Undeformed Sediments in Oceanic Trenches with Sea Floor Spreading

Abstract

THE study of magnetic anomalies associated with mid-ocean ridges indicates that sea floor spreading is active in the vicinity of these ridges1. Mid-ocean ridges are thought likely to be the sites of upwelling convection currents and oceanic trenches to be the sites of descending currents2. Most trenches are associated with intense seismic activity3–6. The hypocentres of earthquakes occur along almost planar zones (Benioff zones) which may be the result of movement of the sea floor under continents or island arcs. Seismic refraction studies of oceanic trenches, however, indicate that the sediments within the trenches are essentially undeformed and that there is no large accumulation of deformed sediments with a low seismic velocity associated with these trenches7–10. The absence of deformed low velocity sediments has been cited as evidence that the sea floor is not descending in the vicinity of these trenches7,8,10. Scholl et al.10 assume that deformed pelagic sediments would be expected within the trenches if the sea floor is actively underthrusting a continent or island arc. The centre of the Benioff zone, however, typically intersects the ocean floor landward of the axes of oceanic trenches4–6. This indicates that downward movement of the sediments occurs landward of the axes of the trenches rather than directly under the axes of the trenches.