Interpretation of Seismic-Reflection Data of the Middle America Trench offshore Guatemala

A geophysical and geological survey conducted over the landward slope of the Middle America Trench offshore Guatemala, together with published well information from the outer shelf and Leg 67 drilling results from the toe of the slope indicate that imbricate slices of oceanic crust were emplaced in the landward slope offshore Guatemala in the Paleocene or early Eocene. Since that time, sediment apparently has accumulated on the landward slope primarily as a sediment apron blanketing an older, tectonically deformed prism of sediments and crustal slices. There is little or no evidence for continued tectonic accretion seaward of the volcanic arc during the late Tertiary. Seismic reflection and refraction surveys have revealed landward-dipping reflections that are associated with high compressional wave velocities, large magnetic anomalies, and basic-ultrabasic rock. Multifold seismic reflection data reveal that the edge of the continental shelf is a structural high of Cretaceous and Paleocene rock against which Eocene and younger sediments of the shelf basin onlap and pinch out. The upper part of the continental slope is covered in most places by a 0.5- to 1.0-km-thick sediment apron with seismic velocities of 1.8 to 2.6 km/s. The base of the sediment apron commonly coincides with the base of a gas hydrate zone where water is 1500 to 2300 meters deep. Immediately beneath the sediment apron an irregular surface is the top of an interval with velocities greater than 4 km/s. Within this interval, landward-dipping reflections are traced to about 6 km below sea level. These reflections coincide with the top of seismic units having oceanic crust velocities and thicknesses. The sediment apron pinches out on the lower continental slope where refraction results indicate only a few hundred meters of 2.5-km/s material lying over about a kilometer of 3.0-km/s sediment. Between the 3.0-km/s sediment and a landward continuation of ocean crust, an interval of 4.1- to 4.7-km/s material occurs that thins seaward. Near the interface between the 4 +-km/s material and oceanic crust with velocities of 6.5 to 6.8 km/s, reflection records indicate a landward-dipping horizon that can be followed about 30 km landward from the Trench axis. Coring on the continental slope returned gravels of unweathered metamorphosed basalt, serpentine, and chert, unlike rock generally found onshore in Guatemalan drainage basins feeding the Pacific coast. These gravels, which were probably derived from local subsea outcrops, are similar to lithologies found on the Nicoya Peninsula farther south. A canyon cut in the outer continental shelf and upper continental slope may be associated with faulting, as indicated by an offset of linear magnetic anomalies at the shelf edge. In a general way our observations are consistent with previous suggestions that slices of rock, some of which may have oceanic crustal lithologies, are imbedded in the upper slope. However, the reflection data collected for the Deep Sea Drilling Project site survey do not show the many concave upward landward-dipping reflections that have been reported from other areas offshore Guatemala. The lower slope is probably a tectonically deformed and consolidated sediment wedge overlying oceanic crust, but it is not clear that it is organized into a series of landward thinning wedges. The structures within the landward slope may have originated during the late Paleocene to early Eocene tectonic event and may not be the result of an ongoing steady-state process of sediment accretion by sediment offscraping at the toe of the slope or by underplating of sediment at the base of the sediment wedge beneath the continental slope and shelf