Abstract

Hemipelagic sediment layers inter-bedded with silt turbidite units were recovered from an elongate basin seaward of the axis of the Peru Trench. The basin is elevated 700 m above the shallowest depth of the present trench axis between lat 7°20′ and 8°30′ S. Micro-faunal content, mineralogy, and topography point to a source for this turbidite on the continental shelf to the northeast of the basin. A radiocarbon date of 5,100 B.P. was obtained for the youngest turbidite in the core, which dates the beginning of uplift in the basin. Although the nature of vertical motion is difficult to determine, the data suggest that there has been a combination of uplift of the seaward wall of the trench and subsidence of the trench axis. These rapid vertical movements may be a consequence of rupturing and decoupling of the Nazca plate into two segments that are being subducted at differential rates or different angles. If the seaward wall is under compressional stress as we suggest, we have an apparent contradiction with earthquake first motions, which indicate extensional stress on the seaward wall of this and other trenches.

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