Understanding the erosive and accretionary nature of convergent margins is significant to understand tectonics and the crustal mass balance at subduction zones. The Costa Rican margin is commonly regarded as an archetypal example of an erosive margin, where subduction of sediments and basal removal of the upper plate in the subduction zone have occurred for most of the Cainozoic. This view is supported by structural constraints from 3D seismic reflection data in the outer forearc, as well as periods of forearc subsidence at ODP/DSDP/IODP drill sites. However, determining the origin of the Upper Eocene-Miocene Osa Mélange that is exposed in south Costa Rica only 10-30 km from today's trench offers another opportunity to constrain the long-term erosive, accretionary, and/or nonerosive evolution of the margin. Existing models for formation of the mélange propose that it resulted from (i) accretion of arc-derived trench-fill sediments, (ii) punctual accretion of the clastic apron of an ocean islands system, (iii) local dismemberment of the margin due to tectonic erosion, or (iv) in-situ deformation of a forearc sedimentary cover. To test the validity of these models and provide new constraints on the accretionary and/or erosive nature of the margin we studied the provenance of volcaniclastic material in the Upper Eocene San Pedrillo Unit of the Osa Mélange using geochemical analysis of detrital pyroxenes, amphiboles and igneous rocks. This innovative approach to determine the origin(s) of dismembered sedimentary sequences reveals that the volcaniclastic fraction of the mélange is, without ambiguity, predominantly composed of forearc material that preserves an assemblage of arc basement, proto-arc and arc sequences of the pre-Oligocene Costa Rican margin. This result and previous geological constraints show that the Osa Mélange formed through accretion of arc-derived trench-fill deposits in the Late Eocene to Miocene, with possibly minor tectonic incorporation of intra-oceanic material (ocean floor and seamount sequences). Therefore, consistently with recent seismic observations in south Costa Rica that document a phase of accretion within the past ∼5m.yr., preservation of arc-derived sediments in the Osa Mélange shows that the margin was predominantly non-erosive over the past ∼35m.yr. Cycles of subsidence and uplift in the south Costa Rican forearc during this period could represent a tectonic response to episodic subduction of seamounts formed at the Galapagos Hotspot, without causal link to short-term subduction erosion or accretion. Fundamentally, new results from the Osa Mélange show that large (10 km-thick) accretionary complexes can form durably due to local sedimentary recycling of the forearc in seamount collisional settings. The recycled component of the NW Osa Mélange exemplifies that quantifying the extent of local recycling vs net crustal addition through accretion of intra-oceanic sequences or net crustal loss due to tectonic erosion poses a serious challenge to determine the crustal mass balance at sub...