Tholeiitic Basalt Ridge in the Peru Trench

A prominent ridge occurs in the axis of the Peru Trench between 7°25′ to 7°50′ and 8°30′ to 9°30′ S. It rises as much as 900 m above the turbidite fill in the trench axis at 9° 20′ and consists of relatively fresh tholeiitic basalt which is overlain by Pleistocene trench turbidites. The tholeiitic basalt was derived from oceanic layer 2 which has been faulted and uplifted relative to the trench floor as the Nazca lithospheric plate descended into the Peru Trench. A K-Ar date of 8.7 m.y. was obtained on the basalt. This date allows three possible interpretations: (1) that the oceanic crust along the eastern edge of the Nazca plate is younger than indicated by magnetic anomalies; (2) that midplate volcanism has occurred on the older oceanic crust of the Nazca plate; or (3) that the basalt is more weathered and hence older than our data suggest. The second interpretation is preferred within the framework of the existing data.