SUMMARY AND CONCLUSIONS

This study investigated the hydrological system of the Costa Rica subduction zone based on data from Legs 170 and 205. In detail, 1-D numerical transport-reaction models were developed to quantify the geochemical fluxes and timescales associated with (1) the flow of cold modern seawater in the basement and (2) fluid flow in the sediments of the prism wedge. The latter fluid flow has two sources: vertical dewatering of the compacted underthrust sediments and lateral flow along conduits parallel to the décollement with thermogenic origin.

Finally, the main results of this study can be summarized as follows:

  1. Only a small fraction of the cold seawater circulating in the oceanic basement is released through the overlying sediment, either by advection or diffusion. Thus, most of this water will be subducted.
  2. Based on the sulfate profile of the oceanic sediments, the basement aquifer has been active for the past 240 k.y. to 1 m.y., depending on the respective transport process.
  3. The propane profile in the prism wedge suggests vertical dewatering due to compaction across the décollement as well as deep-sourced fluid flow along and parallel to the décollement.
  4. Lateral fluid flow in the prism sediments is not a steady-state process. The current flow pulse in the upper conduit and the décollement zone has been active for 2000 and 4000 yr, respectively.
  5. Short-term pulses on the order of weeks or months, resulting from seismic activity, likely do not have significant impacts on the overall fluid expulsion.
  6. The AMO reaction front between the prism and underthrust sediments remobilizes considerable amounts of sulfur, and potentially the methane demand will even lead to the decomposition of gas hydrates above the décollement.

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