The importance of structural softening for the evolution and architecture of passive margins

T. Duretz (1) , B. Petri (2, 1) , G. Mohn (3) , S. M. M Schmalholz (1) , F. L. L Schenker (4) , O. Müntener (1)

1 UNIL - Université de Lausanne = University of Lausanne
2 IPGS-Dylbas - Dynamique de la lithosphère et des bassins sédimentaires (IPGS)
3 GEC - Laboratoire Géosciences et Environnement Cergy
4 Servizio ricerca USI/SUPSI

Lithospheric extension can generate passive margins that bound oceans worldwide. Detailed geological and geophysical studies in present and fossil passive margins have highlighted the complexity of their architecture and their multi-stage deformation history. Previous modeling studies have shown the significant impact of coarse mechanical layering of the lithosphere (2 to 4 layer crust and mantle) on passive margin formation. We built upon these studies and design high-resolution (~100–300 m) thermo-mechanical numerical models that incorporate finer mechanical layering (kilometer scale) mimicking tectonically inherited heterogeneities. During lithospheric extension a variety of extensional structures arises naturally due to (1) structural softening caused by necking of mechanically strong layers and (2) the establishment of a network of weak layers across the deforming multi-layered lithosphere. We argue that structural softening in a multi-layered lithosphere is the main cause for the observed multi-stage evolution and architecture of magma-poor passive margins. Passive continental margins are the focus of intense research activity since decades 1–3 , enhanced by the inten-sification of both industrial and academic geophysical surveys associated with ocean drilling programs (ODP). These research programs brought evidence of hyper-extended domains associated with severely thinned crust (< 10 km thick), exhumed subcontinental mantle and crustal allochthons 4. These observations led to new conceptual and mechanical models for the formation of rift basins and magma-poor margins 5,6. Rifting eventually leading to continental break-up develops in lithospheres that exhibit a complex pre-rift evolution characterized especially by orogenic to post-orogenic processes 7. For example, Mesozoic rifting in Europe occurred in a litho-sphere affected by the Variscan orogeny 8 , while Northern Atlantic rifting took place in a lithosphere affected by the Caledonian orogeny 9 and South China Sea rifting succeeded the Yenshanian orogeny 10. During these pre-rift events, interaction of magmatic, metamorphic and tectonic processes generates a heterogeneous continental lith-osphere (Fig. 1a). The mechanical heterogeneity derived from (i) mechanical anisotropies 11,12 , (ii) a layered crust with a variably metamorphosed upper crust, a migmatitic middle crust and a granulitic lower crust 13 sometimes hosting underplated mafic plutons 14,15 occasionally associated with shallower granitic plutons 16 , and (iii) a layered mantle with structural (e.g. shear zones) or compositional remnants (e.g. pyroxenite) from ancient collisional events 17. Such tectonic inheritance results in an overall layered continental lithosphere 18,19 where mechanical heterogeneities range from a few meters to several kilometers in thickness (Fig. 1a). The extension of small-scale power-law viscous or visco-plastic layered materials can result in shear localiza-tion and asymmetric extension 20. This behavior is the result of structural softening due to necking of strong layers and formation of a network of interconnected weak layers. Structural softening is hence caused by internal reorganizations within heterogeneous materials in order to minimize deformational work 21. Structural softening differs from material softening that requires an a priori defined alteration of material properties with increasing strain and which has been implemented in numerical models to explain the structures of rifted margins 22. The role of coarse mechanical layering of the lithosphere, inherited dipping heterogeneities, and the role of bimineralic crust and mantle on rifting processes has been addressed in previous studies 5,23–31. Here we present high-resolution numerical models that incorporate layering at finer scale, which show that structural softening arises during the extension of lithosphere and controls the development of the key structures observed in passive margins.

Domaines: Tectonique; Océanographie

T. Duretz, B. Petri, G. Mohn, S. M. M Schmalholz, F. L. L Schenker, et al.. The importance of structural softening for the evolution and architecture of passive margins. Scientific Reports, 2016, 6 (1), ⟨10.1038/srep38704⟩. ⟨hal-01670440⟩