Visual observations of the sea floor subduction line in the middle-america trench

Four dives were made to the floor of the Middle-America Trench with the U.S. Navy's deep research submersible DSV Turtle. The area investigated is located between Costa Rica and the Cocos Ridge where the depth of the trench floor does not exceed the 2000-meter capability of the submersible. At the axis of the trench floor a series of steep northeast-facing scarps 10 to 20 meters high lie parallel to the trench axis. Here oceanic crust appears to have been carried down by near-vertical normal vaults of small displacement. Between these small scarps and the landward wall of the trench a narrow line of recent deformation interrupts a smooth apron. Unconsolidated sediments are thrust in sharply serrated piles and cut by sharp-edged chasms. This line of deformation is interpreted as the present sea floor trace of crustal subduction.