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 conc...
Seismic refraction, 24-fold, common-depth-point reflection and magnetic field profiles were obtained during the summer of 1977 on the inner slopes of the Middle America Trench off Guatemala. In addition, piston cores and dredge samples were collected along the midslope region. Five unreversed refraction profiles between 20-and 80-km length were recorded by Texas ocean bottom seismographs using air guns, Maxipulse, and conventional explosives as the sound sources. Travel time-distance curves based on first and later arrivals show velocities ranging between 1.6 km/s and 8.1 km/s. Some of the refracting horizons are observed as reflectors in the multichannel reflection sections. Relatively high velocities (4+ km/s) occur at shallow depth in the midslope region and may indicate imbricate slices of indurated sediment within the slope. From the combination of the reflection and refraction data and previous refraction work we infer the presence of at least two separate landward dipping slabs of oceanic crust within the upper slope. The presence of serpentine, basalt, chert, and mudstone in dredge and core samples collected near midslope is consistent with this interpretation. The top of the oceanic crust, with a velocity of 6.5 km/s, can be traced as a refractor approximately 30 km landward from the trench axis. The mantle with a velocity of 8.1 km/s is found as a refractor at a depth ranging from 13 to 17 km near the trench axis and can be traced landward to increasing depth, for about 15 km.
Results from refraction experiments have been combined with local earthquake data to produce a preliminary model for the structure of the New Hebrides arc‐trench system. Maximum crustal thickness of about 28 km occurs beneath the island ridge. The crust is unusually thick (about 10 km) in the zone seaward (westward) of the trench, where the velocity of the deepest layer is 7.0 km/s. The crustal thickness of the Fiji plateau, east of the New Hebrides ridge, varies between 5 and 7 km. The velocities of the materials that form the inner trench wall are similar to those found in the island ridge. A zone of relatively high velocity (4.1 km/s) is encountered at a depth of about 1 km beneath the arc‐trench slope. This zone thickens toward the trench and is overlain by material with velocity typical of surficial sediments (≈ 2 km/s). This may be an accretionary deposit, or it may be of volcanic origin.
A total of 301 earthquakes were recorded in the vicinity of the Orozco fracture zone by seven Texas ocean bottom seismograph stations during the 2-week period of the Rivera Ocean Seismic Experiment (ROSE) (phase II). Using data from the entire ROSE array, hypocenters of 50 earthquakes were determined. These revealed two distinct zones of seismic activity within the fracture zone. In addition to these earthquake families, many very small events were detected by a station located very close to the spreading center of the East Pacific Rise. The magnitudes of these earthquakes, defined by their duration times, were so small that most of them were recorded only at this station (station 14) in continual or swarmlike occurrences. The slope of the frequency-magnitude distribution of these events is significantly larger than those of other earthquake groups detected during the experiment, i.e., they appear to have an unusually high b value. These results suggest that this new population of earthquakes may be associated with volcanic activity or stress release within highly fractured crustal material. Refraction studies in the fracture zone reveal the presence of a rather high-velocity crustal layer (6.9-7.0 km/s) beneath the experiment zone. The Moho velocity and the crustal thickness are estimated at 7.8 km/s and 6.2 km, respectively.
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