Martin, Erin L.1,3, Cawood, Peter A.1, Murphy, J. Brendan2,3
1School of Earth, Atmosphere and Environment Science, Monash University, Clayton, Australia, 2Department of Earth Sciences, St Francis Xavier University, Antigonish, Canada, 3Earth Dynamics Research Group, The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, Perth, Australia
The amalgamation of continental fragments into supercontinents can occur by processes of introversion, involving the closure of interior oceans, extroversion, in which the exterior ocean closes, or orthoversion, entailing formation 90° from the centroid of the previous supercontinent. However, individual supercontinents are often defined as forming by contradictory mechanisms; for example, Pangea has been argued as forming by introversion and by extroversion. Conflicting interpretations arise, in part, from attempting to define an ocean as interior or exterior based on paleogeography, or the age of oceanic crust relative to the time of supercontinent breakup. We argue that interior and exterior oceans should be defined relative to the peripheral subduction ring and its associated accretionary orogens that surround the amalgamated supercontinent. The subduction ring broadly divides the Earth into two cells, which conform to the spherical harmonic degree-2 structure of the mantle: one associated with supercontinent assembly, and therefore dominated by continental crust with only minor oceanic crust, and the other containing almost exclusively oceanic crust, which is subducted to create peripheral accretionary orogens at the margin of the supercontinent. All oceans within the cell that contains continental blocks are interior oceans, as they are interior to the continental cell of the degree-2 planform. By contrast, the exterior ocean is the oceanic cell antipodal to the continental cell, separated by the subduction ring. Interior oceans close following asymmetrical subduction and collisional orogenesis. However, for the exterior ocean to close, the subduction ring must collapse upon itself, leading to the juxtaposition of long-lived accretionary orogens within the core of the supercontinent. Employing this geodynamic definition for interior and exterior oceans, Rodinia formed by extroversion, but all other supercontinents formed by introversion which cannot occur without orthoversion.
Erin Martin is a research associate working with as part of the Pulse of the Earth ARC Laureate Fellowship team at Monash University. Erin completed her PhD at Curtin University with the Earth Dynamics Research Group.