Integrated geophysical and petrological characterization of mud volcanoes at the Morrocan Atlantic margin: Linking morphology to fluid flow
Depreiter, D.; Van Rensbergen, P.; Poort, J.; De Boever, E.; Swennen, R.; Henriet, J. (2005). Integrated geophysical and petrological characterization of mud volcanoes at the Morrocan Atlantic margin: Linking morphology to fluid flow. Eos, Trans. (Wash. D.C.) AGU Fall Meet. Suppl. 86(52): abstract T13B-0456
In: Eos, Transactions, American Geophysical Union. American Geophysical Union: Washington. ISSN 0096-3941; e-ISSN 2324-9250, more
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Document type: Summary
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| Abstract |
Detailed geophysical, geochemical and petrological data over a cluster of large mud volcanoes at the Moroccan North Atlantic margin document the activity of sea floor mud volcanoes in relation to its morphology and structural setting. Mud volcanoes are often long-lived systems; their changing morphology bears witness of the evolution of fluid flow expulsion. The El Arraiche mud volcano field is a cluster of 9 mud volcanoes.It was discovered in 2002 at the Morrocan Atlantic margin in water depths from 200 m to 700 m. The largest mud volcano in the field is 255 m high and 5.4 km wide. Marine surveys between 2002 and 2005 yielded detailed geophysical, geochemical, sedimentological, and petrological data. The geophysical data include multibeam bathymetry, high-resolution seismics, deep-tow sub bottom profiles and side-scan sonar mosaics.Video imagery lines, video guided grab samples, dredge samples, gravity cores, and box cores were collected for groundtruthing purposes.Petrological and geochemical analysis of authigenic carbonates provided a record of hydrocarbon sources, fluid characteristics, processes of mixing and the mode of venting. The El Arraiche mud volcanoes cluster around two subparallel anticlines and are associated active extensional faults. Extruded rock clasts and regional seismic data locate the El Arraiche field over a Late Miocene to Pliocene extensional basin. The onset of mud volcanic activity is estimated at about 2.4 Ma and probably roots in the Cretacous to Miocene accretionary wedge. Stacked outflows are visible up to a depth of about 500 m below the sea floor. Stratigraphic correlation of the outflow lenses over the entire mud volcano field indicate that although large outflow events are not synchronized between the individual mud volcanoes, eruptions occurred more frequently during periods of active extensive tectonics. The morphology of the sea floor mud volcanoes is the result of a combination of extrusive and intrusive processes. Extrusive debris flow with low yield strength and high fluid content create long mud flows that accumulate at the base of the mud volcanic cone and tend to reduce the overall slope angle. Extrusive debris flows with high yield strength form short, irregular, and thick flow deposits that accumulate on the steep mud volcano slopes. The resulting slope profile is steep with a linear to convex profile. Smaller mud flows and fluidized sand deposits occur within the crater area. Besides the obvious extrusive processes, diapiric (buoyant) rise within earlier mud volcanic deposits is interpreted to be a main contribution in shaping large cone-shaped mud volcanoes. Large subsidence depressions at the base of the mud volcanoes are attributed to sediment removal whereas crater collapse is caused by dewatering and degassing of the intrusive sediment. The characteristics of mud volcanic activity are also reflected by geochemical, mineralogical and carbon isotope signatures of authigenic carbonates. These analyses allow distinguishing differences in the feeder system, the interference by gas hydrates, and differences in discharge rate at the surface. Other indications about the venting process are obtained from the type of mud breccia (size and abundance of clasts), pore water geochemistry, and the distribution of benthic fauna. |
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