachment faults extend for 75 km along one flank of the spreading axis. The detachment faults are apparently all currently active and at various stages of development. A field of extinct core complexes extends away from the axis for at least 100 km. Our observations reveal the topographic characteristics of actively forming core complexes and their evolution from initiation within the axial valley floor to maturity and eventual inactivity. Within the surrounding region there is a strong correlation between detachment fault morphology at the ridge axis and high rates of hydroacoustically recorded earthquake seismicity. Preliminary examination of seismicity and seafloor morphology farther north along the Mid-Atlantic Ridge suggests that active detachment faulting is occurring in many segments and that detachment faulting is more important in the generation of ocean crust at this slow-spreading ridge than previously suspected. PMID:16871215

  18. Two Vent Fields Discovered at the Ultraslow Spreading Arctic Ridge System

    NASA Astrophysics Data System (ADS)

    Pedersen, R. B.; Thorseth, I. H.; Hellevang, B.; Schultz, A.; Taylor, P.; Knudsen, H. P.; Steinsbu, B. O.

    2005-12-01

    Two high-temperature vent fields were discovered at the Mohns Ridge during an expedition with the Norwegian research vessel "G.O. Sars" in July 2005. Both vent fields are located within the southernmost segment of the Mohns Ridge approximately 50 km north of the West Jan Mayen Fracture Zone. Water depths along this segment range from 3800 meters close to the fracture zone to ~500 meters at the segment centre where the vent fields are located. The largest field - named "Gallionella Garden" - is situated within a rift graben where high- and low-temperature venting occurs along ridge-parallel normal faults and fissures. Presently we have documented high- and low-temperature venting along more then 2 km of the fault and fissure system in the area. The high-temperature venting takes place at around 550 mbsl at the base of a 100 meter high fault wall and was traced ~500 meters along strike. The field consists of at least 10 major vent sites, each composed of multiple chimneys that are up to 5-10 meters tall. There are also large areas of diffuse flow. The temperature of the vent fluids was measured to be above 260°C at a chimney orifice. This is at the boiling point of seawater at these water depths, and gas bubbling was observed at several of the vent sites. A sample of the top of a chimney consists of anhydrite, barite, sphalerite and pyrite. Outside the high-temperature vent area mounds of ferric iron are abundant. Such deposits have presently been traced along ~2 km of the faults and fissure system in the area. The deposits are predominantly made up of branching and twisted stalks comparable to those formed by the iron oxidizing bacteria Gallionella ferruginea showing that the precipitation is mediated by microbial activity. The temperatures below the upper crust of a mound were measured to be one degree above the ambient water temperature. The Fe-oxyhydroxides show Nd-isotope compositions similar to the basaltic crust and Sr-isotope compositions close to that of

  19. Hotspot activity and plume pulses recorded by geometry of spreading axes

    NASA Astrophysics Data System (ADS)

    Abelson, Meir; Agnon, Amotz

    2001-06-01

    Anomalous plan view geometry (planform) of spreading axes is shown to be a faithful indicator of hotspot influence, possibly capable of detecting pulses of hotspot discharge. A planform anomaly (PA) occurs when the orientation of second-order ridge segments is prominently oblique to the spreading direction. PA is found in the vicinity of hotspots at shallow ridges (<1.5 km), suggesting hotspot influence. In places the PA and shallow bathymetry are accompanied by geochemical anomalies, corroborating hotspot influence. This linkage is best expressed in the western Gulf of Aden, where the extent of the PA from the Afar hotspot coincides with the extent of La/Sm and Sr isotopic anomalies. Using fracture mechanics we predict PA to reflect overpressurized melt that dominates the stresses in the crust, consistent with hotspot regime. Accordingly, the temporal variations of the planform previously inferred from magnetic anomalies around the Kolbeinsey Ridge (KR), north of Iceland, record episodes of interaction with the hotspot and major pulses of the plume. This suggestion is corroborated by temporal correlation of episodes showing PA north of Iceland with plume pulses previously inferred by the V-shaped ridges around the Reykjanes Ridge (RR), south of Iceland. In contrast to the RR, the temporal correlation suggests simultaneous incidence of the plume pulses at Iceland and KR, hundreds of kilometers to the north. A deep northward branch of the Iceland plume active during pulse-periods may explain these observations.

  20. Comparison of Oceanic and Continental Ultramafic Hosted Hydrothermal Sulfide Deposits under Slow-spreading Mid-ocean Ridge Setting

    NASA Astrophysics Data System (ADS)

    Li, Honglin; Li, Jianghai; Zhang, Huatian

    2013-04-01

    Recently, slow-spreading mid-ocean ridges have attracted lots of researchers, especially in the MAR (Mid-Atlantic Ridge) and the Indian Ocean Ridge. People have found many hydrothermal vents or hydrothermal sulfide deposits around MAR, such as TAG, Rainbow and Lost City. The slow-spreading ridges are characterized by variations in magmatic, tectonic, and alteration processes along ridge segments. Because of the difficulties of the seafloor exploration, we need an example on the continent for compare with the hydrothermal sulfide deposits on mid-ocean ridge. So we found De'erni Cu (Co) ore deposit on the north of Tibet. De'erni Cu (Co) ore deposit is a typical VHMS developing in the north of Tibet, China. The ore body is hosted by the ultramafic rocks of the A'nyemagen ophiolite suite, which is the symbol of the residual crust of Paleo-Tethys Ocean. Through the detailed geological analysis to De'erni Cu (Co) ore deposit, lots of reminded geological records of submarine hydrothermal system, including: 1) thin-layer exhalative rock covering on the ore body; 2) the colloform structure, raspberry-like structure and breccia structure reserved in the porous-type ores; 3) the main mineral composition; 4) the calcite and felsic cement in the synchronization with the pyrite clast; 4) the ore zonality similar to the TAG hydrothermal sulfide deposit. According to the TiO2 content in the MORB basalts, the approximate half-spreading rate is 1.1-2.5cm/a, of the Paleo-Tethys Ocean represented by the De'erni ophiolite. Comparing to the mineralization processes of present mid-ocean hydrothermal sulfide deposits, we insist that De'erni Cu (Co) deposit has experienced three stages: submarine exhalation stage, cooling deposition and subduction emplacement. And the OCC (Oceanic Core Complex) may be the host setting of the submarine hydrothermal exhalation stage. Compared to other similar sulfide deposits on the continent in the world, De'erni Cu (Co) sulfide deposit has a younger age

  1. Intra-segment Variations in Geologic Characteristics Along the Eastern Lau Spreading Center and Valu Fa Ridge

    NASA Astrophysics Data System (ADS)

    Sleeper, J. D.; Martinez, F.

    2012-12-01

    Backarc spreading centers located within ~150 km of the arc volcanic front display systematic departures from mid-ocean ridge (MOR) characteristics. The close association of these departures with slab-derived materials in erupted lavas suggests that subduction-related chemical effects are their primary cause. We examine variations in geologic characteristics at the second- and third-order scale along six segments of the arc-proximal Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR) in the Lau backarc basin, southwest Pacific, using ship multibeam and deep-towed side-scan sonars. Together, the ELSC and VFR constitute a first-order spreading center which progressively approaches the Tofua arc volcanic front southward from ~100-30 km as spreading rates decrease from ~97-39 mm/yr. Second-order segments within VFR (VFR1/VFR2), ~30-50 km from the arc, have axial high morphologies indicative of excess magmatism compared to MOR's spreading at similar rates and erupt increasingly hydrous, vesicular and silicic lavas as the axis approaches the arc. Along-axis topographic highs indicative of locally increased magma supply are located at second-order segment ends, in contrast to the classic MOR model. Along VFR, third-order segments are defined by constructional volcanic ridges, following the classic model of increasing depth toward segment ends, and along VFR2 appear to erupt volcaniclastics at segment centers and lava flows near the ends. Along the southernmost second-order segment of the ELSC (ELSC4), located ~50-60 km from the arc, axial morphology becomes lower relief and much more variable, but still follows the general VFR pattern, where segment ends are shallower and higher relief than the center. Third-order segments form both volcanic ridges that deepen toward the ends, and tectonic grabens that shoal toward the ends, opposite of the MOR model. Beyond ~70 km from the arc, second-order segments become abruptly deeper, erupt less vesicular basaltic lavas, and

  2. Seismicity and active tectonic processes in the ultra-slow spreading Lena Trough, Arctic Ocean

    NASA Astrophysics Data System (ADS)

    Läderach, C.; Schlindwein, V.; Schenke, H.-W.; Jokat, W.

    2011-03-01

    With its remote location in the ice-covered Fram Strait, Lena Trough is a poorly known segment of the global mid-ocean ridge system. It is a prominent member of the ultra-slow spreading mid-ocean ridges but its spreading mechanisms are not well understood. We relocalized teleseismically recorded earthquakes from the past five decades to identify tectonic processes in Lena Trough and the adjacent Spitsbergen Fracture Zone (FZ). During two cruises with RV Polarstern in 2008 and 2009 we deployed seismic arrays on ice floes to record the local seismicity of Lena Trough. We could identify and localize microseismic events which we assume to be present in the entire rift valley. In contrast, our relocalization of teleseismically recorded earthquakes shows an asymmetric epicentre distribution along Lena Trough with earthquakes occurring predominately along the western valley flanks of Lena Trough. In 2009 February/March, several high-magnitude earthquakes peaking in an Mb 6.6 event occurred in an outside-corner setting of the Spitsbergen FZ. This is the strongest earthquake which has ever been recorded in Fram Strait and its location at the outside-corner high of the ultra-slow spreading ridge is exceptional. Comparing the seismicity with the magnetic anomalies and high-resolution multibeam bathymetry, we divide Lena Trough in a symmetrically spreading northern part and an asymmetrically spreading southern part south of the South Lena FZ. We propose that a complex interaction between the former De Geer Megashear zone, which separated Greenland from Svalbard starting at Late Mesozoic/Early Cenozoic times, and the developing rift in the southern Lena Trough resulted an increasing eastward dislocation towards the Spitsbergen FZ between older spreading axes and the recent active spreading axis which we believe to be located west of the bathymetric rift valley flanks in a wide extensional plain.

  3. Tectonics and geology of spreading ridge subduction at the Chile Triple Junction: a synthesis of results from Leg 141 of the Ocean Drilling Program

    USGS Publications Warehouse

    Behrmann, J.H.; Lewis, S.D.; Cande, S.C.

    1994-01-01

    An active oceanic spreading ridge is being subducted beneath the South American continent at the Chile Triple Junction. This process has played a major part in the evolution of most of the continental margins that border the Pacific Ocean basin. A combination of high resolution swath bathymetric maps, seismic reflection profiles and drillhole and core data from five sites drilled during Ocean Drilling Program (ODP) Leg 141 provide important data that define the tectonic, structural and stratigraphic effects of this modern example of spreading ridge subduction. A change from subduction accretion to subduction erosion occurs along-strike of the South American forearc. This change is prominently expressed by normal faulting, forearc subsidence, oversteepening of topographic slopes and intensive sedimentary mass wasting, overprinted on older signatures of sediment accretion, overthrusting and uplift processes in the forearc. Data from drill sites north of the triple junction (Sites 859-861) show that after an important phase of forearc building in the early to late Pliocene, subduction accretion had ceased in the late Pliocene. Since that time sediment on the downgoing oceanic Nazca plate has been subducted. Site 863 was drilled into the forearc in the immediate vicinity of the triple junction above the subducted spreading ridge axis. Here, thick and intensely folded and faulted trench slope sediments of Pleistocene age are currently involved in the frontal deformation of the forearc. Early faults with thrust and reverse kinematics are overprinted by later normal faults. The Chile Triple Junction is also the site of apparent ophiolite emplacement into the South American forearc. Drilling at Site 862 on the Taitao Ridge revealed an offshore volcanic sequence of Plio-Pleistocene age associated with the Taitao Fracture Zone, adjacent to exposures of the Pliocene-aged Taitao ophiolite onshore. Despite the large-scale loss of material from the forearc at the triple junction

  4. Oceanic Core Complexes on the Mohns Ridge

    NASA Astrophysics Data System (ADS)

    Denny, A. R.; Pedersen, R. B.

    2013-12-01

    The Mohns Ridge, an ultra-slow spreading ridge in the Arctic Mid-Ocean Ridge system, is host to multiple volcanic and tectonic spreading segments. This oblique-spreading ridge is hotspot influenced at its southern terminus and is bound to the north by a curvilinear contact with the highly oblique Knipovich Ridge. This study examines EM120 multibeam bathymetry of the Mohns Ridge collected from 1999-2001 and gridded to 50 m cell size. Geomorphic interpretation of near-axis and off-axis structures reveals multiple expressions of potential oceanic core complexes (OCCs) along the 550 km long spreading axis. The OCCs form only on the western side of the spreading axis, consistent with the increased tectonic vs. volcanic morphology of the western flank of the Mohns Ridge. In the southern Mohns Ridge OCCs occur adjacent to on-axis active volcanic spreading centers. In the northern Mohns Ridge OCCs appear related to both ';V' shaped northern-propagating ridge spreading centers and spreading-parallel strings of core complexes extending at least 60 km off axis in the direction of spreading. This geomorphic interpretation should be further refined by dedicated ship-based investigations to fully describe this unique oblique-spreading Arctic Ridge system.

  5. Hybrid shallow on-axis and deep off-axis hydrothermal circulation at fast-spreading ridges.

    PubMed

    Hasenclever, Jörg; Theissen-Krah, Sonja; Rüpke, Lars H; Morgan, Jason P; Iyer, Karthik; Petersen, Sven; Devey, Colin W

    2014-04-24

    Hydrothermal flow at oceanic spreading centres accounts for about ten per cent of all heat flux in the oceans and controls the thermal structure of young oceanic plates. It also influences ocean and crustal chemistry, provides a basis for chemosynthetic ecosystems, and has formed massive sulphide ore deposits throughout Earth's history. Despite this, how and under what conditions heat is extracted, in particular from the lower crust, remains largely unclear. Here we present high-resolution, whole-crust, two- and three-dimensional simulations of hydrothermal flow beneath fast-spreading ridges that predict the existence of two interacting flow components, controlled by different physical mechanisms, that merge above the melt lens to feed ridge-centred vent sites. Shallow on-axis flow structures develop owing to the thermodynamic properties of water, whereas deeper off-axis flow is strongly shaped by crustal permeability, particularly the brittle-ductile transition. About 60 per cent of the discharging fluid mass is replenished on-axis by warm (up to 300 degrees Celsius) recharge flow surrounding the hot thermal plumes, and the remaining 40 per cent or so occurs as colder and broader recharge up to several kilometres away from the axis that feeds hot (500-700 degrees Celsius) deep-rooted off-axis flow towards the ridge. Despite its lower contribution to the total mass flux, this deep off-axis flow carries about 70 per cent of the thermal energy released at the ridge axis. This combination of two flow components explains the seismically determined thermal structure of the crust and reconciles previously incompatible models favouring either shallower on-axis or deeper off-axis hydrothermal circulation. PMID:24759413

  6. Hybrid shallow on-axis and deep off-axis hydrothermal circulation at fast-spreading ridges

    NASA Astrophysics Data System (ADS)

    Hasenclever, Jörg; Theissen-Krah, Sonja; Rüpke, Lars H.; Morgan, Jason P.; Iyer, Karthik; Petersen, Sven; Devey, Colin W.

    2014-04-01

    Hydrothermal flow at oceanic spreading centres accounts for about ten per cent of all heat flux in the oceans and controls the thermal structure of young oceanic plates. It also influences ocean and crustal chemistry, provides a basis for chemosynthetic ecosystems, and has formed massive sulphide ore deposits throughout Earth's history. Despite this, how and under what conditions heat is extracted, in particular from the lower crust, remains largely unclear. Here we present high-resolution, whole-crust, two- and three-dimensional simulations of hydrothermal flow beneath fast-spreading ridges that predict the existence of two interacting flow components, controlled by different physical mechanisms, that merge above the melt lens to feed ridge-centred vent sites. Shallow on-axis flow structures develop owing to the thermodynamic properties of water, whereas deeper off-axis flow is strongly shaped by crustal permeability, particularly the brittle-ductile transition. About 60 per cent of the discharging fluid mass is replenished on-axis by warm (up to 300 degrees Celsius) recharge flow surrounding the hot thermal plumes, and the remaining 40 per cent or so occurs as colder and broader recharge up to several kilometres away from the axis that feeds hot (500-700 degrees Celsius) deep-rooted off-axis flow towards the ridge. Despite its lower contribution to the total mass flux, this deep off-axis flow carries about 70 per cent of the thermal energy released at the ridge axis. This combination of two flow components explains the seismically determined thermal structure of the crust and reconciles previously incompatible models favouring either shallower on-axis or deeper off-axis hydrothermal circulation.

  7. Rapid cooling rates at an active mid-ocean ridge from zircon thermochronology

    NASA Astrophysics Data System (ADS)

    Schmitt, Axel K.; Perfit, Michael R.; Rubin, Kenneth H.; Stockli, Daniel F.; Smith, Matthew C.; Cotsonika, Laurie A.; Zellmer, Georg F.; Ridley, W. Ian; Lovera, Oscar M.

    2011-02-01

    Oceanic spreading ridges are Earth's most productive crust generating environment, but mechanisms and rates of crustal accretion and heat loss are debated. Existing observations on cooling rates are ambiguous regarding the prevalence of conductive vs. convective cooling of lower oceanic crust. Here, we report the discovery and dating of zircon in mid-ocean ridge dacite lavas that constrain magmatic differentiation and cooling rates at an active spreading center. Dacitic lavas erupted on the southern Cleft segment of the Juan de Fuca ridge, an intermediate-rate spreading center, near the intersection with the Blanco transform fault. Their U-Th zircon crystallization ages (29.3 - 4.6 + 4.8 ka; 1σ standard error s.e.) overlap with the (U-Th)/He zircon eruption age (32.7 ± 1.6 ka) within uncertainty. Based on similar 238U- 230Th disequilibria between southern Cleft dacite glass separates and young mid-ocean ridge basalt (MORB) erupted nearby, differentiation must have occurred rapidly, within ~ 10-20 ka at most. Ti-in-zircon thermometry indicates crystallization at 850-900 °C and pressures > 70-150 MPa are calculated from H 2O solubility models. These time-temperature constraints translate into a magma cooling rate of ~ 2 × 10 - 2 °C/a. This rate is at least one order-of-magnitude faster than those calculated for zircon-bearing plutonic rocks from slow spreading ridges. Such short intervals for differentiation and cooling can only be resolved through uranium-series ( 238U- 230Th) decay in young lavas, and are best explained by dissipating heat convectively at high crustal permeability.

  8. Discovery and Distribution of Black Smokers on the Western Galapagos Spreading Center: Implications for Spatial and Temporal Controls on High Temperature Venting at Ridge/Hotspot Intersections

    NASA Astrophysics Data System (ADS)

    Haymon, R. M.; Anderson, P. G.; Baker, E. T.; Resing, J. A.; White, S. M.; MacDonald, K. C.

    2006-12-01

    Though nearly one-fifth of the mid-ocean ridge (MOR) lies on or near hotspots, it has been debated whether hotspots increase or decrease MOR hydrothermal flux, or affect vent biota. Despite hotspot enhancement of melt supply, high-temperature vent plumes are enigmatically sparse along two previously-surveyed ridge- hotspot intersections [Reykjanes Ridge (RR), Southeast Indian Ridge (SEIR)]. This has been attributed to crustal thickening by excess volcanism. During the 2005-06 GalAPAGoS expedition, we conducted nested sonar, plume, and camera surveys along a 540 km-long portion of the Galapagos Spreading Center (GSC) where the ridge intersects the Galapagos hotspot at lon. 94.5 -89.5 deg. W. Although MOR hydrothermal springs were first found along the eastern GSC crest in 1977 near lon. 86 deg. W, the GalAPAGoS smokers are the first active high-temperature vents to be found anywhere along the Cocos-Nazca plate boundary. Active and/or recently-inactive smokers were located beneath plumes at 5 sites on the seafloor between lon. 91 deg. W and 94.5 deg. W (see Anderson et al., this session) during near-bottom, real-time fiber-optic Medea camera surveys. Smokers occur along eruptive seafloor fissures atop axial volcanic ridges near the middles of ridge segments, mainly in areas underlain by relatively shallow, continuous axial magma chamber (AMC) seismic reflectors. These findings (1) support magmatic, rather than tectonic, control of GSC smoker distribution; (2) demonstrate that thick crust at MOR-hotspot intersections does not prevent high-temperature hydrothermal vents from forming; and, (3) appear to be inconsistent with models suggesting that enhanced hydrothermal cooling causes abrupt deepening of the AMC and transition from non-rifted to rifted GSC morphology near lon. 92.7 deg. W. The widely-spaced smoker sites located on different GSC segments exhibit remarkably similar characteristics and seafloor settings. Most sites are mature or extinct, and are on lava

  9. Time-clustering behavior of spreading-center seismicity between 15 and 35°N on the Mid-Atlantic Ridge: observations from hydroacoustic monitoring

    NASA Astrophysics Data System (ADS)

    Bohnenstiehl, DelWayne R.; Tolstoy, Maya; Smith, Deborah K.; Fox, Christopher G.; Dziak, Robert P.

    2003-07-01

    An earthquake catalog derived from the detection of seismically-generated T-waves is used to study the time-clustering behavior of moderate-size (≳3.0 M) earthquakes between 15 and 35°N along the Mid-Atlantic Ridge (MAR). Within this region, the distribution of inter-event times is consistent with a non-periodic, non-random, clustered process. The highest degrees of clustering are associated temporally with large mainshock-aftershock sequences; however, some swarm-like activity also is evident. Temporal fluctuations characterized by a power spectral density P( f) that decays as 1/ fα are present within the time sequence, with α ranging from 0.12 to 0.55 for different regions of the spreading axis. This behavior is negligible at time scales less than ˜5×10 3 s, and earthquake occurrence becomes less clustered (smaller α) as increasing size thresholds are applied to the catalog. A power-law size-frequency scaling for Mid-Atlantic Ridge earthquakes also can be demonstrated using the distribution of acoustic magnitudes, or source levels. Although fractal seismic behavior has been linked to the structure of the underlying fault population in other environments, power-law fault size distributions have not been observed widely in the mid-ocean ridge setting.

  10. Contribution of myosin II activity to cell spreading dynamics.

    PubMed

    Nisenholz, Noam; Paknikar, Aishwarya; Köster, Sarah; Zemel, Assaf

    2016-01-14

    Myosin II activity and actin polymerization at the leading edge of the cell are known to be essential sources of cellular stress. However, a quantitative account of their separate contributions is still lacking; so is the influence of the coupling between the two phenomena on cell spreading dynamics. We present a simple analytic elastic theory of cell spreading dynamics that quantitatively demonstrates how actin polymerization and myosin activity cooperate in the generation of cellular stress during spreading. Consistent with experiments, myosin activity is assumed to polarize in response to the stresses generated during spreading. The characteristic response time and the overall spreading time are predicted to determine different evolution profiles of cell spreading dynamics. These include, a (regular) monotonic increase of cell projected area with time, a non-monotonic (overshooting) profile with a maximum, and damped oscillatory modes. In addition, two populations of myosin II motors are distinguished based on their location in the lamella; those located above the major adhesion zone at the cell periphery are shown to facilitate spreading whereas those in deeper regions of the lamella are shown to oppose spreading. We demonstrate that the attenuation of myosin activity in the two regions may result in reciprocal effects on spreading. These findings provide important new insight into the function of myosin II motors in the course of spreading. PMID:26481613

  11. Seafloor hydrothermal activity and spreading rates: the Eocene carbon dioxide greenhouse revisted.

    PubMed

    Kasting, J F; Richardson, S M

    1985-01-01

    A suggestion has been made that enhanced rates of hydrothermal activity during the Eocene could have caused a global warming by adding calcium to the ocean and pumping CO2 into the atmosphere (Owen and Rea, 1984). This phenomenon was purported to be consistent with the predictions of the CO2 geochemical cycle model of Berner, Lasaga and Garrels (1983) (henceforth BLAG). In fact, however, the BLAG model predicts only a weak connection between hydrothermal activity and atmospheric CO2 levels. By contrast, it predicts a strong correlation between seafloor spreading rates and pCO2, since the release rate of CO2 from carbonate metamorphism is assumed to be proportional to the mean spreading rate. The Ecocene warming can be conveniently explained if the BLAG model is extended by assuming that the rate of carbonate metamorphism is also proportional to the total length of the midocean ridges from which the spreading originates. PMID:11539654

  12. Seafloor hydrothermal activity and spreading rates - The Eocene carbon dioxide greenhouse revisited

    NASA Technical Reports Server (NTRS)

    Kasting, J. F.; Richardson, S. M.

    1985-01-01

    A suggestion has been made that enhanced rates of hydrothermal activity during the Eocene could have caused a global warming by adding calcium to the ocean and pumping CO2 into the atmosphere (Owen and Rea, 1984). This phenomenon was purported to be consistent with the predictions of the CO2 geochemical cycle model of Berner, Lasaga and Garrels (1983) (henceforth BLAG). In fact, however, the BLAG model predicts only a weak connection between hydrothermal activity and atmospheric CO2 levels. By contrast, it predicts a strong correlation between seafloor spreading rates and pCO2, since the release rate of CO2 from carbonate metamorphism is assumed to be proportional to the mean spreading rate. The Eocene warming can be conveniently explained if the BLAG model is extended by assuming that the rate of carbonate metamorphism is also proportional to the total length of the midocean ridges from which the spreading originates.

  13. Seafloor hydrothermal activity and spreading rates: the Eocene carbon dioxide greenhouse revisted

    NASA Technical Reports Server (NTRS)

    Kasting, J. F.; Richardson, S. M.

    1985-01-01

    A suggestion has been made that enhanced rates of hydrothermal activity during the Eocene could have caused a global warming by adding calcium to the ocean and pumping CO2 into the atmosphere (Owen and Rea, 1984). This phenomenon was purported to be consistent with the predictions of the CO2 geochemical cycle model of Berner, Lasaga and Garrels (1983) (henceforth BLAG). In fact, however, the BLAG model predicts only a weak connection between hydrothermal activity and atmospheric CO2 levels. By contrast, it predicts a strong correlation between seafloor spreading rates and pCO2, since the release rate of CO2 from carbonate metamorphism is assumed to be proportional to the mean spreading rate. The Ecocene warming can be conveniently explained if the BLAG model is extended by assuming that the rate of carbonate metamorphism is also proportional to the total length of the midocean ridges from which the spreading originates.

  14. Topographic Expression of the Gulf of Aden Spreading System and its Tectonics: Hotspots-Ridge Interaction in Gulf of Aden

    NASA Astrophysics Data System (ADS)

    Nakanishi, M.; Tamaki, K.; Fujimoto, H.; Okino, K.; Curewitz, D.; Huchon, P.; Khanbari, K. M.

    2001-12-01

    Gulf of Aden is a young ocean basin formed by the rifting of Arabia away from Somalia (NE Africa). The Arabian plate moves away from Africa in a NE direction, at a rate of about 2 cm/yr. The rifting started from Early Miocene. After the, the Gulf of Aden spreding system is propagating westward into the Arabia-Africa continent (Manighetti et al., 1997). It reached the Afar hotspot area about 10 Ma (Manighetti et al., 1998). The spreading system continues to interact with the hotspot up to the present. Tamsett and Searle (1988) exposed that strike of segmentations of the spreading center in Gulf of Aden is NW-SE, although the trend of the spreading system is ENE. Their survey showed the general topographic features of the spreading system, but more detailed survey is necessary to reveal the tectonics of the Gulf of Aden, especially hotspot-ridge interaction. The bathymetric survey by the SEA BEAM 2120 multi-narrow beam echo sounders aboard the R/V Hakuho-maru was carried out along the spreading system of the Gulf of Aden between 45\\deg30'E and 50° 20'E from December 2000 to January 2001. The interval of the track lines is 4-6 km. The trend of the track lines is N70\\degE. Geomagnetic and gravity fields were measured during the bathymetric. The topographic expression of the spreading centers east of N46° 20'E is an axial rift valley offset by transform faults similar to that observed at slow spreading centers. The trend of the axial rift valley is N65\\degW. The offset of the spreading centers increases in length to the east as Tamsett and Searle (1988) indicated. The axial rift valley west of N46° 20'E, which has an east-west direction, is not offset by any prominent transform faults. N65\\deg$W trending en-echelon basins are situated in the rift valley similar to that of the western Gulf of Aden spreading system (Dauteuil, et al. 2001).

  15. Reconstruction of the Flanks of the Mid-Atlantic Ridge, 28° to 29° N: Implications for Evolution of Young Oceanic Lithosphere at Slow-Spreading Centers

    NASA Astrophysics Data System (ADS)

    Sloan, H.; Patriat, P.

    2004-12-01

    We reconstruct the flanks of the Mid-Atlantic Ridge between 28° and 29° N from 1 to 10 Ma at intervals of 1 Myr for the purpose of investigating evolution of young oceanic lithosphere morphology and its variation through time using an innovative method that combines seafloor subsidence correction with interpolated isochrons and rotation poles. Reconstruction results are consistent with formation of abyssal hills every 1 to 2 Myr in 2-3 Myr old lithosphere at the outer edge of the ridge mountains as a result of transition from dynamic regime near the axis to isostatic regime of the flanks. The oblique passage of structures formed at the axial valley walls through this transition zone may play a role in the development of inside corner high bathymetry. Asymmetric juxtaposition of abyssal hill morphology in reconstruction is indicative of independent formation and evolution of the morphotectonic fabric on opposing flanks. The two major factors affecting asymmetric ridge flank morphology are found to be sense of axial offset and fluctuation of magmatic activity at the segment scale. Sense of axial offset determines the relative distribution of inside and outside corner bathymetry on the flanks. Enhanced magma supply and associated segment propagation may contribute to half-spreading rate asymmetry, accretion of thicker crust, and formation of larger abyssal hills on the faster-spreading flank. Consistent alignment in reconstruction of the base of the steep walls bounding inside corner bathymetry confirms that they mark the boundaries between segments on the flanks and that the deep sediment filled basins, typically identified as discordant zones, are attributable to outside corner bathymetry.

  16. Age-Related Changes in Spreading Activation during Infancy

    ERIC Educational Resources Information Center

    Barr, Rachel; Walker, Joanne; Gross, Julien; Hayne, Harlene

    2014-01-01

    The concept of spreading activation describes how retrieval of one memory cues retrieval of other memories that are associated with it. This study explored spreading activation in 6-, 12-, and 18-month-old infants. Infants (n = 144) learned two tasks within the same experimental session; one task, deferred imitation (DI), is typically remembered…

  17. Gabbroic xenoliths from the northern Gorda Ridge: implications for magma chamber processes under slow spreading centers

    USGS Publications Warehouse

    Davis, A.S.; Clague, D.A.

    1990-01-01

    Abundant gabbroic xenoliths in porphyritic pillow basalt were dredged from the northern Gorda Ridge. The host lava is a moderately fractionated, normal mid-ocean ridge basalt with a heterogeneous glass rind (Mg numbers 56-60). Other lavas in the vicinity range from near primary (Mg number 69) to fractionated (Mg number 56). On the basis of textures and mineral compositions, the xenoliths are divided into five types. The xenoliths are not cognate to the host lava, but they are genetically related. Chemistry of mineral phases in conjunction with textural features suggests that the xenoliths formed in different parts of a convecting magma chamber that underwent a period of closed system fractionation. The chamber was filled with a large proportion of crystalline mush when new, more primitive, and less dense magma was injected and mixed incompletely with the contents in the chamber, forming the hybrid host lava. -from Authors

  18. Volcanism and massive sulfide formation at a sedimented spreading center, Escanaba Trough, Gorda Ridge, northeast Pacific.

    USGS Publications Warehouse

    Morton, J.L.; Holmes, M.L.; Koski, R.A.

    1987-01-01

    Seismic-reflection profiles over the sediment-filled Escanaba Trough at the southern Gorda Ridge reveal a series of volcanic centers that pierce the sediment. The volcanic edifices are 3 to 6 km in diameter and are spaced at 15 to 20 km intervals along the axis of the trough. Composition and form of sulfide samples obtained from the bank suggest significant interaction between hydrothermal fluids and sediment at depth, and deposition of sulfide within the sediment pile.-from Authors

  19. Spatial and temporal distribution of the seismicity along two mid-oceanic ridges with contrasted spreading rates in the Indian Ocean

    NASA Astrophysics Data System (ADS)

    Tsang-Hin-Sun, E.; Perrot, J.; Royer, J. Y.

    2015-12-01

    The seismicity of the ultra-slow spreading Southwest (14 mm/y) and intermediate spreading Southeast (60 mm/y) Indian ridges was monitored from February 2012 to March 2013 by the OHASISBIO array of 7 autonomous hydrophones. A total of 1471 events were located with 4 instruments or more, inside the array, with a median location uncertainty < 5 km and a completeness magnitude of mb = 3. Both ridges display similar average rates of seismicity, suggesting that there is no systematic relationship between seismicity and spreading rates. Accretion modes do differ, however, by the along-axis distribution of the seismic events. Along the ultra-slow Southwest Indian Ridge, events are sparse but regularly spaced and scattered up to 50 km off-axis. Along the fast Southeast Indian Ridge, events are irregularly distributed, focusing in narrow regions near the ridge axis at segment ends and along transform faults, whereas ridge-segment centers generally appear as seismic gaps (at the level of completeness of the array). Only two clusters, 6 months apart, are identified in a segment-center at 29°S. From the temporal distribution of the clustered events and comparisons with observations in similar mid-oceanic ridge setting, both clusters seem to have a volcanic origin and to be related to a dike emplacement or a possible eruption on the seafloor. Their onset time and migration rate are comparable to volcanic swarms recorded along the Juan de Fuca Ridge. Overall, the rate of seismicity along the two Indian spreading ridges correlates with the large-scale variations in the bathymetry and shear-wave velocity anomaly in the upper mantle, suggesting that the distribution of the low-magnitude seismicity is mainly controlled by along-axis variations in the lithosphere rheology and temperature.

  20. Spreading Dynamics Following Bursty Activity Patterns

    NASA Astrophysics Data System (ADS)

    Vazquez, Alexei

    The dynamics of many social, technological and economic phenomena are driven by individual human actions, turning the quantitative understanding of human behavior into a central question of modern science. Recent empirical evidence indicates that the timing of individual human actions follow non-Poisson statistics, characterized by bursts of rapidly occurring events separated by long periods of inactivity. In this work we analyze how this bursty dynamics impacts the dynamics of spreading processes in computer and social systems. We demonstrate that the non-Poisson nature of the contact dynamics results in prevalence decay times significantly larger than predicted by the standard Poisson process based models. Thanks to this slow dynamics the spreading entity, namely a virus, rumor, etc., can persist in the system for long times.

  1. Ocean-continent-transition and oceanic ridge structural evolution (eastern Gulf of Aden): Implications for rift to seafloor spreading processes

    NASA Astrophysics Data System (ADS)

    D'Acremont, E.; Leroy, S. D.; Beslier, M.; Autin, J.; Watremez, L.; Maia, M. A.; Gente, P.

    2009-12-01

    The rifting between Arabia and Somalia, which started around 35 Ma ago, is followed by oceanic accretion from at least 17.6 Ma leading to the present Gulf of Aden. The transition between the thinned continental and the oceanic crusts is characterized, in space and time, by an ocean-continent transition (OCT). Here, we use bathymetry, gravity, seismic reflection and magnetism from the Encens-Sheba and Encens cruises in order to constrain the structure and segmentation of the conjugate OCT as well as the oceanic ridge between two main fracture zones (Alula-Fartak and Socotra-Hadbeen). The segmentation of the initial oceanic spreading centers seem directly related to the margin structure. Then, magmatic processes and kinematics change strongly influenced the evolution of the segmentation. The OCT and the oceanic domain can be divided into two distinct areas in the study area. The Eastern area is characterized by an extremely thin OCT and oceanic crusts (< 4km), a ~30 km wide and tectonized OCT with isolated continental blocks and short axial segments. In the western area, thicker OCT and oceanic crusts (>5km), a ~15 km wide OCT with a volcanic ridge, and a 6 km thick underplated mafic body in the northern margin suggest a high melt supply. The magmatic supply observed in the western domain is probably due to an off-axis thermal anomaly located below the southern flank of the Sheba ridge, at 75 km east of the major Alula-Fartak transform fault. This suggests that the OCT and the axial ridge morphology of this domain are perturbed by post-rift volcanism, which is due to a combination of the spreading rate, a thermal anomaly, and the cold edge effect of the Alula-Fartak transform fault. The presence of the inherited Mesozoic basins (Jezar-Qamar-Gardafui basin) located on this western domain can also explain, the difference in both the structure and the nature of the OCT between the two domains. The nature of the OCT could be either (or both) exhumed lower crust or

  2. The axial melt lens as a processor of evolved melts at fast-spreading mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Loocke, M. P.; Lissenberg, J. C. J.; MacLeod, C. J.

    2015-12-01

    The axial melt lens is a steady-state, generally magma-rich body located at the dyke-gabbro transition at mid-crustal levels beneath intermediate- and fast-spreading ridges. It is widely believed to be the reservoir from which mid-ocean ridge basalt (MORB) erupts. Using a remotely-operated vehicle, cruise JC21 to the Hess Deep Rift recovered the first comprehensive sample suite of the uppermost plutonics from a fast-spreading ridge. We present the results of a detailed microanalytical investigation of 23 samples (8 dolerites, 14 gabbronorites, and 1 gabbro) recovered by ROV dive 78 from a section traversing the transition from the uppermost gabbros into the sheeted dykes. With the exception of a single olivine-bearing sample (78R-6), dive 78 is dominated by evolved, varitextured (both in hand sample and thin section) oxide gabbronorites. Full thin section quantitative element maps were acquired on serial thin sections from each sample using the analytical scanning electron microscope in the at Cardiff University. The resulting maps were post-processed in MatlabTM to determine the full distribution of plagioclase compositions across entire thin sections (typically 500,000 analyses per sample); an approach we term 'quantitative assessment of compositional distribution' (QACD). By so doing we are able to conduct the first fully rigorous assessment of gabbro compositions, and, by extension, melt compositions present at this level beneath the ridge axis. Critically, we only found 2 grains of high-An plagioclase (An>80) in all of the samples (N = 51). These occur as cores within a sample dominated by lower-An plagioclase. Instead, the vast majority (75%) of plagioclase within the samples have compositions of An65 or lower; compositions too evolved to be in equilibrium with MORB. The most primitive sample, 78R-6, is an olivine-bearing gabbronorite with Fo67 olivine, and plagioclase ranging from An52-77 (median An = 65). These data are difficult to reconcile with models in

  3. Modes and implications of mantle and lower-crust denudation at slow-spreading mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Schroeder, Timothy John

    Slow-spreading mid-ocean ridges (<5 cm/yr) have intermittent magma supply, and accommodate spreading by a combination of magmatism and tectonic extension (Smith and Cann, 1993, Cannat, 1993). Extension at mid-ocean ridges is most commonly manifested by slip on high angle (˜60°) normal faults that dip into, and define the rift valley walls (Smith and Cann, 1993). Less commonly, extension occurs by long periods of slip along low-angle normal faults that penetrate to structurally deep levels of oceanic lithosphere and denude gabbro and/or pendotite to the seafloor in domal massifs termed "oceanic core complexes" (Dick et al., 1981; Dick et al., 1991; Tucholke et al., 1998; Mutter and Karson, 1992; Cann et al., 1997; MacLeod et al., 2002). This dissertation addresses processes and implications of tectonic extension at two oceanic core complexes. Atlantis Massif (30°N, Mid-Atlantic Ridge) is formed dominantly of serpentinized peridotite with lesser gabbro, and Atlantis Bank (57°E, Southwest Indian Ridge) is dominated by gabbro. Localization of brittle strain at Atlantis Massif occurred by reaction-softening processes associated with metasomatic alteration of peridotite and serpentmite to amphibole-, chlorite- and talc-bearing assemblages. Ductile strain at Atlantis Massif and Atlantis Bank is localized into intervals of highly-fractionated, oxide-rich gabbro. Two-oxide geothermometry of gabbro indicates that it was not penetratively deformed below ˜500°C. Denuded peridotite at Atlantis Massif is host to hydrothermal circulation driven in part by exothermic serpentinization reactions. Serpentinization decreases the seismic velocity of peridotite and leads to acquisition of a magnetic signature. Venting of highly-alkaline, methane- and hydrogen-rich serpentinization-derived fluids leads to lithification of seafloor carbonate ooze by precipitation of carbonate cement in a zone of mixing with "normal" seawater. This process may be the primary depositional mechanism of

  4. Magma system along fast-spreading centers controlled by ridge segmentation: Evidence from the northern Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Miyashita, Sumio; Adachi, Yoshiko

    2013-04-01

    Mid-ocean ridges are segmented at various scales with a hierarchy, from the biggest 1st- order to the smallest 4th-order segments. These segment structures control magmatic processes beneath the mid-ocean ridges such as mantle upwelling, partial melting of the upper mantle, and magma delivery system to form the oceanic crust (Macdonald, 1998). However, systematic studies on the segment control for magmatic processes are rare at modern mid-ocean ridges due to the difficulty of obtaining in-situ samples from different crustal-lithospheric depths. Sampling at ocean floors is generally exclusively limited only to the surface (i.e. the seafloor). Furthermore, the samples obtained from the surface of the ocean floor may likely represent the products of off-axis magmatism (Kusano et al., 2012). Therefore, studies of ocean ridge segmentation in ophiolites provide important constraints for the magmatic processes beneath seafloor spreading centers, because the precise 3-D architecture of the upper mantle and the crust (all the way to the uppermost extrusive layer) and their lateral variations could be observed and investigated in ophiolites. We have studied the northern Oman ophiolite where a complete succession from the upper mantle peridotites to the uppermost extrusive rocks is well exposed. Miyashita et al. (2003), Adachi and Miyashita (2003) and Umino et al. (2003) proposed a segment structure in the northern Oman ophiolite; the Wadi Fizh area is regarded as a northward propagating tip of a mid-ocean ridge based on geological evidence (Adachi and Miyashita. 2003). On the other hand, the Wadi Thuqbah area, about 25 km south of Wadi Fizh, is regarded as a segment center based on the thickest Moho transition zone, well developed EW-trending lineations in the MTZ and layered gabbro, and the comparatively primitive compositions of the layered gabbros. Furthermore, the southern margin of the Hilti block (Salahi block), about 40 km south of Wadi Thuqbah, is inferred to be the

  5. Hawaiian Hotspot - Spreading Ridge Interaction in the Late Cretaceous: A Fair and Balanced Look at the Evidence

    NASA Astrophysics Data System (ADS)

    Keller, R.

    2004-12-01

    As is so often the case in years divisible by 4, reality turns out to be quite different from reputation. The Hawaiian hotspot, often righteously promoted as the hotspot that the rest should strive to emulate, was not as stable nor as free from interactions with plate boundaries as some supporters suggest. Mounting geochemical and geophysical evidence shows that in its youth the hotspot not only inhaled, but probably snorted and did shots as well. The purpose of this presentation is to summarize what we know about the Late Cretaceous interaction between a spreading ridge and the Hawaiian hotspot from recent work on the Emperor Seamount chain. At the time of this writing, facts are a commodity to be fabricated, deleted, spun, denied, and denied-that-you-denied; but by the time of this presentation, we (hopefully) will be looking toward the future: can the Hawaiian hotspot's checkered past be treated as a bonus rather than a burden? Plate reconstructions of the Late Cretaceous northwest Pacific place a seafloor spreading center very close to, or even directly on top of, the Hawaiian hotspot. The geochemical effects of this hotspot-ridge interaction are now well documented by work on Ocean Drilling Program samples from Detroit Seamount, the next-to-oldest remaining Emperor Seamount. Basalts recovered from ODP Site 883 partway up the east side of Detroit Seamount have trace element and isotopic characteristics more akin to MORB than to Hawaiian Islands basalts. Basalts from ODP Site 884 at the eastern foot of the seamount are highly depleted tholeiites unlike anything else found so far in the Hawaiian-Emperor chain (Keller et al. 2000, Nature). Their trace element and radiogenic isotope values are essentially indistinguishable from MORB values (Keller et al. 2000), although triple-spike Pb isotope data are distinct from modern EPR MORB data (Regelous et al. 2003, J. Pet.). These characteristics were the result of the hotspot melting a greater proportion of a depleted

  6. Moytirra: Discovery of the first known deep-sea hydrothermal vent field on the slow-spreading Mid-Atlantic Ridge north of the Azores

    NASA Astrophysics Data System (ADS)

    Wheeler, A. J.; Murton, B.; Copley, J.; Lim, A.; Carlsson, J.; Collins, P.; Dorschel, B.; Green, D.; Judge, M.; Nye, V.; Benzie, J.; Antoniacomi, A.; Coughlan, M.; Morris, K.

    2013-10-01

    Geological, biological, morphological, and hydrochemical data are presented for the newly discovered Moytirra vent field at 45oN. This is the only high temperature hydrothermal vent known between the Azores and Iceland, in the North Atlantic and is located on a slow to ultraslow-spreading mid-ocean ridge uniquely situated on the 300 m high fault scarp of the eastern axial wall, 3.5 km from the axial volcanic ridge crest. Furthermore, the Moytirra vent field is, unusually for tectonically controlled hydrothermal vents systems, basalt hosted and perched midway up on the median valley wall and presumably heated by an off-axis magma chamber. The Moytirra vent field consists of an alignment of four sites of venting, three actively emitting "black smoke," producing a complex of chimneys and beehive diffusers. The largest chimney is 18 m tall and vigorously venting. The vent fauna described here are the only ones documented for the North Atlantic (Azores to Reykjanes Ridge) and significantly expands our knowledge of North Atlantic biodiversity. The surfaces of the vent chimneys are occupied by aggregations of gastropods (Peltospira sp.) and populations of alvinocaridid shrimp (Mirocaris sp. with Rimicaris sp. also present). Other fauna present include bythograeid crabs (Segonzacia sp.) and zoarcid fish (Pachycara sp.), but bathymodiolin mussels and actinostolid anemones were not observed in the vent field. The discovery of the Moytirra vent field therefore expands the known latitudinal distributions of several vent-endemic genera in the north Atlantic, and reveals faunal affinities with vents south of the Azores rather than north of Iceland.

  7. Diversity of microbial communities of Loki's Castle black smoker field at the ultra-slow spreading Arctic Mid-Ocean Ridge

    NASA Astrophysics Data System (ADS)

    Jaeschke, A.; Bernasconi, S. M.; Thorseth, I. H.; Pedersen, R.; Früh-Green, G.

    2010-12-01

    Here we present an organic geochemical study of Loki’s Castle, a black smoker field recently discovered at the Arctic Mid-Ocean Ridge (AMOR) in the Norwegian-Greenland Sea at around 73.2°N. Located at the Mohn-Knipovich Ridge, which is one of the slowest spreading ridge segments on Earth, Loki’s Castle is the most northerly major hydrothermal vent field known to date. The vent field is composed of five actively venting (320°C) black-smoker chimneys that tower on top of a large mound of hydrothermal sulfide deposits. Loki’s Castle is a basalt-hosted hydrothermal system, but high methane and ammonium contents in the vent fluids strongly indicate a sedimentary component below the volcanic ridge. In 2009, another site of low-temperature hydrothermal venting hosting numerous barite chimneys was discovered in the vicinity of the black smokers, which probably results from subsurface mixing of diffuse hydrothermal fluid with seawater. In our study, variations in microbial communities associated with the formation of actively venting, sulfide and sulfate chimneys in this essentially unexplored ultraslow spreading ridge system are assessed based on biomarker lipid and compound-specific carbon isotope analyses. Lipid extracts from an active, high-temperature sulfide chimney yielded abundant archaeal di- and tetraether lipids as well as irregular isoprenoidal hydrocarbons (PMIs) that are associated with archaeal methanogens and methanotrophs. Predominant archaeal biomarker lipids include archaeol, sn-2-hydroxyarchaeol as well as glycerol dialkyl glycerol tetraethers (GDGTs) containing 0-4 cyclopentyl moieties. In addition, GDGTs with an additional covalent bond between the isoprenoid hydrocarbon chains, so-called H-shaped GDGTs, containing 0-4 cyclopentyl rings were also found to be abundant components and are indicative of hyperthermophilic methanogens. Biomarkers characteristic of eukaryotes (sterols) and bacteria (fatty acids and hopanoids) were less prevalent in

  8. Iron-Oxidizing Bacteria Found at Slow-Spreading Ridge: a Case Study of Capelinhos Hydrothermal Vent (Lucky Strike, MAR 37°N)

    NASA Astrophysics Data System (ADS)

    Henri, P. A.; Rommevaux, C.; Lesongeur, F.; Emerson, D.; Leleu, T.; Chavagnac, V.

    2015-12-01

    Iron-oxidizing bacteria becomes increasingly described in different geological settings from volcanically active seamounts, coastal waters, to diffuse hydrothermal vents near seafloor spreading centers [Emerson et al., 2010]. They have been mostly identified and described in Pacific Ocean, and have been only recently found in hydrothermal systems associated to slow spreading center of the Mid-Atlantic Ridge (MAR) [Scott et al., 2015]. During the MoMARSAT'13 cruise at Lucky Strike hydrothermal field (MAR), a new hydrothermal site was discovered at about 1.5 km eastward from the lava lake and from the main hydrothermal vents. This active venting site, named Capelinhos, is therefore the most distant from the volcano, features many chimneys, both focused and diffuses. The hydrothermal end-member fluids from Capelinhos are different from those of the other sites of Lucky Strike, showing the highest content of iron (Fe/Mn≈3.96) and the lowest chlorinity (270 mmol/l) [Leleu et al., 2015]. Most of the chimneys exhibit rust-color surfaces and bacterial mats near diffuse flows. During the MoMARSAT'15 cruise, an active chimney, a small inactive one, and rust-color bacterial mat near diffuse flow were sampled at Capelinhos. Observations by SEM of the hydrothermal samples revealed the presence of iron oxides in an assemblage of tubular "sheaths", assembled "stalks", helical "stalks" and amorphous aggregates. These features are similar to those described from the Loihi iron-mats deposits and argue for the occurrence of iron-oxidizing bacteria. Cultures under micro-aerobic and neutral pH conditions allowed us to isolate strains from the small inactive chimney. Pyrosequencing of the 16S rRNA gene of the isolates and environmental samples will soon be performed, which should confirm the presence of iron-oxidizing bacteria and reveal the organization of bacterial communities in this original and newly discovered hydrothermal site of the slow spreading Mid-Atlantic Ridge. Emerson

  9. Distribution of mega fauna on sulfide edifices on the Eastern Lau Spreading Center and Valu Fa Ridge

    NASA Astrophysics Data System (ADS)

    Sen, Arunima; Becker, Erin L.; Podowski, Elizabeth L.; Wickes, Leslie N.; Ma, Shufen; Mullaugh, Katherine M.; Hourdez, Stéphane; Luther, George W.; Fisher, Charles R.

    2013-02-01

    Hydrothermal vent sulfide edifices contain some of the most extreme thermal and chemical conditions in which animals are able to live. As a result, sulfide edifices in the East Pacific Rise, Juan de Fuca Ridge, and Mid Atlantic Ridge vent systems often contain distinct faunal assemblages. In this study, we used high-resolution imagery and in-situ physico-chemical measurements within the context of a Geographic Information System (GIS) to examine community structure and niche differentiation of dominant fauna on sulfide edifices in the Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR) in the Western Pacific Ocean. Our results show that ELSC and VFR sulfide edifices host two distinct types of communities. One type, that covers the majority of sulfide edifice faces, is overall very similar to nearby lava communities and biomass is dominated by the same chemoautotrophic symbiont-containing molluscs that dominate lava communities, namely the provannid gastropods Alviniconcha spp. and Ifremeria nautilei and the mytilid bivalve Bathymodiolus brevior. The spatial distribution of the dominant molluscs is often a variation of the pattern of concentric rings observed on lavas, with Alviniconcha spp. at the tops of edifices where exposure to vent flow is the highest, and I. nautilei and B. brevior below. Our physico-chemical measurements indicate that because of rapid dispersion of vent fluid, habitable area for symbiont-containing fauna is quite limited on sulfide edifices, and the realized niches of the mollusc groups are narrower on sulfide edifices than on lavas. We suggest that competition plays an important role in determining the realized distributions of the mollusc groups on edifices. The other habitat, present in small patches of presumably hot, new anhydrite, is avoided by the dominant symbiont-containing molluscs and inhabited by crabs, shrimp and polynoids that are likely more heat tolerant. The ratio of sulfide concentration to temperature anomaly of

  10. Mid-Ocean Ridge Hydrothermal Vent Fluid Chemistry at Ultrafast Spreading Rates: Control by Phase Separation and Water-Rock Equilibrium

    NASA Astrophysics Data System (ADS)

    O'Grady, K. M.; Von Damm, K. L.

    2001-12-01

    Phase separation, overprinted by water-rock equilibration are the major controls on the chemical composition of hydrothermal vent fluids sampled from two morphologically distinct areas (18\\deg 24-26'S and 21\\deg 24-27'S) along the ultrafast spreading ( ~15 cm/yr full rate) Southern East Pacific Rise (SEPR) during the 1998 SouEPR Cruise. This conclusion, along with the growing evidence that phase separation and water-rock equilibrium also control the composition of previously sampled hydrothermal vent fluids from slower-spreading ridges, indicates that to a first approximation neither spreading rate nor ridge morphology can be directly related to hydrothermal fluid compositions. Hydrothermal fluids from ultrafast spreading centers therefore do not form a unique subset in the global range of known chemical compositions. Previous geophysical surveys and submersible observations suggested that the hydrothermal system located at 21\\deg 24-27'S, the SouEPR Area, was dominated by tectonic activity (Renard et al., 1985; Tufar, 1995; Krasnov et al., 1997). Submersible observations and hydrothermal vent fluid chemistry indicated that the N. Hump Area, experienced volcanic activity shortly before the 1993 NADUR Cruise (Charlou et al., 1996). The N. Hump Area vent fluids sampled during the 1998 SouEPR Cruise displayed a relatively uniform chlorinity (616-670 mmol/kg Cl) that is greater than seawater. The Si and Cl data from the N. Hump Area vent fluids suggest reaction zone conditions up to ~360 bars (~1 km below the seafloor) and ~430\\deg C, indicating supercritical phase separation. The unusually large chlorinity variation (113-803 mmol/kg Cl) in the SouEPR Area hydrothermal vent fluids covers almost the entire range of sampled mid-ocean ridge (MOR) hydrothermal vent fluid chemistries worldwide (30.5-1245 mmol/kg Cl). The Si and Cl data from the SouEPR Area vent fluids suggest reaction zone conditions up to ~410 bars ( ~1.3 km below the seafloor) and ~450\\deg C. The

  11. Spreading Sea Floors and Fractured Ridges. Crustal Evaluation Education Project. Teacher's Guide [and] Student Investigation.

    ERIC Educational Resources Information Center

    Stoever, Edward C., Jr.

    Crustal Evolution Education Project (CEEP) modules were designed to: (1) provide students with the methods and results of continuing investigations into the composition, history, and processes of the earth's crust and the application of this knowledge to man's activities and (2) to be used by teachers with little or no previous background in the…

  12. [In Situ Analysis of Element Geochemistry in Submarine Basalt in Hydrothermal Areas from Ultraslow Spreading Southwest Indian Ridge].

    PubMed

    Wang, Yan; Sun, Xiao-ming; Xu, Li; Liang, Ye-heng; Wu, Zhong-wei; Fu, Yu; Huang, Yi

    2015-03-01

    In this study, we analyze element geochemistry of submarine basalt in situ, which is sampled in hydrothermal areas from ultraslow spreading Southwest Indian Ridge, including the fresh basalt rocks (B19-9, B15-13) and altered basalt (B5-2). And we can confirm that altered mineral in B5-2 is celadonite by microscope and Raman Spectrum. Furthermore, amygdaloidal celadonites are analyzed by electron microprobe (EPMA) and EDS-line scanning. The results show that K-contents decrease and Na-contents increase from the core to the edge in these altered minerals, indicating the transition from celadonite to saponite. Celadonite is an altered minerals, forming in low temperature (< 50 degrees C) and oxidizing condition, while saponite form in low water/rock and more reducing condition. As a result, the transition from celadonite to saponite suggests environment change from oxidizing to reducing condition. Using the result of EPMA as internal standard, we can analyze rare earth elements (REE) in altered mineral in situ. Most of result show positive Eu anomaly (Δ(Eu)), indicating hydrothermal fluid transform from oxidizing to reducing, and reducing fluid rework on the early altered minerals. Comparison with REE in matrix feldspar both in altered and unaltered zoning, we find that reducing fluid can leach REE from the matrix feldspar, leading to lower total REE concentrations and positive Eu anomaly. So leaching process play an important role in hydrothermal system. PMID:26117900

  13. Evidence for a Slow Spreading Ocean Ridge in the Southern Rockall Trough From Satellite Gravity Inversion and Seismic Data

    NASA Astrophysics Data System (ADS)

    Chappell, A. R.; Kusznir, N. J.

    2005-12-01

    The southern Rockall Trough, located to the west of Ireland and the UK in the NE Atlantic, has been interpreted as both a Mesozoic intra-continental rift basin (O'Reilly 1995) and a mid Cretaceous ocean basin (e.g. Roberts et al. 1980). The continental rift hypothesis (O'Reilly 1995) requires differential stretching of the upper and lower crust and syn-tectonic cooling to mechanically explain the formation of 5-6km thick continental crust and allow serpentinisation of the upper mantle. In this model serpentinisation of the upper mantle is needed to explain low upper mantle seismic velocities. The serpentinisation has also been required to fit gravity modelling of seismic transects to the observed gravity (e.g. Shannon 1999). We use satellite gravity inversion to map Moho depth and crustal thickness (Chappell & Kusznir 2005) for the Rockall Trough area. The satellite gravity inversion is a 3D spectral method incorporating a correction for the residual lithosphere thermal gravity anomaly present in continental rifted margin lithosphere and oceanic lithosphere. The gravity inversion predicts Moho depth and geometry in agreement with wide-angle seismic estimates without invoking the extensive serpentinisation of the upper-mantle needed by the intra-continental rift hypothesis (O'Reilly 1995). Recent seismic modelling (Morewood 2005) suggests that the thin crust in the southern Rockall Trough does not have the seismic layering associated with oceanic crust formed at intermediate or fast spreading rates. Also, wide-angle seismic data shows low upper mantle seismic velocities are present and spatially associated with the thin 5-6km crust (Shannon 1999). These observations are consistent with models and observations of oceanic crust formed at slow spreading ocean ridges (Cannat 1996, Jokat 2003). Such models are based on a proportion of melt being retained in the upper mantle, producing low seismic velocities, and a reduced supply of melt to the crust, resulting in thin

  14. Major off-axis hydrothermal activity on the northern Gorda Ridge

    SciTech Connect

    Rona, P.A. ); Denlinger, R.P. ); Fisk, M.R.; Howard, K.J.; Taghon, G.L. ); Klitgord, K.D. ); McClain, J.S. ); McMurray, G.R. ); Wiltshire, J.C. )

    1990-06-01

    The first hydrothermal field on the northern Gorda Ridge, the Sea Cliff hydrothermal field, was discovered and geologic controls of hydrothermal activity in the rift valley were investigated on a dive series using the DSV Sea Cliff. The Sea Cliff hydrothermal field was discovered where predicted at the intersection of axis-oblique and axis-parallel faults at the south end of a linear ridge at mid-depth (2700 m) on on the east wall. Preliminary mapping and sampling of the field reveal: a setting nested on nearly sediment-free fault blocks 300 m above the rift valley floor 2.6 km from the axis; a spectrum of venting types from seeps to black smokers; high conductive heat flow estimated to be equivalent to the convective flux of multiple black smokers through areas of the sea floor sealed by a caprock of clastic breccia primarily derived from basalt with siliceous cement and barite pore fillings; and a vent biota with Juan de Fuca Ridge affinities. These findings demonstrate the importance of off-axis hydrothermal activity and the role of the intersection of tectonic lineations in controlling hydrothermal sites at sea-floor spreading centers.

  15. Information Retrieval by Constrained Spreading Activation in Semantic Networks.

    ERIC Educational Resources Information Center

    Cohen, Paul R.; Kjeldsen, Rick

    1987-01-01

    Describes GRANT, an expert system for finding sources of funding given research proposals. The architecture of GRANT and the implementation of constrained spreading activation (a modified search algorithm based on semantic memory) are described, and recall and precision rates are analyzed. (Author/LRW)

  16. Inward spread of activation in vertebrate muscle fibres

    PubMed Central

    González-Serratos, H.

    1971-01-01

    1. A method for detecting the activation of individual myofibrils or groups of myofibrils within an isolated muscle fibre is described. It consists in making all the myofibrils wavy by setting the fibre in gelatine and compressing it longitudinally; active shortening of myofibrils can then be recognized by the straightening out of the waves. 2. The time course of this straightening during a twitch was found by high-speed ciné micrography. 3. There is a delay of activation between the superficial and central myofibrils, from which the velocity of inward spread of activation can be found. 4. This velocity has a Q10 of 2, and is about 7 cm/sec at 20° C. The mechanism of the inward spread of activation is discussed. 5. On relaxation the waves reappear, showing that there is a spontaneous elongation of the myofibrils. ImagesPlate 1Plate 2Plate 3Plate 4 PMID:5557071

  17. Time-Clustering Behavior of Spreading-Center Seismicity Between 15-35 N on the Mid-Atlantic Ridge: Observations from Hydroacoustic Monitoring

    NASA Astrophysics Data System (ADS)

    Bohnenstiehl, D. R.; Tolstoy, M.; Smith, D. K.; Fox, C. G.; Dziak, R. P.

    2002-12-01

    An earthquake catalog derived from the detection of seismically-generated Tertiary (T) waves is used to study the time-clustering behavior of moderate-size (> 3.0 M) earthquakes along the north-central Mid-Atlantic Ridge. Because T-waves propagate efficiently within the ocean's sound channel, these data represent a significant improvement relative to the detection capabilities of land-based seismic stations. In addition, hydroacoustic monitoring overcomes many of the spatial and temporal limitations associated with ocean-bottom seismometer data, with the existing array being deployed continuously between 15-35 degrees N during the period February 1999-Februrary 2001.Within this region, the distribution of inter-event times is consistent with a non-random clustered process, with a coefficient of variation greater than 1.0. The clustered behavior is power-law in nature with temporal fluctuations characterized by a power spectral density that decays as 1/fα . Using Allan Factor analysis, α is found to range from 0.12-0.55 for different regions of the spreading axis. This scaling is negligible at time scales less than 3.5 x 103 s, and earthquake occurrence becomes less clustered (smaller α ) as increasing size thresholds are applied to the catalog. The highest degrees of clustering are associated temporally with large mainshock-aftershock sequences; however, some swarm-like activity also is evident. The distribution of acoustic magnitudes, or source levels, is consistent with a power-law size-frequency scaling for earthquakes. Although such behavior has been linked closely to the fractal nature of the underlying fault population in other environments, power-law fault size distributions have not been widely observed in the mid-ocean ridge setting.

  18. Three-dimensional inversion of marine magnetic anomalies on the equatorial Atlantic Ridge (St. Paul Fracture Zone): Delayed magnetization in a magmatically starved spreading center?

    NASA Astrophysics Data System (ADS)

    Sichler, Bertrand; HéKinian, Roger

    2002-12-01

    The St. Paul Fracture Zone (FZ) in the equatorial Atlantic is interrupted by three intratransform ridge (ITR) spreading centers. A detailed magnetic survey, corrected for the diurnal variations using a moored magnetic station, six submersible dives, and three bottom-towed video camera tracks provide data on the most eastern ITR (0°37'N, 25°27'W). Visual observations and submersible sampling displayed a high ultramafic/volcanic ratio, supporting the assumption that the ITR is in a magmatically starved state. Volcanics were mainly found on the rift valley floor from 4700 to 4000 m and as a thin cap (<160 m) on the top of the eastern rift crest (2700 m). Most of the rift walls consist essentially of serpentinized peridotites and gabbros. The magnetic data show a well-defined ridge centered anomaly. A generalized inversion method was applied to the field data to calculate the crustal equivalent magnetization, assuming that the seafloor is broken down into elementary cells of 1 × 1 × 0.5 km3 which fit the topography. The average of absolute value of equivalent magnetization is 2.7 A m-1. The width of the central normal polarity (Brunhes epoch) is wider (at least 34 km) than that indicated by the NUVEL-1 kinematic model (24.5 km). This 40% excess is believed to be significant and is thought to be the result of prolonged chemical remanent magnetization acquired during the serpentinization of peridotites. In a magmatically starved accretion segment, we suggest that peridotites could continue to acquire magnetization as long as tectonic activities facilitate the circulation of seawater in the upper mantle.

  19. Deep pyroclastic deposits and evidence for explosive volcanism on the ultraslow spreading Gakkel Ridge at 85E

    NASA Astrophysics Data System (ADS)

    Pontbriand, C. W.; Soule, S. A.; Sohn, R. A.; Humphris, S. E.

    2008-12-01

    Seafloor surveys conducted during the 2007 Arctic Gakkel Vents (AGAVE) expedition provide evidence for widespread explosive volcanism within the axial valley of the ultraslow spreading Gakkel Ridge at 85°E. We have used high-definition video and high-resolution bathymetry to map out the extent of the deposits as well as lava flows. The video imagery reveals that unconsolidated pyroclastic material lightly blankets the axial valley at 85°E with thicknesses up to ~10cm over an area 10km2. The bathymetric data show that the axial valley contains ubiquitous cratered volcanoes, that we interpret as potential source vents for the clastic material. We collected detailed visual imagery from one of these volcanoes, and found that the crater center as well as the proximal portions of the rim and outer flanks are covered with talus, suggesting the possibility that Vulcanian explosions played a role in crater formation and pyroclast deposition. We collected samples of the pyroclasts from two locations within the axial valley. The pyroclasts are dominated by low vesicularity angular fragments, with a small weight fraction (~ 12%) of bubble-wall fragments (limu o Pele). Many bubble-wall fragments have fluidal morphologies and stretched vesicles. The morphology of the clasts help constrain multiple models of fragmentation that may have occurred. The distribution of clasts suggests explosive discharge from multiple source vents within the axial valley over a prolonged period of time (i.e, not a single eruption in 1999). In order to explain the generation of pyroclastic material in water depths of ~3800 m (well below the critical pressure for steam generation), we present a model wherein volatiles exsolve from ascending magmas and are sequestered and stored in a lithospheric reservoir before being explosively discharged during a volcanic eruption. The long inter-eruption interval (100s to 1000s of years) and strong spatial heterogeneity of melt delivery associated with ultra

  20. Quantitative and phylogenetic study of the Deep Sea Archaeal Group in sediments of the Arctic mid-ocean spreading ridge

    PubMed Central

    Jørgensen, Steffen L.; Thorseth, Ingunn H.; Pedersen, Rolf B.; Baumberger, Tamara; Schleper, Christa

    2013-01-01

    In marine sediments archaea often constitute a considerable part of the microbial community, of which the Deep Sea Archaeal Group (DSAG) is one of the most predominant. Despite their high abundance no members from this archaeal group have so far been characterized and thus their metabolism is unknown. Here we show that the relative abundance of DSAG marker genes can be correlated with geochemical parameters, allowing prediction of both the potential electron donors and acceptors of these organisms. We estimated the abundance of 16S rRNA genes from Archaea, Bacteria, and DSAG in 52 sediment horizons from two cores collected at the slow-spreading Arctic Mid-Ocean Ridge, using qPCR. The results indicate that members of the DSAG make up the entire archaeal population in certain horizons and constitute up to ~50% of the total microbial community. The quantitative data were correlated to 30 different geophysical and geochemical parameters obtained from the same sediment horizons. We observed a significant correlation between the relative abundance of DSAG 16S rRNA genes and the content of organic carbon (p < 0.0001). Further, significant co-variation with iron oxide, and dissolved iron and manganese (all p < 0.0000), indicated a direct or indirect link to iron and manganese cycling. Neither of these parameters correlated with the relative abundance of archaeal or bacterial 16S rRNA genes, nor did any other major electron donor or acceptor measured. Phylogenetic analysis of DSAG 16S rRNA gene sequences reveals three monophyletic lineages with no apparent habitat-specific distribution. In this study we support the hypothesis that members of the DSAG are tightly linked to the content of organic carbon and directly or indirectly involved in the cycling of iron and/or manganese compounds. Further, we provide a molecular tool to assess their abundance in environmental samples and enrichment cultures. PMID:24109477

  1. Composition of plume-influenced mid-ocean ridge lavas and glasses from the Mid-Atlantic Ridge, East Pacific Rise, Galápagos Spreading Center, and Gulf of Aden

    NASA Astrophysics Data System (ADS)

    Kelley, Katherine A.; Kingsley, Richard; Schilling, Jean-Guy

    2013-01-01

    The global mid-ocean ridge system is peppered with localities where mantle plumes impinge on oceanic spreading centers. Here, we present new, high resolution and high precision data for 40 trace elements in 573 samples of variably plume-influenced mid-ocean ridge basalts from the Mid-Atlantic ridge, the Easter Microplate and Salas y Gomez seamounts, the Galápagos spreading center, and the Gulf of Aden, in addition to previously unpublished major element and isotopic data for these regions. Included in the data set are the unconventional trace elements Mo, Cd, Sn, Sb, W, and Tl, which are not commonly reported by most geochemical studies. We show variations in the ratios Mo/Ce, Cd/Dy, Sn/Sm, Sb/Ce, W/U, and Rb/Tl, which are expected not to fractionate significantly during melting or crystallization, as a function of proximity to plume-related features on these ridges. The Cd/Dy and Sn/Sm ratios show little variation with plume proximity, although higher Cd/Dy may signal increases in the role of garnet in the mantle source beneath some plumes. Globally, the Rb/Tl ratio closely approximates the La/SmN ratio, and thus provides a sensitive tracer of enriched mantle domains. The W/U ratio is not elevated at plume centers, but we find significant enrichments in W/U, and to a lesser extent the Mo/Ce and Sb/Ce ratios, at mid-ocean ridges proximal to plumes. Such enrichments may provide evidence of far-field entrainment of lower mantle material that has interacted with the core by deeply-rooted, upwelling mantle plumes.

  2. Crustal structure across the post-spreading magmatic ridge of the East Sub-basin in the South China Sea: Tectonic significance

    NASA Astrophysics Data System (ADS)

    He, Enyuan; Zhao, Minghui; Qiu, Xuelin; Sibuet, Jean-Claude; Wang, Jian; Zhang, Jiazheng

    2016-05-01

    The 140-km wide last phase of opening of the South China Sea (SCS) corresponds to a N145° direction of spreading with rift features identified on swath bathymetric data trending N055° (Sibuet et al., 2016). These N055° seafloor spreading features of the East Sub-basin are cut across by a post-spreading volcanic ridge oriented approximately E-W in its western part (Zhenbei-Huangyan seamounts chain). The knowledge of the deep crustal structure beneath this volcanic ridge is essential to elucidate not only the formation and tectonic evolution of the SCS, but also the mechanism of emplacement of the post-spreading magmatism. We use air-gun shots recorded by ocean bottom seismometers to image the deep crustal structure along the N-S oriented G8G0 seismic profile, which is perpendicular to the Zhenbei-Huangyan seamounts chain but located in between the Zhenbei and Huangyan seamounts, where topographic changes are minimum. The velocity structure presents obvious lateral variations. The crust north and south of the Zhenbei-Huangyan seamounts chain is ca. 4-6 km in thickness and velocities are largely comparable with those of normal oceanic crust of Atlantic type. To the south, the Jixiang seamount with a 7.2-km thick crust, seems to be a tiny post-spreading volcanic seamount intruded along the former extinct spreading ridge axis. In the central part, a 1.5-km thick low velocity zone (3.3-3.7 km/s) in the uppermost crust is explained by the presence of extrusive rocks intercalated with thin sedimentary layers as those drilled at IODP Site U1431. Both the Jixiang seamount and the Zhenbei-Huangyan seamounts chain started to form by the intrusion of decompressive melt resulting from the N-S post-spreading phase of extension and intruded through the already formed oceanic crust. The Jixiang seamount probably formed before the emplacement of the E-W post-spreading seamounts chain.

  3. A reduced crustal magnetization zone near the first observed active hydrothermal vent field on the Southwest Indian Ridge

    NASA Astrophysics Data System (ADS)

    Zhu, Jian; Lin, Jian; Chen, Yongshun J.; Tao, Chunhui; German, Christopher R.; Yoerger, Dana R.; Tivey, Maurice A.

    2010-09-01

    Inversion of near-bottom magnetic data reveals a well-defined low crustal magnetization zone (LMZ) near a local topographic high (37°47‧S, 49°39‧E) on the ultraslow-spreading Southwest Indian Ridge (SWIR). The magnetic data were collected by the autonomous underwater vehicle ABE on board R/V DaYangYiHao in February-March 2007. The first active hydrothermal vent field observed on the SWIR is located in Area A within and adjacent to the LMZ at the local topographic high, implying that this LMZ may be the result of hydrothermal alteration of magnetic minerals. The maximum reduction in crustal magnetization is 3 A/M. The spatial extent of the LMZ is estimated to be at least 6.7 × 104 m2, which is larger than that of the LMZs at the TAG vent field on the Mid-Atlantic Ridge (MAR), as well as the Relict Field, Bastille, Dante-Grotto, and New Field vent-sites on the Juan de Fuca Ridge (JdF). The calculated magnetic moment, i.e., the product of the spatial extent and amplitude of crustal magnetization reduction is at least -3 × 107 Am2 for the LMZ on the SWIR, while that for the TAG field on the MAR is -8 × 107 Am2 and that for the four individual vent fields on the JdF range from -5 × 107 to -3 × 107 Am2. Together these results indicate that crustal demagnetization is a common feature of basalt-hosted hydrothermal vent fields at mid-ocean ridges of all spreading rates. Furthermore, the crustal demagnetization of the Area A on the ultraslow-spreading SWIR is comparable in strength to that of the TAG area on the slow-spreading MAR.

  4. A study of epidemic spreading on activity-driven networks

    NASA Astrophysics Data System (ADS)

    Zou, Yijiang; Deng, Weibing; Li, Wei; Cai, Xu

    2016-03-01

    The epidemic spreading was explored on activity-driven networks (ADNs), accounting for the study of dynamics both on and of the ADN. By employing the susceptible-infected-susceptible (SIS) model, two aspects were considered: (1) the infection rate of susceptible agent (depending on the number of its infected neighbors) evolves due to the temporal structure of ADN, rather than being a constant number; (2) the susceptible and infected agents generate unequal links while being activated, namely, the susceptible agent gets few contacts with others in order to protect itself. Results show that, in both cases, the larger epidemic threshold and smaller outbreak size were obtained.

  5. A History of Classified Activities at Oak Ridge National Laboratory

    SciTech Connect

    Quist, A.S.

    2001-01-30

    The facilities that became Oak Ridge National Laboratory (ORNL) were created in 1943 during the United States' super-secret World War II project to construct an atomic bomb (the Manhattan Project). During World War II and for several years thereafter, essentially all ORNL activities were classified. Now, in 2000, essentially all ORNL activities are unclassified. The major purpose of this report is to provide a brief history of ORNL's major classified activities from 1943 until the present (September 2000). This report is expected to be useful to the ORNL Classification Officer and to ORNL's Authorized Derivative Classifiers and Authorized Derivative Declassifiers in their classification review of ORNL documents, especially those documents that date from the 1940s and 1950s.

  6. Relationship between ridge segmentation and Moho transition zone structure from 3D multichannel seismic data collected over the fast-spreading East Pacific Rise at 9°50'N

    NASA Astrophysics Data System (ADS)

    Aghaei, O.; Nedimovic, M. R.; Canales, J.; Carton, H. D.; Carbotte, S. M.; Mutter, J. C.

    2010-12-01

    We present stack and migrated stack volumes of a fast-spreading center produced from the high-resolution 3D multichannel seismic (MCS) data collected in summer of 2008 over the East Pacific Rise (EPR) at 9°50’N during cruise MGL0812. These volumes give us new insight into the 3D structure of the lower crust and Moho Transition Zone (MTZ) along and across the ridge axis, and how this structure relates to the ridge segmentation at the spreading axis. The area of 3D coverage is between 9°38’N and 9°58’N (~1000 km2) where the documented eruptions of 1990-91 and 2005-06 occurred. This high-resolution survey has a nominal bin size of 6.25 m in cross-axis direction and 37.5 m in along-axis direction. The prestack processing sequence applied to data includes 1D and 2D filtering to remove low-frequency cable noise, offset-dependent spherical divergence correction to compensate for geometrical spreading, surface-consistent amplitude correction to balance abnormally high/low shot and channel amplitudes, trace editing, velocity analysis, normal moveout (NMO), and CMP mute of stretched far offset arrivals. The poststack processing includes seafloor multiple mute to reduce migration noise and poststack time migration. We also will apply primary multiple removal and prestack time migration to the data and compare the results to the migrated stack volume. The poststack and prestack migrated volumes will then be used to detail Moho seismic signature variations and their relationship to ridge segmentation, crustal age, bathymetry, and magmatism. We anticipate that the results will also provide insight into the mantle upwelling pattern, which is actively debated for the study area.

  7. Active and relict sea-floor hydrothermal mineralization at the TAG hydrothermal field, Mid-Atlantic Ridge

    SciTech Connect

    Rona, P.A. . Atlantic Oceanographic and Meteorological Labs.); Hannington, M.D. ); Raman, C.V. ); Thompson, G.; Tivey, M.K.; Humphris, S.E. ); Lalou, C. . Lab. CNRS-CEA); Petersen, S. Aachen Univ. of Technology )

    1993-12-01

    The TAG hydrothermal field is a site of major active and inactive volcanic-hosted hydrothermal mineralization in the rift valley of the slow-spreading Mid-Atlantic Ridge at 26[degree]N. The axial high is the principal locus of present magmatic intrusions. The TAG field contains three main areas of present and past hydrothermal activity: (1) an actively venting high-temperature sulfide mound; (2) two former high-temperature vent areas; (3) a zone of low-temperature venting and precipitation of Fe and Mn oxide deposits. The volcanic centers occur at the intersections between ridge axis-parallel normal faults and projected axis-transverse transfer faults. The intersections of these active fault systems may act as conduits both for magmatic intrusions from sources beneath the axial high that build the volcanic centers and for hydrothermal upwelling that taps the heat sources. Radiometric dating of sulfide samples and manganese crusts in the hydrothermal zones and dating of sediments intercalated with pillow lava flows in the volcanic center adjacent to the active sulfide mound indicate multiple episodes of hydrothermal activity throughout the field driven by heat supplied by episodic intrusions over a period of at least 140 [times] 10[sup 3] yr. The sulfide deposits are built by juxtaposition and superposition during relatively long residence times near episodic axial heat sources counterbalanced by mass wasting in the tectonically active rift valley of the slow-spreading oceanic ridge. Hydrothermal reworking of a relict hydrothermal zone by high-temperature hydrothermal episodes has recrystallized sulfides and concentrated the first visible primary gold reported in a deposit at an oceanic ridge.

  8. The temporal and spatial distribution of magmatism during lower crustal accretion at an ultraslow-spreading ridge: High-precision U-Pb zircon dating of ODP Holes 735B and 1105A, Atlantis Bank, Southwest Indian Ridge

    NASA Astrophysics Data System (ADS)

    Rioux, Matthew; Cheadle, Michael J.; John, Barbara E.; Bowring, Samuel A.

    2016-09-01

    Ocean Drilling Program Hole 735B at Atlantis Bank on the Southwest Indian Ridge sampled 1508 m of plutonic oceanic crust, hosted in the footwall of an oceanic detachment fault. We present new high-precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon dates from samples spanning the length of Hole 735B, and from the shallower adjacent Hole 1105A (158 m). The new dates provide the most complete and precise record of both the spatial and temporal distribution of magmatism during accretion of the lower oceanic crust to date. Whole rock and mineral geochemistry from Hole 735B define three main igneous series. Weighted mean 206Pb/238U dates suggest each igneous series intruded beneath the preceding series. Weighted mean 206Pb/238U dates range from 12.175 to 11.986 Ma in Series 1; 11.974 to 11.926 Ma in Series 2; and 11.936 to 11.902 Ma in Series 3 (±0.015 to 0.069 Ma). Weighted mean 206Pb/238U dates from Hole 1105A range from 11.9745 to 11.9573 Ma (±0.0082 to 0.0086 Ma). The Hole 1105A dates are coeval with Series 2 in Hole 735B, consistent with previous correlations of Fe-Ti oxide-rich layers between the two holes, suggesting individual magmatic series formed sheet-like bodies that were ≥250 m thick and extended ≥1.1 km parallel to the ridge axis (E-W) and ≥0.48 km in the spreading direction (N-S). The data suggest a total duration of magmatism in Hole 735B of ≥ 0.214 ± 0.032 Ma, corresponding to accretion over a horizontal distance of ≥ 2.6 ± 0.4 km. The crust at Atlantis Bank was formed during active detachment faulting, and the successive underplating of each magmatic unit may have been favored in this environment. The combined U-Pb dates, and reported Ti-in-zircon temperatures, are consistent with magmatic cooling rates of 103-104 °C/Ma over the temperature interval of 900-700 °C.

  9. The temporal and spatial distribution of magmatism during lower crustal accretion at an ultraslow-spreading ridge: High-precision U-Pb zircon dating of ODP Holes 735B and 1105A, Atlantis Bank, Southwest Indian Ridge

    NASA Astrophysics Data System (ADS)

    Rioux, Matthew; Cheadle, Michael J.; John, Barbara E.; Bowring, Samuel A.

    2016-09-01

    Ocean Drilling Program Hole 735B at Atlantis Bank on the Southwest Indian Ridge sampled 1508 m of plutonic oceanic crust, hosted in the footwall of an oceanic detachment fault. We present new high-precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon dates from samples spanning the length of Hole 735B, and from the shallower adjacent Hole 1105A (158 m). The new dates provide the most complete and precise record of both the spatial and temporal distribution of magmatism during accretion of the lower oceanic crust to date. Whole rock and mineral geochemistry from Hole 735B define three main igneous series. Weighted mean 206Pb/238U dates suggest each igneous series intruded beneath the preceding series. Weighted mean 206Pb/238U dates range from 12.175 to 11.986 Ma in Series 1; 11.974 to 11.926 Ma in Series 2; and 11.936 to 11.902 Ma in Series 3 (±0.015 to 0.069 Ma). Weighted mean 206Pb/238U dates from Hole 1105A range from 11.9745 to 11.9573 Ma (±0.0082 to 0.0086 Ma). The Hole 1105A dates are coeval with Series 2 in Hole 735B, consistent with previous correlations of Fe-Ti oxide-rich layers between the two holes, suggesting individual magmatic series formed sheet-like bodies that were ≥250 m thick and extended ≥1.1 km parallel to the ridge axis (E-W) and ≥0.48 km in the spreading direction (N-S). The data suggest a total duration of magmatism in Hole 735B of ≥ 0.214 ± 0.032 Ma, corresponding to accretion over a horizontal distance of ≥ 2.6 ± 0.4 km. The crust at Atlantis Bank was formed during active detachment faulting, and the successive underplating of each magmatic unit may have been favored in this environment. The combined U-Pb dates, and reported Ti-in-zircon temperatures, are consistent with magmatic cooling rates of 103-104 °C/Ma over the temperature interval of 900-700 °C.

  10. Insights on the Biology of the Eastern Lau Spreading Center from Studies on the East Pacific Rise and Juan de Fuca Ridge

    NASA Astrophysics Data System (ADS)

    Fisher, C. R.; Sen, A.; Becker, E.

    2011-12-01

    A primary goal of the Ridge 2000 program was to conduct comparable interdisciplinary studies at a few fundamentally different sites that would facilitate comparisons among sites and development of concepts with broad application across deep ocean ridge spreading centers. Although the Eastern Lau Spreading Center (ELSC) was the least known of the three Integrated Study Sites, we were able to make amazingly fast progress on understanding this system because we could draw on technology and experience developed during the RIDGE program to plan and conduct the work, and now interpret our findings in the context of the rich literature and Ridge 2000 studies on East Pacific Rise (EPR) , Juan de Fuca Ridge (JdFR), and Mid Atlantic Ridge communities. The ELSC communities not only house different species than those of the other regions, but unlike the often tubeworm dominated E. Pacific vents, they are structured by symbiont reliant species that are mobile; snails and mussels. Although there is some variation with lava type on the ELSC, the 4 species of large, symbiont-containing snails largely occupy the niches filled by tubeworms and mussels on the EPR, while the niche of the mussel in the W. Pacific vents is quite different from that of its EPR cousin. Although we have not observed any significant tectonic or magmatic events during our studies of the ELSC, 4 years of study considered in the context of what we have learned on the EPR and JdFR allow us to formulate and begin testing hypotheses about temporal change and succession in these very different and much less visited ecosystems. Furthermore, athough Paralvinella fijiensis are only found in limited areas on some chimneys and flanges, unlike the situation on the EPR and JdFR ,the ELSC chimney communities are largely composed of the same species as are found in diffuse flow on the lavas. The ELSC chimney communities are also remarkably stable, hosting some of the largest and apparently oldest individuals found on the

  11. Continental rifting and metamorphic core complex formation ahead of the Woodlark spreading ridge, D'Entrecasteaux Islands, Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Little, Timothy A.; Baldwin, S. L.; Fitzgerald, P. G.; Monteleone, B.

    2007-02-01

    We evaluate the role of a metamorphic core complex (MCC) on Normanby Island in the Woodlark rift. Located <30 km from an active mid-ocean ridge (MOR), a >1 km thickness of blueschist-derived mylonites formed in a midcrustal shear zone during the Pliocene at ˜400-500°C. This top-to-the-north zone appears to have reactivated the gently dipping base of the Papuan ophiolite (Papuan Ultramafic Body, PUB), and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the MCC and rapid subsidence of the Woodlark Rise. Mylonites in the MCC's lower plate have been exhumed along a detachment as a result of >50 km of slip at rates of >12 mm/yr. The inactive, back-tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the Plio-Pleistocene plate motion. A second SE vergent detachment has been established on the opposite flank of this rolling-hinge style MCC, probably since <0.5 Ma. Centimeters per year slip rates on these two faults can account for most of the Pleistocene plate motion in this eastern sector of the Woodlark rift, and confirm the important role of MCCs in exhuming very young HP rocks in this rift. Paleopiezometry of mylonites using recrystallized quartz grain size indicates flow stresses of ˜30 MPa before the rocks were overprinted by extension fractures. These results imply high pore fluid pressures (λ > 0.8) at depth, and provide a sufficient mechanism for activating low-angle normal faults in the rift. MCC inception was not localized to the tip of the Woodlark MOR. Instead, extreme crustal thinning near the MCC preconditioned later continental breakup. The lower crust appears to be weak, thickening beneath unloaded footwalls to uplift MCCs above sea level, and flowing laterally to even out regional crustal thickness contrasts on a 1-6 m.y. timescale. Deep-seated transforms separate rheologically distinct domains in which extension has been localized along the weak PUB

  12. The relationships between volcanism, tectonism and hydrothermal activity on the Mid-Atlantic Ridge south of the equator

    NASA Astrophysics Data System (ADS)

    Devey, C. W.; German, C. R.; Haase, K. M.; Lackschewitz, K. S.; Melchert, B.; Connelly, D.; Parson, L. M.

    2009-04-01

    Using data from the complete bathymetric and side-scan (TOBI) coverage of the Mid-Atlantic Ridge 2-14 °S collected since 2004 in conjunction with the results of extensive prospecting for hydrothermal systems in this area we attempt to formulate a general model for the interplay between volcanism, tectonics and hydrothermalism on a slow-spreading ridge. The model defines three basic types of ridge morphology with specific hydrothermal characteristics: (a) A deep, tectonically-dominated rift valley where hydrothermalism is seldom associated with volcanism and much more likely confined to long-lived bounding faults (b) a shallower, segment-centre bulge where a combination of repeated magmatic activity and tectonism results in repeated, possibly temporally overlapping periods of hydrothermal activity on the ridge axis and (c) a very shallow, inflated axis beneath which temperatures in all but the uppermost crust are so high that deformation is ductile, inhibiting the formation of high-porosity deep fractures and severely depressing hydrothermal circulation. This model is used together with predicted bathymetry to provide forecasts of the best places to look for hydrothermal sites in the remaining unexplored regions of the South Atlantic

  13. Epidemic spreading and immunization in node-activity networks

    NASA Astrophysics Data System (ADS)

    Wu, Qingchu; Chen, Shufang

    2015-09-01

    In this paper, we study the epidemic spreading in node-activity networks, where an individual participates in social networks with a certain rate h. There are two cases for h: the state-independent case and the state-dependent case. We investigate the epidemic threshold as a function of h compared to the static network. Our results suggest the epidemic threshold cannot be exactly predicted by using the analysis approach in the static network. In addition, we further propose a local information-based immunization protocol on node-activity networks. Simulation analysis shows that the immunization can not only eliminate the infectious disease, but also change the epidemic threshold via increasing the immunization parameter.

  14. Reconstruction of the flanks of the Mid-Atlantic Ridge, 28° to 29°N: Implications for evolution of young oceanic lithosphere at slow-spreading centers

    NASA Astrophysics Data System (ADS)

    Sloan, Heather; Patriat, Philippe

    2004-09-01

    We reconstruct the flanks of the Mid-Atlantic Ridge (MAR) between 28° and 29°N from 1 to 10 Ma at intervals of 1 Myr for the purpose of investigating evolution of young oceanic lithosphere morphology and its variation through time using an innovative method that combines seafloor subsidence correction with interpolated isochrons and rotation poles. Reconstruction results are consistent with formation of abyssal hills every 1 to 2 Myr in 2-3 Myr old lithosphere at the outer edge of the ridge mountains as a result of transition from dynamic regime near the axis to isostatic regime of the flanks. The oblique passage of structures formed at the axial valley walls through this transition zone may play a role in the development of inside corner high bathymetry. Asymmetric juxtaposition of abyssal hill morphology in reconstruction is indicative of independent formation and evolution of the morphotectonic fabric on opposing flanks. The two major factors affecting asymmetric ridge flank morphology are found to be sense of axial offset and fluctuation of magmatic activity at the segment scale. Sense of axial offset determines the relative distribution of inside and outside corner bathymetry on the flanks. Enhanced magma supply and associated segment propagation may contribute to half-spreading rate asymmetry, accretion of thicker crust, and formation of larger abyssal hills on the faster spreading flank. Consistent alignment in reconstruction of the base of the steep walls bounding inside corner bathymetry confirms that they mark the boundaries between segments on the flanks and that the deep sediment-filled basins, typically identified as discordant zones, are attributable to outside corner bathymetry.

  15. Cortical spreading depression activates and upregulates MMP-9

    PubMed Central

    Gursoy-Ozdemir, Yasemin; Qiu, Jianhua; Matsuoka, Norihiro; Bolay, Hayrunnisa; Bermpohl, Daniela; Jin, Hongwei; Wang, Xiaoying; Rosenberg, Gary A.; Lo, Eng H.; Moskowitz, Michael A.

    2004-01-01

    Cortical spreading depression (CSD) is a propagating wave of neuronal and glial depolarization and has been implicated in disorders of neurovascular regulation such as stroke, head trauma, and migraine. In this study, we found that CSD alters blood-brain barrier (BBB) permeability by activating brain MMPs. Beginning at 3–6 hours, MMP-9 levels increased within cortex ipsilateral to the CSD, reaching a maximum at 24 hours and persisting for at least 48 hours. Gelatinolytic activity was detected earliest within the matrix of cortical blood vessels and later within neurons and pia arachnoid (≥3 hours), particularly within piriform cortex; this activity was suppressed by injection of the metalloprotease inhibitor GM6001 or in vitro by the addition of a zinc chelator (1,10-phenanthroline). At 3–24 hours, immunoreactive laminin, endothelial barrier antigen, and zona occludens-1 diminished in the ipsilateral cortex, suggesting that CSD altered proteins critical to the integrity of the BBB. At 3 hours after CSD, plasma protein leakage and brain edema developed contemporaneously. Albumin leakage was suppressed by the administration of GM6001. Protein leakage was not detected in MMP-9–null mice, implicating the MMP-9 isoform in barrier disruption. We conclude that intense neuronal and glial depolarization initiates a cascade that disrupts the BBB via an MMP-9–dependent mechanism. PMID:15146242

  16. Patterns of volcanism and tectonism at a slow-spreading segment of the Mid-Atlantic Ridge (Lucky Strike, 37N): preliminary results from near-bottom geological and geophysical surveys

    NASA Astrophysics Data System (ADS)

    Escartin, J.; Soule, S.; Bezos, A.; Cannat, M.; Fornari, D. J.; Ballu, V.; Humphris, S.

    2006-12-01

    Patterns of volcanism and tectonism and the mechanisms that influence them are not well understood at slow- spreading plate boundaries. Is magma supply persistent or episodic? Is tectonic strain symmetric or asymmetric? Are volcanism and tectonism distributed across the rift valley or localized along narrow bands of crust? Systematic, segment-scale observations, measurements, and sampling are needed to address these questions. During a recent cruise (GRAVILUCK, Aug. 2006) we conducted near-bottom surveys across the axial valley of the Lucky Strike segment of the Mid-Atlantic Ridge (37°N) to determine where active volcanic and tectonic processes are occurring within the rift valley and their relationship with a recently recognized mid-crustal magma body at the segment center beneath Lucky Strike volcano [Singh et al., 2006]. Using a deep-towed digital imaging system (WHOI TowCam) and a submersible (Nautile) we collected still and video imagery, high-resolution bathymetry, magnetic data, and rock samples along 11 across-axis and 3 along-axis profiles covering ~80 km. Preliminary interpretation of the seafloor imagery shows that the majority of the most recent volcanic activity is in the form of jumbled sheet flows that are concentrated within a discontinuous narrow graben along the axis of the rift valley that bisects Lucky Strike volcano. In rare cases we observe young (i.e., less sedimented and unfaulted) pillow ridges up to 3 km from the axis of the rift valley. Recent volcanic activity appears to be more prevalent south of the volcano, and cuts across distinct geologic terrains characterized by extensive sheet flows near the segment center and axial volcanic ridges to the north and south of Lucky Strike Volcano. We will present a preliminary interpretation of the distribution and relative ages of volcanic deposits and fault characteristics across the rift valley, lava compositions, and magnetic intensities. We compare these observations with existing sidescan

  17. Lava Morphology Classification of a Fast-Spreading Ridge Using Deep-Towed Sonar Data: East Pacific Rise

    NASA Astrophysics Data System (ADS)

    Meyer, J.; White, S.

    2005-05-01

    Classification of lava morphology on a regional scale contributes to the understanding of the distribution and extent of lava flows at a mid-ocean ridge. Seafloor classification is essential to understand the regional undersea environment at midocean ridges. In this study, the development of a classification scheme is found to identify and extract textural patterns of different lava morphologies along the East Pacific Rise using DSL-120 side-scan and ARGO camera imagery. Application of an accurate image classification technique to side-scan sonar allows us to expand upon the locally available visual ground reference data to make the first comprehensive regional maps of small-scale lava morphology present at a mid-ocean ridge. The submarine lava morphologies focused upon in this study; sheet flows, lobate flows, and pillow flows; have unique textures. Several algorithms were applied to the sonar backscatter intensity images to produce multiple textural image layers useful in distinguishing the different lava morphologies. The intensity and spatially enhanced images were then combined and applied to a hybrid classification technique. The hybrid classification involves two integrated classifiers, a rule-based expert system classifier and a machine learning classifier. The complementary capabilities of the two integrated classifiers provided a higher accuracy of regional seafloor classification compared to using either classifier alone. Once trained, the hybrid classifier can then be applied to classify neighboring images with relative ease. This classification technique has been used to map the lava morphology distribution and infer spatial variability of lava effusion rates along two segments of the East Pacific Rise, 17 deg S and 9 deg N. Future use of this technique may also be useful for attaining temporal information. Repeated documentation of morphology classification in this dynamic environment can be compared to detect regional seafloor change.

  18. Hydrothermal activity and core complex formation at the Arctic Mid-Ocean Ridge: An overview of preliminary results of the H2DEEP expedition to the southern Knipovich Ridge at 73N

    NASA Astrophysics Data System (ADS)

    Pedersen, R. B.; Thorseth, I. H.; Olson, E.; Hellevang, H.; Okland, I.; Baumberger, T.; Lilley, M.; Bruvoll, V.; Mjelde, R.; Haflidason, H.

    2007-12-01

    The oblique spreading Mohns Ridge passes into the highly oblique spreading Knipovich Ridge through a near 90 degree bend in the ridge axis at 73-74N. Multibeam mapping of this area shows that a 30 km long axial volcanic ridge (AVR) with a 500m high summit occupy the rift valley floor in the central part of the bend where the axis is perpendicular to the spreading direction. The volcanic activity decreases northwards as the obliquity of the spreading increases and this is associated with an increase in the water depth from 2000-2500 m to 3000-3500 m. A hydrothermal plume was located at the eastern side of the AVR with methane values reaching 260 nmol/l and hydrogen values 53 nmol/l. These anomalies are associated with small positive temperature anomaly, but no significant particle anomaly could be detected. The rift valley in the area is partly filled with sediments derived from the nearby Bjornoya fan, and reflection seismic profiles across the ridge demonstrate that sediments thicknesses in the rift valley locally exceeds several hundred meters. Gravity cores of the upper 4 m of these sediments show multiple glass- and iron-rich laminas and layers, demonstrating that these sediments represent a record of hydrothermal and volcanic activity in the area. Seamounts at the western flank of the ridge extend to 600 m below sea level and are limited by low angle detachment faults to the east. A flat summit area suggests that one of the seamounts has been at or close to sea level. Gabbros, troctolites and serpentinites were sampled from the fault surfaces, demonstrating that lower crust and mantle rocks have been exhumed in the area, and bathymetry data indicates that this oceanic core complex cover an area of about 500 km2. A younger detachment fault appears to develop at the inner rift wall just west of the AVR. The rift valley sediments are affected by the faulting and the seismic stratigraphy of these sediments constrain the history of core complex formation.

  19. Data Sharing Report for the Quantification of Removable Activity in Various Surveillance and Maintenance Facilities at the Oak Ridge National Laboratory Oak Ridge TN

    SciTech Connect

    King, David A

    2013-12-12

    The U.S. Department of Energy (DOE) Oak Ridge Office of Environmental Management (OR-EM) requested that Oak Ridge Associated Universities (ORAU), working under the Oak Ridge Institute for Science and Education (ORISE) contract, provide technical and independent waste management planning support using American Recovery and Reinvestment Act (ARRA) funds. Specifically, DOE OR-EM requested that ORAU plan and implement a sampling and analysis campaign targeting potential removable radiological contamination that may be transferrable to future personal protective equipment (PPE) and contamination control materials—collectively referred to as PPE throughout the remainder of this report—used in certain URS|CH2M Oak Ridge, LLC (UCOR) Surveillance and Maintenance (S&M) Project facilities at the Oak Ridge National Laboratory (ORNL). Routine surveys in Bldgs. 3001, 3005, 3010, 3028, 3029, 3038, 3042, 3517, 4507, and 7500 continuously generate PPE. The waste is comprised of Tyvek coveralls, gloves, booties, Herculite, and other materials used to prevent worker exposure or the spread of contamination during routine maintenance and monitoring activities. This report describes the effort to collect and quantify removable activity that may be used by the ORNL S&M Project team to develop radiation instrumentation “screening criteria.” Material potentially containing removable activity was collected on smears, including both masselin large-area wipes (LAWs) and standard paper smears, and analyzed for site-related constituents (SRCs) in an analytical laboratory. The screening criteria, if approved, may be used to expedite waste disposition of relatively clean PPE. The ultimate objectives of this effort were to: 1) determine whether screening criteria can be developed for these facilities, and 2) provide process knowledge information for future site planners. The screening criteria, if calculated, must be formally approved by Federal Facility Agreement parties prior to use for

  20. Surficial Expressions of Deeper Processes- Ridge 2000 Spurs Understanding of Mantle-Hydrothermal Connections and the Role of Crustal Processes at Oceanic Spreading Centers

    NASA Astrophysics Data System (ADS)

    Blackman, D. K.

    2011-12-01

    A decade ago the Ridge 2000 (R2K) program began implementing the Integrated Studies Site (ISS) strategy as a means to advance understanding of the linked magmatic/tectonic/hydrothermal systems that dictate the structure and ecosystems observed in young crust along the spreading axis. Through comparison amongst ISSs and other well-studied sites, where controlling factors such as spreading rate or tectonic/thermal setting differ, a number of new insights have been gained. I will review progress on 3 aspects, emphasizing R2K contributions but also noting a few other recent results: the pattern of magma supply, along and across axis; ridge segmentation and crust/mantle interplay; threshold behavior and limiting processes that are manifested in crustal properties. The results are derived from petrological/geochemical, seismic (imaging, seismicity, compliance), electromagnetic, modeling, and mapping investigations, so I will touch on each of these types of constraint. The breadth of the melt supply zone is an example where R2K results document that influx to at least the lower crust can extend out several km beyond the axial graben. Such knowledge addresses a fundamental problem in Earth Sciences- how magmatism and faulting interact and the potential for hydrothermal circulation to both influence, and be influenced by, their distributions. In addition to briefly summarizing work already completed, I will highlight efforts on the mantle portions of the Juan de Fuca and Lau ISS that are currently underway, using data/modeling from the final phase of R2K, to tease out further connections between mantle processes and crustal structure, within which the now-known-to-be-ubiquitous hydrothermal systems develop.

  1. Experimental quantification of P-T conditions of mantle refertilisation at shallow depth under spreading ridges and formation of plagioclase + spinel lherzolite

    NASA Astrophysics Data System (ADS)

    Chalot-Prat, Françoise; Falloon, Trevor J.; Green, David H.

    2014-05-01

    We studied the first-order melting process of differentiation in the Earth, and the major process of rejuvenation of the upper mantle after melting related to plate spreading (Chalot-Prat et al, 2010; 2013). We conducted experiments at High Pressure (0.75 and 0.5 GPa) and High Temperature (1260-1100°C) to obtain magma compositions in equilibrium with the mineral assemblages of a plagioclase + spinel lherzolite. These PT conditions prevail at 17-30km below axial oceanic spreading ridges. We used a "trial and error" approach in a system involving nine elements (Cr-Na-Fe-Ca-Mg-Al-Si-Ti-Ni). This approaches as closely as possible a natural mantle composition, Cr being a key element in the system. Our objectives were : • to determine experimentally the compositions of melts in equilibrium with plagioclase + spinel lherzolite, with emphasis on the role of plagioclase composition in controlling melt compositions; • to test the hypothesis that MORB are produced at shallow depth (17-30kms) • to quantify liquid- and mantle residue compositional paths at decreasing T and low P to understand magma differentiation by "percolation-reaction" at shallow depth in the mantle; • to compare experimental mantle mineral compositions to those of re-fertilised oceanic mantle lithosphere outcropping at the axis of oceanic spreading ridges, enabling quantification of the pressure (i.e. depth) and temperature of the re-fertilisation process that leads to formation of plagioclase and indicates the minimum thickness of the lithosphere at ridge axes. In the normative basalt tetrahedron, liquids plot on two parallel cotectic lines from silica-oversaturated (basaltic andesite at 0.75 GPa or andesite at 0.5 GPa) at the calcic end to silica-undersaturated compositions (trachyte) at the sodic end. The lower the pressure, the greater the silica oversaturation. Besides the plagioclase solid solution has a dominant role in determining the solidus temperature of plagioclase + spinel lherzolites

  2. Protracted construction of gabbroic crust at a slow spreading ridge: Constraints from 206Pb/238U zircon ages from Atlantis Massif and IODP Hole U1309D (30°N, MAR)

    USGS Publications Warehouse

    Grimes, Craig B.; John, Barbara E.; Cheadle, Michael J.; Wooden, Joseph L.

    2008-01-01

    Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages of 24 samples from oceanic crust recovered in Integrated Ocean Drilling Program (IODP) Hole U1309D and from the surface of Atlantis Massif, Mid-Atlantic Ridge (MAR) (30°N) document a protracted history of accretion in the footwall to an oceanic detachment fault. Ages for 18 samples of evolved Fe-Ti oxide gabbro and felsic dikes collected 40–1415 m below seafloor in U1309D yield a weighted mean of 1.20 ± 0.03 Ma (mean square of weighted deviates = 7.1). However, the ages range from 1.08 ± 0.07 Ma and 1.28 ± 0.05 Ma indicating crustal construction occurred over a minimum of 100–200 ka. The zircon ages, along with petrologic observations, indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka. The oldest ages are observed below 600 mbsf, an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault. The data are most consistent with a “multiple sill” model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1.4 km. Zircon ages from broadly spaced samples collected along the southern ridge of Atlantis Massif yield a detachment fault slip rate of 28.7 ± 6.7 mm/a and imply significant asymmetric plate spreading (up to 100% on the North American plate) for at least 200 ka during core complex formation.

  3. Oceanic core complex and newly-formed basalts in axial part of Mid-Atlantic Ridge (5-7oN): Implications for formation and evolution of slow-spreading ridges

    NASA Astrophysics Data System (ADS)

    Bortnikov, N. S.; Sharkov, E. V.

    2011-12-01

    Data on petrography, mineralogy and U-Pb and Sr isotope studies of rocks exposed in the axial valley of the Mid-Atlantic Ridge between 5oN to 7oN are discussed. Samples were collected in 10th cruise of R/V "Academik Ioffe" (2001-2002) and 22nd cruise of R/V "Professor Logachev" (2003). Dredged rocks are serpentinized mantle peridotites, gabbros, including ferrogabbros and trondhjemites as well as fresh basalts with chilled glassy crusts. Plutonic rocks are strongly altered and tectonized. The association of rocks is identical to that defined as oceanic core complexes (OCC), described by Tucholke et al. (1998), Escartin et al. (2003), Ildefonse et al. (2007), MacLeod et al. (2009). Two complexes of altered gabbros: (1) primitive magnesian gabbros, derived from MORB, and (2) hornblende-bearing ferrogabbros defined as siliceous Fe-Ti-oxide series were found. The latter are saturated and supersaturated with silica under relatively high water content. Such features are usually typical of subduction-related magmas, but they have high contents of Ti, Fe, Nb, Ta and P, which characterize plume-related magmas. SHRIMP-studies of the magmatic zircon grains with oscillatory zoning, extracted from the gabbros, showed that their U-Pb age range from 0.7 to 2.3 Ma. Newly-formed fresh basaltic flows covered protrusions in the rift valleys and fill in dips in axial valleys where they overlap both altered gabbros and serpentinites. Fresh basalts are close in composition to oceanic platobasalts. These basalts are considered to be derived due to crystallizing differentiation in shallow transitional magmatic chambers (intrusions). This assumes the present-day magmatic activity beneath the spreading zone in this area. Several centimeters-scale Sr-isotopic heterogeneity in basalts was discovered. It is considered as a result of incomplete dissolution in basaltic magmas of gabbro fragments, captured during rapid ascent of the melt through the lower crust. This is evidence that a magmatic

  4. Axial magnetic anomalies over slow-spreading ridge segments: insights from numerical 3-D thermal and physical modelling

    NASA Astrophysics Data System (ADS)

    Gac, Sébastien; Dyment, Jérôme; Tisseau, Chantal; Goslin, Jean

    2003-09-01

    The axial magnetic anomaly amplitude along Mid-Atlantic Ridge segments is systematically twice as high at segment ends compared with segment centres. Various processes have been proposed to account for such observations, either directly or indirectly related to the thermal structure of the segments: (1) shallower Curie isotherm at segment centres, (2) higher Fe-Ti content at segment ends, (3) serpentinized peridotites at segment ends or (4) a combination of these processes. In this paper the contribution of each of these processes to the axial magnetic anomaly amplitude is quantitatively evaluated by achieving a 3-D numerical modelling of the magnetization distribution and a magnetic anomaly over a medium-sized, 50 km long segment. The magnetization distribution depends on the thermal structure and thermal evolution of the lithosphere. The thermal structure is calculated considering the presence of a permanent hot zone beneath the segment centre. The `best-fitting' thermal structure is determined by adjusting the parameters (shape, size, depth, etc.) of this hot zone, to fit the modelled geophysical outputs (Mantle Bouguer anomaly, maximum earthquake depths and crustal thickness) to the observations. Both the thermoremanent magnetization, acquired during the thermal evolution, and the induced magnetization, which depends on the present thermal structure, are modelled. The resulting magnetic anomalies are then computed and compared with the observed ones. This modelling exercise suggests that, in the case of aligned and slightly offset segments, a combination of higher Fe-Ti content and the presence of serpentinized peridotites at segment ends will produce the observed higher axial magnetic anomaly amplitudes over the segment ends. In the case of greater offsets, the presence of serpentinized peridotites at segment ends is sufficient to account for the observations.

  5. Modeling lithosphere hydration processes at slow spreading ridges: What are the parameters controlling the formation of low temperature ultramafic hydrothermal vents?

    NASA Astrophysics Data System (ADS)

    Gouze, P.; Lods, G.; Godard, M.; Andreani, M.

    2008-12-01

    The numerous ultramafic-hosted hydrothermal systems discovered during the last ten years emphasized the importance of peridotite alteration (serpentinization) as means of chemical exchange between the mantle and the ocean at slow- and ultra-slow spreading ridges. Hydration of the cooling ultramafic lithosphere results from the thermal retraction of the poly-mineral mantle assemblage and/or faulting of stretched lithosphere; it induces in return strong changes in the permeability field. A better understanding of this highly complex system is essential for constraining mass and energy transfers at ridge axis. In order to characterize the roles of these various parameters as well as their feedback effects and to explore different hydro-geothermal initial and boundary conditions, we carried out a series of a two-dimensional flow/reaction simulations. In the model, thermal convection flow is coupled with serpentinization, which is described by the thermo- kinetic-controlled reaction of the hydration of olivine and orthopyroxene (assumed to be pure pure forsterite and enstatite respectively). The changes in porosity and permeability associated with (i) thermal retraction of the solid while cooling and (ii) serpentinization processes are computed including energy production and water consumption during reaction. These highly coupled processes generate a non-linear evolution of the flow regimes, inducing (transient) focalized fluid outputs (events) triggered by the thermal convection. The comparison between the results of numerical simulations and field observations (distribution of the events, outlet fluid composition and distribution of the serpentine at depth) is used to discuss the model parameters and determine which ones control the hydrodynamic processes in natural hydrothermal systems.

  6. The Masked Semantic Priming Effect Is Task Dependent: Reconsidering the Automatic Spreading Activation Process

    ERIC Educational Resources Information Center

    de Wit, Bianca; Kinoshita, Sachiko

    2015-01-01

    Semantic priming effects are popularly explained in terms of an automatic spreading activation process, according to which the activation of a node in a semantic network spreads automatically to interconnected nodes, preactivating a semantically related word. It is expected from this account that semantic priming effects should be routinely…

  7. Masked Priming Effects in Aphasia: Evidence of Altered Automatic Spreading Activation

    ERIC Educational Resources Information Center

    Silkes, JoAnn P.; Rogers, Margaret A.

    2012-01-01

    Purpose: Previous research has suggested that impairments of automatic spreading activation may underlie some aphasic language deficits. The current study further investigated the status of automatic spreading activation in individuals with aphasia as compared with typical adults. Method: Participants were 21 individuals with aphasia (12 fluent, 9…

  8. Chemical characteristics of magma and related seafloor sulfide deposits on back-arc spreading center and off-ridge volcanoes in Southern Mariana Trough

    NASA Astrophysics Data System (ADS)

    Urabe, T.; Kanamori, S.; Ishibashi, J.; Kentaro, K.; Sato, H.; Kato, S.; Toyoda, S.

    2012-12-01

    The back-arc basalt in Mariana Trough is characterized by fluid-dominated components (Stolper and Newman, 1994). They suggested that the H2O-enriched magma of the Mariana Trough is formed as melting mixture between MORB-type mantle source and H2O-rich component which is likely to be derived from the subducting slab. Four active and one inactive hydrothermal sites were found within a distance of 5 km in Southern Mariana Trough; that is, Snail site (12o57.19'N, 143o37.16'E, depth:2861m) and Yamanaka site (12o56.64'N, 143o36.80'E, depth: 2823m) on the spreading-axis, Archean site (12o56.35'N, 143o37.89'E, depth: 2986m), and Pika+Urashima sites (12o55.13'N, 143o38.92'E, depth: 2773m) on the off-axis seamount, respectively. We conducted nine BMS (Benthic Multi-coring System) drillings during the Hakurei-Maru No.2 cruise of TAIGA project (see below) in June 2010. Both basalt glasses and associated seafloor massive sulfide ores from these sites are cored and served for ICP-MS analyses. Multi-element plot of basalt glass indicates that both on-axis and off-axis basalts have similar pattern and are categorized as differentiated MORB and basaltic andesite which cannot be produced by fractionation of MORB, respectively. Sulfide ores at on-axis and off-axis sites show similar mineral assemblage of pyrite/marcasite, sphalerite, chalcopyrite, barite, and limited occurrence of galena only at on-axis site. Fluid-mobile elements such as As, Ba, Pb and others in sulfide ores show systematic increase at off-axis sites which reflect the influence of subduction zone fluids towards the Mariana arc. The sulfur isotope composition of pyrite/marcasite from on-axis sites shows values (+6.4 - +7.9 permil) typically observed in arc magma-related hydrothermal deposits (Suzuki, unpubl. data). On the other hand, those observed at off-axis sites (Archean; +3.6 - +6.9 permil, Pika; +0.8 - +3.5 permil) are similar to the composition of sulfides on mid-ocean ridges where the influence of sulfur

  9. Glacial-interglacial trench supply variation, spreading-ridge subduction, and feedback controls on the Andean margin development at the Chile triple junction area (45-48°S)

    NASA Astrophysics Data System (ADS)

    Bourgois, Jacques; Guivel, Christele; Lagabrielle, Yves; Calmus, Thierry; BoulèGue, Jacques; Daux, ValéRie

    2000-04-01

    During the Chile triple junction (CTJ) cruise (March-April 1997), EM12 bathymetry and seismic reflection data were collected in the vicinity of the Chile triple junction (45-480S), where an active spreading ridge is being subducted beneath the Andean continental margin. Results show a continental margin development shaped by tectonic processes spanning a spectrum from subduction-erosion to subduction-accretion. The Andean continental margin and the Chile trench exhibit a strong segmentation which reflects the slab segmentation and the Chile triple junction migration. Three segments were identified along the Andean continental margin: the presubduction, the synsubduction, and the postsubduction segments, from north to south. Both climate-induced variations of the sediment supply to the trench and the tectonic reorganization at the Nazca-Antarctica plate boundary involving postsubduction ridge jump are the two main factors that control the tectonic regime of this continental margin. Along the survey area we infer the succession of two different periods during the last glacial-interglacial cycle: a glacial period with ice-rafted detrital discharges restricted to the shoreline area and low river output and a warmer period during which the Andean ice cap retreat allowed the Andes to be drained off. During these warm periods, rapid increase in trench deposition caused the margin to switch from subductionerosion or nonaccretion to subduction-accretion: (1) along the presubduction segment after the last deglaciation and (2) along the postsubduction segment after the interglacial episode at 130-117 ka. Conversely, a nonaccretion or subduction-érosion mode characterized the presubduction and postsubduction segments during glacial maximums. The major effects of subduction of the buoyant Chile ridge include a shallow trench which diverts trench sediment supply and tectonic instabilities at the Nazca-Antarctica plate boundary. We suggest that a postsubduction westward jump of

  10. Hydrothermal plumes over spreading ridges and related deposits in the northeast Pacific Ocean: The East Pacific Rise near 11 degrees north and 21 degrees north, Explorer Ridge and J. Tuzo Wilson Seamounts

    SciTech Connect

    McConachy, T.F.

    1988-01-01

    Hydrothermal plumes emanating from hot springs over spreading ridges in the north east Pacific Ocean have been mapped and sampled using the submersible ALVIN and equipment deployed from surface ships. The geologic setting and polymetallic sulfides of the vent field producing the hydrothermal plume at 11{degree}N have also been examined. At 11{degree}N, two distinct metalliferous components are delivered to the intermediate to far-field from high temperature black smoker discharge as a result of the physical and chemical processes that occur in the lower 32 m of the plume. About 60 volume % of this material is estimated to settle within a 6-km-radius of the vent field, based on the results of SEM-IPS grain-size analyses and their application to a published particle settling model. The second component delivered to the far-field consists of the remaining 40 volume % of fine-grained sulfides and non-sulfides, 10% of dissolved Fe which will eventually precipitate as oxyhydroxides, and {>=} 80% of the hydrothermally injected Mn, Si, and probably Ba. At 21{degree} N by contrast, only 20 volume % of the smoke particles is conservatively estimated to settle within a 8-km-radius of the NGS vent due to their finer grain size and a higher terminal height of the buoyant hydrothermal plume.

  11. The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge

    NASA Astrophysics Data System (ADS)

    Cluzel, Dominique; Jourdan, Fred; Meffre, SéBastien; Maurizot, Pierre; Lesimple, StéPhane

    2012-06-01

    Amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of New Caledonia (termed Peridotite Nappe) recrystallized in the high-temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the Eocene metamorphic complex. Amphibolites mostly display the geochemical features of MORB with a slight Nb depletion and thus are similar to the youngest (Late Paleocene-Eocene) BABB components of the allochthonous Poya Terrane. Thermochronological data from hornblende (40Ar/39Ar), zircon, and sphene (U-Pb) suggest that these mafic rocks recrystallized at ˜56 Ma. Using various geothermobarometers provides a rough estimate of peak recrystallization conditions of ˜0.5 GPa at ˜800-950°C. The thermal gradient inferred from the metamorphic assemblage (˜60°C km-1), geometrical relationships, and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction/obduction system and recrystallized when they subducted below young and hot oceanic lithosphere. They were detached from the down-going plate and finally thrust onto unmetamorphosed Poya Terrane basalts. This and the occurrence of slab melts at ˜53 Ma suggest that subduction inception occurred at or near to the spreading ridge of the South Loyalty Basin at ˜56 Ma.

  12. Plutonic foundation of a slow-spreading ridge segment: Oceanic core complex at Kane Megamullion, 23°30'N, 45°20'W

    NASA Astrophysics Data System (ADS)

    Dick, Henry J. B.; Tivey, Maurice A.; Tucholke, Brian E.

    2008-05-01

    We mapped the Kane megamullion, an oceanic core complex on the west flank of the Mid-Atlantic Ridge exposing the plutonic foundation of a ˜50 km long, second-order ridge segment. The complex was exhumed by long-lived slip on a normal-sense detachment fault at the base of the rift valley wall from ˜3.3 to 2.1 Ma (Williams, 2007). Mantle peridotites, gabbros, and diabase dikes are exposed in the detachment footwall and in outward facing high-angle normal fault scarps and slide-scar headwalls that cut through the detachment. These rocks directly constrain crustal architecture and the pattern of melt flow from the mantle to and within the lower crust. In addition, the volcanic carapace that originally overlay the complex is preserved intact on the conjugate African plate, so the complete internal and external architecture of the paleoridge segment can be studied. Seafloor spreading during formation of the core complex was highly asymmetric, and crustal accretion occurred largely in the footwall of the detachment fault exposing the core complex. Because additions to the footwall, both magmatic and amagmatic, are nonconservative, oceanic detachment faults are plutonic growth faults. A local volcano and fissure eruptions partially cover the northwestern quarter of the complex. This volcanism is associated with outward facing normal faults and possible, intersecting transform-parallel faults that formed during exhumation of the megamullion, suggesting the volcanics erupted off-axis. We find a zone of late-stage vertical melt transport through the mantle to the crust in the southern part of the segment marked by a ˜10 km wide zone of dunites that likely fed a large gabbro and troctolite intrusion intercalated with dikes. This zone correlates with the midpoint of a lineated axial volcanic high of the same age on the conjugate African plate. In the central region of the segment, however, primitive gabbro is rare, massive depleted peridotite tectonites abundant, and dunites

  13. Annual report on surveillance and maintenance activities at Oak Ridge National Laboratory, Oak Ridge, Tennessee, fiscal year 1996

    SciTech Connect

    1996-11-01

    In fiscal year (FY) 1995, the sites and facilities from both the Remedial Action (RA) and Decontamination and Decommissioning (D and D) programs were combined to form the Oak Ridge National Laboratory (ORNL) Environmental Restoration (ER) Surveillance and Maintenance (S and M) Program. Surveillance and Maintenance activities were conducted throughout FY 1996 at the RA facilities. Overall, the RA S and M Program consists of approximately 650 acres that include 14 waste area groupings with approximately 200 sites. These sites include 46 major facilities, several leak and contaminated soil sites, 38 inactive tanks, approximately 50 environmental study areas and approximately 2,973 wells and boreholes. Site inspections were conducted at established frequencies on appropriate sites in the RA S and M Program in accordance with the established S and M FY 1996 Incentive Task Order (ITO).

  14. Impact of Non-Poissonian Activity Patterns on Spreading Processes

    NASA Astrophysics Data System (ADS)

    Vazquez, Alexei; Rácz, Balázs; Lukács, András; Barabási, Albert-László

    2007-04-01

    Halting a computer or biological virus outbreak requires a detailed understanding of the timing of the interactions between susceptible and infected individuals. While current spreading models assume that users interact uniformly in time, following a Poisson process, a series of recent measurements indicates that the intercontact time distribution is heavy tailed, corresponding to a temporally inhomogeneous bursty contact process. Here we show that the non-Poisson nature of the contact dynamics results in prevalence decay times significantly larger than predicted by the standard Poisson process based models. Our predictions are in agreement with the detailed time resolved prevalence data of computer viruses, which, according to virus bulletins, show a decay time close to a year, in contrast with the 1 day decay predicted by the standard Poisson process based models.

  15. Implications of the Iceland Deep Drilling Project for improving understanding of hydrothermal processes at slow spreading mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Elders, Wilfred A.; Friðleifsson, Guðmundur Ómar

    The Iceland Deep Drilling Project (IDDP) is investigating producing geothermal energy from magma-hydrothermal systems at supercritical conditions. This requires drilling to depths of 4 to 5 km where temperatures should be in the range of 450-600°C or higher. Modeling studies suggest producing superheated steam from supercritical temperatures >450°C could increase power output tenfold relative to steam produced from a 300°C reservoir. The first IDDP well was drilled at Krafla within a volcanic caldera in the active rift zone of NE Iceland. At the end of June 2009, drilling was terminated at only 2100 m depth when a >900°C rhyolitic magma flowed into the drill hole. The well was completed with a casing cemented a few meters above the magma. Depending on the result of a planned flow test, there is the possibility of creating the world's hottest "Engineered Geothermal System" by injecting water in a nearby well to produce superheated steam from the magma. An advantage of such a strategy would be that the acidic gases likely to be given off by the magma could be neutralized by injecting suitably treated water. Two new wells, ˜4 km deep, are planned to be drilled during 2010-2012 at the Hengill and the Reykjanes geothermal fields in SW Iceland to explore for supercritical zones. The Reykjanes geothermal system produces hydrothermally modified seawater. This presents an ideal situation to study a high-temperature magma-hydrothermal system at depth analogous to those responsible for the black smokers at submarine divergent plate margins.

  16. SAN-RL: combining spreading activation networks and reinforcement learning to learn configurable behaviors

    NASA Technical Reports Server (NTRS)

    White, J.; Gaines, D. M.; Wilkes, M.; Kusumalnukool, K.; Thongchai, S.; Kawamura, K.

    2001-01-01

    This approach provides the agent with a causal structure, the spreading activation network, relating goals to the actions that can achieve those goals. This enables the agent to select actions relative to the goal priorities.

  17. 4D Time-Lapse Seismic Analysis of Active Gas Seepage Systems on the Vestnesa Ridge, Offshore W-Svalbard

    NASA Astrophysics Data System (ADS)

    Bunz, S.; Hurter, S.; Plaza-Faverola, A. A.; Mienert, J.

    2014-12-01

    Active gas venting occurs on the Vestnesa Ridge, an elongated sediment drift north of the Molloy Transform and just east of the Molloy Ridge, one of the shortest segments of the slow spreading North-Atlantic Ridge system. The crest of the Vestnesa Ridge at water depth between 1200-1300 m is pierced with fluid-flow features. Seafloor pockmarks vary in size up to 1 km in diameter with significant morphological features consisting of small ridges, diapiric structures and small pits. Detailed hydro-acoustic surveying shows that gas mostly emanates from the small-scale pits, where also hydrates have been recovered by sediment sampling. High-resolution P-Cable 3D seismic data acquired in 2012 show vertical focused fluid flow features beneath the seafloor pockmarks. These co-called chimneys extend down to the free-gas zone underneath a bottom-simulating reflection (BSR). Here, they link up with small fault systems that might provide pathways to the deeper subsurface. The chimney features show a high variability in their acoustic characteristics with alternating blanked or masked zones and high-amplitude anomalies scattered through the whole vertical extent of the chimneys. The amplitude anomalies indicate high-impedance contrasts due to the likely presence of gas or a high-velocity material like gas hydrates or carbonates. In most cases, the high-amplitude anomalies line up along specific vertical pathways that connect nicely with the small-scale pits at the surface where gas bubbles seep from the seafloor. We re-acquired the 3D seismic survey in 2013 for time-lapse seismic studies in order to better understand the origin of the amplitude anomalies and in order to track potentially migrating gas fronts up along the chimney structure. The time-lapse seismic analysis indicates several areas, where gas migration may have led to changes in acoustic properties of the subsurface. These areas are located along chimney structures and the BSR. This work provides a basis for better

  18. Influence of Geologic Setting on the Morphology, Mineralogy, and Geochemistry of Vent Deposits Along the Eastern Lau Spreading Center and Valu Fa Ridge

    NASA Astrophysics Data System (ADS)

    Tivey, M. K.; Evans, G. N.; Ferrini, V. L.

    2014-12-01

    Establishment of links between lithology, vent fluid chemistry, and vent deposit characteristics along the Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR) was made possible using deep submergence vehicles and technology. ROV Jason was used to collect ultrahigh-resolution (submeter) bathymetric data sufficient to quantify characteristics of volcanic, tectonic and hydrothermal features; differences within vent fields from north to south include a change from low-relief volcanic domes cut by faults and fissures to higher aspect ratio volcanic domes dominated by aa-type lava morphologies (Ferrini et al., G-cubed, 2008). Highest temperature fluids are associated with crosscutting faults at all but Mariner vent field where faults are not observed. The detailed maps were used to target areas within vent fields for observations and sampling. Vent deposit morphologies are similar at the northernmost vent fields (Kilo Moana, TowCam, Tahi Moana), with black smokers and diffusers present on branched edifices. Vent deposits at the more southerly ABE, Tui Malila and Mariner vent fields vary in morphology, despite similar substrate lithology. Examples include abundant flanges at ABE and Tui Malila and ~20m-tall spires and squat barite-rich edifices at Mariner. Geochemical analyses and petrographic observations document the influence of lithology, fluid temperature, pH, and extents of seawater mixing on deposit formation. Concentrations of As, which increase from north to south, reflect lithologic control. Sb, Pb, and Ba concentrations also reflect lithologic control, but are affected as well by low pH and/or extents of seawater mixing. The significant differences in Mariner deposits reflect formation from very high temperature, low pH (<3 vs >4) fluids that keep Zn in solution, combined with local subsurface mixing. Overall, results document the influence of the Tonga Subduction Zone on vent deposits through its affects on lithology and vent fluid composition.

  19. Mid-Atlantic Ridge at 13-14N: Evidence of Unstable Seafloor Spreading Processes From Deep-Towed Magnetic Measurements

    NASA Astrophysics Data System (ADS)

    Searle, R.; Mallows, C.; Cipcigan, F.; Party, J. S.

    2007-12-01

    During cruise JC007 in March-April 2007 we recorded total magnetic field anomalies over two active and one defunct oceanic core complex (OCC) and the intervening seafloor. Measurements were made by towed magnetometer at the sea surface, and by the TOBI deep-towed vehicle approximately 400 m above seafloor, along 13 E-W lines about 60 km long and spaced 3 to 6 km apart. Sea-surface data show a fairly coherent central anomaly on most lines, though on some it is significantly displaced from the spreading axis as indicated by bathymetry and side-scan sonar data. Modelling in terms of a standard, simple (but probably unrealistic), continuous reversal sequence requires total spreading rates ranging from about 15 to 40 km/Myr with offsets of the axis up to 20 km and highly asymmetric spreading. The deep-towed data were corrected for the heading-dependent magnetic effects of the TOBI vehicle before inversion to crustal magnetisation using the 2D Parker & Huestis (1974) procedure. These results were checked by comparing with inversions of the sea-surface field, which shows similar features at reduced resolution. The deep-towed inversion results show a rather incoherent magnetisation pattern. The central magnetisation high is nowhere more than 13 km wide, only 70% of the expected width of the Brunhes here, and several profiles yield apparently negative magnetisation over areas we expect to be of Brunhes age based on sonar and bathymetry data. This may due to a combination of destruction of magnetisation by faulting (Hussenoeder at al., 1996), departure from the 2D geometry assumed for the inversions, and departure (via tectonic rotation) from the assumed constant magnetisation direction. We are now carrying out fully 3D inversions and forward modelling guided by the structural evidence provided by sidescan and bathymetry. These results will be presented and discussed in relation to the seafloor spreading history and structure of the region.

  20. The p-wave upper mantle structure beneath an active spreading centre - The Gulf of California

    NASA Technical Reports Server (NTRS)

    Walck, M. C.

    1984-01-01

    Over 1400 seismograms of earthquakes in Mexico are analyzed and data sets for the travel time, apparent phase velocity, and relative amplitude information are utilized to produce a tightly constrained, detailed model for depths to 900 km beneath an active oceanic ridge region, the Gulf of California. The data are combined by first inverting the travel times, perturbing that model to fit the p-delta data, and then performing trial and error synthetic seismogram modelling to fit the short-period waveforms. The final model satisfies all three data sets. The ridge model is similar to existing upper mantle models for shield, tectonic-continental, and arc-trench regimes below 400 km, but differs significantly in the upper 350 km. Ridge model velocities are very low in this depth range; the model 'catches up' with the others with a very large velocity gradient from 225 to 390 km.

  1. Activity of nodes reshapes the critical threshold of spreading dynamics in complex networks

    NASA Astrophysics Data System (ADS)

    Liu, Chen; Zhou, Li-xin; Fan, Chong-jun; Huo, Liang-an; Tian, Zhan-wei

    2015-08-01

    In this paper, we investigate spreading dynamics on complex networks with active nodes based on SIR (Susceptible-Infected-Removed) model. Different from previous studies, each node of the network rotates between active state and inactive state according to certain probabilities. An active susceptible node can be infected by all its infected neighbors, while an inactive susceptible node can only be infected by its active infected neighbors. By means of ​mean-field approach and numerical simulations, we explore the critical phenomenon by the combined effects of activity rate and infection rate on spreading dynamics. We show that the critical threshold of infection rate is increased by node activity, and node activity also shows a critical phenomenon given certain infection rate. On the whole, there exists a critical curve consists of pairs of critical activity rate and infection rate. We also analyze theoretically the impact of activity rate and infection rate on the final size of spreading dynamics, which is verified by numerical simulations. This work complements our understanding of spreading dynamics with active nodes and may be used to develop more feasible and more economical methods to control spreading dynamics.

  2. Forced sea-level change in a forearc basin related to subduction of a spreading ridge: the Fossil Bluff Group (Jurassic-Cretaceous), Alexander Island, Antarctic Peninsula

    NASA Astrophysics Data System (ADS)

    Macdonald, David

    2015-04-01

    During the Mesozoic, the Antarctic Peninsula was the site of an active volcanic arc related to the eastwards subduction of proto-Pacific oceanic crust. Alexander Island is the largest of the many islands that lie on the western (fore-arc) side of the Antarctic Peninsula; it forms one of the best-exposed ancient fore-arcs in the world. The pre-Tertiary rocks can be divided into two main units. The LeMay Group (Jurassic-Tertiary) forms the structural basement to Alexander Island and comprises greenschist-facies metasedimentary rocks. It is interpreted as a Mesozoic accretionary prism. The Fossil Bluff Group unconformably overlies and is faulted against the LeMay Group; it represents the sedimentary fill of a coeval fore-arc basin. Subduction ceased due to a series of Cenozoic ridge-trench collisions which began off Alexander Island at 50 Ma and got progressively younger to the north. However, the approach of the ridge can be inferred from the Mesozoic deposits of the Fossil Bluff Group (Jurassic-Cretaceous) in Alexander Island. In this paper, I will show that the ocean floor being subducted became progressively shallower through Jurassic and Cretaceous time (by at least 1,000 m). The result in the forearc basin was a sudden shallowing in water depths from at least 1,000 m at 125 Ma, to emergent at 100 Ma. This forced shallowing ended sedimentation in the basin and resulted in considerable topography on Alexander Island that persists to the present day.

  3. Mapping oceanic ridge segments in Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Nicolas, A.; Boudier, F.

    1995-04-01

    This paper presents the results of detailed mapping of high-temperature flow structures in the mantle and crust of two massifs of the Oman ophiolite. In these massifs, the dominant structures, including large-scale folds, shear zones, and fractures, were generated at elevated temperatures and are ascribed to the ridge or ridge environment activity; this means that the structural maps presented can be viewed as those of partly dissected ridge segments. It has been possible in the two massifs to locate the paleoaxis of the oceanic ridge which created this crust. This location, which is constrained by several independent tests, is a prerequisite to reconstruct the structure and to investigate the dynamics of a fast spreading ridge. In the Nakhl-Rustaq massif, high temperature tectonic activity at the ridge rotated the Moho toward a vertical altitude and folded the layered gabbros on the scale of several hundred meters. This tectonism is attributed to a propagating ridge deforming a slightly older lithosphere. The propagating ridge segment extends in the field from a diapir area to a domain located along strike some 20 km away, where the sheeted dike complex roots directly in the mantle, without layered gabbros in between. The diapir area represents the mantle feeder for the ridge segment, and the rooted dikes represent the propagating tip. Other results include the detailed mapping of two mantle diapirs and of the diverging mantle flow issued from them. Magma chambers are centered over diapirs and are tent-shaped, in accord with our previous models.

  4. Spread of activation and deactivation in the brain: does age matter?

    PubMed Central

    Gordon, Brian A.; Tse, Chun-Yu; Gratton, Gabriele; Fabiani, Monica

    2014-01-01

    Cross-sectional aging functional MRI results are sometimes difficult to interpret, as standard measures of activation and deactivation may confound variations in signal amplitude and spread, which however, may be differentially affected by age-related changes in various anatomical and physiological factors. To disentangle these two types of measures, here we propose a novel method to obtain independent estimates of the peak amplitude and spread of the BOLD signal in areas activated (task-positive) and deactivated (task-negative) by a Sternberg task, in 14 younger and 28 older adults. The peak measures indicated that, compared to younger adults, older adults had increased activation of the task-positive network, but similar levels of deactivation in the task-negative network. Measures of signal spread revealed that older adults had an increased spread of activation in task-positive areas, but a starkly reduced spread of deactivation in task-negative areas. These effects were consistent across regions within each network. Further, there was greater variability in the anatomical localization of peak points in older adults, leading to reduced cross-subject overlap. These results reveal factors that may confound the interpretation of studies of aging. Additionally, spread measures may be linked to local connectivity phenomena and could be particularly useful to analyze age-related deactivation patterns, complementing the results obtained with standard peak and region of interest analyses. PMID:25360115

  5. 78 FR 69447 - Agency Information Collection Activities; Existing Collection, Comments Requested: Friction Ridge...

    Federal Register 2010, 2011, 2012, 2013, 2014

    2013-11-19

    ... From the Federal Register Online via the Government Publishing Office DEPARTMENT OF JUSTICE Federal Bureau of Investigation Agency Information Collection Activities; Existing Collection, Comments Requested: Friction Ridge Cards: Arrest and Institution; Applicant; Personal Identification; FBI Standard Palm Print; Supplemental Finger and Palm...

  6. ARCO moves to spark U. S. activity, spread risk

    SciTech Connect

    Petzet, G.A.

    1992-07-13

    This paper reports that one US major oil company has taken the offensive to combat tightness of drilling capital in the US and the overseas flight of interest in elephant hunting. ARCO Oil and Gas Co., with substantial acreage and seismic data covering most of the US Lower 48 states, s communicating with a broad audience to try to make activity happen on its properties. ARCO is looking to bring others in on its prospects, hawk seismic data, and take deals from others. On some acreage that internal funds are not available to evaluate, ARCO was offering 100% farmounts.

  7. Morphology of the Knipovich Ridge Area

    NASA Astrophysics Data System (ADS)

    Zarayskaya, Y.; Abramova, A.; Dobrolyubova, K.; Mazarovich, A.; Moroz, E.

    2014-12-01

    Knipovich Ridge is the northernmost part of the Mid-Atlantic Ridge system. It is located between Mohns and Molloy spreading centers in the Greenland Sea. The scientific team of the R/V "Akademik Nikolaj Strakhov" (Geological Institute RAS, Russia) surveyed this area in 2006, 2007, 2009 and 2010 using the deep-water multibeam echosounder RESON Seabat 7150 with working frequency 12 kHz. The total surveyed area is up to 82000 km² including 65000 km² covering rift valley and flanges of the ridge. Knipovich ridge is classified as an ultra-slow oblique spreading center with spreading rate around 1,4 cm/y. Its large-scale morphological features are reduced in number comparing to other mid-ocean ridges. Eastern flange is buried under the continental slope sediments and only the rare highest peaks rise above this cover. Western flange is fully developed and consists of several ridges prolonged parallel to the rift valley. Ridges are supplemented with individual highs. Rift valley is 20-40 km wide and 500 km long. Its depth is 3300-3700 m. Valley slopes have terraces and ledges of different amplitudes. The bottom of the valley is echeloned by 5 volcanic axial highs rising 400 - 1000 m above it. Spreading obliquity is imprinted in the ridge morphology. The global models predict a plate motion vector of 307º. The main ridge axis has general orientation of 350º. Rift valley follows this direction from the South, and on the half way to the North turns to azimuth of 2º. The detailed bathymetry shows that small-scale features orientation differs. Rift axial highs and individual flange highs are prolonged NW-SE under azimuth of 30º. This orientation is sub-perpendicular to the plate motion vector (83º) and oblique to the ridge axes (40º). The multibeam bathymetry shows no sing of transform faults or non-transform discontinuities along the Knipovich ridge rift valley. There is one strong lineation in the northern part of the ridge. It includes flange and axial highs and

  8. Hybrid spreading mechanisms and T cell activation shape the dynamics of HIV-1 infection.

    PubMed

    Zhang, Changwang; Zhou, Shi; Groppelli, Elisabetta; Pellegrino, Pierre; Williams, Ian; Borrow, Persephone; Chain, Benjamin M; Jolly, Clare

    2015-04-01

    HIV-1 can disseminate between susceptible cells by two mechanisms: cell-free infection following fluid-phase diffusion of virions and by highly-efficient direct cell-to-cell transmission at immune cell contacts. The contribution of this hybrid spreading mechanism, which is also a characteristic of some important computer worm outbreaks, to HIV-1 progression in vivo remains unknown. Here we present a new mathematical model that explicitly incorporates the ability of HIV-1 to use hybrid spreading mechanisms and evaluate the consequences for HIV-1 pathogenenesis. The model captures the major phases of the HIV-1 infection course of a cohort of treatment naive patients and also accurately predicts the results of the Short Pulse Anti-Retroviral Therapy at Seroconversion (SPARTAC) trial. Using this model we find that hybrid spreading is critical to seed and establish infection, and that cell-to-cell spread and increased CD4+ T cell activation are important for HIV-1 progression. Notably, the model predicts that cell-to-cell spread becomes increasingly effective as infection progresses and thus may present a considerable treatment barrier. Deriving predictions of various treatments' influence on HIV-1 progression highlights the importance of earlier intervention and suggests that treatments effectively targeting cell-to-cell HIV-1 spread can delay progression to AIDS. This study suggests that hybrid spreading is a fundamental feature of HIV infection, and provides the mathematical framework incorporating this feature with which to evaluate future therapeutic strategies. PMID:25837979

  9. A Dual-Porosity, In Situ Crystallisation Model For Fast-Spreading Mid-Ocean Ridge Magma Chambers Based Upon Direct Observation From Hess Deep

    NASA Astrophysics Data System (ADS)

    MacLeod, C. J.; Lissenberg, C. J.

    2014-12-01

    We propose a revised magma chamber model for fast-spreading mid-ocean ridges based upon a synthesis of new data from a complete section of lower crust from the East Pacific Rise, reconstructed from samples collected from the Hess Deep rift valley during cruise JC21. Our investigation includes detailed sampling across critical transitions in the upper part of the plutonic section, including the inferred axial melt lens (AML) within the dyke-gabbro transition. We find that an overall petrological progression, from troctolite and primitive gabbro at the base up into evolved (oxide) gabbro and gabbronorite at the top of the lower crustal section, is mirrored by a progressive upward chemical fractionation as recorded in bulk rock and mineral compositions. Crystallographic preferred orientations measured using EBSD show that the downward increase in deformation of mush required in crystal subsidence models is not observed. Together these observations are consistent only with a model in which crystallisation of upward migrating evolving melts occurs in situ in the lower crust. Over-enrichment in incompatible trace element concentrations and ratios above that possible by fractional crystallisation is ubiquitous. This implies redistribution of incompatible trace elements in the lower crust by low porosity, near-pervasive reactive porous flow of interstitial melt moving continuously upward through the mush pile. Mass balance calculations reveal a significant proportion of this trace element enriched melt is trapped at mid-crustal levels. Mineral compositions in the upper third to half of the plutonic section are too evolved to represent the crystal residues of MORB. Erupted MORB therefore must be fed from melts sourced in the deeper part of the crystal mush pile, and which must ascend rapidly without significant modification in the upper plutonics or AML. From physical models of mush processes we posit that primitive melts are transported through transient, high porosity

  10. The persistence of equatorial spread F - an analysis on seasonal, solar activity and geomagnetic activity aspects

    NASA Astrophysics Data System (ADS)

    Sreeja, V.; Devasia, C. V.; Ravindran, Sudha; Sridharan, R.

    2009-02-01

    The persistence (duration) of Equatorial Spread F (ESF), which has significant impact on communication systems, is addressed. Its behavior during different seasons and geomagnetic activity levels under the solar maximum (2001) and minimum (2006) conditions, is reported using the data from the magnetic equatorial location of Trivandrum (8.5° N; 77° E; dip 0.5° N) in India. The study reveals that the persistence of the irregularities can be estimated to a reasonable extent by knowing the post sunset F region vertical drift velocity (Vz) and the magnetic activity index Kp. Any sort of advance information on the possible persistence of the ionospheric irregularities responsible for ESF is important for understanding the scintillation morphology, and the results which form the first step in this direction are presented and discussed.

  11. Influence of emotional valence and arousal on the spread of activation in memory.

    PubMed

    Jhean-Larose, Sandra; Leveau, Nicolas; Denhière, Guy

    2014-11-01

    Controversy still persists on whether emotional valence and arousal influence cognitive activities. Our study sought to compare how these two factors foster the spread of activation within the semantic network. In a lexical decision task, prime words were varied depending on the valence (pleasant or unpleasant) or on the level of emotional arousal (high or low). Target words were carefully selected to avoid semantic priming effects, as well as to avoid arousing specific emotions (neutral). Three SOA durations (220, 420 and 720 ms) were applied across three independent groups. Results indicate that at 220 ms, the effect of arousal is significantly higher than the effect of valence in facilitating spreading activation while at 420 ms, the effect of valence is significantly higher than the effect of arousal in facilitating spreading activation. These findings suggest that affect is a sequential process involving the successive intervention of arousal and valence. PMID:24715543

  12. 4D seismic study of active gas seepage systems on the Vestnesa Ridge, offshore W-Svalbard

    NASA Astrophysics Data System (ADS)

    Bünz, Stefan; Plaza-Faverola, Andreia; Hurter, Sandra; Mienert, Jürgen

    2014-05-01

    Active gas venting occurs on the Vestnesa Ridge, an elongated sediment drift north of the Molloy Transform and just east of the Molloy Ridge, one of the shortest segments of the slow spreading North-Atlantic Ridge system. The crest of the Vestnesa Ridge at water depth between 1200-1300 m is pierced with fluid-flow features. Seafloor pockmarks vary in size up to 1 km in diameter. High-resolution P-Cable 3D seismic data acquired in 2012 show vertical focused fluid flow features beneath the seafloor pockmarks. These co-called chimneys extend down to the free-gas zone underneath a bottom-simulating reflection. Here, they link up with small fault systems that might provide pathways to the deeper subsurface. The chimney features show a high variability in their acoustic characteristics with alternating blanked or masked zones and high-amplitude anomalies scattered through the whole vertical extent of the chimneys. The amplitude anomalies indicate high-impedance contrasts due to the likely presence of gas or a high-velocity material like gas hydrates or carbonates. We re-acquired the 3D seismic survey in 2013 for time-lapse seismic studies in order to better understand the origin of the amplitude anomalies and in order to track potentially migrating gas fronts up along the chimney structure. Here, we will present the preliminary results of this time-lapse analysis, which will allow us to better understand gas migration and seafloor plumbing systems in continental margins. This work is part of CAGE - Centre of Excellence for Arctic Gas Hydrate, Environment and Climate. Details on the CAGE research plan and organization can be found on www.cage.uit.no to foster opportunities for cross-disciplinary collaboration. Based in Tromsø, at the world's northernmost University, CAGE establishes the intellectual and infrastructure resources for studying the amount of methane hydrate and magnitude of methane release in Arctic Ocean environments on time scales from the Neogene to the

  13. DATA SHARING REPORT CHARACTERIZATION OF POPULATION 7: PERSONAL PROTECTIVE EQUIPMENT, DRY ACTIVE WASTE, AND MISCELLANEOUS DEBRIS, SURVEILLANCE AND MAINTENANCE PROJECT OAK RIDGE NATIONAL LABORATORY OAK RIDGE, TENNESSEE

    SciTech Connect

    Harpenau, Evan M

    2013-10-10

    The U.S. Department of Energy (DOE) Oak Ridge Office of Environmental Management (EM-OR) requested that Oak Ridge Associated Universities (ORAU), working under the Oak Ridge Institute for Science and Education (ORISE) contract, provide technical and independent waste management planning support under the American Recovery and Reinvestment Act (ARRA). Specifically, DOE EM-OR requested that ORAU plan and implement a sampling and analysis campaign targeting certain URS|CH2M Oak Ridge, LLC (UCOR) surveillance and maintenance (S&M) process inventory waste. Eight populations of historical and reoccurring S&M waste at the Oak Ridge National Laboratory (ORNL) have been identified in the Waste Handling Plan for Surveillance and Maintenance Activities at the Oak Ridge National Laboratory, DOE/OR/01-2565&D2 (WHP) (DOE 2012) for evaluation and processing to determine a final pathway for disposal. Population 7 (POP 7) consists of 56 containers of aged, low-level and potentially mixed S&M waste that has been staged in various locations around ORNL. Several of these POP 7 containers primarily contain personal protective equipment (PPE) and dry active waste (DAW), but may contain other miscellaneous debris. This data sharing report addresses the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA) specified waste in a 13-container subpopulation (including eight steel boxes, three 55-gal drums, one sealand, and one intermodal) that lacked sufficient characterization data for possible disposal at the Environmental Management Waste Management Facility (EMWMF) using the approved Waste Lot (WL) 108.1 profile.

  14. Epidemic spreading with activity-driven awareness diffusion on multiplex network.

    PubMed

    Guo, Quantong; Lei, Yanjun; Jiang, Xin; Ma, Yifang; Huo, Guanying; Zheng, Zhiming

    2016-04-01

    There has been growing interest in exploring the interplay between epidemic spreading with human response, since it is natural for people to take various measures when they become aware of epidemics. As a proper way to describe the multiple connections among people in reality, multiplex network, a set of nodes interacting through multiple sets of edges, has attracted much attention. In this paper, to explore the coupled dynamical processes, a multiplex network with two layers is built. Specifically, the information spreading layer is a time varying network generated by the activity driven model, while the contagion layer is a static network. We extend the microscopic Markov chain approach to derive the epidemic threshold of the model. Compared with extensive Monte Carlo simulations, the method shows high accuracy for the prediction of the epidemic threshold. Besides, taking different spreading models of awareness into consideration, we explored the interplay between epidemic spreading with awareness spreading. The results show that the awareness spreading can not only enhance the epidemic threshold but also reduce the prevalence of epidemics. When the spreading of awareness is defined as susceptible-infected-susceptible model, there exists a critical value where the dynamical process on the awareness layer can control the onset of epidemics; while if it is a threshold model, the epidemic threshold emerges an abrupt transition with the local awareness ratio α approximating 0.5. Moreover, we also find that temporal changes in the topology hinder the spread of awareness which directly affect the epidemic threshold, especially when the awareness layer is threshold model. Given that the threshold model is a widely used model for social contagion, this is an important and meaningful result. Our results could also lead to interesting future research about the different time-scales of structural changes in multiplex networks. PMID:27131489

  15. Epidemic spreading with activity-driven awareness diffusion on multiplex network

    NASA Astrophysics Data System (ADS)

    Guo, Quantong; Lei, Yanjun; Jiang, Xin; Ma, Yifang; Huo, Guanying; Zheng, Zhiming

    2016-04-01

    There has been growing interest in exploring the interplay between epidemic spreading with human response, since it is natural for people to take various measures when they become aware of epidemics. As a proper way to describe the multiple connections among people in reality, multiplex network, a set of nodes interacting through multiple sets of edges, has attracted much attention. In this paper, to explore the coupled dynamical processes, a multiplex network with two layers is built. Specifically, the information spreading layer is a time varying network generated by the activity driven model, while the contagion layer is a static network. We extend the microscopic Markov chain approach to derive the epidemic threshold of the model. Compared with extensive Monte Carlo simulations, the method shows high accuracy for the prediction of the epidemic threshold. Besides, taking different spreading models of awareness into consideration, we explored the interplay between epidemic spreading with awareness spreading. The results show that the awareness spreading can not only enhance the epidemic threshold but also reduce the prevalence of epidemics. When the spreading of awareness is defined as susceptible-infected-susceptible model, there exists a critical value where the dynamical process on the awareness layer can control the onset of epidemics; while if it is a threshold model, the epidemic threshold emerges an abrupt transition with the local awareness ratio α approximating 0.5. Moreover, we also find that temporal changes in the topology hinder the spread of awareness which directly affect the epidemic threshold, especially when the awareness layer is threshold model. Given that the threshold model is a widely used model for social contagion, this is an important and meaningful result. Our results could also lead to interesting future research about the different time-scales of structural changes in multiplex networks.

  16. Measurement of ridge-spreading movements (Sackungen) at Bald Eagle Mountain, Lake County, Colorado, II : continuation of the 1975-1989 measurements using a Global Positioning System in 1997 and 1999

    USGS Publications Warehouse

    Varnes, David J.; Coe, J.A.; Godt, J.W.; Savage, W.Z.; Savage, J.E.

    2000-01-01

    Measurements of ridge-spreading movements at Bald Eagle Mountain in north-central Colorado were reported in USGS Open-File Report 90-543 for the years 1975-1989. Measurements were renewed in 1997 and 1999 using the Global Positioning System (GPS). Movements are generally away from a ridge-top graben and appear to be concentrated along 3 or 4 trenches with uphill facing scarps that are parallel with slope contours. A point just below the lowest trench has moved the most? a total of 8.3 cm horizontally and slightly downward from 1977 to 1999 relative to an assumed stable point on the periphery of the graben. Movements from 1997 to 1999 are less than 1 cm or within the error of measurement.

  17. Large-scale organization of rat sensorimotor cortex based on a motif of large activation spreads.

    PubMed

    Frostig, Ron D; Xiong, Ying; Chen-Bee, Cynthia H; Kvasnák, Eugen; Stehberg, Jimmy

    2008-12-01

    Parcellation according to function (e.g., visual, somatosensory, auditory, motor) is considered a fundamental property of sensorimotor cortical organization, traditionally defined from cytoarchitectonics and mapping studies relying on peak evoked neuronal activity. In the adult rat, stimulation of single whiskers evokes peak activity at topographically appropriate locations within somatosensory cortex and provides an example of cortical functional specificity. Here, we show that single whisker stimulation also evokes symmetrical areas of suprathreshold and subthreshold neuronal activation that spread extensively away from peak activity, effectively ignoring cortical borders by spilling deeply into multiple cortical territories of different modalities (auditory, visual and motor), where they were blocked by localized neuronal activity blocker injections and thus ruled out as possibly caused by "volume conductance." These symmetrical activity spreads were supported by underlying border-crossing, long-range horizontal connections as confirmed with transection experiments and injections of anterograde neuronal tracer experiments. We found such large evoked activation spreads and their underlying connections regardless of whisker identity, cortical layer, or axis of recorded responses, thereby revealing a large scale nonspecific organization of sensorimotor cortex based on a motif of large symmetrical activation spreads. Because the large activation spreads and their underlying horizontal connections ignore anatomical borders between cortical modalities, sensorimotor cortex could therefore be viewed as a continuous entity rather than a collection of discrete, delineated unimodal regions, an organization that could coexist with established specificity of cortical organization and that could serve as a substrate for associative learning, direct multimodal integration and recovery of function after injury. PMID:19052219

  18. Large scale organization of rat sensorimotor cortex based on a motif of large activation spreads

    PubMed Central

    Frostig, Ron D.; Xiong, Ying; Chen-Bee, Cynthia H.; Kvašňák, Eugen; Stehberg, Jimmy

    2008-01-01

    Parcellation according to function (e.g., visual, somatosensory, auditory, motor) is considered a fundamental property of sensorimotor cortical organization, traditionally defined from cytoarchitectonics and mapping studies relying on peak evoked neuronal activity. In the adult rat, stimulation of single whiskers evokes peak activity at topographically appropriate locations within somatosensory cortex and provides an example of cortical functional specificity. Here, we show that single whisker stimulation also evokes symmetrical areas of supra- and sub-threshold neuronal activation that spread extensively away from peak activity, effectively ignoring cortical borders by spilling deeply into multiple cortical territories of different modalities (auditory, visual and motor), where they were blocked by localized neuronal activity blocker injections and thus ruled out as possibly due to ‘volume conductance’. These symmetrical activity spreads were supported by underlying border-crossing, long-range horizontal connections as confirmed with transection experiments and injections of anterograde neuronal tracer experiments. We found such large evoked activation spreads and their underlying connections irrespective of whisker identity, cortical layer, or axis of recorded responses, thereby revealing a large scale nonspecific organization of sensorimotor cortex based on a motif of large symmetrical activation spreads. Because the large activation spreads and their underlying horizontal connections ignore anatomical borders between cortical modalities, sensorimotor cortex could therefore be viewed as a continuous entity rather than a collection of discrete, delineated unimodal regions – an organization that could co-exist with established specificity of cortical organization and that could serve as a substrate for associative learning, direct multimodal integration and recovery of function following injury. PMID:19052219

  19. Gravitational spreading of mountain ridges coeval with Late Weichselian deglaciation: impact on glacial landscapes in Tröllaskagi, northern Iceland

    NASA Astrophysics Data System (ADS)

    Coquin, Julien; Mercier, Denis; Bourgeois, Olivier; Cossart, Etienne; Decaulne, Armelle

    2015-01-01

    During the Late Weichselian deglaciation, the coastal mountains of northern central Iceland have experienced significant paraglacial readjustment processes in the form of conspicuous rock slope failures and deep-seated gravitational slope deformation (DSGSD). Local topographic slopes and ridges were deeply reshaped by these large scale paraglacial processes. Located on the eastern side of Skagafjörður, one of the largest fjords of northern Iceland, the Óslandshlíðarfjöll and Hnjúkar ridges (65°49N, 19°14W) exhibit geomorphic evidence of spectacular DSGDS. Several series of DSGSD-induced landforms such as crestal graben and troughs initiated by ridge-top splitting were investigated over a 30-km2 area. On the basis of geomorphological mapping we recognized: (i) a ridge-top splitting event mainly controlled by glacial debuttressing induced by a minimal 300 m lowering of the glacier surface in the Deildardalur valley; (ii) a rapid Late Weichselian deglaciation of the Deildardalur valley spanning a few thousands of years (ice-free stage probably reached around 14,000 years cal. BP); (iii) ridge-top splitting having an influence on large-scale glacial patterns by guiding and facilitating glacial erosion along ridge-top grabens, resulting in accelerated trough widening. Based on these interpretations, we propose an evolutionary sequence of both the kinematic stages of the DSGSD and the Late Weichselian deglaciation at the valley scale. This work provides new insights into (i) the patterns of the Late Weichselian deglaciation in the Skagafjörður area, especially in tributary valleys of the fjord, (ii) the timing of large-scale paraglacial ridge-top deformations in relation to the post-LGM deglaciation and (iii) the influence of paraglacial DSGSD features on large-scale glacial erosional patterns.

  20. Activity of Wind-Blown Sand and the Formation of Feathered Sand Ridges in the Kumtagh Desert, China

    NASA Astrophysics Data System (ADS)

    Liao, Kongtai; Qu, Jianjun; Tang, Jinnian; Ding, Feng; Liu, Hujun; Zhu, Shujuan

    2010-05-01

    We study the activity of wind-blown sand and its effects on the evolution of feathered sand ridges in the Kumtagh Desert, China, and attempt to reveal the formation process of feathered sand ridges using wind-tunnel experiments, remote sensing data, and detailed field observations from 2005 to 2008. The prevailing wind direction in the Kumtagh Desert is easterly in winter and north-easterly in other seasons. The average annual wind speed is 5.9 ms-1, and winds sufficiently strong to entrain sand occur on 143 days per annum. The sand transport rate within 0.4 m of the ground is strongly influenced by local landforms, and is related to wind speed by a power function. Wind erosion occurs on the crest, the windward slope of crescent sand ridges and inter-ridge sand strips, where the blowing sand cloud is in an unsaturated state; in contrast, sand accumulation occurs on the leeward slope of the crescent sand ridges, where the blowing sand cloud is in an over-saturated state. These results indicate that the development of feathered sand ridges in the Kumtagh Desert is mainly controlled by the local wind regime. The dominant winds (from the north, north-north-east and north-east) and additional winds (from the east-north-east, east and east-south-east) determine the development of crescent sand ridges, but winds that are approximately parallel to sand ridges form the secondary inter-ridge sand strips.

  1. Ridge-trench collision in Archean and Post-Archean crustal growth: Evidence from southern Chile

    NASA Technical Reports Server (NTRS)

    Nelson, E. P.; Forsythe, R. D.

    1988-01-01

    The growth of continental crust at convergent plate margins involves both continuous and episodic processes. Ridge-trench collision is one episodic process that can cause significant magmatic and tectonic effects on convergent plate margins. Because the sites of ridge collision (ridge-trench triple junctions) generally migrate along convergent plate boundaries, the effects of ridge collision will be highly diachronous in Andean-type orogenic belts and may not be adequately recognized in the geologic record. The Chile margin triple junction (CMTJ, 46 deg S), where the actively spreading Chile rise is colliding with the sediment-filled Peru-Chile trench, is geometrically and kinematically the simplest modern example of ridge collision. The south Chile margin illustrates the importance of the ridge-collision tectonic setting in crustal evolution at convergent margins. Similarities between ridge-collision features in southern Chile and features of Archean greenstone belts raise the question of the importance of ridge collision in Archean crustal growth. Archean plate tectonic processes were probably different than today; these differences may have affected the nature and importance of ridge collision during Archean crustal growth. In conclusion, it is suggested that smaller plates, greater ridge length, and/or faster spreading all point to the likelihood that ridge collision played a greater role in crustal growth and development of the greenstone-granite terranes during the Archean. However, the effects of modern ridge collision, and the processes involved, are not well enough known to develop specific models for the Archean ridge collison.

  2. Human factors activities in teleoperator development at the Oak Ridge National Laboratory

    SciTech Connect

    Draper, J.V.; Herndon, J.N.

    1986-01-01

    The Consolidated Fuel Reprocessing Program (CFRP) at the Oak Ridge National Laboratory is developing advanced teleoperator systems for maintenance of future nuclear reprocessing facilities. Remote maintenance systems developed by the CFRP emphasize man-in-the-loop teleoperation. Consequently, human factors issues which affect teleoperator performance must be addressed. This papers surveys research and development activities carried out by the human factors group within the Remote Control Engineering Task of the CFRP.

  3. Detailed leak detection test plan and schedule for the Oak Ridge National Laboratory LLLW active tanks

    SciTech Connect

    Douglas, D.G.; Maresca, J.W. Jr. )

    1993-03-01

    This document provides a detailed leak detection test plan and schedule for leak testing many of the tanks that comprise the active portion of the liquid low-level waste (LLLW) system at the Oak Ridge National Laboratory (ORNL). This plan was prepared in response to the requirements of the Federal Facility Agreement (FFA) between the US Department of Energy (DOE) and two other agencies, the US Environmental Protection Agency (EPA) and the Tennessee Department of Environment and Conservation (TDEC).

  4. Reactive spreading in ceramic/metal systems

    SciTech Connect

    Saiz, Eduardo; Cannon, Rowland M.; Tomsia, Antoni P.

    2000-11-06

    Reactive spreading, in which a chemically active element is added to promote wetting of noble metals on nonmetallic materials, is evaluated mechanistically. Theories for the energetics and kinetics of the steps involved in spreading are outlined to permit comparison to the steps in the compound formation that typically accompanies reactive wetting. These include: fluid flow, active metal adsorption, including nonequilibrium effects, and triple line ridging. They can all be faster than compound nucleation under certain conditions. This analysis plus assessment of recently reported experiments on metal/ceramic systems lead to a focus on those conditions under which spreading proceeds ahead of the actual formation of a new phase at the interface. This scenario may be more typical than commonly believed, and perhaps is the most effective situation leading to enhanced spreading. A rationale for the slow spreading rates plus the pervasive variability and hysteresis observed during high temperature wetting also emerges.

  5. Ischaemia triggered by spreading neuronal activation is inhibited by vasodilators in rats

    PubMed Central

    Dreier, Jens P; Petzold, Gabor; Tille, Katrin; Lindauer, Ute; Arnold, Guy; Heinemann, Uwe; Einhäupl, Karl M; Dirnagl, Ulrich

    2001-01-01

    It has been previously shown that spreading neuronal activation can generate a cortical spreading ischaemia (CSI) in rats. The purpose of the present study was to investigate whether vasodilators cause CSI to revert to a normal cortical spreading depression (CSD). A KCl-induced CSD travelled from an open cranial window to a closed window where the cortex was superfused with physiological artificial cerebrospinal fluid (ACSF). At the closed window, recordings revealed a short-lasting negative slow potential shift accompanied by a variable, small and short initial hypoperfusion followed by hyperaemia and then oligaemia. In contrast, spreading neuronal activation locally induced CSI at the closed window when ACSF contained a NO. synthase (NOS) inhibitor, NG-nitro-l-arginine, and an increased K+ concentration ([K+]ACSF). CSI was characterised by a sharp and prolonged initial cerebral blood flow decrease to 29 ± 11 % of the baseline and a prolonged negative potential shift. Co-application of a NO. donor, S-nitroso-N-acetylpenicillamine, and NOS inhibitor with high [K+]ACSF re-established a short-lasting negative potential shift and spreading hyperaemia typical of CSD. Similarly, the NO.-independent vasodilator papaverine caused CSI to revert to a pattern characteristic of CSD. In acute rat brain slices, NOS inhibition and high [K+]ACSF did not prolong the negative slow potential shift compared to that induced by high [K+]ACSF alone. The data indicate that the delayed recovery of the slow potential was caused by vasoconstriction during application of high [K+]ACSF and a NOS inhibitor in vivo. This supports the possibility of a vicious circle: spreading neuronal activation induces vasoconstriction, and vasoconstriction prevents repolarisation during CSI. Speculatively, this pathogenetic process could be involved in migraine-induced stroke. PMID:11230523

  6. Spreading Activation in an Attractor Network with Latching Dynamics: Automatic Semantic Priming Revisited

    ERIC Educational Resources Information Center

    Lerner, Itamar; Bentin, Shlomo; Shriki, Oren

    2012-01-01

    Localist models of spreading activation (SA) and models assuming distributed representations offer very different takes on semantic priming, a widely investigated paradigm in word recognition and semantic memory research. In this study, we implemented SA in an attractor neural network model with distributed representations and created a unified…

  7. The Roles of Spreading Activation and Retrieval Mode in Producing False Recognition in the DRM Paradigm

    ERIC Educational Resources Information Center

    Meade, Michelle L.; Watson, Jason M.; Balota, David A.; Roediger, Henry L., III

    2007-01-01

    The nature of persisting spreading activation from list presentation in eliciting false recognition in the Deese-Roediger-McDermott (DRM) paradigm was examined in two experiments. We compared the time course of semantic priming in the lexical decision task (LDT) and false alarms in speeded recognition under identical study and test conditions. The…

  8. Relatedness Proportion Effects in Semantic Categorization: Reconsidering the Automatic Spreading Activation Process

    ERIC Educational Resources Information Center

    de Wit, Bianca; Kinoshita, Sachiko

    2014-01-01

    Semantic priming effects at a short prime-target stimulus onset asynchrony are commonly explained in terms of an automatic spreading activation process. According to this view, the proportion of related trials should have no impact on the size of the semantic priming effect. Using a semantic categorization task ("Is this a living…

  9. Mantle plume capture, anchoring and outflow during ridge interaction

    NASA Astrophysics Data System (ADS)

    Gibson, S. A.; Richards, M. A.; Geist, D.

    2015-12-01

    Geochemical and geophysical studies have shown that >40% of the world's mantle plumes are currently interacting with the global ridge system and such interactions may continue for up to 180 Myr[1]. At sites of plume-ridge interaction up to 1400 km of the spreading centre is influenced by dispersed plume material but there are few constraints on how and where the ridge-ward transfer of deep-sourced material occurs, and also how it is sustained over long time intervals. Galápagos is an archetypal example of an off-axis plume and sheds important light on these mechanisms. The Galápagos plume stem is located ~200 km south of the spreading axis and its head influences 1000 km of the ridge. Nevertheless, the site of enriched basalts, greatest crustal thickness and elevated topography on the ridge, together with active volcanism in the archipelago, correlate with a narrow zone (~150 km) of low-velocity, high-temperature mantle that connects the plume stem and ridge at depths of ~100 km[2]. The enriched ridge basalts contain a greater amount of partially-dehydrated, recycled oceanic crust than basalts elsewhere on the spreading axis, or indeed basalts erupted in the region between the plume stem and ridge. The presence of these relatively volatile-rich ridge basalts requires flow of plume material below the peridotite solidus (i.e.>80 km). We propose a 2-stage model for the development and sustainment of a confined zone of deep ridge-ward plume flow. This involves initial on-axis capture and establishment of a sub-ridge channel of plume flow. Subsequent anchoring of the plume stem to a contact point on the ridge during axis migration results in confined ridge-ward flow of plume material via a deep network of melt channels embedded in the normal spreading and advection of the plume head[2]. Importantly, sub-ridge flow is maintained. The physical parameters and styles of mantle flow we have defined for Galápagos are less-well known at other sites of plume-ridge

  10. Determination of Wetting Behavior, Spread Activation Energy, and Quench Severity of Bioquenchants

    NASA Astrophysics Data System (ADS)

    Prabhu, K. Narayan; Fernandes, Peter

    2007-08-01

    An investigation was conducted to study the suitability of vegetable oils such as sunflower, coconut, groundnut, castor, cashewnut shell (CNS), and palm oils as quench media (bioquenchants) for industrial heat treatment by assessing their wetting behavior and severity of quenching. The relaxation of contact angle was sharp during the initial stages, and it became gradual as the system approached equilibrium. The equilibrium contact angle decreased with increase in the temperature of the substrate and decrease in the viscosity of the quench medium. A comparison of the relaxation of the contact angle at various temperatures indicated the significant difference in spreading of oils having varying viscosity. The spread activation energy was determined using the Arrhenius type of equation. Oils with higher viscosity resulted in lower cooling rates. The quench severity of various oil media was determined by estimating heat-transfer coefficients using the lumped capacitance method. Activation energy for spreading determined using the wetting behavior of oils at various temperatures was in good agreement with the severity of quenching assessed by cooling curve analysis. A high quench severity is associated with oils having low spread activation energy.

  11. Effects of heterogeneity on active spreading strategies to remediate contaminated groundwater

    NASA Astrophysics Data System (ADS)

    Kasprzyk, J. R.; Piscopo, A. N.; Neupauer, R.

    2015-12-01

    The effectiveness of in situ chemical oxidation (ISCO) to remediate contaminated aquifers is constrained by the amount of contact between the groundwater contaminant and the injected oxidant. Contaminant degradation during ISCO can be enhanced using innovative active spreading strategies, which involve injecting and extracting water at wells in the vicinity of the plume to generate flow fields that spread the contaminant and oxidant plumes in a manner that increases their contact. Because aquifer heterogeneity affects the transport of the contaminant and oxidant during injection and extraction, aquifer heterogeneity also affects the amount of contact and the degree of contaminant degradation achieved using active spreading strategies during ISCO. Consequently, we can improve the effectiveness of active spreading strategies by generating sequences of injection and extraction that take the aquifer heterogeneity into account. In this study, we optimize sequences of injections and extractions to maximize contaminant degradation in aquifers with zonal and spatially-correlated heterogeneity for three contaminant-oxidant pairings with different reaction kinetics. Analysis of the transport and degradation corresponding to the optimal sequences of injection and extraction demonstrates that the underlying aquifer and contaminant properties are reflected by the optimal sequences.

  12. Staphylococcus aureus forms spreading dendrites that have characteristics of active motility

    PubMed Central

    Pollitt, Eric J. G.; Crusz, Shanika A.; Diggle, Stephen P.

    2015-01-01

    Staphylococcus aureus is historically regarded as a non-motile organism. More recently it has been shown that S. aureus can passively move across agar surfaces in a process called spreading. We re-analysed spreading motility using a modified assay and focused on observing the formation of dendrites: branching structures that emerge from the central colony. We discovered that S. aureus can spread across the surface of media in structures that we term ‘comets’, which advance outwards and precede the formation of dendrites. We observed comets in a diverse selection of S. aureus isolates and they exhibit the following behaviours: (1) They consist of phenotypically distinct cores of cells that move forward and seed other S. aureus cells behind them forming a comet ‘tail’; (2) they move when other cells in the comet tail have stopped moving; (3) the comet core is held together by a matrix of slime; and (4) the comets etch trails in the agar as they move forwards. Comets are not consistent with spreading motility or other forms of passive motility. Comet behaviour does share many similarities with a form of active motility known as gliding. Our observations therefore suggest that S. aureus is actively motile under certain conditions. PMID:26680153

  13. Staphylococcus aureus forms spreading dendrites that have characteristics of active motility.

    PubMed

    Pollitt, Eric J G; Crusz, Shanika A; Diggle, Stephen P

    2015-01-01

    Staphylococcus aureus is historically regarded as a non-motile organism. More recently it has been shown that S. aureus can passively move across agar surfaces in a process called spreading. We re-analysed spreading motility using a modified assay and focused on observing the formation of dendrites: branching structures that emerge from the central colony. We discovered that S. aureus can spread across the surface of media in structures that we term 'comets', which advance outwards and precede the formation of dendrites. We observed comets in a diverse selection of S. aureus isolates and they exhibit the following behaviours: (1) They consist of phenotypically distinct cores of cells that move forward and seed other S. aureus cells behind them forming a comet 'tail'; (2) they move when other cells in the comet tail have stopped moving; (3) the comet core is held together by a matrix of slime; and (4) the comets etch trails in the agar as they move forwards. Comets are not consistent with spreading motility or other forms of passive motility. Comet behaviour does share many similarities with a form of active motility known as gliding. Our observations therefore suggest that S. aureus is actively motile under certain conditions. PMID:26680153

  14. Extinct mid-ocean ridges and insights on the influence of hotspots at divergent plate boundaries

    NASA Astrophysics Data System (ADS)

    MacLeod, Sarah; Dietmar Müller, R.; Williams, Simon; Matthews, Kara

    2016-04-01

    We review all global examples of confirmed or suspected extinct mid-ocean ridges that are preserved in present-day ocean basins. Data on their spreading rate prior to extinction, time of cessation, length of activity, bathymetric and gravity signature are analysed. This analysis identifies some differences between subgroups of extinct ridges, including microplate spreading ridges, back-arc basin ridges and large-scale mid-ocean ridges. Crustal structure of extinct ridges is evaluated using gravity inversion to seek to resolve a long-standing debate on whether the final stages of spreading leads to development of thinned or thickened crust. Most of the ridges we assess have thinner crust at their axes than their flanks, yet a small number are found to have a single segment that is overprinted by an anomalous feature such as a seamount or volcanic ridge. A more complex cessation mechanism is necessary in these cases. The location of spreading centres at their time of cessation relative to hotspots was also evaluated using a global plate reconstruction. This review provides strong evidence for the long-term interaction of spreading centres with hotspots and plate boundaries have been frequently modified within the radius of a hotspot zone of influence.

  15. Divergent Ridge Features on the Juan de Fuca and Gorda Ridges

    NASA Astrophysics Data System (ADS)

    Eaton, M. E.; Sautter, L.; Steele, M.

    2014-12-01

    Multibeam data collected using a Kongsberg EM122 sonar system on the NOAA ship R/V Marcus G. Langseth led by chief scientist Douglas Toomey (University of Oregon) in 2009 and with a Simrad EM302 sonar system on two NOAA ship Okeanos Explorer cruises led by chief scientists James Gardner (University of New Hampshire) and Catalina Martinez (University of Rhode Island) in 2009 show the morphology of the Juan de Fuca and Gorda Ridges, as well as the Blanco and Mendocino Fracture Zones. These ridges and fracture zones comprise the divergent plate boundary of the eastern edge of the Pacific Plate and the western edges of the Juan de Fuca and Gorda Plates. Both plates are being subducted beneath the western edge of the North American Plate. CARIS HIPS 8.1 software was used to process the multibeam data and create bathymetric images. The ridge axes, located off the coast of Washington and Oregon (USA) adjacent to the Cascadia Basin, indicate obvious signs of spreading, due to the series of faults and rocky ridges aligned parallel to the plate boundaries. Fault and ridge orientations are used to compare the direction of seafloor spreading, and indicate that both the Juan de Fuca Plate and Gorda Plate are spreading in a southeastern direction. Younger ridges from the Gorda Ridge system mapped in the study run parallel to the boundary, however older ridges do not show the same orientation, indicating a change in spreading direction. The presence of hydrothermal vents along the Juan de Fuca Ridge is also evidence of the active boundary, as the vent chimneys are composed of minerals and metals precipitated from the hot water heated by magma from beneath the spreading seafloor. In this study, the data are used to compare and contrast earthquake seismicity and ridge morphologies at a depth range of approximately 762 to 2134 meters. The diverging Pacific, Juan de Fuca, and Gorda Plates along with the San Andreas Fault have potential to increase seismic and volcanic activity around

  16. Mid-ocean ridge basalt generation along the slow-spreading, South Mid-Atlantic Ridge (5-11°S): Inferences from 238U-230Th-226Ra disequilibria

    NASA Astrophysics Data System (ADS)

    Turner, Simon; Kokfelt, Thomas; Hauff, Folkmar; Haase, Karsten; Lundstrom, Craig; Hoernle, Kaj; Yeo, Isobel; Devey, Colin

    2015-11-01

    U-series disequilibria have provided important constraints on the physical processes of partial melting that produce basaltic magma beneath mid-ocean ridges. Here we present the first 238U-230Th-226Ra isotope data for a suite of 83 basalts sampled between 5°S and 11°S along the South Mid-Atlantic Ridge. This section of the ridge can be divided into 5 segments (A0-A4) and the depths to the ridge axis span much of the global range, varying from 1429 to 4514 m. Previous work has also demonstrated that strong trace element and radiogenic isotope heterogeneity existed in the source regions of these basalts. Accordingly, this area provides an ideal location in which to investigate the effects of both inferred melt column length and recycled materials. 226Ra-230Th disequilibria indicate that the majority of the basalts are less than a few millennia old such that their 230Th values do not require any age correction. The U-Th isotope data span a significant range from secular equilibrium up to 32% 230Th excess, also similar to the global range, and vary from segment to segment. However, the (230Th/238U) ratios are not negatively correlated with axial depth and the samples with the largest 230Th excesses come from the deepest ridge segment (A1). Two sub-parallel and positively sloped arrays (for segments A0-2 and A3 and A4) between (230Th/238U) and Th/U ratios can be modelled in various ways as mixing between melts from peridotite and recycled mafic lithologies. Despite abundant evidence for source heterogeneity, there is no simple correlation between (230Th/238U) and radiogenic isotope ratios suggesting that at least some of the trace element and radiogenic isotope variability may have been imparted to the source regions >350 kyr prior to partial melting to produce the basalts. In our preferred model, the two (230Th/238U) versus Th/U arrays can be explained by mixing of melts from one or more recycled mafic lithologies with melts derived from chemically heterogeneous

  17. Impact of Network Activity on the Spread of Infectious Diseases through the German Pig Trade Network

    PubMed Central

    Lebl, Karin; Lentz, Hartmut H. K.; Pinior, Beate; Selhorst, Thomas

    2016-01-01

    The trade of livestock is an important and growing economic sector, but it is also a major factor in the spread of diseases. The spreading of diseases in a trade network is likely to be influenced by how often existing trade connections are active. The activity α is defined as the mean frequency of occurrences of existing trade links, thus 0 < α ≤ 1. The observed German pig trade network had an activity of α = 0.11, thus each existing trade connection between two farms was, on average, active at about 10% of the time during the observation period 2008–2009. The aim of this study is to analyze how changes in the activity level of the German pig trade network influence the probability of disease outbreaks, size, and duration of epidemics for different disease transmission probabilities. Thus, we want to investigate the question, whether it makes a difference for a hypothetical spread of an animal disease to transport many animals at the same time or few animals at many times. A SIR model was used to simulate the spread of a disease within the German pig trade network. Our results show that for transmission probabilities <1, the outbreak probability increases in the case of a decreased frequency of animal transports, peaking range of α from 0.05 to 0.1. However, for the final outbreak size, we find that a threshold exists such that finite outbreaks occur only above a critical value of α, which is ~0.1, and therefore in proximity of the observed activity level. Thus, although the outbreak probability increased when decreasing α, these outbreaks affect only a small number of farms. The duration of the epidemic peaks at an activity level in the range of α = 0.2–0.3. Additionally, the results of our simulations show that even small changes in the activity level of the German pig trade network would have dramatic effects on outbreak probability, outbreak size, and epidemic duration. Thus, we can conclude and recommend that the network activity

  18. Significant Centers of Tectonic Activity as Identified by Wrinkle Ridges for the Western Hemisphere of Mars

    NASA Technical Reports Server (NTRS)

    Anderson, R.C.; Haldemann, A. F. C.; Golombek, M. P.; Franklin, B. J.; Dohm, J. M.; Lias, J.

    2000-01-01

    The western hemisphere region of Mars has been the site of numerous scientific investigations regarding its tectonic evolution. For this region of Mars, the dominant tectonic region is the Tharsis province. Tharsis is characterized by an enormous system of radiating grabens and a circumferential system of wrinkle ridges. Past investigations of grabens associated with Tharsis have identified specific centers of tectonic activity. A recent structural analysis of the western hemisphere region of Mars which includes the Tharsis region, utilized 25,000 structures to determine the history of local and regional centers of tectonic activity based primarily on the spatial and temporal relationships of extensional features. This investigation revealed that Tharsis is more structurally complex (heterogeneous) than has been previously identified: it consists of numerous regional and local centers of tectonic activity (some are more dominant and/or more long lived than others). Here we use the same approach as Anderson et al. to determine whether the centers of tectonic activity that formed the extensional features also contributed to wrinkle ridge (compressional) formation.

  19. Crevasse-squeeze ridge corridors: Diagnostic features of late-stage palaeo-ice stream activity

    NASA Astrophysics Data System (ADS)

    Evans, David J. A.; Storrar, Robert D.; Rea, Brice R.

    2016-04-01

    A 200-km-long and 10-km-wide linear assemblage of till-filled geometrical ridges on the bed of the Maskwa palaeo-ice stream of the late Wisconsinan southwest Laurentide Ice Sheet are interpreted as crevasse-squeeze ridges (CSR) developed during internal flow unit reorganization, immediately prior to ice stream shutdown. Ridge orientations are predominantly orientated WNW-ESE, with a subordinate WSW-ENE alignment, both indicative of ice fracture development transverse to former ice stream flow, as indicated by NNE-SSW aligned MSGL. Subglacial till injection into basal and/or full depth, mode I and II crevasses occurred at the approximate centreline of the ice stream, in response to extension and fracturing. Landform preservation indicates that this took place during the final stages of ice streaming, immediately prior to ice stream shutdown. This linear zone of ice fracturing therefore likely represents the narrowing of the fast-flowing trunk, similar to the plug flow identified in some surging valley glaciers. Lateral drag between the final active flow unit and the slower moving ice on either side is likely recorded by the up-ice bending of the CSR limbs. The resulting CSR corridor, here related to an individual ice stream flow unit, constitutes a previously unreported style of crevasse infilling and contrasts with two existing CSR patterns: (1) wide arcuate zones of CSRs related to widespread fracturing within glacier surge lobes; and (2) narrow concentric arcs of CSRs and recessional push moraines related to submarginal till deformation at active temperate glacier lobes.

  20. High-resolution bathymetry reveals contrasting landslide activity shaping the walls of the Mid-Atlantic Ridge axial valley

    NASA Astrophysics Data System (ADS)

    Cannat, Mathilde; Mangeney, Anne; OndréAs, HéLèNe; Fouquet, Yves; Normand, Alain

    2013-04-01

    Axial valleys are found along most slow-spreading mid-ocean ridges and are one of the most prominent topographic features on Earth. In this paper, we present the first deep-tow swath bathymetry for the axial valley walls of the Mid-Atlantic Ridge. These data allow us to analyze axial valley wall morphology with a very high resolution (0.5 to 1 m compared to ≥ 50 m for shipboard multibeam bathymetry), revealing the role played by landslides. Slow-spreading ridge axial valleys also commonly expose mantle-derived serpentinized peridotites in the footwalls of large offset normal faults (detachments). In our map of the Ashadze area (lat. 13°N), ultramafic outcrops have an average slope of 18° and behave as sliding deformable rock masses, with little fragmentation. By contrast, the basaltic seafloor in the Krasnov area (lat. 16°38'N) has an average slope of 32° and the erosion of the steep basaltic rock faces leads to extensive fragmentation, forming debris with morphologies consistent with noncohesive granular flow. Comparison with laboratory experiments suggests that the repose angle for this basaltic debris is > 25°. We discuss the interplay between the normal faults that bound the axial valley and the observed mass wasting processes. We propose that, along axial valley walls where serpentinized peridotites are exposed by detachment faults, mass wasting results in average slopes ≤ 20°, even in places where the emergence angle of the detachment is larger.

  1. Spreading Topsoil Encourages Ecological Restoration on Embankments: Soil Fertility, Microbial Activity and Vegetation Cover

    PubMed Central

    Rivera, Desirée; Mejías, Violeta; Jáuregui, Berta M.; López-Archilla, Ana Isabel; Peco, Begoña

    2014-01-01

    The construction of linear transport infrastructure has severe effects on ecosystem functions and properties, and the restoration of the associated roadslopes contributes to reduce its impact. This restoration is usually approached from the perspective of plant cover regeneration, ignoring plant-soil interactions and the consequences for plant growth. The addition of a 30 cm layer of topsoil is a common practice in roadslope restoration projects to increase vegetation recovery. However topsoil is a scarce resource. This study assesses the effects of topsoil spreading and its depth (10 to 30 cm) on two surrogates of microbial activity (β-glucosidase and phosphatase enzymes activity and soil respiration), and on plant cover, plant species richness and floristic composition of embankment vegetation. The study also evaluates the differences in selected physic-chemical properties related to soil fertility between topsoil and the original embankment substrate. Topsoil was found to have higher values of organic matter (11%), nitrogen (44%), assimilable phosphorous (50%) and silt content (54%) than the original embankment substrate. The topsoil spreading treatment increased microbial activity, and its application increased β-glucosidase activity (45%), phosphatase activity (57%) and soil respiration (60%). Depth seemed to affect soil respiration, β-glucosidase and phosphatase activity. Topsoil application also enhanced the species richness of restored embankments in relation to controls. Nevertheless, the depth of the spread topsoil did not significantly affect the resulting plant cover, species richness or floristic composition, suggesting that both depths could have similar effects on short-term recovery of the vegetation cover. A significant implication of these results is that it permits the application of thinner topsoil layers, with major savings in this scarce resource during the subsequent slope restoration work, but the quality of topsoil relative to the

  2. Using Hydrothermal Plumes and Their Chemical Composition to Identify and Understand Hydrothermal Activity at Explorer Ridge

    NASA Astrophysics Data System (ADS)

    Resing, J.; Lebon, G.; Baker, E.; Walker, S.; Nakamura, K.; Silvers, B.

    2002-12-01

    During June and July, 2002, an extensive survey of the hydrothermal systems of the Explorer Ridge was made aboard the R/V Thomas Thompson. This survey employed hydrocasts and the Autonomous Benthic Explorer (ABE) to locate and map hydrothermal vent fields. A total of 28 hydrocasts (17 verticals and 11 tow-yos) were used to search for hydrothermal activity from 49.5°N to 50.3°N on the Explorer Ridge. During the hydrocasts continuous measurements were made of conductivity, temperature, pressure, light backscatter, eH, Fe, Mn, and pH. Discrete samples were collected for total dissolved Fe and Mn, methane, pH, total CO2, and particulate matter. Most of the strong hydrothermal venting was near the Magic Mountain area of the Explorer Ridge at ~49.76° N, 130.26° W, where strong particulate backscatter signals (~0.130 NTUs) and moderate temperature anomalies (~ 0.05 °C) were detected. The particulate matter causing the backscatter was made up primarily of volatile particulate sulfur (PS) with little to no hydrothermal PFe. PS:PFe ratios exceeded 25 in the areas of most intense venting, . These PFe and PS data suggest that the hydrothermal Fe, if any, is deposited as sulfide minerals beneath the sea floor and that S is far in excess of Fe in the hydrothermal fluids. In the most intense plumes,total dissolvable Fe and Mn were between 20 and 30 nM, pH anomalies exceeded 0.025 pH units (indicating an increase of ~10uM CO2), and methane reached 16nM. These results suggest that the fluids exiting the sea floor are metal-poor and moderately gas-rich.

  3. Kindlin-2 cooperates with talin to activate integrins and induces cell spreading by directly binding paxillin

    PubMed Central

    Theodosiou, Marina; Widmaier, Moritz; Böttcher, Ralph T; Rognoni, Emanuel; Veelders, Maik; Bharadwaj, Mitasha; Lambacher, Armin; Austen, Katharina; Müller, Daniel J; Zent, Roy; Fässler, Reinhard

    2016-01-01

    Integrins require an activation step prior to ligand binding and signaling. How talin and kindlin contribute to these events in non-hematopoietic cells is poorly understood. Here we report that fibroblasts lacking either talin or kindlin failed to activate β1 integrins, adhere to fibronectin (FN) or maintain their integrins in a high affinity conformation induced by Mn2+. Despite compromised integrin activation and adhesion, Mn2+ enabled talin- but not kindlin-deficient cells to initiate spreading on FN. This isotropic spreading was induced by the ability of kindlin to directly bind paxillin, which in turn bound focal adhesion kinase (FAK) resulting in FAK activation and the formation of lamellipodia. Our findings show that talin and kindlin cooperatively activate integrins leading to FN binding and adhesion, and that kindlin subsequently assembles an essential signaling node at newly formed adhesion sites in a talin-independent manner. DOI: http://dx.doi.org/10.7554/eLife.10130.001 PMID:26821125

  4. Low-latitude equinoctial spread-F occurrence at different longitude sectors under low solar activity

    NASA Astrophysics Data System (ADS)

    Pezzopane, M.; Zuccheretti, E.; Abadi, P.; de Abreu, A. J.; de Jesus, R.; Fagundes, P. R.; Supnithi, P.; Rungraengwajiake, S.; Nagatsuma, T.; Tsugawa, T.; Cabrera, M. A.; Ezquer, R. G.

    2013-02-01

    We present the results of a comparative study of spread-F signatures over five low-latitude sites: Chiangmai (CGM; 18.8° N, 98.9° E, mag. Lat. 8.8° N), Thailand; Tanjungsari (TNJ; 6.9° S, 107.6° E, mag. Lat. 16.9° S), Indonesia; Palmas (PAL; 10.2° S, 311.8° E, mag. Lat. 0.9° S) and São José Dos Campos (SJC; 23.2° S, 314.1° E, mag. Lat. 14.0° S), Brazil; and Tucumán (TUC; 26.9° S, 294.6° E, mag. Lat. 16.8° S), Argentina. The investigation was based on simultaneous ionograms recorded by an FMCW (frequency-modulated continuous-wave) at CGM, an IPS-71 (digital ionosonde from KEL aerospace) at TNJ, a CADI (Canadian Advanced Digital Ionosonde) at PAL and SJC, and an AIS-INGV (Advanced Ionospheric Sounder - Istituto Nazionale di Geofisica e Vulcanologia) at TUC, during the equinoctial periods March-April (R12 = 2.0 and R12 = 2.2) and September-October (R12 = 6.1 and R12 = 7.0) 2009, for very low solar activity. Spread-F signatures were categorized into two types: the range spread-F (RSF) and the frequency spread-F (FSF). The study confirms that the dynamics and the physical processes responsible for these phenomena are actually complicated. In fact, the features that arise from the investigation are different, depending on both the longitude sector and on the hemisphere. For instance, TUC, under the southern crest of the ionospheric equatorial ionization anomaly (EIA), shows a predominance of RSF signatures, while both SJC, under the southern crest of EIA but in a different longitude sector, and CGM, under the northern crest of EIA, show a predominance of FSF signatures. Moreover, the spread-F occurrence over the longitude sector that includes CGM and TNJ is significantly lower than the spread-F occurrence over the longitude sector of PAL, SJC, and TUC.

  5. Discoidin domain receptor 1 activation suppresses alpha2beta1 integrin-dependent cell spreading through inhibition of Cdc42 activity.

    PubMed

    Yeh, Yi-Chun; Wang, Chau-Zen; Tang, Ming-Jer

    2009-01-01

    Upregulation and overexpression of discoidin domain receptor 1 (DDR1) have been implied in the regulation of kidney development and progression of cancers. Our previous studies with Mardin-Darby canine kidney (MDCK) cells showed that overexpression of DDR1 inhibited cell spreading, whereas dominant negative DDR1 promoted cell spreading on collagen-coated dish. Cell spreading is an important characteristic for cell differentiation and survival. However, little is known about the molecular mechanisms underlying the role of DDR1 in cell spreading. We have found here a novel signaling pathway of DDR1 consisting of Cdc42 that regulates the assembly and disassembly of cytoskeleton and cell spreading in MDCK cells. Cell spreading involves the organization of cytoskeleton that is mainly regulated by Rho-family GTPases. We assessed the activity of Rho-family GTPases and transfected MDCK cells with constitutively active or dominant negative GTPases, and quantified the extent of cell spreading. These results showed that DDR1 decreased the filamentous actin ratio and Rac1/Cdc42 activities, but had no effects on RhoA activity. Neither constitutively active nor dominant negative Rac1 altered DDR1-inhibited cell spreading. Constitutively active Cdc42 could rescue the DDR1-inhibited cell spreading, whereas dominant negative Cdc42 inhibited cell spreading, indicating that DDR1-inhibited cell spreading is Cdc42 dependent. With the use of alpha(2)beta(1) integrin blocking antibody, we showed that collagen-induced Cdc42 activation was mediated by alpha(2)beta(1) integrin. Moreover, ectopic FAK expression enhanced the Cdc42 activity. Reducing FAK activity by dominant negative FAK (FRNK) markedly abolished the Cdc42 activity. These findings show that DDR1a/b activation inhibits cell spreading through suppressing alpha(2)beta(1) integrin-mediated Cdc42 activation. PMID:18780290

  6. Geodynamic environments of ultra-slow spreading

    NASA Astrophysics Data System (ADS)

    Kokhan, Andrey; Dubinin, Evgeny

    2015-04-01

    Ultra-slow spreading is clearly distinguished as an outstanding type of crustal accretion by recent studies. Spreading ridges with ultra-slow velocities of extension are studied rather well. But ultra-slow spreading is characteristic feature of not only spreading ridges, it can be observed also on convergent and transform plate boundaries. Ultra-slow spreading is observed now or could have been observed in the past in the following geodynamic environments on divergent plate boundaries: 1. On spreading ridges with ultra-slow spreading, both modern (f.e. Gakkel, South-West Indian, Aden spreading center) and ceased (Labrador spreading center, Aegir ridge); 2. During transition from continental rifting to early stages of oceanic spreading (all spreading ridges during incipient stages of their formation); 3. During incipient stages of formation of spreading ridges on oceanic crust as a result of ridge jumps and reorganization of plate boundaries (f.e. Mathematicians rise and East Pacific rise); 4. During propagation of spreading ridge into the continental crust under influence of hotspot (Aden spreading center and Afar triple junction), under presence of strike-slip faults preceding propagation (possibly, rift zone of California Bay). Ultra-slow spreading is observed now or could have been observed in the past in the following geodynamic environments on transform plate boundaries: 1. In transit zones between two "typical" spreading ridges (f.e. Knipovich ridge); 2. In semi strike-slip/extension zones on the oceanic crust (f.e. American-Antarctic ridge); 3. In the zones of local extension in regional strike-slip areas in pull-apart basins along transform boundaries (Cayman trough, pull-apart basins of the southern border of Scotia plate). Ultra-slow spreading is observed now or could have been observed in the past in the following geodynamic environments on convergent plate boundaries: 1. During back-arc rifting on the stage of transition into back-arc spreading (central

  7. The mode of transverse spread of contraction initiated by local activation in single frog muscle fibers.

    PubMed

    Sugi, H; Ochi, R

    1967-10-01

    Isolated single frog muscle fibers were locally activated by applying negative current pulses to a pipette whose tip was in contact with the fiber surface. In contrast to the graded inward spread of contraction initiated by a moderate depolarization, the contraction in response to a strong negative current was observed to spread transversely around the whole perimeter but not through the center of the fiber. This response was elicited only with pipettes of more than 6 micro diameter. The response was still present if the sodium of the Ringer solution was replaced by choline, or the chloride was replaced by nitrate or propionate. The duration of the response appeared to be independent of the duration of stimulating current in fresh fibers, while the contraction lasted as long as the current went on in deteriorated fibers. The contraction was first initiated at the area of fiber surface covered by the pipette, and spread around the perimeter of the fiber with a velocity of 0.8-6 cm/sec. Possible mechanisms of the response are discussed in connection with the properties of the transverse tubular system, the possibility of some self-propagating process along the walls of the tubules being suggested. PMID:6064146

  8. Cell spreading analysis with directed edge profile-guided level set active contours.

    PubMed

    Ersoy, I; Bunyak, F; Palaniappan, K; Sun, M; Forgacs, G

    2008-01-01

    Cell adhesion and spreading within the extracellular matrix (ECM) plays an important role in cell motility, cell growth and tissue organization. Measuring cell spreading dynamics enables the investigation of cell mechanosensitivity to external mechanical stimuli, such as substrate rigidity. A common approach to measure cell spreading dynamics is to take time lapse images and quantify cell size and perimeter as a function of time. In our experiments, differences in cell characteristics between different treatments are subtle and require accurate measurements of cell parameters across a large population of cells to ensure an adequate sample size for statistical hypothesis testing. This paper presents a new approach to estimate accurate cell boundaries with complex shapes by applying a modified geodesic active contour level set method that directly utilizes the halo effect typically seen in phase contrast microscopy. Contour evolution is guided by edge profiles in a perpendicular direction to ensure convergence to the correct cell boundary. The proposed approach is tested on bovine aortic endothelial cell images under different treatments, and demonstrates accurate segmentation for a wide range of cell sizes and shapes compared to manual ground truth. PMID:18979769

  9. Optimal design of active spreading systems to remediate sorbing groundwater contaminants in situ

    NASA Astrophysics Data System (ADS)

    Piscopo, Amy N.; Neupauer, Roseanna M.; Kasprzyk, Joseph R.

    2016-07-01

    The effectiveness of in situ remediation to treat contaminated aquifers is limited by the degree of contact between the injected treatment chemical and the groundwater contaminant. In this study, candidate designs that actively spread the treatment chemical into the contaminant are generated using a multi-objective evolutionary algorithm. Design parameters pertaining to the amount of treatment chemical and the duration and rate of its injection are optimized according to objectives established for the remediation - maximizing contaminant degradation while minimizing energy and material requirements. Because groundwater contaminants have different reaction and sorption properties that influence their ability to be degraded with in situ remediation, optimization was conducted for six different combinations of reaction rate coefficients and sorption rates constants to represent remediation of the common groundwater contaminants, trichloroethene, tetrachloroethene, and toluene, using the treatment chemical, permanganate. Results indicate that active spreading for contaminants with low reaction rate coefficients should be conducted by using greater amounts of treatment chemical mass and longer injection durations relative to contaminants with high reaction rate coefficients. For contaminants with slow sorption or contaminants in heterogeneous aquifers, two different design strategies are acceptable - one that injects high concentrations of treatment chemical mass over a short duration or one that injects lower concentrations of treatment chemical mass over a long duration. Thus, decision-makers can select a strategy according to their preference for material or energy use. Finally, for scenarios with high ambient groundwater velocities, the injection rate used for active spreading should be high enough for the groundwater divide to encompass the entire contaminant plume.

  10. Optimal design of active spreading systems to remediate sorbing groundwater contaminants in situ.

    PubMed

    Piscopo, Amy N; Neupauer, Roseanna M; Kasprzyk, Joseph R

    2016-07-01

    The effectiveness of in situ remediation to treat contaminated aquifers is limited by the degree of contact between the injected treatment chemical and the groundwater contaminant. In this study, candidate designs that actively spread the treatment chemical into the contaminant are generated using a multi-objective evolutionary algorithm. Design parameters pertaining to the amount of treatment chemical and the duration and rate of its injection are optimized according to objectives established for the remediation - maximizing contaminant degradation while minimizing energy and material requirements. Because groundwater contaminants have different reaction and sorption properties that influence their ability to be degraded with in situ remediation, optimization was conducted for six different combinations of reaction rate coefficients and sorption rates constants to represent remediation of the common groundwater contaminants, trichloroethene, tetrachloroethene, and toluene, using the treatment chemical, permanganate. Results indicate that active spreading for contaminants with low reaction rate coefficients should be conducted by using greater amounts of treatment chemical mass and longer injection durations relative to contaminants with high reaction rate coefficients. For contaminants with slow sorption or contaminants in heterogeneous aquifers, two different design strategies are acceptable - one that injects high concentrations of treatment chemical mass over a short duration or one that injects lower concentrations of treatment chemical mass over a long duration. Thus, decision-makers can select a strategy according to their preference for material or energy use. Finally, for scenarios with high ambient groundwater velocities, the injection rate used for active spreading should be high enough for the groundwater divide to encompass the entire contaminant plume. PMID:27153361