Crustal emplacement of exhuming (ultra)high-pressure rocks: Will that be pro- or retro-side?

(Ultra) high-pressure [(U)HP] rocks form and exhume from deep within subduction channels, but subsequent horizontal transport in the shallower orogenic crust makes it difficult to reconstruct their tectonic histories. We use a conceptual framework and numerical models to show that buoyant exhumation from within a subduction conduit formed during one-sided subduction may lead to emplacement of (U) HP rocks into either the lower plate (prowedge) or upper plate (retrowedge) of an orogen, depending on whether the upper plate crust deforms or acts as a backstop during exhumation. Both modes may operate at different positions or different times within an orogen, leading to emplacement of (U) HP rocks into both plates without changing subduction geometry. We propose that retrotransport during exhumation may explain some (U) HP rocks (e.g., Liverpool Land) situated in the upper plate of the Greenland-Norwegian Caledonides.