Subduction invasion polarity switch (SIPS): A new mechanism of subduction initiation, with an application to the Scotia Sea region

The initiation of subduction remains an enigmatic process and a variety of conceptual models has been proposed to explain such initiation. Conceptual models have been tested with geodynamic models and have been applied to various subduction settings around the globe. None of these tested models, however, are applicable to the Scotia subduction zone in the Southern Atlantic (also referred to as South Sandwich subduction zone), where subduction started in the Late Cretaceous/Early Cenozoic in a pristine ocean basin setting devoid of other subduction/collision zones. How this subduction zone initiated remains intensely debated, as exemplified by the variability of published plate tectonic reconstructions. We present new tectonic reconstructions of the Scotia region involving a relatively simple middle-Late Cretaceous plate boundary configuration that involves a new mechanism of subduction initiation, Subduction Invasion Polarity Switch (SIPS). SIPS involves a long-lived, wide and deep subduction zone (South American-Antarctic subduction zone) that imposes major horizontal trench-normal compressive deviatoric stresses on the overriding plate. The overriding plate consists of a narrow continental lithospheric (land) bridge at the trench (Cretaceous-Early Cenozoic Antarctica-South America land bridge) with oceanic lithosphere behind it (Weddell Sea-Atlantic Ocean). The stresses cause shortening and thrusting at the continent-ocean boundary in the backarc region of the overriding plate, forcing oceanic lithosphere under continental lithosphere, starting the subduction initiation process, and eventually leading to a new, self-sustaining, subduction zone (Scotia subduction zone) with an opposite polarity (dipping westward) compared to the long-lived subduction zone (dipping eastward). The model thus involves invasion of a new subduction zone into a pristine ocean basin (Atlantic Ocean), with the primary driver being a long-lived subduction zone in another ocean basin (Pacific Ocean). To test the physical viability of the SIPS model, we have conducted numerical geodynamic simulations of buoyancy-driven subduction. Numerical results demonstrate that the SIPS model is viable, with compressive stresses in the overriding plate resulting from strong trenchward basal drag induced by subduction-driven whole-mantle poloidal return flow and compression at the subduction zone plate boundary due to the high resistance of the subduction zone hinge of the long-lived subduction zone to retreat westward. Subduction initiation starts in the overriding plate after ~100 Myr of long-lived subduction, eventually resulting in the formation of a new, opposite-dipping, subduction zone. Notably, this new subduction zone develops at the continent-ocean boundary for models without and with a pre-imposed weak zone. Apart from the Scotia Sea region, the SIPS model might also be applicable to subduction initiation that has occurred elsewhere in the geological past (e.g. the New Caledonia, Lesser Antilles-Puerto Rico, Rocas Verdes and Arperos subduction zones), and that is presently in a very early stage of development in the Japan Sea.