Role of Lithospheric Buoyancy Forces in Driving Deformation in East Africa From 3D Geodynamic Modeling

Abstract

The East African Rift (EAR, Figure 1a) is the dominantly continental portion of the East African Rift System, which separates the Nubian and Somalian Plates and is the largest continental rift on Earth. The EAR is traditionally divided into three distinct rift segments: the northern EAR (Afar region), the Eastern Branch, and the Western Branch. Its spatial extent, heterogeneous lithospheric structure, and variable rates of extension across the EAR produce a wide range of deformation styles. At present, the origin of forces driving extension across the EAR remains highly debated. The absence of regional slab-pull forces requires dominant contributions from lithospheric buoyancy and horizontal mantle flow. Lithospheric buoyancy arises from a combination of changes in lithospheric structure (composition, geothermal gradients) and surface topography (Fleitout, 1991; Fleitout & Froidevaux, 1982; Flesch et al., 2000; Jones et al., 1996), with the latter partially supported by vertical mantle flow (i.e., dynamic topography; Flesch et al., 2007). When isostatic compensation is assumed, lithospheric buoyancy can be quantified through the integration of lithostatic pressure to a compensation depth (i.e., gravitational potential energy ∼GPE), with regions of locally high GPE undergoing extension (e.g., Coblentz & Sandiford, 1994). While lithospheric buoyancy generates internal stresses driving deformation, mantle flow gives rise to both vertical and horizontal tractions acting at the base of the lithosphere (e.g.

A Geodynamic Investigation of Plume‐Lithosphere Interactions Beneath the East African Rift

“…Thus, our model is derived from Rajaronarison et al. (2021) in which the lithospheric buoyancy forces are implemented using ETOPO1 (Amante & Eakins, 2009) for the surface topography, CRUST1.0 for the laterally varying crustal structures and densities (lower, middle, and upper crust; Laske et al., 2013), and isostatically compensated to 100 km depth for the mantle lithosphere density. We neglect the effect of dynamic GPE that may arise from changes in lithospheric thickness since, based on the findings of Ghosh et al.…”

“…Details of the calculation of our lithospheric geothermal gradient is described in Rajaonarison et al. (2021; Supporting Information; Section 5.3) and 3D diagram of the temperature field is shown in Figure S3 in Supporting Information S1.…”

“…Significantly, all of the aforementioned studies approximate deformation within the lithosphere using a depth‐independent 2D (i.e., thin shell or sheet) modeling approach. In contrast, recent 3D thermomechanical modeling (Rajaonarison et al., 2021) suggests that the E‐W extension of the EAR, (i.e., the rigid plates and microplates rotation and their velocity magnitudes) is dominated by lithospheric buoyancy forces and that additional forces, such as viscous coupling to mantle flow, are only needed to explain the anomalous observed northward surface motions within the deforming zones as revealed by GNSS velocities (Figure 1a, Stamps et al., 2018).…”

“…Here, we expand on the work of Rajaonarison et al. (2021) to test if plume‐lithosphere interactions are a plausible explanation for the rift‐parallel deformation observed along the EAR, where plume‐lithosphere interactions have been suggested to be important in the system's long‐term evolution and dynamics (Koptev et al., 2016; Koptev, Burov, et al., 2018; Koptev, Calais, et al., 2018; Koptev, Cloetingh, et al., 2018). Numerous seismic tomography studies have imaged low‐velocity anomalies beneath the EAR and associated them with the presence of one or more upwelling thermal anomalies (e.g., Bagley & Nyblade, 2013; Chang et al., 2020, 2011; Ebinger & Sleep, 1998; Emry et al., 2019; S. E. Hansen et al., 2012; Montelli et al., 2006).…”

3D geodynamic-geomorphologic modelling of deformation and exhumation at curved plate boundaries: Implications for the southern Alaskan plate corner

“…Although the setup of our experiments captures the main features of the subducting plate configuration, the laterally homogeneous overriding plate does not encompass the various elements of the crustal and lithospheric structure of the studied area 38 , 46 . The contribution of lithospheric buoyancy forces to internal lithospheric stresses and deformations is known from previous global 82 – 84 and regional 85 – 87 numerical geodynamic models and, therefore, warrants thorough further investigation in upcoming studies with the application of cutting-edge technologies such as coupled geodynamic-geomorphologic modelling. Moreover, lateral differences in subducting plate properties may also be important: for example, oceanic plateaus are known to have a thicker crust 88 , resulting in higher lithospheric buoyancy relative to the surrounding oceanic lithosphere 89 .…”