Abstract
Abstract. Transform margins represent ∼ 30 % of non-convergent margins worldwide. Their formation and evolution have traditionally been addressed through kinematic models that do not account for the mechanical behaviour of the lithosphere. In this study, we use high-resolution 3D numerical thermo-mechanical modelling to simulate and investigate the evolution of intra-continental strain localization under oblique extension. The obliquity is set through velocity boundary conditions that range from 15∘ (high obliquity) to 75∘ (low obliquity) every 15∘ for rheologies of strong and weak lower continental crust. Numerical models show that the formation of localized strike-slip shear zones leading to transform continental margins always follows a thinning phase during which the lithosphere is thermally and mechanically weakened. For low- (75∘) to intermediate-obliquity (45∘) cases, the strike-slip faults are not parallel to the extension direction but form an angle of 20∘ to 40∘ with the plate motion vector, while for higher obliquities (30∘ to 15∘) the strike-slip faults develop parallel to the extension direction. Numerical models also show that during the thinning of the lithosphere, the stress and strain re-orient while boundary conditions are kept constant. This evolution, due to the weakening of the lithosphere, leads to a strain localization process in three major phases: (1) initiation of strain in a rigid plate where structures are sub-perpendicular to the extension direction; (2) distributed deformation with local stress field variations and formation of transtensional and strike-slip structures; (3) formation of highly localized plate boundaries stopping the intra-continental deformation. Our results call for a thorough re-evaluation of the kinematic approach to studying transform margins.
Highlights
Transform margins represent ∼ 30 % of non-convergent margins worldwide (Mercier de Lépinay et al, 2016; Philippon and Corti, 2016)
Transform continental margin refers to the continent–ocean transition derived from a transform plate boundary that accommodates, or has accommodated, ocean spreading (Basile, 2015; Mascle and Blarez, 1987)
Kinematic interpretations of transform margins are mainly based on a conceptual model (Basile, 2015; Basile et al, 2013; Mascle and Blarez, 1987; Scrutton, 1979)
Summary
Transform margins represent ∼ 30 % of non-convergent margins worldwide (Mercier de Lépinay et al, 2016; Philippon and Corti, 2016). Based on natural examples, Bellahsen et al (2013) and Basile (2015) highlighted the fact that transform faults can form synchronously with syn-rift structures and may reactivate or cross-cut inherited structures (e.g. equatorial Atlantic, Gulf of California; Fig. 1), or they can develop after the oceanic spreading starts to connect offset oceanic ridges (e.g. Woodlark basin; Taylor et al, 2009) This conceptual model and its offspring based on rigid plate tectonics do not reflect the whole intra-continental deformation phase associated with progressive strain localization and structure re-orientation (Ammann et al, 2017; Brune, 2014; Brune and Autin, 2013; Mondy et al, 2018; Le Pourhiet et al, 2017). We compare the model results to emblematic natural examples of transform margins and propose a simplified tectonic evolution model for the formation of transform margins undergoing intermediate and highly oblique extension
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