Journey to Subduction Zones: Microcontinents vs Oceanic Plateaus

It is well-known that terranes, either having a continental or oceanic origin, drift to subduction zones within the oceanic lithosphere. However, it is less certain what the dynamics around this process are i.e.: what controls subduction or accretion of these allochthons; and how do they deform during this journey? Our recent work has revealed that continental terranes, namely microcontinents, may undergo significant extension while drifting to subduction zones in the pre-collisional episode due to a slab-pull force, which differs from the previously suggested terrane extension models. Further, we have extended our numerical geodynamic models to include oceanic plateaus in addition to microcontinents. We have tested a variety of plateau sizes (120-600 km) under 0 to 2 cm/yr imposed convergence rates. The crustal thicknesses of the oceanic plateaus and microcontinents were 15 km and 30 km, respectively. Our preliminary results show that oceanic plateaus may also undergo pre-collisional extension while they drift to subduction zones, although the magnitude of extension is not as great as microcontinental ones (~20% less). Most importantly, the oceanic plateaus tend to subduct for the chosen crustal thickness when they reach subduction zones and do not e the subduction process. On the contrary, according to our models, microcontinents always accrete to the overriding plate and cease subduction resulting in a slab-break off. We conclude that the differences in the magnitude of pre-collisional extension of oceanic plateaus compared to microcontinents are most likely related to having stronger rheology and thinner crust. Further, having thinner and denser crust makes oceanic plateaus more prone to subduct rather than accrete in contrast to microcontinents.