accretion vs. erosion at subductio zones

Here is a figure from Clift and Vannucchi (2004) showing which subduction zones are adding material to the continental crust (accretion), and which are actively scraping away the continental crust (erosion) and dragging it into the depths.

Accretion is occurring in the Cascadia (forming the coast range), and eroding in the central Andes (exposing magmatic rocks from Jurassic subduction).

Clift and Vannuchi explain that accretion occurs where rivers introduce large volumes of sediments into the subduction trench, such as the Columbia river in Cascadia. In arid regions such as the central Andean coast, sediments are few, and continental erosion dominates. 

Clift, P., Vannucchi, P., 2004, Controls on tectonic accretion versus erosion in subduction zones: Implications for the origin and recycling of the continental crust, Review of Geophysics, 42.