Continental breakup style of Marginal Seas

Marginal Seas are extensional basins formed in a convergent setting near active subduction zones. They are characterized by a short life (<25 Ma), as well as unstable and changing directions of seafloor spreading. However, the underlying processes involved in their formation from rifting to seafloor spreading initiation are still debated (supra-subduction convection/extension, slab-pull). This problem is further compounded by the fact that our understanding of continental breakup is primarily derived from the evolution of magma-poor and magma-rich Continent-Ocean Transitions (COT) of the Atlantic margins.

In this contribution, we characterize the tectono-magmatic processes acting during continental breakup by investigating the COT structures of three main Marginal Seas located in the Western Pacific, namely the South China Sea, the Coral Sea and the Woodlark Basin. All three examples formed under rapid extension rates and propagation of seafloor spreading. Although each marginal basin has its uniqueness, we show that these three marginal basins are characterized by a narrow COT (typically <~20 km), documenting the sharp juxtaposition of continental crust against igneous oceanic crust. The COT of the three basins shows that final extension is accommodated by the activity of one major low-angle normal fault. This extension is contemporaneous with important magmatic activity expressed by volcanic edifices, dykes and sills emplaced in the distalmost part of these margins. Such narrow COT suggests that a rapid shift from rifting to spreading.

The rapid localization of extensional deformation in a narrow area has major implications for partial melting generation. The evolution of extensional structures is controlled by the interplay of lithospheric thinning, asthenosphere upwelling and decompression melting. High extension rate prevents conductive cooling and lead to focus volcanic activity in a narrow area evolving rapidly in space and time to magmatic accretion. Causes for the fast extensions rates of Marginal Sea rifting are likely controlled by kinematic boundary conditions directly or indirectly controlled by nearby subduction zones. Such mode of breakup is probably not limited to marginal Seas but only enhanced in such settings.