Analysis of a reprocessed seismic reflection line and results from Deep Sea Drilling Project Leg 66 off the southwest coast of Mexico reveal a bottom simulating reflector (BSR) associated with the equilibrium phase boundary of methane hydrate. Several seismic processing techniques were used to accentuate the lateral continuity of the BSR and to delineate the top and base of a 200-700 m thick concentrated hydrate layer. These results suggest the concentrated hydrate layer extends about 20 km parallel to the slope of the inner trench wall in water depths ranging from 2 250-4 500 m. Direct seismic indicators below the BSR and geochemical evidence imply small amounts of free gas may be trapped beneath the base of the hydrate layer and suggest that low‐permeability hydrated sediments can act as a seal for reservoirs.
Seismic line MX-16 transects the Middle America Trench on the Pacific coast of Mexico. The bathymetric map reveals a narrow continental shelf, a steep trench slope, and an axial trench about 5 km below sea level. A large submarine canyon cuts the upper slope in the vicinity of line MX-16. This area was extensively drilled during Leg 66 of the Deep Sea Drilling Project (Watkins et al., 1982).
Subduction in the area has been active intermittently during the last 100 m.y. (Karig et al., 1978). The convergence rate of the underthrusting Cocos plate relative to the overriding North American plate is inferred to be about 7 cm/yr (Minster and Jordan, 1978). Leg 66 drilling results suggest the continental margin in the area was truncated between 10 and 35 m.y.B.P., perhaps by transcurrent faulting or subduction erosion (Moore et al., 1982). A small accreted wedge subsequently developed by sediment underplating and offscraping (Moore et al., 1982). No forearc basin such as those seen on many trench slopes is evident on seismic line MX-16.
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