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 indentation causes the uplift of the Osa mélange which we interpret to represent an exhumed major channel for the transport of tectonically eroded material down into the subduction zone. We present evidence that, similar to the Nicoya segment of the Costa Rica convergent margin, subduction erosion rather than accretion has been the dominant process along the plate boundary. The composition of the Osa mélange is dominated by tectonized material of the upper‐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 temperatures > 200°C. We thus interpret the Osa mélange to be a product of subduction erosion at the base of the outer arc wedge structure.

Precise provenance analysis of andesite and dacite pebbles from conglomerates in the Eastern Alpine Molasse zone, using geochemical and geochronological methods, provides evidence for a synorogenic volcanic chain in the Eastern Alps which is completely eroded today. This volcanism was related to Periadriatic magmatism along the Periadriatic lineament and took place in the Palaeogene, roughly between 40 and 30 Ma. The occurrence of remnants of these volcanic rocks together with other marker lithologies in the Eastern Alpine Molasse, implies an early to middle Miocene drainage system which was, in some respects, similar to the present Inn river system, but had a considerably larger catchment area, reaching farther south. The Palaeo‐Inn drained the central and eastern sections of the Periadriatic magmatic belt to the northern foreland basin.