Site 494: Middle America Trench Lower Slope

Abstract

Principal results (Holes 494 and 494A): The recovered section casts doubt on the concept of simple, orderly accretion along the seaward edge of the Guatemala convergent margin. Site 494, on the lower slope, is about 3 km from the trench axis and 580 meters above it and is situated on a small terrace that is about 2 km wide ( Fig. 1). On the basis of site survey data obtained prior to drilling at Site 494, this terrace was thought to be at least 18 km long and to have been formed by emergence of a large imbricate thrust. The bathymetry collected during a deep-tow survey (Moore et al., this volume) shows, however, that this terrace extends only about 4 km along the trench. Reentrants separate this small terrace from similar-sized terraces to the northwest and southeast, terraces that are at depths about 100 meters shallower and 100 meters deeper, respectively. Thus the bathymetry gathered from conventional reflection profiles suggests that the terrace at Site 494 is one of a series of benches that might be individual features, perhaps slump blocks. Seabeam bathymetry of Aubouin et al. (this volume) indicates that the terrace is a relatively continuous feature and that reentrants are of minor importance. A major objective of the drilling at Site 494 was to test if the terraces are related to proposed imbricate thrusts or to smaller slump blocks.The recovered lithologies (Fig. 2) Gaseous hydrocarbons in the section are mainly methane, and only minor amounts of ethane were detected. BACKGROUND AND OBJECTIVESOn the landward slope of the Middle America Trench in about 5520 meters of water, Site 494 is located on a narrow terrace about 580 meters above and 3 km from the adjacent trench axis (Fig. 1). The multichannel seismic records off Guatemala show strong reflections dipping to the north-northeast under the outer continental shelf and the upper and middle slope, which might represent an extension of the Nicoya Complex that crops out in Costa Rica. For Site 494, located on the lower slope, seismic records are highly obscured by diffractions. Nevertheless, three reflections may be traced across the trench and perhaps under the landward slope-the uppermost, possibly marking the lower boundary of the sedimentary apron on the slope; the second, a boundary between the sediments filling the trench and the deep ocean basin sediment; and the third reflection, the top of the igneous ocean crust that can be traced a short distance under the continental margin. Thus Site 494 is situated in a zone where the model of an accretionary prism could be tested, if the drilling were sufficiently deep to penetrate through the slope deposits. Likewise, it would be possible to sample the underlying deposits inferred to be trench and perhaps deep ocean basin sediment. Another goal of Leg 67 was to test if the terraces are related to proposed imbricate thrusts or to smaller slump blocks. OPERATIONSThe Glomar Challenger, operating its underway seismic reflection, magnetic, and 12-Khz and 3.5-Khz, transducer geophysical instruments, was in ...