Abstract

A complex set of lithologies, including calcareous oozes, hemipelagites, and turbidites, was recovered from the landward wall of the Peru Trench at its intersection with the Nazca Ridge. The sequence occurs at a water depth of 4,900 m and overlies an acoustic basement in the lowermost continental slope. Early Pliocene calcareous ooze overlies Pliocene to Quaternary ooze; both of these deposits are sandwiched between late Pleistocene (⩽ 400,000 yr), organic-rich turbidites and hemipelagic deposits typical of the Peru Trench and margin.

Planktonic and benthic foraminiferal assemblages indicate that the early Pliocene ooze originally was deposited on the Nazca Ridge above the calcium carbonate compensation depth (4,000 m) and to the west of the cool Peru-Chile Current. The Pliocene-Pleistocene ooze contains a temperate fauna associated with the Peru-Chile Current. Block faulting at the terminus of the Nazca Ridge displaced the calcareous ooze 1,900 m from the top of the ridge to the trench below. Apparently these lithologies were then folded against or thrust beneath the lower continental slope within the past 400,000 yr.

The stratigraphic sequence and the physiographic setting of the Nazca Ridge–Peru Trench intersection indicate convergence of the Nazca Ridge with the South American block. A minimum convergence rate of 0.8 cm/yr is calculated for the Pleistocene based upon the past and present geographic positions of the calcareous ooze. The best estimate of the rate is 2.8 cm/yr.

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