What crustal features can get swept into a subduction zone, but will fail to get subducted into the mantle

The upper tectonic plate.

Explanation:

There are two tectonic plates which takes part in the process of subduction. The upper tectonic plate will fail to get subducted into the mantle because it is present in the upper side while other plate is remain beneath the upper tectonic plate. This lower plate subducted into the mantle due to high force of gravitation while the upper plate remain safe from sinking into the mantle.