Site 28

Abstract

The Shipboard Scientific Party 1 SETTING AND PURPOSEThe Outer Ridge, which lies between the north wall of the Puerto Rican Trench and the Nares Basin to the north, is covered for the most part with thick, acoustically "transparent" sediments, diminishing in thickness northward where they dip beneath turbidites of the Nares Abyssal Plain (Ewing and Ewing, 1962; Ewing etal, 1965).Along the southern portion of the Outer Ridge, reflection profiles reveal that the basement layer reaches a maximum elevation, while the sediment cover diminishes to less than 0.2 second acoustical thickness near the north wall of the Puerto Rican Trench. Seismic refraction measurements show that the Outer Ridge crustal layers are unusually thin and that the 8.2 km/sec mantle reaches within less than 5 kilometers of the ocean bottom in this area. This thinning of the crust has been cited by Worzel (1965) as evidence of extensional forces on the seaward side of island arc trenches. The same effect is shown by Ludwig et al. (1966) for the Japan Trench. Isaacs et al. (1968) show that extensional stresses are predicted on the convex side of the bend of the downmoving lithosphere beneath island arcs, even though the principal stress, deeper in the lithosphere, may be compressional. Hersey (1966) described the typical Outer Ridge sedimentary section (from seismic refraction and reflection measurements) as having an upper zone of unconsolidated, acoustically "transparent" sediments separated from basement rock by a semi-transparent layer having a considerably higher compressional wave velocity (4.2 km/sec), assumed to be layered rock or sediment. Figure 1 shows the topography of the Outer Ridge, the position of Site 28, and the location of a reflection profiler transect which is presented in Figure 2. Figure 3, from Ewing and Ewing (1962), shows a section drawn