The accretionary wedge sediments thicken landwards by a combination of two processes: vertical thickening and bulk shortening through tectonic compression, and thrust faulting that emplaces initially high-porosity sediments at greater depths. As the equilibrium porosity-depth profile is reestablished, the sediments are consolidated and fluids are expelled (Hyndman and Wang, 1993). Hyndman and Davis (1992) suggested that hydrate layers at and above the BSR are formed when methane is removed from these upward-moving fluids as they pass into the hydrate stability field. This mechanism accounts for the common occurrence of widespread BSRs in sand-dominated accretionary prisms. A similar formation mechanism likely occurs on some passive margins, except that the fluid flow results from rapid deposition of sediments which are initially underconsolidated.
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