81 research outputs found

    The break-up of continents and the formation of new ocean basins

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    Rifted continental margins are the product of stretching, thinning and ultimate break-up of a continental plate into smaller fragments, and the rocks lying beneath them store a record of this rifting process. Earth scientists can read this record by careful sampling and with remote geophysical techniques. These experimental studies have been complemented by theoretical analyses of continental extension and associated magmatism. Some rifted margins show evidence for extensive volcanic activity and uplift during rifting; at these margins, the record of the final stages of rifting is removed by erosion and obscured by the thick volcanic cover. Other margins were underwater throughout their formation and showed rather little volcanic activity; here the ongoing deposition of sediment provides a clearer record. During the last decade, vast areas of exhumed mantle rocks have been discovered at such margins between continental and oceanic crust. This observation conflicts with the well-established idea that the mantle melts to produce new crust when it is brought close to the Earth's surface. In contrast to the steeply dipping faults commonly seen in zones of extension within continental interiors, faults with very shallow dips play a key role in the deformation immediately preceding continental break-up. Future progress in the study of continental break-up will depend on studies of pairs of margins which were once joined and on the development of computer models which can handle rigorously the complex transition from distributed continental deformation to sea-floor spreading focused at a mid-ocean ridge

    The microstructure of sediment-hosted hydrates: evidence from effective medium modelling of laboratory and borehole seismic data

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    Much of our knowledge of hydrate distribution in the subsurface comes frominterpretations of remote seismic measurements. A key step in such interpretations isan effective medium theory that relates the seismic properties of a given sediment toits hydrate content. A variety of such theories have been developed; these theoriesgenerally give similar results if the same assumptions are made about the extent towhich hydrate contributes to the load-bearing sediment frame. We have furtherdeveloped and modified one such theory, the self-consistentapproximation/differential effective medium approach, to incorporate additionalempirical parameters describing the extent to which both the sediment matrix material(clay or quartz) and the hydrate are load-bearing. We find that a single choice ofthese parameters allows us to match well both P and S wave velocity measurementsfrom both laboratory and in situ datasets, and that the inferred proportion of hydratethat is load-bearing varies approximately linearly with hydrate saturation. Thisproportion appears to decrease with increasing hydrate saturation for gas-richlaboratory environments, but increase with hydrate saturation when hydrate is formedfrom solution and for an in situ example

    Lithospheric controls on melt production during continental breakup at slow rates of extension: Application to the North Atlantic

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    Rifted margins form from extension and breakup of the continentallithosphere. If this extension is coeval with a region of hotter lithosphere,then it is generally assumed that a volcanic margin would follow. Herewe present the results of numerical simulations of rift margin evolution byextending continental lithosphere above a thermal anomaly. We find that unlessthe lithosphere is thinned prior to the arrival of the thermal anomalyor half spreading rates are more than ? 50mmyr?1, the lithosphere actsas a lid to the hot material. The thermal anomaly cools significantly by conductionbefore having an effect on decompression melt production. If the lithosphereis thinned by the formation of extensional basins then the thermalanomaly advects into the thinned region and leads to enhanced decompressionmelting. In the North Atlantic a series of extensional basins off the coastof northwest Europe and Greenland provide the required thinning. This observationsuggests that volcanic margins that show slow rates of extension,only occur where there is the combination of a thermal anomaly and previousregional thinning of the lithosphere

    Crustal structure of the Murray Ridge, northwest Indian Ocean, from wide-angle seismic data

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    The Murray Ridge/Dalrymple Trough system forms the boundary between the Indian and Arabian plates in the northern Arabian Sea. Geodetic constraints from the surrounding continents suggest that this plate boundary is undergoing oblique extension at a rate of a few millimetres per year. We present wide-angle seismic data that constrains the composition of the Ridge and of adjacent lithosphere beneath the Indus Fan. We infer that Murray Ridge, like the adjacent Dalrymple Trough, is underlain by continental crust, while a thin crustal section beneath the Indus Fan represents thinned continental crust or exhumed serpentinized mantle that forms part of a magma-poor rifted margin. Changes in crustal structure across the Murray Ridge and Dalrymple Trough can explain short-wavelength gravity anomalies, but a long-wavelength anomaly must be attributed to deeper density contrasts that may result from a large age contrast across the plate boundary. The origin of this fragment of continental crust remains enigmatic, but the presence of basement fabrics to the south that are roughly parallel to Murray Ridge suggests that it separated from the India/Seychelles/Madagascar block by extension during early breakup of Gondwana

    Seismic data reveal eastern Black Sea Basin structure

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    Rifted continental margins are formed by progressive extension of the lithosphere. The development of these margins plays an integral role in the plate tectonic cycle, and an understanding of the extensional process underpins much hydrocarbon exploration. A key issue is whether the lithosphere extends uniformly, or whether extension varies\ud with depth. Crustal extension may be determined using seismic techniques. Lithospheric extension may be inferred from the waterloaded subsidence history, determined from\ud the pattern of sedimentation during and after rifting. Unfortunately, however, many rifted margins are sediment-starved, so the subsidence history is poorly known.\ud To test whether extension varies between the crust and the mantle, a major seismic experiment was conducted in February–March 2005 in the eastern Black Sea Basin (Figure 1), a deep basin where the subsidence history is recorded\ud by a thick, post-rift sedimentary sequence. The seismic data from the experiment indicate the presence of a thick, low-velocity zone, possibly representing overpressured sediments. They also indicate that the basement and\ud Moho in the center of the basin are both several kilometers shallower than previously inferred. These initial observations may have considerable impact on thermal models of the petroleum system in the basin. Understanding\ud the thermal history of potential source rocks is key to reducing hydrocarbon exploration risk. The experiment, which involved collaboration between university groups in the United Kingdom, Ireland, and Turkey, and BP and\ud Turkish Petroleum (TPAO), formed part of a larger project that also is using deep seismic reflection and other geophysical data held by the industry partners to determine the subsidence history and hence the strain evolution of\ud the basin

    Anisotropic Physical Properties of Mafic and Ultramafic Rocks From an Oceanic Core Complex

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    We analyzed the physical properties of altered mafic and ultramafic rocks drilled at the Atlantis Massif (Mid‐Atlantic Ridge, 30°N; Integrated Ocean Discovery Program Expeditions 304‐305 and 357). Our objective was to find a physical property that allows direct distinction between these lithologies using remote geophysical methods. Our data set includes the density, the porosity, P and S wave velocities, the electrical resistivity, and the permeability of mafic and ultramafic samples under shallow subsurface conditions (confining pressure up to 50 MPa equivalent to ~2‐km depth). In shallow subsurface conditions, mafic and ultramafic samples showed distinct differences in the density, the seismic wave velocities, and the electrical resistivity (mafic samples: 2,840 to 2,860 kg/m3, 5.92 to 6.70 km/s, and 60 to 221 Ω m; ultramafic samples: 2,370 to 2,790 kg/m3, 3.36 and 3.62 km/s, and 8 to 44 Ω m). However, we observed an overlap between physical properties of mafic and ultramafic rocks when we compared our measurements with those acquired from similar environments. The anisotropic homogeneous electrical resistivity inversion shows transverse isotropy symmetry, which is typical of a foliated microstructure. In both the inversion results and the thin sections, the direction of high resistivity axes of ultramafic rock samples is systematically perpendicular to the equivalent axes in mafic rock samples analyzed in this study. Our sample scale study suggests that electrical resistivity anisotropy may allow us to distinguish mafic and ultramafic lithologies via controlled source electromagnetic surveys. When surface conduction is negligible, the electrical resistivity can be used as proxy for permeability

    Cruise report: RRS James Clark Ross 42. A Seismic Tomographic and Hydroacoustic Study of Ascension Island, 2nd-18th May 1999, Montevideo-Ascension

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    The structure of the Earth’s crust at oceanic volcanic islands is of interest for the information it provides both about the processes involved in the formation of these islands and about the rheology of the underlying oceanic lithosphere, which deforms in response to the island load. Since some of the acoustic energy from large underwater explosions is commonly coupled into the oceanic sound channel as so called “T-phases”, hydrophones around several such islands and broad-band seismic stations on the islands are being used for monitoring of the Comprehensive Test Ban Treaty. Cruise JR42 involved a four-day seismic experiment around the island of Ascension, a 4-km-high volcano some 90 km west of the Mid-Atlantic Ridge. During the experiment, a 6186 cu. in. airgun array was fired at one-minute intervals for about two days along a series of lines extending up to 45 km from the coast of the island. These shots were received by hydrophones and sonobuoys offshore and seismometers onshore. The shots were used to locate and calibrate three permanently installed hydrophones, to study the coupling of seismic energy into the island slope, and to study the structure of the crust beneath the island. The experiment was funded by the US Department of Energy (through Lawrence Livermore National Laboratory (LLNL)), the US Office of Naval Research, the UK Natural Environment Research Council, the United Nations (through the Comprehensive Test Ban Treaty Office in Vienna), IFREMER’s Centre de Brest, and funds held at the University of Southampton by T. A. Minshull. On the passage from Montevideo to Ascension Island, an unrelated scientific party conducted oceanographic measurements as part of the “Atlantic Meridional Transect”

    Methane release from warming-induced hydrate dissociation in the West Svalbard continental margin: timing, rates, and geological controls

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    Hundreds of plumes of methane bubbles, first observed in 2008, emanate from an area of the seabed off West Svalbard that has become 1°C warmer over the past 30 years. The distribution of the plumes, lying close to and upslope from the present upper limit of the methane hydrate stability zone, indicates that methane in the plumes could come from warming-induced hydrate dissociation, a process commonly invoked as contributing to rapid climate change. We used numerical modeling to investigate the response of hydrate beneath the seabed to changes in bottom-water temperature over periods of up to 1000 years B.P. The delay between the onset of warming and emission of gas, resulting from the time taken for thermal diffusion, hydrate dissociation, and gas migration, can be less than 30 years in water depths shallower than the present upper limit of the methane hydrate stability zone, where hydrate was initially several meters beneath the seabed and fractures increase the effective permeability of intrinsically low-permeability glacigenic sediment. At the rates of warming of the seabed that have occurred over the past two centuries, the enthalpy of hydrate dissociation limits the rate of gas release to moderate values. Cycles of warming and cooling can create and sustain hydrate close to the seabed where there is locally a supply of methane of tens of mol·m–2 yr–1. This rate of gas flow can be achieved where stratigraphic and structural heterogeneity focus gas migration, although the regional rate of methane supply could be much less

    An integrated kinematic and geochemical model to determine lithospheric extension and mantle temperature from syn-rift volcanic compositions

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    We present an integrated kinematic and geochemical model that determines the composition of melts and their residual source rocks generated by decompression melting of the mantle during continental rifting. Our approach is to construct a unified numerical solution that merges an established lithospheric stretching model which determines the rate and depth at which melting occurs, with several compositional parameterisations of mantle melting to predict the composition of primary melts. We also incorporate a parameterisation for the rare earth elements. Using our approach, we are able to track the composition of the melt fractions and mantle residues as melting progresses. Our unified model shows that primary melt composition is sensitive to rift duration and mantle temperature, with rapid rifting and higher mantle temperatures producing larger melt fractions, at a greater mean pressure of melting, than slower/cooler rifting. Comparison of the model results with primitive basalts recovered from oceanic spreading ridges and rifted margins in the North Atlantic indicates that rift duration and synrift mantle temperature can be inferred independently from the appropriate geochemical data
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