Crustal-scale fluid migration and dewatering of the Costa Rica subduction zone

Abstract:

In 2011 we acquired a large 3D seismic reflection data volume with the R/V Langseth to examine the structure of the Costa Rica margin, NW of the Osa Peninsula. Multibeam bathymetry and backscatter data also acquired during the cruise reveal mud mounds, pockmarks, and gas plumes that indicate numerous, previously unknown, seafloor vent-related features that extend from the lower slope to the outer shelf region (Kluesner et al., 2013). These features imply active or recently active seafloor vents; however, the processes of fluid focusing into vents are not apparent from the seafloor data alone. The 3D seismic reflection data reveal indirect indicators of fluids below the seafloor. In the slope cover sediment, within the uppermost 1000 m of the seafloor, we calculated the RMS amplitude of the seismic reflections to identify vent systems from anomalously high seismic reflection amplitudes. We attribute anomalously high-amplitude zones to fluid-filled fractures and concentrated free gas directly associated with vents. Along the lower slope where a BSR is present, vents inferred from amplitude anomalies are also coincident with local shallow BSR depth anomalies. Fluids feeding these shallow vent systems appear to originate from the underlying fault-cored fold and thrust systems of the margin wedge. High amplitude anomalies within the slope cover lie directly over margin wedge thrust anticlines that develop within the 2-to-8 km thick margin wedge sequence. These anticlines are typically cored by thrusts, many of which themselves have high amplitude, reversed polarity seismic reflections that we interpret as fault zone dilation and active fluid migration. Fluids originating from the plate interface appear to be migrating up toward the seafloor along these thrust faults, as well as along the imbricately stacked stratigraphic horizons within the margin wedge. Fluids become focused into the crest of fold anticlines and directed to the seafloor vent-related features. This crustal scale fluid migration system extends from the lower slope and across much of the shelf implying broad, active dewatering of the seaward most 50 km of the overriding plate. We interpret this large scale fluid migration system as an actively dewatering fold and thrust sequence driven by recent thickening of sediment sequences within the margin wedge.