Melange formation by subduction erosion: the case of the Osa melange in southern Costa Rica

The convergent plate margin off the Osa peninsula in southern Costa Rica is characterized by the indentation of the Cocos ridge at 4-5 Ma. The indenta tion causes the uplift of the Osa melange which we interpret to represent a n exhumed major channel for the transport of tectonically eroded material d own into the subduction zone. We present evidence that, similar to the Nico ya segment of the Costa Rica convergent margin, subduction erosion rather t han accretion has been the dominant process along the plate boundary. The c omposition of the Osa melange is dominated by tectonized material of the up per-plate Nicoya ophiolite complex (basalt, radiolarite, limestone). Strong deformation is concentrated in numerous discrete shear zones and produced the layered fabric of large rock volumes, which partly experienced temperat ures > 200 degrees C. We thus interpret the Osa melange to be a product of subduction erosion at the base of the outer are wedge structure.