1,721,028 research outputs found
RRS Charles Darwin Cruise 167, 23 Nov - 21 Dec 2004. Sub-seafloor physical properties at Saldanha Seamount, Mid-Atlantic Ridge, and controls on the spatial distribution of hydrothermal venting
Full text linkCharles Darwin 167 (CD167) was a joint research project carried out by UK and Portuguese scientists from the National Oceanography Centre, Southampton, the University of Durham and the University of Lisbon. The task of CD167 was to carry out geophysical and geological studies at the Mid-Atlantic Ridge. The work area was a ridge offset centred on 36º 34’ N, 33º 26’ W - about 200 n.m. south of the island of Flores in the Azores. At this location, the median valley of the ridge is offset by about 20 km right-laterally, forming a non-transform discontinuity. Of particular interest is a site known as the Saldanha Massif, where previous studies have revealed an area of exposed, tectonically-unroofed mantle rocks and unexpectedly significant hydrothermal circulation with venting near the summit of the massif. A three-dimensional controlled source electromagnetic (CSEM) survey of sub-sea-floor electrical resistivity was carried out over a 10 km2 area centred on the Saldanha Massif. Following CSEM data analysis, the resulting images of electrical structure will be translated into constraints on porosity distribution, interconnectedness and pore fluid properties by means of geophysical effective medium modelling methods. This in turn will address the questions of whether the Saldanha vent site owes its existence to the presence of a deep fracture network, whether this network completely penetrates the thin crustal carapace within the ridge offset, and how far it extends downwards into the underlying mantle rocks. A secondary objective was to collect a series of ridge-perpendicular (approximately east-west) profiles of bathymetry and of gravity and magnetic anomalies, to contribute to regional tectonic studies through improving an international European compilation of such data. This was successfully achieved, and the data have been passed on to our international collaborators. A third objective was to collect sea bottom geological samples, by gravity coring and dredging, for analysis at CREMINER in Lisbon. This objective was also achieved
A three-dimensional study of a crustal low velocity region beneath the 9,3'N overlapping spreading center
No full textOverlapping spreading centers (OSCs) play a key role in models of magma distribution at fast spreading ridges. To investigate the relationship between ridge-axis discontinuities and magma supply, we conducted a three-dimensional seismic reflection and tomography experiment at the 9°03?N OSC along the East Pacific Rise. Tomographic analysis imaged a broad mid-crustal low velocity zone (LVZ) beneath parts of the overlapper and the associated overlap basin, demonstrating that it is magmatically robust. The complementary datasets reveal a complex storage and tapping of melt: the LVZ and melt sill at either end of the overlap basin are not simply centered beneath the rise crest but are skewed inwards. The subsequent focussing of the LVZ and sill beneath the axis of the eastern limb appears to be due to melt migration toward the tip. The OSC western limb is less magmatically robust and may be in the process of dying
Three-dimensional shallow crustal emplacement at 9,03'N overlapping spreading center on the East Pacific Rise: correlations between magnetization and tomographic images
No full textWe report a three-dimensional (3-D) seismic reflection and tomographic survey conducted at the 9°03'N overlapping spreading center (OSC) on the East Pacific Rise to understand crustal accretion at this feature. Inversions of travel time data from 19 ocean bottom hydrophones provide a 3-D image of the shallow velocity structure beneath the nontransform offset and associated discordance zone. Seismic analysis indicates that layer 2A thickness varies between 100 and 900 m and averages 430 m throughout the study area. The heterogeneous upper crustal structure at the OSC region contrasts with the simpler symmetric structure flanking the midsegments of the East Pacific Rise. The crust affected by the OSC migration carries evidence for the complex accretion at the axial discontinuity where the overlap basin may act as a lava pond. An area of thick layer 2A covers the southern half of the overlap basin and the propagating ridge tip and shows good correlation with a high magnetization region. Comparison of the magnetic field anomaly derived from the seismic structure model with the observed sea surface magnetic anomaly suggests that a significant portion of the high magnetization can be related to magnetic source thickness variation rather than solely to the geochemistry of the volcanic rocks
RRS James Clark Ross Cruise 269B, 07-24 Jul 2012, Reykjavik – Reykjavik. Arctic hydrate dissociation as a consequence of climate change: determining the vulnerable methane reservoir and gas escape mechanisms
Full text linkCruise JR269B was the second part of a two-leg geophysical programme led by a science party from the School of Ocean and Earth Science, University of Southampton, and carried out on the western margin of the Svalbard archipelago. The overall objective was to investigate gas transport and escape mechanisms and quantify gas and hydrate saturation values, in shallow sediments along the margin in areas where both significant accumulations of methane hydrate and active methane venting through the sea floor are known to occur. During the first leg, JR269A, which was completed in August-September 2011, the main objective had been to acquire high resolution seismic reflection and wide-angle survey data, working in collaboration with IFREMER’s ‘SYSIF’ team. In contrast, the primary objective of this second (2012) leg, JR269B, was the acquisition of controlled source electromagnetic (CSEM) survey data, to complement the pre-existing seismic data and to provide better constraints on the gas and hydrate saturation values within pore spaces. Secondary objectives during both legs included acquiring additional sub-bottom profiler, water column physical properties, and swath bathymetry data. A key objective of the analysis of data from both legs will be the joint inversion of co-located seismic and CSEM data, in order to optimise in situ estimates of concentrations of methane in free gas and hydrate forms within the subsurface
Life and death of axial volcanic ridges: segmentation and crustal accretion at the Reykjanes Ridge
No full textIn this paper we present a description of ridge segmentation and segment evolution at the slow spreading Reykjanes Ridge, based on the combined and integrated interpretation of several geophysical datasets acquired along this section of the Mid-Atlantic Ridge. At the Reykjanes Ridge, segmentation is manifest at the seabed most clearly as axial volcanic ridges (AVRs). These correspond to third-order segments, while multiple adjacent AVRs collectively represent the larger-scale pattern of second-order segmentation. AVRs are known to undergo a life cycle of multiple phases of magmatic accretion, tectonic extension and dismemberment.Our combined interpretation of the results of the previously independently analysed datasets indicates that magma influx from the mantle to the crust associated with these cycles is initially focused towards second-order segment centres. Adjacent AVRs within a second-order segment are progressively rejuvenated from the segment centre towards the segment tips during each cycle, with the redistribution of magma along-axis occurring within individual AVRs at mid-crustal to upper-crustal level during the magmatic phase. In some cases, offset basins between adjacent AVRs are characterised by significant crustal melt accumulation and increased crustal thickness, indicating that they are currently sites of incipient AVR growth. The initiation of new AVRs within former offset basins, and the abandonment of other AVRs, indicates that third-order segments have a finite and limited life span.As part of this study we have mapped the geometry and location of all abandoned (inactive), relict AVRs preserved off-axis to form the basis of a reconstruction of the pattern of asymmetric spreading. Using this reconstruction we have developed a new model of AVR evolution, spanning 2 Myr of crustal accretion, that reveals nested scales and phases of accretion, in which each AVR undergoes multiple tectonomagmatic cycles before ending its life once it has migrated sufficiently far across the median valley to become isolated from its melt supply. Once extinct, AVRs are rafted further off-axis and are preserved in the morphology of the seabed and the thickness of layer 2a.<br/
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