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

Abstract

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.

Crustal versus mantle core complexes

“…In other terms, contrary to models of fully asymmetrical rifting controlled by structural or strain softening (e.g. Brune et al, 2016;Duretz et al, 2016), there is not a single and permanent detachment fault that, after separation of the two continental crusts, would accommodate extension in the lithospheric mantle along the whole rifting process. At the beginning of the amplification stage, when the mantle starts to exhume, the tip of the strongly thinned continental crust is still submitted to extension with new high-angle normal faults cutting through both crust and the brittle underlying mantle (Fig.…”

“…involves crustal thickness variations (Harry and Grandel, 2007), strain softening (Brune et al, 2014;2017) or structural softening of the crust and sub-Moho mantle (Duretz et al, 2016). In absence of one of these factors, the overall deformation pattern is symmetrical in both analogue (Brun andBeslier, 1996, Nestola et al, 2013; and numerical (Nagel and Buck, 2004;Lavier and Manatchal, 2006;Burov, 2007b;Weinberg et al, 2007;Beaumont and Ings, 2012) models.…”

“…Décollement versus detachment in section 4.2). Two exceptions are Huismans and Beaumont (2007) where there is no potential zone of décollement in the crust and Duretz et al (2016) where the whole crust and the upper part of lithospheric mantle are made of a kilometer-scale alternation of high strength (frictional down to 50 km and viscous below) and extremely weak layers (i.e. strong mechanical anisotropy at lithosphere scale).…”

“…Extension is localized in the sub-Moho mantle and distributed in the upper crust. In models, whose bulk deformation pattern is asymmetrical, the décollement is connected to a detachment that cuts downward the sub-Moho mantle giving a ramp-flat geometry at lithosphere scale (Brune et al, 2014;Duretz et al, 2016).…”

Breakup Without Borders: How Continents Speed Up and Slow Down During Rifting

“…Several lithospheric weakening mechanisms are not accounted for such as strain softening, intrusion-related heating, shear heating, and structural softening (Duretz et al, 2015(Duretz et al, , 2016Huismans et al, 2005). Several lithospheric weakening mechanisms are not accounted for such as strain softening, intrusion-related heating, shear heating, and structural softening (Duretz et al, 2015(Duretz et al, , 2016Huismans et al, 2005).…”