This week in News & Views, Anne Glerum, postdoc at GFZ Potsdam, discusses how her numerical models support a lithosphere-driven mechanism for the rotation of large continental microplates, like Victoria in the East African Rift System.
The East African Rift System (EARS) is a newly forming divergent boundary between the Nubian and Somalian plates (Fig. 1). The plate boundary system includes several continental rift arms and one or more smaller microplates. According to GPS data (e.g. Saria et al. 2014), while Somalia and the other microplates rotate clockwise relative to Nubia, the Victoria microplate is rotating counter-clockwise.
Previous hypotheses suggested that this distinctive rotation is driven by the interaction of a mantle plume with the microplate’s cratonic keel and the rift system (Calais et al. 2006, Koptev et al. 2015, 2016). While plumes can help localize deformation at crustal and lithospheric heterogeneities (e.g. Beniest et al. 2017), with the help of 3D time-dependent upper-scale numerical models, we propose that the configuration of weaker and stronger lithospheric regions predominantly controls the rotation of continental microplates and that of Victoria in particular.
In the EARS, Proterozoic mobile belts, like the Mozambique belt (Fig. 1), guide the localization and propagation of the rift arms. The particular configuration of the mechanically weaker mobile belts led to the curved, partly overlapping Eastern and Western rift branches of the rift system. At the same time, rheologically stronger regions are formed by for example the failed Anza rift and the Tanzania craton, at which the Eastern rift branch diverges. This particular configuration of lithospheric strength under the ~E-W extensional motion of the major tectonic plates, transmitted to the microplate along the stronger areas, induces Victoria’s rotation.
We demonstrated this edge-driven mechanism (Schouten et al. 1993) for large continental microplates with generic numerical models on the scale of the EARS (2100x2700x300 km). We then tested what particular geometry of partly overlapping mobile belts leads to the fastest rotation – an ellipsoidal geometry of EARS proportions. With a mobile belt configuration modelled after the EARS (Fig. 2), we then investigated the additional effect of stronger than average lithosphere on the rotation as well as that of a gradual decrease in prescribed extension velocity towards the south, as seen in the EARS (black arrows in Fig. 1a). The combination of EARS belt configuration, stronger craton and failed rift regions, and a gradual velocity decrease (Fig. 2f) predicts a rotation pole of the model Victoria microplate closest to the recent rotation pole of Saria et al. (2014) based on GPS data as well as earthquake slip directions and other geological indicators (Fig. 2g).
The good match between predicted and observed Victoria rotation poles brought us to compare our stress predictions to the World Stress Map (Heidbach et al. 2016; Fig. 3a&c). Both show predominantly normal faulting, with the maximum horizontal compressive stress direction changing in orientation along the curved rift branches. Notably, the oblique mobile belts (with respect to the overall extension direction) and the induced rotation lead to stress orientations that locally deviate from the N-S directions expected from the prescribed E-W regional extension. Only in the most oblique section of the Western Branch, transient phases of strike-slip faulting are predicted.
The model evolution allows us to synthesise several kinematic interpretations of the EARS (e.g. Morley 2010, Chorowicz 2006, Delvaux et al. 2012) as follows: In the overall E–W extending system of partly overlapping arcuate rift branches that follow lithospheric weak zones, drag of the major plates along stronger sides of the microplate intrinsically leads to local WNW–ESE extension directions in the rift segments. In the oblique sections, normal faulting thus occurs at small angles to the trend of the rift and the pre-existing weaknesses, with some strike-slip motion. Nevertheless, motion is predominantly accommodated by normal faulting. Small-scale sources such as basement fabric can locally deflect the stress field (Morley 2010) such that even the nonoverlapping, oblique Tanganyika-Rukwa-Malawi segment deforms under a mostly normal-faulting regime.
These insights provide exciting opportunities for. As the heterogeneity of the lithosphere plays such a large role in the deformation of East Africa, we are currently working on incorporating observational data directly into our model setup, such that we can zoom in on second-order dynamic processes affecting crust and lithosphere tectonics. At the same time, we are using the lithosphere-scale edge-driven mechanism to interpret the rotations of some of the many more continental microplates on Earth.
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