The tectonics of introversion and extroversion: redefining interior and exterior oceans in the supercontinent cycle

Abstract

Supercontinent amalgamation is described by the end-member kinematic processes of introversion – closure of interior oceans; extroversion – closure of exterior oceans; or orthoversion – amalgamation 90° from the centroid of the previous supercontinent. However, supercontinent formations are often ascribed to contra-dictory mechanisms; for example, Pangaea has been argued to have formed by introversion from Pannotia/ Gondwana, and extroversion from Rodinia. Conflicting interpretations arise partly from attempting to define oceans as interior or exterior based on palaeogeography or the age of the oceanic lithosphere relative to the time of supercontinent breakup. We define interior and exterior oceans relative to the external subduction ring, and associated accretionary orogens that surround amalgamated supercontinents. All oceans within the continental dominated cell and internal to the subduction ring are interior oceans. The exterior ocean is separated from the interior oceans by the subduction ring and bordered by external accretionary orogens. Wilson cycle tectonics dominate the interior continental cell, conversely, subduction of the exterior ocean is doubly vergent and lacks continent–continent collision. For the exterior ocean to close, the subduction ring must collapse upon itself, leading to the collision of external accretionary orogens. Employing this definition, Rodinia formed by extroversion, but all other supercontinents formed by introversion.