Subduction invasion polarity switch from the Pacific to the Atlantic Ocean: A new geodynamic model of subduction initiation based on the Scotia Sea region

Subduction zones and their associated slabs are the main drivers of plate tectonics and mantle flow, but how these zones initiate remains enigmatic. In the Scotia Sea region, 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. Despite such variability, several works argue for a subduction initiation mechanism, in which a South America-Antarctica relative plate motion change, in combination with a particular plate boundary geometry in the western Weddell Sea, caused convergence across a transform plate boundary segment that subsequently evolved into a subduction zone. Here we discuss this kinematic model of subduction initiation, and, following geometric and kinematic arguments, highlight several unsolved issues that call for alternative explanations. Furthermore, we present new tectonic reconstructions of the Scotia region involving a simpler middle-Late Cretaceous plate boundary configuration, which avoid the geometric and kinematic problems of earlier reconstructions and that call for a new mechanism of subduction initiation. We refer to this mechanism as Subduction Invasion Polarity Switch (SIPS), which involves a long-lived and wide subduction zone (South American-Antarctic subduction zone) with lower mantle slab penetration, which imposes major horizontal trench-normal compressive deviatoric stresses on the overriding plate. This 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 compared to the long-lived subduction zone. 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. 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. 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. This new subduction zone develops at the continent-ocean boundary for models without and with a pre-imposed weak zone. We further propose that the SIPS model might explain subduction initiation elsewhere, including the New Caledonia subduction zone in the Southwest Pacific, the Lesser Antilles-Puerto Rico subduction zone in the Caribbean region, and the subduction zones that consumed the Rocas Verdes and Arperos backarc basins in South America and Central America, respectively. We further postulate that active backarc shortening in the Japan Sea, with eastward under-thrusting of Japan Sea oceanic lithosphere below the Japan arc, represents an early stage of SIPS.