SEISMIC REFRACTION AND REFLECTION MEASUREMENTS IN THE MIDDLE AMERICA TRENCH OFFSHORE GUATEMALA

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 velocitis 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 regon 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.-Authors