Middle America Trench: Topography and Structure

Abstract

From 1952 to 1959, during nine expeditions of the Scripps Institution of Oceanography and one of the U. S. Navy Electronics Laboratory, research vessels recorded 31,950 miles of echo-sounding traverses in and adjacent to the Middle America Trench, which extends from the Islas Tres Marias off western Mexico to the Cocos Ridge southwest of Costa Rica. The Middle America Trench is continuous at depths greater than 2400 fathoms (4400 m) for 1260 miles, except off Manzanillo and Zihuatanejo, Mexico, where submarine mountains lie in the trench. It is deeper than 3000 fathoms (5500 m) for 380 miles as the Guatemala Deep. Northwest of Acapulco it is generally U-shaped in cross section, with a steeper shoreward flank and a flat bottom suggesting sedimentary fill. From Acapulco southeast to the west side of the Gulf of Tehuantepec, the trench shoals, in a series of basins, to 2700 fathoms (5000 m). To the southeast it widens and deepens abruptly to a maximum 3500 fathoms (6400 m) off western Guatemala, then shoals gradually to merge into the sea floor off Costa Rica. The southeast segment is also asymmetrical in cross section but is V-shaped with irregular bottom. A northeast-trending band of ridge-and-trough topography, 60 miles wide, separates the 1800- to 1900-fathom sea floor outside the trench off southern Mexico from the 2100- to 2200-fathom Guatemala Basin. This zone has been traced from several hundred miles offshore to an intersection with the trench near the west side of the Gulf of Tehuantepec. Seismic-refraction studies reported in an accompanying paper (Shor and Fisher, 1961) were employed in determining the trench structure. Three refraction stations were taken along the axis of the trench west of Acapulco and two along its axis off Guatemala and El Salvador. Another station was shot on the shelf and one 60 miles seaward of the trench off Guatemala. Thick sediments were found in the Tres Marias Basin off Manzanillo and at the shelf station off Guatemala. Arrivals from rock with compressional wave velocity of 4–6 km/sec were observed at the Tres Marias Basin and Guatemala shelf stations. Off Guatemala, on a section normal to the trench, the depth below sea level to the M discontinuity is interpreted from these seismic data as about 9 km (Pacific Basin), 10 km (outer ridge), 16 km (trench), and 17 km (shelf). Below the sea floor the crust thickens from 5–7 to 10–17 km along this section. The M discontinuity is deeper and the crust below the sediments thicker under the two southern stations than under the two central trench stations. The mantle is deeper under the Tres Marias Basin, where thick (1½ km) sediments are found, than under the central stations. The Gulf of Tehuantepec marks a major change in trench configuration and possibly in age. Northwest of Tehuantepec the flat trench bottom developed in most places suggests a greater age. Southeast of the gulf the deep V-shaped trench, with thicker crustal layers but very little fill, borders a volcanically active coast. The zone of ridge-and-trough topography trending southwest from Tehuantepec may be another evidence of this boundary.