Reflection profiles across the Middle America Trench

Abstract

In the summer of 1963 reflection profiles were made across the Middle America trench between Tres Marias Islands and the coast of Panama using an electric sound source (the ‘Arcer’). A syncline underlies the continental shelf in the Gulf of Tehuantepec and the Gulf of Panama. A topographic bench frequently found part way down the landward flank of the trench may be continuous for most of the length of the trench. Underlying the bench is a basin partially or completely filled with sediment held by a linear ridge on the seaward side. The sediments in this basin, 400 meters or more thick, show evidence of folding, faulting, and slumping. Little evidence of sediment, other than occasional slumped blocks, was observed on the slope between the perched basin and the trench bottom, although cores in the northern part of the trench indicate at least a thin veneer of sediment on the slope. The trench bottom shows varying amounts of sediment (100 to more than 700 m); reflections from the base of the sediment were not always observed. Sediments in the trench bottom are folded and in many places dip down toward the landward side. At one location a partially buried hill or ridge has ponded the sediments in the trench bottom. On the seaward side of the trench, sediments are ponded in the valleys between the hills. There is evidence that the trench, interrupted by the Cocos ridge, may continue beneath a sedimentary cover as far southeast as Cape Mala, Panama. The shelf and trench structure off the Gulf of Tehuantepec resembles the sedimentary troughs found off the northeastern coast of North America.