Normal fault reactivation during multiphase extension: Analogue models and application to the Turkana depression, East Africa

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

“…We start with the basic formula addressing the stress ratios between model and nature (σ model /σ nature ) : σ * = ρ * × h * × g * , where ρ * , h * and g * are density, length and gravity ratios, respectively (Hubbert, 1937;Ramberg, 1981). We can then acquire the strain rate ratioε * from the stress and viscosity ratios σ * and η * (Weijermars and Schmeling, 1986):ε * = σ * /η * . Knowing the strain rate ratio, the following equations yield the velocity and time ratios (v * and t * ) :ε * = v/h * = 1/t * .…”

“…Handin, 1969;Jaeger and Cook, 1976;Twiss and Moore, 1992) but is acceptable, since in nature, the lithosphere is generally weakened by several phases of deformation. When assessing viscous materials, the Ramberg number R m applies (Weijermars and Schmeling, 1986): R m = gravitational stress / viscous strength = (ρ × g × h 2 )/(η × v). The Ramberg number for both the model and natural materials is 17.…”

“…The varying impacts of the different types of crustal weaknesses on rift structures concur with previous models. For example, Wang et al (2021) show how the amount of extension in a previous deformation phase determines the magnitude of the inherited weaknesses. In their analogue models, smaller amounts of previous extension (e = ca.…”

“…Extension is generally considered to be (sub-)perpendicular to the strike of normal faults; hence multiple structural orientations within a rift basin may indicate a change in extension direction over time, as for instance proposed for the North Sea Rift, the Main Ethiopian Rift and Turkana Depression in the East African Rift, and the Labrador Sea (e.g. Bonini et al, 1997;Erratt et al, 1999Erratt et al, , 2010Heron et al, 2019;Wang et al, 2021). Yet it has also been shown that this assumption is often not fully valid due to the influence of structural inheritance, oblique extension and associated regional stress field distributions (e.g.…”

“…However, these modelling efforts also either focused on structural inheritance in the crust or on the effect of mantle weaknesses (e.g. Brun and Tron, 1993;Le Calvez and Vendeville, 2002;Bellahsen and Daniel, 2005;Autin et al, 2010Autin et al, , 2013Agostini et al, 2011;Zwaan et al, 2016Zwaan et al, , 2018Zwaan et al, , 2019Molnar et al, 2017Molnar et al, , 2018Molnar et al, , 2019Wang et al, 2021). To our knowledge, only Molnar et al (2020) simulated differently oriented mantle and crustal weaknesses in their experiments to reproduce the Red Sea rift.…”