Complex rift patterns, a result of interacting crustal and mantle weaknesses, or multiphase rifting? Insights from analogue models

Abstract

Abstract. During lithospheric extension, localization of deformation often occurs along structural weaknesses inherited from previous tectonic phases. Such weaknesses may occur in both the crust and mantle, but the combined effects of these weaknesses on rift evolution remain poorly understood. Here we present a series of 3D brittle–viscous analogue models to test the interaction between differently oriented weaknesses located in the brittle upper crust and/or upper mantle. We find that crustal weaknesses usually express first at the surface, with the formation of grabens parallel to their orientation; then, structures parallel to the mantle weakness overprint them and often become dominant. Furthermore, the direction of extension exerts minimal control on rift trends when inherited weaknesses are present, which implies that present-day rift orientations are not always indicative of past extension directions. We also suggest that multiphase extension is not required to explain different structural orientations in natural rift systems. The degree of coupling between the mantle and upper crust affects the relative influence of the crustal and mantle weaknesses: low coupling enhances the influence of crustal weaknesses, whereas high coupling enhances the influence of mantle weaknesses. Such coupling may vary over time due to progressive thinning of the lower crustal layer, as well as due to variations in extension velocity. These findings provide a strong incentive to reassess the tectonic history of various natural examples.

Competition between 3D structural inheritance and kinematics during rifting: insights from analogue models

“…Our reference model results are consistent with the observations from previous modelling studies. These earlier studies show that, without a seed, a double graben forms due to the presence of shallow-dipping shear zones originating from the VD or mantle discontinuity (e.g., Tron & Brun 1991;Michon & Merle 2000;, Dyksterhuis et al 2007Zwaan et al 2019;2021a;Oliveira et al in review) (Figs. 7a, 8a).…”

“…Moving the sidewall and base plate furthermore creates a so-called velocity discontinuity (VD) along the edge of the mobile base plate. This VD has often been used to represent a weakness or fault/shear zone in the strong lithospheric mantle (e.g., Tron & Brun 1991;Brun & Tron 1993;Bonini et al, 1997;Keep and McClay, 1997;Michon and Merle, 2000;Zwaan et al 2019;2021a). Applying different plate geometries and thus different VD orientations allowed us to test different mantle weakness orientation with respect to the general divergence direction, as defined by angle θ VD (Fig.…”

“…These seeds were semicircular bars (ø 5 mm) made of the same viscous material as used for the lower crustal layer. Above these seeds the brittle sand layer was locally thinner, causing a 44% weakening of the brittle layer (Zwaan et al 2021a), leading to the localization of faulting (e.g., Le Calvez et al 2002;Zwaan et al 2016;2020d;Molnar et al 2018Molnar et al , 2020. Similar to the VD, we also applied different orientations for these simulated crustal weaknesses, defined as angle θ S (Fig.…”

“…The authors pointed out, as was previously suggested by Reeve et al (2015), that the reactivation of pre-existing crustal and mantle weaknesses during a single phase of rifting could establish a rift system with structural trends that would otherwise suggest a multiphase rifting history involving changes in largescale plate divergence directions over time. Yet Zwaan et al (2021a) did not test the impact of multiphase rifting, of which previous work has shown the occurrence and importance during continental rifting (Bonini et al 1997;Dubois et al (2002); Henza et al 2010Henza et al , 2011Withjack et al (2017); Heron et al 2019;Wang et al 2021). Traditionally, multiphase rifting is associated with changing plate divergence directions, as has been proposed for the North Sea (Erratt et al 1999;2010), the Main Ethiopian Rift (Bonini et al 1997), the Turkana Depression in East Africa (Wang et a.…”

“…A recent analogue modelling study by Molnar et al (2020) showed that mantle weaknesses may determine the general rift trend, whereas crustal weaknesses may segment or partition the rift structure on a smaller scale. In a subsequent publication, Zwaan et al (2021a) improved upon this study by systematically testing how mantle and crustal weaknesses interact under constant kinematic settings. Their model results revealed the development of complex rift structures with different structural orientations under a constant kinematic setting, showing that structural weaknesses can be a highly dominant factor in a rift system.…”