A High Resolution Geophysical Survey of Deep Sea Drilling Project Leg 84 Site 570

Abstract

Numerous small canyons and gullies begin or end on the middle and upper continental slope off Guatemala suggesting rapidly varying depositional centers and erosional sites. Seismic data indicate downslope sediment transport and deposition on the midslope above a high-amplitude diffractive acoustic basement. Near Hole 570 acoustic basement is at a position and depth that corresponds to recovered serpentinites and serpentinized peridotites. The topographic high west of Site 570 is the source of a high-gradient magnetic anomaly, and on the basis of seismic and magnetic data it is interpreted to be an exposed outcrop of acoustic basement. INTRODUCTIONWe present the results of a detailed bathymetric, magnetic, and seismic survey of the seafloor surrounding Site 570, located on the continental slope 70 km off the coast of Guatemala, 60 km northwest of San José Canyon, and 40 km upslope from the Middle America Trench. The survey (Figs. 1 and 2) was part of a high resolution study of sediment accretion and subduction in the Middle America Trench conducted aboard the Thomas Washington during the Ariadne Leg 3 (SIO-Scripps Institution of Oceanography) cruise, April 1982. The purposes of the survey were to determine the geologic setting of Site 570, to better measure the magnetic field in order to determine the source of a high-gradient anomaly, and to obtain more information on slope sedimentary processes. DATA BathymetryBathymetric swaths (Fig. 3) recorded by a multibeam echo sounder (Seabeam) were merged with corrected satellite navigation and plotted at a 20-m contour interval. There were seven satellite-position fixes recorded during the survey with an average 50% error ellipse of 150 × 320 m. In several instances contours of adjacent and crossing swaths were matched by shifting ship tracks within the limits of the position errors.The seafloor along the upper continental slope (200-1000 m depth) has high relief with deep (200-500 m) furrows aligned perpendicularly to the shelf edge. Numerous small canyons and gullies, 400-2000 m wide and 14°00N von Huene, R., Aubouin, J., et al