The Formation of Continental Fragments in Subduction Settings: The Importance of Structural Inheritance and Subduction System Dynamics

Microcontinents and continental fragments are pieces of continental lithosphere, formed by extension and breakup, followed by plate boundary relocations. Microcontinents or continental fragments affiliated with passive margins are well documented, but those close to active margins are less studied. We use dynamic two- and three-dimensional numerical experiments to investigate how preexisting weaknesses within a continental upper plate affect extension and the possible formation of continental fragments. Our parametric study of the configuration (width and viscosity) of this imposed weakness indicates that stress localization and breakup of the upper plate are most efficient for narrow weak zones and a viscosity contrast between the weak zone and the surrounding crust of at least 1 order of magnitude. Moreover, upper plate extension and breakup occurs only if extension has a rotational component, here caused by the presence of a continental indenter on the downgoing plate. The width of the indenter relative to oceanic part of the downgoing plate controls differential slab pull that triggers trench retreat and upper plate deformation. A downgoing plate with a relatively large continental indenter yields large enough slab rotation to detach a continental block from the overriding plate and form wide back-arc basins. Variations in the weak zone angle with respect to the trench result in different basin geometries. We successfully modeled the first step in breakup of active continental margins and determined the settings that may facilitate microcontinent formation in a subduction framework.