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

Abstract

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.

Genetic Relations Between the Aves Ridge and the Grenada Back‐Arc Basin, East Caribbean Sea

“…Collision of an arc (here the Great Arc of the Caribbean) with a buoyant crustal body (e.g., Bahamas Bank) can generate rapid forearc rotation leading to back-arc rifting away from the point of collision (van den Broek et al, 2020;Wallace et al, 2009). In this sense, Aitken et al (2011) mention the collision at the south and north ends of the Great Arc of the Caribbean as a possible mechanism for the acceleration of slab rollback and arc rotation during the Paleocene and Eocene.…”

Microcontinents and Continental Fragments Associated With Subduction Systems

“…The first category proposes that plate boundary relocations (ridge jumps) assisted by mantle heterogeneities can lead to microcontinent formation (Abera et al, 2016; Müller et al, 2001). A second category emphasizes the role of inherited tectonic structures and the need for a strike‐slip motion, or a rotational component in the breakup processes resulting in the isolation of a small fragment of continental crust (Molnar et al, 2018; Nemčok et al, 2016; Péron‐Pinvidic & Manatschal, 2010; van den Broek et al, 2020). The microcontinents described in this paper appear to have mainly formed via mechanisms of the second category.…”

“…Recent analogue and numerical work indicates that this combination of rotational kinematics with inherited structures is a requirement for continental rifting and microcontinent formation (Molnar et al, 2018; van den Broek et al, 2020). The differential motion applied to different segments of the crust induces differential stress, which then localizes in an inherited weak zone.…”

“…The differential motion applied to different segments of the crust induces differential stress, which then localizes in an inherited weak zone. Formation of the Corsica‐Sardinia block is likely to be the result of this kind of mechanism (van den Broek et al, 2020).…”

Geochemistry and geochronology of Pengco subduction‐related ophiolites, Tibet: Implications for Dongkaco microcontinent in the Bangong–Nujiang suture zone

“…Microcontinents generally originated from continental breakup and subsequent seafloor spreading, and they are considered as small pieces of a continental block surrounded by oceanic lithospheric mantle Broek, Magni, Gaina, & Buiter, 2020;Molnar, Cruden, & Betts, 2018). During the course of geological movement, microcontinents contribute to terrane gathering and the formation of a larger continent (Li et al, 2018;Zhou, Wilde, Zhao, & Han, 2018), and also affect the process of oceanic subduction (Hegner et al, 2019;Jiang et al, 2014).…”