Terrane geodynamics: Evolution on the subduction conveyor from pre-collision to post-collision and implications on Tethyan orogeny

Terranes are passengers within drifting oceanic plate, and ride with the plate to subduction plate boundaries. Although oceanic lithosphere readily subducts into the mantle at the plate boundary, terranes may resist sinking or not, depending on a variety of controlling factors that are not very well understood. Further, the tectonic development of terranes prior to their arrival at a subduction zone is not well documented. We performed numerical experiments to explore these unknowns of terrane geodynamics during the whole pre- to post-collisional period. Our analyses reveal that terranes can undergo considerable extension prior to their arrival to subduction plate boundaries owing to the pull force exerted by sinking oceanic slabs. Increasing terrane crustal thickness, decreasing terrane width or imposed convergence velocity, and an intra-oceanic subduction setting aggrandizes the pre-collisional terrane extension. The numerical models identify increasing terrane crustal thickness and width, and decreasing imposed convergence velocity as factors that promote terrane accretion. Additionally, having a continental overriding plate at the subduction boundary increases the propensity for terrane accretion. Some intra-oceanic subduction experiments demonstrate ablative subduction and subduction polarity reversal events in connection with terrane collisions/subduction. We compare our models to the evolution of a controversial Tethyan terrane, the Nilufer oceanic plateau. Our models suggest that the Nilufer terrane may have undergone pre-collisional extension owing to the pull of the sinking Tethys Ocean plate. Further, uninterrupted subduction of the Tethys Ocean, despite the accretion of the Nilufer terrane, is illustrated by our results which have implications on other regions along the Tethyan orogenic belt.