Zonation of North Alex Mud Volcano highlighted by 3-D active and passive seismic data.

Bialas, Jörg , Lefeldt, Marten R., Klaeschen, Dirk , Papenberg, Cord A. and Brückmann, Warner (2010) Zonation of North Alex Mud Volcano highlighted by 3-D active and passive seismic data. [Talk] In: AGU Fall Meeting 2010. , 13.12.-17.12.2010, San Francisco, California, USA .

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Abstract

OS53A-1343
The West Nile Delta forms part of the source of the large turbiditic Nile Deep Sea Fan. Since the late Miocene sediments have formed an up to 10 km thick pile, which includes about 1 - 3 km of Messinian evaporates. The sediment load of the overburden implies strong overpressures and salt-related tectonic deformation. Both are favourable for fluid migration towards the seafloor guided by the fractured margin. The western deltaic system, Rosetta branch, has formed an 80 km wide continental shelf. Here at 700 m water depth the mud volcano North Alex (NA) developed his circular bathymetric feature, which proved to be an active gas and mud-expelling structure. A 3-D high-resolution multichannel seismic survey (IFM-GEOMAR P-Cable system) was completed across the mud volcano. 3-D time migration provided a 3-D data cube with a 6.25 m grid. Vertical seismic sections did reveal a large set of faults located within the main mud volcano as well as surrounding the structure. Internal faults are mainly related to episodic mud expulsion processes and continuous gas and fluid production. Deep cutting external faults surround the structure in a half circle shape. Horizontal amplitude maps (time slices) of indicate recent activity of these faults even up to the seafloor. High gas saturation of the sediments is indicated by inverted reflection events. In the centre the gas front cuts into the seafloor reflection while it dips down with increasing radius. Only with the small grid resolution inward dipping reflections become visible, which form an upward opened concave reflector plane underlying the top gas front. The interpretation assumes an oval lens shaped body (conduit) saturated with gas at the top of the mud volcano. It provides the upper termination of the mud chimney. This separation is further supported by passive seismic observations. Distant earthquakes can stimulate long-period harmonic oscillations in mud volcanoes. Such oscillations are detectable with three-component ocean bottom seismometers (OBS) and are best explained by a gas-saturated volume at - or in close proximity to - the surface. The period of these oscillations is directly linked to the composition and dimension of this volume. Further, these oscillations are associated with pressure changes in the gas volume, which are thought to disturb the balance between gas pressure and water pressure strongly enough to cause degassing in the upper sediment layers of the mud volcano. This way, gas transport and release in mud volcanoes might be triggered by external seismic sources. Additionally, tremors of higher frequencies can be observed at NA Mud Volcano, and are most likely generated in the mud chimney beneath the top gas volume. Further evidence for the existence of a rather deep chimney (>750m) comes from S-wave observations of regional earthquakes. Records from OBS that were placed at the volcano’s centre (1), differ from OBS in greater distance (2). S-wave arrivals suggest the existence of a cylindrical-shaped waveguide beneath OBS of type (1). Such features cannot be seen on OBS of type (2). Thus, S-wave velocities need to be lower in the chimney than in the surrounding, which is a reasonable assumption. Modelling of these waveguides can give the dimension of the chimney (width and depth).

Document Type: Conference or Workshop Item (Talk)
Keywords: Seismology; Meeresgeologie; Geophysics; Seismic methods; Gas and hydrate systems; Marine seismics; Seismicity and tectonics; Ocean Sciences
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
Open Access Journal?: Yes
Date Deposited: 28 Jan 2011 14:51
Last Modified: 23 Feb 2012 05:17
URI: http://oceanrep.geomar.de/id/eprint/10909

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