Abstract

New seismic reflection data along closely spaced lines reveal deeper structures than did earlier records. The new records show sediment in the trench continuing as much as 12 km landward under the continental slope, that large slumps and extensional structures are common on the slope, and suggest that in earlier periods the sea floor now under the trench was receiving sediment from an eastern source rather than from the present western source. Large tectonic displacements appear to be absent from oceanic basement beneath the continental margin because undisturbed linear oceanic magnetic anomalies and a fracture zone continue from the abyssal plain at least to the continental shelf. However, since vertical displacements in the overlying sediment are greater than any in the basement, thrust faults are indicated even though they are not visible in the reflection records. No single zone of deformation marks the emergence of the Benioff zone, but instead the zone probably emerges in the whole complex of tectonic structures across the continental margin. There are enough data to estimate roughly the maximum amount of abyssal sediment that may have been plastered against the continent during the Tertiary.

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