Analogue modelling of marginal flexure in Afar, East Africa: Implications for passive margin formation

Frank Zwaan,Derek Keir,Federico Sani,Giacomo Corti

doi:10.1016/j.tecto.2020.228595

Abstract

The Afar region in East Africa is a key locality for studying continental break-up. Within Afar, passive margins are developing, of which the Southern Afar Margin (SAM) contains synthetic (basinward) faulting, whereas crustal flexure, antithetic faulting and marginal grabens occur along the Western Afar Margin (WAM). Numerous conflicting scenarios for the evolution of the WAM exist. In this analogue modelling study we test various factors that may affect the development of a WAM-style passive margin: brittle crustal thickness, (en echelon) rheological contrasts, sedimentation and oblique extension.Our experimental results illustrate how marginal flexure due to a weak lower crust below Afar can elegantly account for the structural features of the WAM. Brittle crustal thickness controls what structures occur: a thinner brittle crust accommodates flexure internally, whereas increasing brittle thicknesses lead to faulting. Large escarpment faults develop early on, followed by late-stage antithetic faulting and marginal grabens. A thicker brittle crust also causes enhanced subsidence, and increased strength contrasts between lower crustal domains leads to more localized deformation. Basin-wide sedimentation causes enhanced subsidence, as well as longer activity along large synthetic (escarpment) faults. Finally, oblique extension clearly prevents the development of marginal grabens, which only form in near-orthogonal extension.These results support a tectonic scenario involving initial oblique extension due to Arabian plate motion, creating echelon synthetic escarpment faults along the WAM. After the Danakil Block started its independent rotation, near-orthogonal extension conditions were established, allowing (enhanced) marginal flexure, antithetic faulting and marginal graben formation along the older en echelon escarpment. Differences in extension obliquity may also explain the differences in structural architectures between the WAM and SAM. The characteristics of the WAM are typical of magma-rich passive margins, and the margin has great potential for studying continental break-up and (magma-rich) passive margin formation.

Highlights

The region is considered a key locality for the study of continental break-up processes, as it contains rift systems in various stages of development, from incipient rifting in the Main Ethiopian Rift (MER), ongoing breakup and passive margin formation in Afar proper, to active oceanic spreading occurs in the Red Sea and Gulf of Aden (e.g. Bosworth et al 2005; Corti 2009, 2012; Zwaan et al 2020a, and references therein)
Note that the flexural domain in models A-D is dominated by large synthetic faults, whereas antithetic faulting is constrained to the marginal graben boundary faults (Figs. 3b-c, 4b-c)
Model L simulates the second phase of Western Afar Margin (WAM) evolution and involves a right-stepping rheological contrast mimicking the right-stepping character of the margin, inherited from the first phase of oblique extension simulated in Model K (Figs. 1a, 2d, 8)

Summary

Introduction

Afar contains the triple junction between the Main Ethiopian Rift (MER), Gulf of Aden and Red Sea rift axes. At the Gulf of Zula in the north, the Red Sea axis steps into Afar and continues SE through the Danakil Depression and along the Dabbahu Manda-Harraro segments (McClusky et al, 2010). The MER, itself the northernmost segment of the East African Rift System, forms the third branch of the triple junction and interacts with the other branches at the Tendaho Goba’ad Discontinuity (Fig. 1a) (Pagli et al, 2019). The region is considered a key locality for the study of continental break-up processes, as it contains rift systems in various stages of development, from incipient rifting in the MER, ongoing breakup and passive margin formation in Afar proper, to active oceanic spreading occurs in the Red Sea and Gulf of Aden The region is considered a key locality for the study of continental break-up processes, as it contains rift systems in various stages of development, from incipient rifting in the MER, ongoing breakup and passive margin formation in Afar proper, to active oceanic spreading occurs in the Red Sea and Gulf of Aden (e.g. Bosworth et al 2005; Corti 2009, 2012; Zwaan et al 2020a, and references therein)

Results

Discussion

Conclusion