Abstract
AbstractMicrocontinents 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.
Highlights
In the context of this paper, we take into account both “microcontinents,” which are defined as pieces of continental lithosphere completely surrounded by oceanic crust, and “continental fragments” which are not completely detached from their parent continent by oceanic crust, but may be in an incipient stage of microcontinent formation or a failed one
We aim to investigate continental fragment formation on the overriding plate focusing on inherited heterogeneities in the continental upper plate and tectonic forces involved in subduction processes
Based on observations made on the evolution of the Corsica‐Sardinia block, we aim to investigate the role of an inherited lithospheric weakness and rotational tectonic forces in continental breakup and subsequent formation of a continental fragment on the overriding plate in a subduction system
Summary
The Formation of Microcontinents and Continental Fragments. Microcontinents are small pieces of continental lithosphere, situated above or below sea level, that are almost completely surrounded by oceanic crust. Continental fragments are defined but are still attached to their parent continent via thinned continental crust (Scrutton, 1976). This continental connection distinguishes continental fragments from microcontinents. In the context of this paper, we take into account both “microcontinents,” which are defined as pieces of continental lithosphere completely surrounded by oceanic crust, and “continental fragments” which are not completely detached from their parent continent by oceanic crust, but may be in an incipient stage of microcontinent formation or a failed one
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