don't forget, you can have sediment buildup at subduction zones. granted, it's a touch wierd, but . . .
you know those mountains on the west coast of california (not the sierra nevada, the other ones)? those are essentially the result of the destruction of the farallon plate (the san andreas transform fault used to be a subduction zone). When plates collide, the sediment doesn't just get subducted, but can build some mountains).
ah, should add, the sierra nevada is from the collision, the extra mountain range is from the sediment. did I confuse people yet?