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Keywords:
Mud volcano
Seafloor Spreading
Décollement
Doming
Mud volcano
Seafloor Spreading
Décollement
Doming
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Doming
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The formation of mud volcanoes/mud diapirs is directly related to oil and gas accumulation and gas-hydrate mineralization. Their eruptive activities easily cause engineering accidents and may increase the greenhouse effect by the eruption of methane gas. Many scholars have performed much research on the developmental characteristics, geochemistry, and carbon emissions of mud diapirs/mud volcanoes, but the formation mechanism of mud diapirs/mud volcanoes is still controversial. Mud diapirs and mud volcanoes are especially developed in the northern South China Sea and are accompanied by abundant oil, gas, and gas-hydrate resources. Based on the mud volcanoes/mud diapirs in the northern South China Sea, the physical simulation experiments of mud diapir/mud volcano formation and evolution under different fluid pressures and tectonic environments have been performed by loading a fluid-input system in traditional sandbox simulation equipment. The genetic mechanism of mud diapirs/mud volcanoes is revealed, and a fluid-leakage model of mud diapirs/mud volcanoes under different geological conditions is established. We believe that in an overpressured environment, the greater the thickness of the overlying strata is, the greater the pressure or power required for the upward migration of muddy fluid to penetrate the overlying strata. Tectonic activity promotes the development of mud volcanos/mud diapirs. To a certain extent, the more intense the tectonic activity is, the more significant the effect of promoting the development of mud volcanoes/mud diapirs and the larger the mud diapirs/mud volcanoes become.
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Mineralogical, petrographic and stable isotope analyses were carried out on sediment and rock samples collected from a deep-sea cold seep province in the Paola Ridge (southeastern Tyrrhenian Sea). The results, coupled with the interpretation of the seafloor backscatter, constrained both the biogeochemical zonation and the spatial distribution of the cold seep habitats in the area. These have proved to change in depth in a range of few meters and laterallywithin narrow zones. The mud volcanoes, characterized by a high backscatter signature, are the site of vigorous gas venting and, in the subsurface, show a rapid transition from the oxic sea water interface toward the methane-sulfate transition zone in the sediments. Intermediate backscatter typifies areas where free venting is hampered by the presence of mudflows at the seafloor. These conditions favor: i) the oxidation of sulfides near the seafloor, ii) the precipitation of siderites a few meters below the seafloor and iii) the formation of sulfides deeper in the sub-seafloor. Faults are likely candidates to act as conduits for sulfates and metal oxides that juxtapose different redox environments. Siderites precipitated in the fast and low venting sites showed enrichment in δ13C and δ18O, which are compatible with their precipitation in the methanogenic zone. The heavy-oxygen isotopic compositions of the siderites are possibly related to the dissociation of gas hydrates, which have not been mapped so far by seismic data in the study area. Mud diapirism is characterized by low backscatter seafloor, large fields of pockmarks and is dissected bynormal faults. In coincidence with the normal faults, authigenic calcites and aragonites are present at or very close to the seafloor. They have the typical isotopic signature indicating formation during sulfate-dependent microbially-mediated anaerobic oxidation of methane.They are associated with Lucinoma borealis, the youngest being dated 640-440 BP. This suggests that the seepage activity in the mud diapirs was likely clogged by either carbonates or activity of the faults only very recently.
Seafloor Spreading
Mud volcano
Authigenic
Cold seep
Clathrate hydrate
Isotopic signature
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Data on ridges with slow spreading rates (1–3 cm/yr), obtained through detailed studies with Deep‐Tow instruments and manned submersibles (French‐American Mid‐Ocean Undersea Study), or where the active structures are not submarine (Afar triangle) yield a precise picture of the axial region. There is evidence for a thinned lithosphere to be present at the axis with a thickness of 4–5 km. The thermal structure and composition of this solid layer can be estimated from seismic data and thermal and petrologic models. Extensional tectonics prevails in a belt some 10 km wide and is expressed at the surface by normal faults and fissures. In most previous mechanical models the existence of the lithosphere at the axis is neglected, and the rise of viscous asthenospheric material in a narrow vertical cleft beneath the axis is considered the main cause for the steady state presence of an axial valley and the development of normal faults. In contrast with these models we suggest here that these features depend on the rheological behavior of the lithosphere at the axis: the lithosphere is continually thinned by tectonic strain but also thickened by cooling at the base and by volcanism at the surface. In the steady state this process can be viewed as a succession of tectonic ‘neckings’ in the central active part of the axial valley (10 km) followed by doming of the lithosphere over the whole width of the axial valley (30 km), in response to isostatic disequilibrium. Creep is likely to control the process at depth and because of the high temperature and large strains will be of the steady state type. The application of experimental flow laws for material constituting layer 3 and the uppermost mantle to this problem, where both temperature profile and strain rate can be estimated, allows an order of magnitude of the ‘strength’ of the lithosphere at a given strain rate to be calculated. With this strength, isostatic recovery in response to vertical shear stresses will occur at a distance of about 8–15 km from the axis. The resulting ‘simple shear’ strain progressively ‘levels off’ the mean topographic slope until it becomes horizontal in the rift mountains. Whereas for rifted ridges or slowly accreting plate boundaries the behavior of the lithosphere controls the mechanics of the axial region, where only small discontinuous transient magma chambers exist, we suggest that in the case of nonrifted ridges the behavior of the asthenosphere is more important, with axial crust in isostatic equilibrium over a large continuous permanent magma chamber.
Lithospheric flexure
Necking
Asthenosphere
Doming
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Abstract Submarine mud volcanoes are important sources of methane to the water column. However, the temporal variability of their mud and methane emissions is unknown. Methane emissions were previously proposed to result from a dynamic equilibrium between upward migration and consumption at the seabed by methane-consuming microbes. Here we show non-steady-state situations of vigorous mud movement that are revealed through variations in fluid flow, seabed temperature and seafloor bathymetry. Time series data for pressure, temperature, pH and seafloor photography were collected over 431 days using a benthic observatory at the active Håkon Mosby Mud Volcano. We documented 25 pulses of hot subsurface fluids, accompanied by eruptions that changed the landscape of the mud volcano. Four major events triggered rapid sediment uplift of more than a metre in height, substantial lateral flow of muds at average velocities of 0.4 m per day, and significant emissions of methane and CO 2 from the seafloor.
Mud volcano
Seafloor Spreading
Seabed
Clathrate hydrate
Submarine volcano
Submarine landslide
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Seafloor Spreading
Mud volcano
Seabed
Reflection
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