Abstract
It is widely accepted that subduction initiation at modern Earth passive margin systems critically depends on the buoyancy and strength of the oceanic lithosphere and requires failure of the load-bearing crustal and mantle layers. As such, subduction initiation upon orthogonal convergence is controlled by the age of the oceanic lithosphere and thus the strength contrast at the margin. However, it is still unclear where along the margin and how subduction initiates. In particular, rheologically-controlled mechanisms are poorly understood and require further investigation. Therefore, this combined analogue and numerical modelling study aims at exploring the effects of first order rheological and kinematic conditions on subduction initiation at passive margins. Our results highlight the sensitivity of early stages of subduction initiation to the initial rheological setup as well as evolving thermo-mechanical feedback mechanisms. Additionally, they provide more insights on the conditions responsible for the locus of subduction initiation. We infer that the locus of subduction is controlled by the rheology of both the crust and the mantle lithosphere at the margin, which is in strong correlation with the thermal age of the oceanic lithosphere. Suitable conditions for subduction initiation at passive margins correspond to an intermediate age (ca. 80 Myr) of the oceanic lithosphere. In all other cases deformation localizes within the oceanic lithosphere or affects the entire continent. We advocate the significance of crust-mantle decoupling at the passive margin for its inversion and possible evolution towards a subduction zone. Additionally, the development of a self-sustaining subduction zone will fail in the absence of weakening mechanisms taking place in the mantle lithosphere such as thermal softening (shear heating) and low temperature plasticity (Peierls mechanism). • Analogue and numerical models are used to infer favorable mechanical conditions for subduction initiation at passive margins. • Ductile continental crust is a key element for localizing deformation during the early stages of subduction initiation. • Subduction initiation is controlled by the strength of the mantle lithosphere at the margin. • Evolution towards sustainable subduction is controlled by both Peierls creep mantle rheology and shear heating.
Highlights
Subduction initiation at passive margin is lacking direct observations, several subduction zones are believed to have been formed at continental margins (Hall, 2019; Karig, 1982; Manzotti et al, 2014; Marroni et al, 2017)
Underthrusting initiates when the down-flexed passive margin is cut by an evolving major thrust structure that accommodates most of the shortening
A selection of experiments, involving both oceanic and continental lithospheres subjected to compression, is used to investigate the role of oceanic and continental lithosphere rheology for scenarios where convergence is orthogonal to the passive margin
Summary
Subduction initiation at passive margin is lacking direct observations, several subduction zones are believed to have been formed at continental margins (Hall, 2019; Karig, 1982; Manzotti et al, 2014; Marroni et al, 2017). Rheological and density contrasts between the oceanic and continental lithospheres have been considered as key parameters for subduction initiation (SI) (Baes et al, 2018; Cloetingh et al, 1989; Erickson, 1993; Faccenna et al, 1999; Goren et al, 2008; Mart et al, 2005; Nikolaeva et al, 2010; Hamai et al, 2018; Yamato et al, 2013) These studies used different approaches, they consistently emphasize that the aging of the oceanic plate is key for providing favorable mechanical conditions for SI at passive margins. This causes deformation of the ductile crust at the margin and deflection of the oceanic lithosphere
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