Plume-driven rifting around a craton without far-field extensional forces: Initiation and asymmetric development of double rifts

The East African Rift System provides a unique tectonic setting to investigate the influences of mantle plumes on continental rifting in the absence of significant far-field extensional stresses. Surrounded by mid-oceanic ridges, the African plate experiences minimal boundary-driven extension, allowing plume-lithosphere interactions to play a dominant role. The system comprises two active rift arms-the Eastern and Western branches encircling the Tanzanian craton. The Eastern Rift initiated earlier and is more evolved, approaching continental breakup and incipient seafloor spreading, while the Western Rift remains at an earlier stage of development. To explore the controls on rift evolution and asymmetry, we construct two-dimensional thermo-mechanical models using ASPECT, an open-source geodynamic simulation code. Two sets of numerical experiments are conducted to evaluate the influences of (1) varying mantle plume parameters including temperature, radius, and position relative to the craton; and (2) time-dependent, asymmetric ridge push forces on rift initiation and propagation. The model domain spans 3000 km horizontally and 660 km vertically. The initial lithospheric thickness is set to 100 km, increasing to 150 km beneath the central craton via a colder geotherm. Randomized initial plastic strain (0.5–1.5) is imposed in the upper 50 km of the lithosphere outside the craton to simulate weakness relative to the craton and inherent heterogeneities. Ridge push forces are applied as time-varying tractions (0–3.5 TN/m) at the lateral boundaries to represent the progressive increase in ridge push associated with the thermal subsidence of aging oceanic lithosphere. These models aim to constrain the mechanisms responsible for double rift development and identify the conditions under which one arm evolves into a successful rift while the other stalls or fails.