High resolution reconstructions of the Southwest Indian Ridge, 52 Ma to present: implications for the breakup and absolute motion of the Africa plate

Abstract

Summary We reconstruct the post-52 Ma seafloor spreading history of the Southwest Indian Ridge at 44 distinct times from inversions of ≈20,000 magnetic reversal, fracture zone, and transform fault crossings, spanning major regional tectonic events such as the Arabia-Eurasia continental collision, the Arabia Peninsula’s detachment from Africa, the arrival of the Afar mantle plume below eastern Africa, and the initiation of rifting in eastern Africa. Best-fitting and noise-reduced rotation sequences for the Nubia-Antarctic, Lwandle-Antarctic, and Somalia-Antarctic plate pairs indicate that spreading rates everywhere along the ridge declined gradually by ≈50 percent from ≈31 Ma to 19-18 Ma. A concurrent similar-magnitude slowdown in the component of the Africa plate’s absolute motion parallel to Southwest Indian Ridge spreading suggests that both were caused by a 31-18 Ma change in the forces that drove and resisted Africa’s absolute motion. Possible causes for this change include the effects of the Afar mantle plume on eastern Africa or the Arabia Peninsula’s detachment from the Somalia plate, which culminated at 20-18 Ma with the onset of seafloor spreading in the Gulf of Aden. At earlier times, an apparently robust but previously unknown ≈6-Myr-long period of rapid kinematic change occurred from 43 Ma to 37 Ma, consisting of a ≈50 percent spreading rate slowdown from 43-40 Ma followed by a full spreading rate recovery and 30-40 ○ clockwise rotation of the plate slip direction from 40-37 Ma. Although these kinematic changes coincided with a reconfiguration of the paleoridge geometry, their underlying cause is unknown. Southwest Indian Ridge abyssal hill azimuths are consistent with the slip directions estimated with our newly derived Somalia-Antarctic and Lwandle-Antarctic angular velocities, adding confidence in their reliability. Lwandle-Antarctica plate motion has closely tracked Somalia-Antarctic plate motion since 50 Ma, consistent with slow-to-no motion between the Lwandle and Somalia plates for much of that time. In contrast, Nubia-Somalia rotations estimated from our new Southwest Indian Ridge rotations indicate that 189±34 km of WNW-ESE divergence between Nubia and Somalia has occurred in northern Africa since 40 Ma, including 70-80 km of WNW-ESE divergence since 17-16 Ma, slow to no motion from 26-17 Ma, and 109±38 km of WNW-ESE divergence from 40 Ma to ≈26 Ma absent any deformation within eastern Antarctica before 26 Ma.

Evolution of the East African Rift System from trap-scale to plate-scale rifting

“…10. Spreading rates along the ridges bounding the Nubia and Somali plates (Merkouriev and DeMets, 2006;Fournier et al, 2010;DeMets and Merkouriev, 2019;DeMets et al, 2020DeMets et al, , 2021Augustin et al, 2021). Note the changes in the spreading rate gradients coeval with the main magmato-tectonic events represented as in Fig.…”

“…The opening of the Red Sea started at 27 Ma synchronously with extension around the area covered by the Ethiopian traps (Bosworth et al, 2005) and slightly before the extension in the Rukwa rift (Roberts et al, 2012b). The onset of seafloor spreading in the Gulf of Aden is dated at 17.5 Ma (Fournier et al, 2010) and the changes of spreading rates along the ridges surrounding the Nubia and Somali plates (DeMets and Merkouriev, 2019;DeMets et al, 2020DeMets et al, , 2021 were all coeval with the initiation of magmatic periods and/or extension in the EARS (Fig. 10).…”

“…The age of 35 Ma corresponds to the onset of rifting in the Gulf of Aden (Bellahsen et al, 2006) and to a sharp increase of the spreading rate in the South-West Indian ridge (SWIR; Fig. 10; DeMets et al, 2021). The opening of the Red Sea started at 27 Ma synchronously with extension around the area covered by the Ethiopian traps (Bosworth et al, 2005) and slightly before the extension in the Rukwa rift (Roberts et al, 2012b).…”

Pliocene-to-Holocene volcano-tectonic activity on Mohéli Island (Comoros archipelago) constrained by new K Ar ages

“…If, as proposed by Famin et al (2020), the Comoros archipelago represents a dextral strike-slip boundary between the Somali and Lwandle plates, then the kinematics of this boundary seems to have changed from ≥3 Ma to ≤2 Ma in the Mohéli area. According to plate motion reconstructions from the spreading of the Southwest Indian Ridge, the Lwandle/Somali relative motion did not experience significant variation during this period (DeMets et al, 2021). Thus, the cause of this kinematic change has to be searched in relation to magmatic processes and the decrease of lithosphere partial melting.…”