Summary—Leg 67, Middle America Trench Transect off Guatemala

The Middle America Trench transect off Guatemala is the second of two International Program of Ocean Drilling (IPOD) geophysical and drilling transects conducted by the Deep Sea Drilling Project (DSDP) across the convergent margins of southern Mexico and Central America. The first, a transect across the Middle America Trench north of the Tehuantepec Ridge off Oaxaca, was drilled on Leg 66 to investigate a convergent margin where the continent is truncated and the front of the margin is an inferred accretionary complex (Moore et al., 1979). Off Guatemala, our principal objective was to study a convergent margin where continuous accretion and imbrication were indicated by geophysical studies and a single deep drill hole (Seely et al., 1974). We hoped to strengthen the tie between onshore and offshore geology, to study modern sediment sequences in the Trench and on the slope, and to recover a continuous chronology of volcanic ash from explosive volcanism along the magmatic arc. The Middle America Trench was recommended for study on the basis of past experience in drilling along other convergent margins. Previous drilling along the Oregon convergent margin, the Aleutian Trench, and the Nankai Trough showed that detailed age resolution is required to define stratigraphic repetitions expected in a sequence of imbricated and accreted modern trench sediment. Fine-scale age resolution is easier to achieve in low rather than high latitudes, because microfaunal diversity is greater in more temperate environments. In high latitudes, moreover, the great influx of terrigenous sediment during glaciation results in transport of terrigenous debris seaward of the trenches and makes it virtually impossible to differentiate much of the oceanbasin deposits from slope deposits by lithologic criteria. Thus it was reasoned that convergent tectonism would best be studied where terrigenous sources are moderate, where faunal diversity is great, and where subduction is relatively rapid. These conditions exist along much of Central and South America. Another factor influencing the choice of the Leg 67 transect was the report by Seely et al. (1974) that included a processed multichannel seismic-reflection record and drill-hole information for the Guatemala segment of the Middle America Trench. These authors made a