Fluid Transport In Fore Arc Basins at Active Margins: Evidence from I-129 and Halogen Data

Active margins are often associated with large fore arc basins, formed by the upper plate during the subduction process. Although these basins generally are not targeted for oil and gas exploration, they can contain considerable amounts of hydrocarbons. One expression of it is the frequent occurrence of gas hydrates in large sections of active margins. We used the I-129 isotopic system together with halogen systematics of pore waters collected from gas hydrate occurrences in order to determine the origin of iodine and, by association, of methane in a number of gas hydrate locations in active margins of the Pacific Rim. The main study areas include Peru Margin (ODP 201), Hydrate Ridge (ODP 204), Cascadia Margin (IODP 311) and several locations in the Nankai Trough. Samples were collected from drill cores covering the hydrate zone to the free gas zone below the BSR. In general, iodine ages found in these pore waters increased with depth and were in most cases considerably older than the age of the host sediments or the subducting marine sediments, with most samples falling into ranges between 30 and 50 Ma. These findings, together with the halogen systematics in the pore waters, indicate that the majority of iodine was transported into the current locations from old formations found in the upper plate and was not derived from local sediments or subducting marine sediments in the gas hydrate locations. Because of the close association between iodine and organic material, the results also suggest that large scale migration of methane occurs in active margins, often over distances of 30 or more km, derived from organic-rich source formations of Eocene age found in the upper plate of active margins.