UNDERTHRUST SEDIMENTS, FLUID MIGRATION PATHS, AND MUD VOLCANOS ASSOCIATED WITH THE ACCRETIONARY WEDGE OFF COSTA-RICA - MIDDLE AMERICA TRENCH

We estimate the volume of fluids and sediment underthrust beneath the toe of an accretionary wedge and describe evidence of fluid migration observed in seismic data in the Middle America Trench near Costa Rica. Reduction in normal incident travel time of the underthrust oceanic plate section arcward of the deformation front may be related to fluid expulsion. The rate of fluid transfer from the underthrust section is highest in the zone 0-4km arcward of the deformation front. The clarity of the reflection associated with the decollement is exceptional and shows a phase reversal, relative to the seafloor. This high-amplitude phase-reversed reflection ends about 4km arcward of the deformation front, suggesting a rapid reduction in the density and/or velocity contrast between the dewatering underthrust section and overlying offscraped sediments at this position. Fluid migration within the wedge produces mud volcanoes or ridges in the midslope region having relief of about 50m and extent of perhaps 500m × 1000m. Some fluids appear to migrate along pathways marked by reflections which extend deep within the accretionary wedge but not always extending into the underthrust section. -from Authors