HR: 10:20h
AN: T12B-01    [Abstracts]
TI: Modes of Fluid Expulsion and its Significance for Forearc Dewatering at Costa Rica Convergent Margin
AU: * Hensen, C
EM: chensen@ifm-geomar.de
AF: IFM-GEOMAR and SFB574, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Wallmann, K
EM: kwallmann@ifm-geomar.de
AF: IFM-GEOMAR and SFB574, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Ranero, C
EM: cranero@icm.csic.es
AF: ICREA at Instituto de Ciencias del Mar, CSIC, Pg. de la Barceloneta 37-49, Barcelona, 08003 Spain
AU: Rehder, G
EM: grehder@ifm-geomar.de
AF: IFM-GEOMAR and SFB574, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Brueckmann, W
EM: wbrueckmann@ifm-geomar.de
AF: IFM-GEOMAR and SFB574, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Grevemeyer, I
EM: igrevemeyer@ifm-geomar.de
AF: IFM-GEOMAR and SFB574, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Reston, T
EM: treston@ifm-geomar.de
AF: IFM-GEOMAR and SFB574, Wischhofstr. 1-3, Kiel, 24148 Germany
AB: The expulsion of chloride-depleted fluids is characteristic for vent sites at Costa Rica continental margin. Oxygen and hydrogen isotope ratios, thermogenic methane as well as elevated heat flow demonstrate that the fluid flow is initiated by mineral dehydration in subducting sediments at about 10-12 km depth. Conspicuous differences in the geochemical composition allow a subdivision of a southern and a northern type of fluids, which may reflect differences in the input or a general south to north decrease in flow rates. Fluids of the southern type are enriched in boron and typically rise at high rates. In contrast, the northern type of fluids is strongly enriched in calcium and barium, which points to significant alteration along the flow path. Fluid venting seems to be an important dewatering process as it occurs at a huge number of mound-like structures, which are carbonate-capped in many places and comprise of mixed types of mud extrusion features, along major slope failures caused by subducting seamounts and at fault-controlled slides. Convergence related seamount subduction and subduction-erosion are the primary reasons for slope instability, resulting in large-scale deformation structures. Fluid expulsion related to seamount subduction is largely unconstrained at present. Whereas seeps are rare at the top of the uplifted sediment bulge, massive discharge of methane-rich fluids is documented by lush tubeworm communities and significant methane plumes at the scarp planes. Recent estimates reveal that up to 65 Mg of methane per year may be released at a single structure, which may prove them as important as the mound structures in terms of fluid recycling. In order to improve our current understanding of fluid recycling, to constrain long-term estimates of fluid flow, to systematize the variability of fluid geochemistry, and to fully understand the role of seamounts in the forearc it is proposed to drill several key sites of the most prominent dewatering structures within IODP (proposal 633 full).
UR: http://www.sfb574.geomar.de/Subprojects/index.html
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3070 Submarine landslides
DE: 4835 Marine inorganic chemistry (1050)
DE: 8045 Role of fluids
SC: Tectonophysics [T]
MN: Fall Meeting 2005