Gas and fluid venting at the Makran accretionary wedge off Pakistan |
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| Authors: | U von Rad U Berner G Delisle H Doose-Rolinski N Fechner P Linke A Lückge H A Roeser R Schmaljohann M Wiedicke SONNE / Scientific Parties M Block V Damm J Erbacher J Fritsch B Harazim J Poggenburg G Scheeder B Schreckenberger N von Mirbach M Drews S Walter A Ali Khan A Inam M Tahir A R Tabrez A H Cheema M Pervaz M Ashraf |
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| Institution: | (1) BGR, P.O. Box 510153, 30631 Hannover, Germany e-mail: U.vonRad@bgr.de, DE;(2) GEOMAR, Wischhofstr. 1–3, 24148 Kiel, Germany, DE;(3) Institut für Meereskunde, Düsternbrooker Weg 20, 24105 Kiel, Germany, DE;(4) BGR, P.O. Box 510153, 30631 Hannover, Germany, DE;(5) GEOMAR, Wischhofstr. 1–3, 24148 Kiel, Germany, DE;(6) Institut für Meereskunde, Düsternbrooker Weg 20, 24105 Kiel, Germany, DE;(7) National Institute of Oceanography, 47 S.T., Karachi, Pakistan, PK;(8) Hydrocarbon Development Institute of Pakistan, P.O. Box 1308, Islamabad, Pakistan, PK;(9) Office of Deputy Hydrographer of Pakistan Navy, 11 Liaquat Barracks, Karachi, Pakistan, PK |
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| Abstract: | The Makran accretionary complex shows a distinct bottom-simulating reflector, indicating a thick gas-hydrate-bearing horizon
between the deformational front and about 1350 m water depth which seals off the upward flow of gas-charged fluids. A field
of presently inactive mud diapirs with elevations up to 65 m was discovered in the abyssal plain seawards of the deformation
front, suggesting that in the past conditions were favorable for periodic but localized vigorous mud diapirism. Regional destabilization
of the gas hydrate leading to focused flow was observed where deep-penetrating, active faults reach the base of the gas-hydrate
layer, as in a deeply incised submarine canyon (2100–2500 m water depth). At this location we discovered seeps of methane
and H2S-rich fluids associated with chemoautotrophic vent faunas (e.g., Calyptogena sp.). Driven by the accretionary wedge dynamics, the landward part of the gas-hydrate layer below the Makran margin is being
progressively uplifted. Due to reduced hydrostatic pressure and rising ocean bottom-water temperatures, gas hydrates are progressively
destabilized and dissociated into hydrate water, methane and H2S. Sediment temperatures lie outside the methane stability field wherever water depth is less than 800 m. Above this depth,
upward migration of fluids to the seafloor is unimpeded, thus explaining the abundance of randomly distributed gas seeps observed
at water depths of 350 to 800 m.
Received: 14 June 1999 / Revision accepted: 6 February 2000 |
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