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  • 1
    Electronic Resource
    Electronic Resource
    Seismic velocities of the uppermost igneous crust versus age (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 124 (1996), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Seismic refraction velocity data from the acoustic basement (called layer 2A) have been compiled for different mid-ocean ridges. The data from post-1970 studies show a strong correlation between velocity and basement age. Importantly, velocities double in less than 10 Myr for all ridges, but for older crust, up to 160 Myr in age, velocities do not increase significantly.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb07041.x
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  • 2
    Electronic Resource
    Electronic Resource
    Hotspot-ridge interaction in the Indian Ocean: constraints from Geosat/ERM altimetry (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 126 (1996), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Upper-mantle structure of Indian Ocean spreading ridges was investigated by track segments of Geosat/ERM altimeter measurements. To determine the upper-mantle structure of the Earth's gravity field, a low-degree and -order spherical harmonic representation of the geoid was removed. A test of several reference fields suggested that a degree 2–25 field with gradually rolled off coefficients (Sandwell & Renkin 1988) offers an adequate representation of the long-wavelength geoidal undulations.Filtered profiles of three individual ridge segments display a strong asymmetry in geoid versus age trends of conjugated rift flanks. The unusually low geoid slopes on one flank can perhaps best be explained as a dynamic or thermal phenomenon reflecting a flow connection between a neighbouring off-axis hotspot plume and the ridge axis, while the other flank simply cools as it spreads away from the axial zone. It seems reasonable to hypothesize that the Southwest Indian Ridge and the Southeast Indian Ridge act as sinks for plumes beneath Agulhas Plateau and Kerguelen Islands, respectively. The Carlsberg Ridge data suggest that the Réunion hotspot contaminated northwestern African lithosphere until 15 Ma. Moreover, symmetric flattening of geoid versus age trends of conjugated ridge flanks offers evidence that plume events affect geoid versus age trends
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb04703.x
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  • 3
    Electronic Resource
    Electronic Resource
    The Aegir Rift: Crustal Structure of an Extinct Spreading Axis (1997)
    Springer
    Marine geophysical researches 19 (1997), S. 1-23 
    Details
    ISSN: 1573-0581
    Keywords: Extinct spreading axis ; oceanic crust ; seismic refraction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Two seismic refraction and gravity lines were obtained along and normal to the axis of the Aegir Rift, an extinct spreading centre in the Norway Basin. Velocity-depth solutions and crustal structure models are derived from ocean-bottom records using two-dimensional ray tracing and synthetic seismogram modelling techniques. Gravity data are used to generate models consistent with the lateral variations in thickness of the layers in the crustal models. The resulting models require considerable degree of lateral inhomogeneity along and perpendicular to the rift axis. Crust within the extinct spreading centre is found to be thinner and of low P-wave velocity when compared with the crust sampled off-axis. To explain reduced velocities of the lower crust we suggest that, due to the relationship between fracturing and seismic velocity, the decreasing spreading rate leading up to extinction let the mechanically strong layer thicken, so that faulting and fracturing extended to greater depths . Low velocities are also observed in the uppermost mantle underlying the extinct spreading ridge. This zone is attributed to hydrothermal alteration of upper mantle peridotites. Furthermore, after spreading ceased 32-26 my ago, ongoing passive hydrothermal circulation was accompanied by the precipitation of alteration products in open void spaces, thereby decreasing the porosity and increasing the velocity. Consequently the typical low velocities of layer 2 found at active mid-ocean ridges have been replaced by values typical of mature oceanic crust.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1004288815129
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  • 4
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    Inversion of Scholte wave dispersion and waveform modeling for shallow structure of the Ninetyeast Ridge (2008)
    Springer
    Details
    Publication Date: 2008-12-19
    Description: The construction of S-wave velocity models of marine sediments down to hundreds of meters below the seafloor is important in a number of disciplines. One of the most significant trends in marine geophysics is to use interface waves to estimate shallow shear velocities which play an important role in determining the shallow crustal structure. In marine settings, the waves trapped near the fluid-solid interface are called Scholte waves, and this is the subject of the study. In 1998, there were experiments on the Ninetyeast Ridge (Central Indian Ocean) to study the shallow seismic structure at the drilled site. The data were acquired by both ocean bottom seismometer and ocean bottom hydrophone. A new type of seafloor implosion sources has been used in this experiment, which successfully excited fast and high frequency (〉500 Hz) body waves and slow, intermediate frequency (〈20 Hz) Scholte waves. The fundamental and first higher mode Scholte waves have both been excited by the implosion source. Here, the Scholte waves are investigated with a full waveform modeling and a group velocity inversion approach. Shear wave velocities for the uppermost layers of the region are inferred and results from the different methods are compared. We find that the full waveform modeling is important to understand the intrinsic attenuation of the Scholte waves between 1 and 20 Hz. The modeling shows that the S-wave velocity varies from 195 to 350 m/s in the first 16 m of the uppermost layer. Depths levels of high S-wave impedance contrasts compare well to the layer depth derived from a P-wave analysis as well as from drilling data. As expected, the P- to S-wave velocity ratio is very high in the uppermost 16 m of the seafloor and the Poisson ratio is nearly 0.5. Depth levels of high S-wave impedance contrasts are comparable to the layer depth derived from drilling data. © The Author(s) 2008.
    Print ISSN: 1383-4649
    Electronic ISSN: 1573-157X
    Topics: Geosciences , Physics
    Published by Springer
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  • 5
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    An implosive seismoacoustic source for seismic experiments on the ocean floor (1998)
    Springer
    In:  Marine Geophysical Researches, 20 (3). pp. 239-247.
    Details
    Publication Date: 2018-02-08
    Description: Bottom shots have been used for a number of years in seismic studies on the ocean floor. Most experiments utilized explosives as the energy source, though researchers have recognized the usefulness of collapsing water voids to produce seismoacoustic signals. Implosive sources, however, suffered generally from a lack of control of source depth. We present a new experimental tool, called SEEBOSEIS, to carry out seismic experiments on the seafloor utilizing hollow glass spheres as controlled implosive sources. The source is a 10-inch BENTHOS float with penetrator. Inside the sphere we place a small explosive charge (two detonators) to destabilize the glass wall. The time of detonation is controlled by an external shooting device. Test measurements on the Ninetyeast Ridge, Indian Ocean, show that the implosive sources can be used in seismic refraction experiments to image the subbottom P- wave velocity structure in detail beyond that possible with traditional marine seismic techniques. Additionally, the implosions permit the efficient generation of dispersed Scholte waves, revealing upper crustal S-wave velocities. The frequency band of seismic energy ranges from less than 1 Hz for Scholte modes up to 1000 Hz for diving P-waves. Therefore, broadband recording units with sampling rates 〉2000 Hz are recommended to sample the entire wave field radiated by implosive sources.
    Type: Article , PeerReviewed
    Format: text
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  • 6
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    Inversion of Scholte wave dispersion and waveform modeling for shallow structure of the Ninetyeast Ridge (2009)
    Springer
    In:  Journal of Seismology, 13 . pp. 543-559.
    Details
    Publication Date: 2017-08-09
    Description: The construction of S-wave velocity models of marine sediments down to hundreds of meters below the seafloor is important in a number of disciplines. One of the most significant trends in marine geophysics is to use interface waves to estimate shallow shear velocities which play an important role in determining the shallow crustal structure. In marine settings, the waves trapped near the fluid-solid interface are called Scholte waves, and this is the subject of the study. In 1998, there were experiments on the Ninetyeast Ridge (Central Indian Ocean) to study the shallow seismic structure at the drilled site. The data were acquired by both ocean bottom seismometer and ocean bottom hydrophone. A new type of seafloor implosion sources has been used in this experiment, which successfully excited fast and high frequency (> 500 Hz) body waves and slow, intermediate frequency (〈 20 Hz) Scholte waves. The fundamental and first higher mode Scholte waves have both been excited by the implosion source. Here, the Scholte waves are investigated with a full waveform modeling and a group velocity inversion approach. Shear wave velocities for the uppermost layers of the region are inferred and results from the different methods are compared. We find that the full waveform modeling is important to understand the intrinsic attenuation of the Scholte waves between 1 and 20 Hz. The modeling shows that the S-wave velocity varies from 195 to 350 m/s in the first 16 m of the uppermost layer. Depths levels of high S-wave impedance contrasts compare well to the layer depth derived from a P-wave analysis as well as from drilling data. As expected, the P- to S-wave velocity ratio is very high in the uppermost 16 m of the seafloor and the Poisson ratio is nearly 0.5. Depth levels of high S-wave impedance contrasts are comparable to the layer depth derived from drilling data.
    Type: Article , PeerReviewed
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  • 7
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    Slope failure on the flanks of the southern Cape Verde Islands (2007)
    Springer
    In:  In: Submarine mass movements and their consequences. , ed. by Lykousis, V., Sakellariou, D. and Locat, J. Springer, Dordrecht, pp. 337-345.
    Details
    Publication Date: 2012-07-05
    Type: Book chapter , PeerReviewed
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  • 8
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    Seamounts (2016)
    Springer
    In:  In: Encyclopedia of Marine Geosciences. , ed. by Harff, J., Meschede, M., Petersen, S. and Thiede, J. Springer, Dordrecht, pp. 754-760. ISBN 978-94-007-6239-4
    Details
    Publication Date: 2019-09-23
    Description: Seamounts are literally mountains rising from the seafloor. More specifically, they are “any geographically isolated topographic feature on the seafloor taller than 100 m, including ones whose summit regions may temporarily emerge above sea level, but not including features that are located on continental shelves or that are part of other major landmasses” (Staudigel et al., 2010). The term “guyot” can be used for seamounts having a truncated cone shape with a flat summit produced by erosion at sea level (Hess, 1946), development of carbonate reefs (e.g., Flood, 1999), or partial collapse due to caldera formation (e.g., Batiza et al., 1984). Seamounts 〈1,000 m tall are sometimes referred to as “knolls” (e.g., Hirano et al., 2008). “Petit spots” are a newly discovered subset of sea knolls confined to the bulge of subducting oceanic plates of oceanic plates seaward of deep-sea trenches (Hirano et al., 2006).
    Type: Book chapter , PeerReviewed
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  • 9
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    Volatile (H2O, CO2, Cl, S) budget of the Central American subduction zone (2014)
    Springer
    In:  International Journal of Earth Sciences, 103 (7). pp. 2101-2127.
    Details
    Publication Date: 2019-09-23
    Description: After more than a decade of multidisciplinary studies of the Central American subduction zone mainly in the framework of two large research programmes, the US MARGINS program and the German Collaborative Research Center SFB 574, we here review and interpret the data pertinent to quantify the cycling of mineral-bound volatiles (H2O, CO2, Cl, S) through this subduction system. For input-flux calculations, we divide the Middle America Trench into four segments differing in convergence rate and slab lithological profiles, use the latest evidence for mantle serpentinization of the Cocos slab approaching the trench, and for the first time explicitly include subduction erosion of forearc basement. Resulting input fluxes are 40–62 (53) Tg/Ma/m H2O, 7.8–11.4 (9.3) Tg/Ma/m CO2, 1.3–1.9 (1.6) Tg/Ma/m Cl, and 1.3–2.1 (1.6) Tg/Ma/m S (bracketed are mean values for entire trench length). Output by cold seeps on the forearc amounts to 0.625–1.25 Tg/Ma/m H2O partly derived from the slab sediments as determined by geochemical analyses of fluids and carbonates. The major volatile output occurs at the Central American volcanic arc that is divided into ten arc segments by dextral strike-slip tectonics. Based on volcanic edifice and widespread tephra volumes as well as calculated parental magma masses needed to form observed evolved compositions, we determine long-term (105 years) average magma and K2O fluxes for each of the ten segments as 32–242 (106) Tg/Ma/m magma and 0.28–2.91 (1.38) Tg/Ma/m K2O (bracketed are mean values for entire Central American volcanic arc length). Volatile/K2O concentration ratios derived from melt inclusion analyses and petrologic modelling then allow to calculate volatile fluxes as 1.02–14.3 (6.2) Tg/Ma/m H2O, 0.02–0.45 (0.17) Tg/Ma/m CO2, and 0.07–0.34 (0.22) Tg/Ma/m Cl. The same approach yields long-term sulfur fluxes of 0.12–1.08 (0.54) Tg/Ma/m while present-day open-vent SO2-flux monitoring yields 0.06–2.37 (0.83) Tg/Ma/m S. Input–output comparisons show that the arc water fluxes only account for up to 40 % of the input even if we include an “invisible” plutonic component constrained by crustal growth. With 20–30 % of the H2O input transferred into the deeper mantle as suggested by petrologic modeling, there remains a deficiency of, say, 30–40 % in the water budget. At least some of this water is transferred into two upper-plate regions of low seismic velocity and electrical resistivity whose sizes vary along arc: one region widely envelopes the melt ascent paths from slab top to arc and the other extends obliquely from the slab below the forearc to below the arc. Whether these reservoirs are transient or steady remains unknown.
    Type: Article , PeerReviewed
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  • 10
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    Seamounts (2015)
    Springer
    Details
    Publication Date: 2023-11-08
    Description: Definition: Seamounts are literally mountains rising from the seafloor. More specifically, they are “any geographically isolated topographic feature on the seafloor taller than 100 m, including ones whose summit regions may temporarily emerge above sea level, but not including features that are located on continental shelves or that are part of other major landmasses” (Staudigel et al., 2010). The term “guyot” can be used for seamounts having a truncated cone shape with a flat summit produced by erosion at sea level (Hess, 1946), development of carbonate reefs (e.g., Flood, 1999), or partial collapse due to caldera formation (e.g., Batiza et al., 1984). Seamounts 〈1,000 m tall are sometimes referred to as “knolls” (e.g., Hirano et al., 2008). “Petit spots” are a newly discovered subset of sea knolls confined to the bulge of subducting oceanic plates of oceanic plates seaward of deep-sea trenches (Hirano et al., 2006).
    Type: Book chapter , NonPeerReviewed
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