The Middle America Trench off Guatemala, which was drilled on DSDP Legs 67 and 84, is part of a nonaccreting convergent margin. The igneous continental framework of Central America extends to the base of the landward trench slope. Ophiolitic rocks, which correspond to the acoustic basement seen in seismic records beneath a cover of Neogene slope deposits, were recovered at five sites (seven holes). This basement represents an ocean crust first tectonized in the pre-Campanian and emplaced in pre-early Eocene time; the basement is not a tectonic product of the present convergent margin. Subduction of sediment on the Cocos plate may result in the development of overpressures which was observed directly at four sites.

On Leg 84, gas hydrate was recovered at three sites and detected at five of the six sites drilled. It occurs dispersed in muddy sediment or in porous lithologies and in fractures. A distinctive gas composition and low salinity pore water were found associated with the gas hydrate. The source of the gas was mainly biogenic, but thermogenic gas apparently was present in fractures of the basement rock.

End_of_Article - Last_Page 1702------------