ALBERT

Description: 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.

Description: Slope deposits drilled during Leg 67 were later detailed in redisplayed seismic-reflection records. These deposits are of significantly lower seismic velocity and probably lower density than the underlying basement. This relationship indicates a contact between rocks of differing consolidation rather than a continuous sedimentary sequence. The slope deposits cover basement terranes of three different topographies. The shelf edge is an arch whose seaward flank forms a steep (up to 15°) upper slope. The midslope area has a rugged topography covered by thick slope deposits. The lower slope is relatively smooth except where broken locally by benches. The upper and middle slope areas are associated with strong magnetic anomalies and rare, landward-dipping reflections truncated by the rough surface. The rough midslope topography may reflect erosion succeeding the Paleocene uplift of this area and at least local subsidence in the early Miocene. Slope deposits covered the Trench landward slope, contemporaneous with the increased arc volcanism indicated by ash layers and with the present period of subduction. The subducting ocean crust and sediment of the Cocos Plate has a linear horst and graben topography of hundreds of meters relief that disappears beneath the landward slope of the Trench without disturbing its topography. This passive assimilation of oceanic material without significant accretion in the late Neogene argues for significant decoupling at the front of the subduction zone. A base of gas-hydrate reflection is evident in many of the redisplayed seismic records off Guatemala. These reflections are most common where slope deposits are thick; the reflections have not been identified in the underlying acoustic basement. This observation is consistent with the geochemical evidence that gas hydrate has its source in the organic- rich slope sediment. The hydrate depth and the temperature measurements in drill holes indicate a temperature gradient of 30°C/km.

Description: Deep-tow data across the Middle America Trench and the lower part of the landward Trench slope off Guatemala provide high-resolution information on the geological setting of four Leg 67 drill sites. Our 6-kHz profiler data resolve stratified sediments seaward of the Trench floor, which we interpret as deposits of Trench turbidites that flowed part way up the seaward slope of the Trench. Reflectors in the Trench fill are laterally continuous and relatively undisturbed. The Trench is segmented into diamond-shaped basins by ridges that intersect it at oblique angles. The acoustic stratigraphy in the Trench fill in these sub-basins is different on either side of an oblique ridge. A mound of variable height and width occurs at the base of the lower Trench slope; the mound could be either deformed Trench strata or an accumulation of debris that slumped from the Trench slope. The lower part of the landward Trench slope is steep and is linear along strike. There is no evidence on our 6-kHz or side-scan sonar data of large gravity slides in this area. Slope sediments are ponded on a bench along the landward Trench slope. They are acoustically stratified, but are not as continuous laterally as are the Trench strata. The structure parallel to the strike of the Trench slope is variable, with many small-scale folds and faults.