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  • 1
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    Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid-Atlantic Ridge 30°N (2011)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 116 (2011): B07103, doi:10.1029/2010JB007931.
    Description: Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100–220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45° rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises ∼70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.
    Keywords: Atlantis Massif ; Integrated Ocean Drilling Program ; Oceanic Core Complex
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 2
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    Hydrothermal vent fields and chemosynthetic biota on the world's deepest seafloor spreading centre (2012)
    Nature Publishing Group
    Details
    Publication Date: 2022-05-25
    Description: © Macmillan Publishers Limited, 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Nature Communications 3 (2012): 620, doi:10.1038/ncomms1636.
    Description: The Mid-Cayman spreading centre is an ultraslow-spreading ridge in the Caribbean Sea. Its extreme depth and geographic isolation from other mid-ocean ridges offer insights into the effects of pressure on hydrothermal venting, and the biogeography of vent fauna. Here we report the discovery of two hydrothermal vent fields on the Mid-Cayman spreading centre. The Von Damm Vent Field is located on the upper slopes of an oceanic core complex at a depth of 2,300 m. High-temperature venting in this off-axis setting suggests that the global incidence of vent fields may be underestimated. At a depth of 4,960 m on the Mid-Cayman spreading centre axis, the Beebe Vent Field emits copper-enriched fluids and a buoyant plume that rises 1,100 m, consistent with 〉 400 °C venting from the world’s deepest known hydrothermal system. At both sites, a new morphospecies of alvinocaridid shrimp dominates faunal assemblages, which exhibit similarities to those of Mid-Atlantic vents.
    Description: This work is supported by a UK NERC award (NE/F017774/1 & NE/F017758/1) to J.T.C., D.P.C., B.J.M., K.S. and P.A.T., Royal Society Travel Grant 2009/R3 to R.C.S., A.M. is supported by SENSEnet, a Marie Curie Initial Training Network (ITN) funded by the European Commission Seventh Framework Programme, Contract Number PITN-GA-2009-237868 and a NASA ASTEP Grant NNX09AB75G to C.R.G. and C.L.V.D., which are gratefully acknowledged.
    Repository Name: Woods Hole Open Access Server
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  • 3
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    Oceanic detachment faults generate compression in extension (2017)
    Geological Society of America
    Details
    Publication Date: 2022-05-26
    Description: © The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geology 45 (2017): 923-926, doi:10.1130/G39232.1.
    Description: In extensional geologic systems such as mid-ocean ridges, deformation is typically accommodated by slip on normal faults, where material is pulled apart under tension and stress is released by rupture during earthquakes and magmatic accretion. However, at slowly spreading mid-ocean ridges where the tectonic plates move apart at rates 〈80 km m.y.–1, these normal faults may roll over to form long-lived, low-angled detachments that exhume mantle rocks and form corrugated domes on the seabed. Here we present the results of a local micro-earthquake study over an active detachment at 13°20′N on the Mid-Atlantic Ridge to show that these features can give rise to reverse-faulting earthquakes in response to plate bending. During a 6 month survey period, we observed a remarkably high rate of seismic activity, with 〉244,000 events detected along 25 km of the ridge axis, to depths of ∼10 km below seafloor. Surprisingly, the majority of these were reverse-faulting events. Restricted to depths of 3–7 km below seafloor, these reverse events delineate a band of intense compressional seismicity located adjacent to a zone of deeper extensional events. This deformation pattern is consistent with flexural models of plate bending during lithospheric accretion. Our results indicate that the lower portion of the detachment footwall experiences compressive stresses and deforms internally as the fault rolls over to low angles before emerging at the seafloor. These compressive stresses trigger reverse faulting even though the detachment itself is an extensional system.
    Description: This research was funded by Natural Environment Research Council (NERC) grants NE/J02029X/1, NE/ J021741/1, and NE/J022551/1, and by U.S. National Science Foundation grant OCE-1458084.
    Repository Name: Woods Hole Open Access Server
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  • 4
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    Seismicity trends and detachment fault structure at 13 degrees N, Mid-Atlantic Ridge (2021)
    Geological Society of America
    Details
    Publication Date: 2022-05-26
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Parnell-Turner, R., Sohn, R. A., Peirce, C., Reston, T. J., MacLeod, C. J., Searle, R. C., & Simao, N. M. Seismicity trends and detachment fault structure at 13 degrees N, Mid-Atlantic Ridge. Geology, 49(3), (2021): 320-324, https://doi.org/10.1130/G48420.1.
    Description: At slow-spreading ridges, plate separation is commonly partly accommodated by slip on long-lived detachment faults, exposing upper mantle and lower crustal rocks on the seafloor. However, the mechanics of this process, the subsurface structure, and the interaction of these faults remain largely unknown. We report the results of a network of 56 ocean-bottom seismographs (OBSs), deployed in 2016 at the Mid-Atlantic Ridge near 13°N, that provided dense spatial coverage of two adjacent detachment faults and the intervening ridge axis. Although both detachments exhibited high levels of seismicity, they are separated by an ∼8-km-wide aseismic zone, indicating that they are mechanically decoupled. A linear band of seismic activity, possibly indicating magmatism, crosscuts the 13°30′N domed detachment surface, confirming previous evidence for fault abandonment. Farther south, where the 2016 OBS network spatially overlapped with a similar survey done in 2014, significant changes in the patterns of seismicity between these surveys are observed. These changes suggest that oceanic detachments undergo previously unobserved cycles of stress accumulation and release as plate spreading is accommodated.
    Description: This work was funded by UK Natural Environmental Research Council (NERC) grants NE/J02029X/1, NE/J022551/1, and NE/J021741/1 and by U.S. National Science Foundation grants OCE-1458084 and OCE-1839727. OBSs were provided by NERC UK Ocean-Bottom Instrumentation Facility (Minshull et al., 2005).
    Repository Name: Woods Hole Open Access Server
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  • 5
    Electronic Resource
    Electronic Resource
    Merged GLORIA sidescan and Hydrosweep pseudo-sidescan: Processing and creation of digital mosaics (1996)
    Springer
    Marine geophysical researches 18 (1996), S. 651-661 
    Details
    ISSN: 1573-0581
    Keywords: GLORIA ; sidescan sonar ; Hydrosweep ; multibeam bathymetry ; image processing ; transform fault ; Juan Fernandez microplate ; East Pacific Rise
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We have replaced the usual band of poor-quality data in the near-nadir region of our GLORIA long-range sidescan-sonar imagery with a shaded-relief image constructed from swath bathymetry data (collected simultaneously with GLORIA) which completely cover the nadir area. We have developed a technique to enhance these “pseudo-sidescan” images in order to mimic the neighbouring GLORIA backscatter intensities. As a result, the enhanced images greatly facilitate the geologic interpretation of the adjacent GLORIA data, and geologic features evident in the GLORIA data may be correlated with greater confidence across track. Features interpreted from the pseudo-sidescan may be extrapolated from the near-nadir region out into the GLORIA range where they may nt have been recognized otherwise, and therefore the pseudo-sidescan can be used to ground-truth GLORIA interpretations. Creation of digital sidescan mosaics utilized an approach not previously used for GLORIA data. Pixels were correctly placed in cartographic space and the time required to complete a final mosaic was significantly reduced. Computer software for digital mapping and mosaic creation is incorporated into the newly-developed Woods Hole Image Processing System (WHIPS) which can process both low- and high-frequency sidescan, and can interchange data with the Mini Image Processing System (MIPS) most commonly used for GLORIA processing. These techniques are tested by creating digital mosaics of merged GLORIA sidescan and Hydrosweep pseudo-sidescan data from the vicinity of the Juan Fernandez microplate along the East Pacific Rise (EPR).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00313879
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  • 6
    Electronic Resource
    Electronic Resource
    Segmentation of the Central Indian Ridge between 12°12′ S and the Indian Ocean Triple Junction (1993)
    Springer
    Marine geophysical researches 15 (1993), S. 265-282 
    Details
    ISSN: 1573-0581
    Keywords: Mid-Ocean Ridge ; Central Indian Ridge ; GLORIA ; segmentation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The present morphology and tectonic evolution of more than 1500 kilometres of the Central Indian Ridge are described and discussed following the integration of GLORIA side-scan sonographs with conventional geophysical datasets. Segmentation of the ridge occurs by a series of ridge axis discontinuities ranging in periodicity along strike from 275 km to less than 30 km. These segment boundaries we have classified into two types: first order fracture zones of offsets greater than 50 km which bound five major (mega-) segments, and smaller scale structures of a variety of offset styles and amplitudes which cut four of these segments. We refer to these as ridge-axis discontinuities. The frequent opposite sense of offset identified between the first order structures and the subordinate discontinuities between these major structures is interpreted as resulting from the adjustment to new kinematic parameters after magnetic anomaly 20. As far as our data allows us to determine, the central major segment is not subdivided by minor ridge axis discontinuities, which we suggest is a result of its proximity to the Rodriguez hotspot.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01982385
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  • 7
    Electronic Resource
    Electronic Resource
    Fault scarp statistics at the Galapagos spreading centre from Deep Tow data (1998)
    Springer
    Marine geophysical researches 20 (1998), S. 183-193 
    Details
    ISSN: 1573-0581
    Keywords: Mid-ocean ridge tectonics ; volcanic flows ; seismicity ; slope stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Much of the relief of the abyssal hills covering the ocean basins is believed to originate from faulting of oceanic crust at mid-ocean ridges. The timescale over which faults grow is controversial, however, with some authors arguing that faults continue to grow in places for 0.5 m.y. or more based on increasing relief of fault scarps with distance from ridge axes. We examine Deep Tow profiler records of the Galapagos Spreading Centre, in which basement reflections allow scarp relief to be measured beneath the sediment cover, and find that relief does not increase but decreases systematically to 40 km off-axis (1.5 Ma seafloor). Since reversal of fault offsets is unlikely in this tectonic setting, we interpret this result as indicating that variations in fault statistics could reflect temporal variations in the tectonic or volcanic state of the ridge crest, not necessarily progressive fault growth with age as previously assumed. Resolving the issue of fault longevity will therefore require independent data on the timing of fault growth and distribution of present growth activity. We suggest some possible alternative indicators of fault longevity and discuss more generally the implications of volcanic flows to studies of faulting at ridges.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1004556813514
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  • 8
    Electronic Resource
    Electronic Resource
    Extensional faulting and segmentation of the Mid-Atlantic Ridge north of the Kane Fracture Zone (24° 00′ N to 24° 40′ N) (1995)
    Springer
    Marine geophysical researches 17 (1995), S. 37-61 
    Details
    ISSN: 1573-0581
    Keywords: Mid-Atlantic Ridge ; ridge segmentation ; spreading structure ; deep-tow side-scan sonar
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The combination of multi-beam echo-sounder swath bathymetry and high-resolution deep-towed sidescan sonar provides a powerful database from which to examine mid-ocean ridge processes. We have used such a database, gathered from the Mid-Atlantic Ridge north of the Kane Fracture Zone (the MARNOK area), to examine the relationship between tectonic, volcanic, and bathymetric segmentation. We have identified structural domains, with different fault distributions, and neovolcanic segments that are distinct from the 2nd or 3rd order bathymetric segmentation. From their mutual relationships, a model is proposed for the magmatic accretion of oceanic crust at slow spreading ridges that relates the local melt supply to the tectonic style. We suggest that these are mutually interactive, and determine whether volcanic extrusion along the ridge is continuous and slow, or episodic and rapid.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01268050
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  • 9
    Electronic Resource
    Electronic Resource
    Analysis of simrad EM12 multibeam bathymetry and acoustic backscatter data for seafloor mapping, exemplified at the Mid-Atlantic Ridge at 45° N (1996)
    Springer
    Marine geophysical researches 18 (1996), S. 663-688 
    Details
    ISSN: 1573-0581
    Keywords: Sonar ; sidescan ; image processing ; mid-ocean ridge
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The EM12 multibeam echosounder can record acoustic backscatter information as well as high resolution bathymetry. The dataset presented, from the axis of the Mid-Atlantic Ridge at 45° N, was the first EM12 survey of a mid-ocean ridge. This paper presents methods for utilising the backscatter information. Data processing enables the production of a mosaic of acoustic backscatter, and visualisation techniques are investigated to provide initial qualitative views of the combined backscatter and bathymetry datasets. The co-registration of the backscatter and bathymetry data enables quantitative analysis of their relationships. Various sites of different geological type have been selected and their angular acoustic backscattering relationships estimated, including the effect on backscatter of incidence angle, its regional variability with bottom type and the influence of bottom slope. Incidence angles and bottom type are shown to affect backscatter to a similar degree, while slopes appear to contribute little. The geometry of hull-mounted systems, such as the EM12, is significantly different from that of conventional sidescan sonars, such as GLORIA, and the backscatter images from the two types differ in various respects. Because of the wide variations in incidence angle that are common with hull-mounted systems, and the importance of incidence angle in determining backscatter strength, it is vital to consider the effect of incidence angle during interpretation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00313880
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  • 10
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    Eruptive hummocks: Building blocks of the upper ocean crust (2012)
    Geological Society of America (GSA)
    Details
    Publication Date: 2012-01-01
    Description: The spreading axis at many slow-spreading mid-ocean ridges is marked by an axial volcanic ridge. In this study, we use a combination of high-resolution remote sensing methods to elucidate the detailed nature of volcanoes in such a ridge. We find that the “hummocks” described in previous sidescan sonar studies are dome- or cone-shaped edifices, 5–150 m high with diameters of 30–330 m. We estimate they form quickly, in single eruptions, each of which may produce several hummocks. Hummock collapse is common and hummocks of all heights are prone to failure. Collapses generally occur down the regional seafloor slope, suggesting control by local topography. Approximately 33% of hummocks lose ~40% of their volume by collapse, so ~12% of all material erupted on the axial volcanic ridge is rapidly converted to talus. The higher porosity of these deposits may increase average upper crustal porosity by several percent, contributing 〉0.5 km s-1 to seismic velocity decrease in the upper oceanic crust, and may be one of the dominant mechanisms for increasing porosity in upper slow-spreading oceanic crust.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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