Subduction-Induced Back-Arc Extension Versus Far-Field Stretching: Contrasting Modes for Continental Marginal Break-Up

Continental break-up is an ingredient of plate tectonics and the Wilson cycle. During the evolution of the Tethyan Realm since the Paleozoic, a series of ribbon-like (micro-)continents are split from the Gondwana and drifted northwards to the Eurasian continent. The initial opening of the Neo-Tethys Ocean follows a typical continental marginal break-up of Cimmeria from Gondwana, which is generally considered to be driven by subduction of the Paleo-Tethyan plate. Yet, the competition between back-arc extension and far-field stretching is matter of a long-standing debate. Therefore, the authors here present the results of systematic 2-D numerical simulations. The results reveal four types of subduction-induced continental marginal break-ups: back-arc extension, far-field stretching, double break-up, and subduction channel opening. A weak relic suture zone is a prerequisite for continental break-up and an important factor controlling the mode selection. The back-arc extension mode occurs for relatively short and shallow dipping highly viscous subducted slabs. The far-field stretching mode, on the other hand, preferably occurs for spatially long and wide subducted slabs without mid-ocean ridges. Combining the geological observations, numerical simulations and force balance analyses, the authors propose that the northward, single-sided Paleo-Tethys oceanic subduction beneath Laurasia, was most likely responsible for the far-field break-up of the Cimmerian terranes from Gondwana. This would have required both pre-existing relic suture zones and the already subducted mid-ocean ridge, which thus played an important role in the opening of the Neo-Tethys Ocean.