The impact of rotational rifting on the evolution of the East African Rift System: an analogue modelling study
Abstract. The East African Rift System (EARS) represents a major tectonic feature splitting the African continent apart into the Nubian Plate situated to the west, and the Somalian Plate to the east. The EARS comprises various rift segments and two microplates (the Victoria and Rovuma plates) and represents a key location for studying rift evolution. Researchers have proposed various scenarios for the evolution of the EARS, but the impact of continental-scale rotational rifting, caused by the rotation of the Somalian Plate, has received only limited attention. In this study we apply analogue models to explore the dynamic evolution of the EARS within the broader rotational rifting framework. Our models show that rotational rifting leads to the lateral propagation of deformation towards the rotation axis, but we must distinguish between the propagation of distributed deformation, which can move very rapidly, and localized deformation, which can significantly lag behind. The various structural weakness arrangements in our models (representing pre-existing structural heterogeneities) lead to a variety of different structures. Laterally overlapping weaknesses are required for localizing parallel rift basins to create rift pass structures, possibly leading to the rotation and segregation of micro-plates, as is the case for the Victoria Plate in the EARS. Additional model observations concern the development of early pairs of rift-bounding faults flanking the rift basins, followed by the localization of deformation along the axes of the most advanced rift basins. Furthermore, the orientation of rift segments with respect to the regional (rotational) plate divergence affects deformation along these segments: oblique rift segments are less wide due to a strike-slip deformation component. Overall, our model results are a good fit with the large-scale features of the EARS, providing constraints on the timing of rift development and on the segregation and rotation of the Victoria plate within the broader rotational rifting framework of the EARS.
Frank Zwaan and Guido Schreurs
Frank Zwaan and Guido Schreurs
Time-lapse imagery, digital image correlation (DIC) and topographic analysis of laboratory experiments simulating the evolution of the East African Rift System https://www.dropbox.com/sh/6k34psul6cij4dh/AABpt-cnM1OyIEdolfTszmmya?dl=0
Frank Zwaan and Guido Schreurs
Viewed (geographical distribution)
Zwaan and Schreurs utilize fully scaled analogue models to investigate the role of rotational stresses on the continental-scale spatial evolution of rift zones, with comparison to rifting patterns in East Africa. The models explore a range of co-eval rift initiation geometries, and then track the time evolution of strain for comparison with the modern geometry of active rift zones in parts of E Africa. The models ignore rifting in the offshore SE rift branch in oceanic lithosphere east and southeast of the Rovuma block, and the SW branch that continues perhaps as far west as Angola around the San block.
My major concern with the paper is the reliance on outdated structural models based on coarse satellite imagery, and lacking information from seismicity and geodesy that shows modern plate motions. Specifically, the papers cited for the role of pre-existing basement shear zones are all based on satellite imagery and lack the 3rd dimension – the dip of structures. Likewise, the age of some lineaments mapped in these publications remains speculative (e.g., Chorowicz, 2005), and papers utilizing displacement of data strata should only be considered.
The comparisons with data in Section 4.15 as well as model constraints need to be re-thought, and omission of processes (e.g., magmatism, GPE, etc) need to be clearly articulated.
Seismicity (Lindenfeld, M., Rümpker, G., Link, K., Koehn, D., & Batte, A. (2012). Fluid-triggered earthquake swarms in the Rwenzori region, East African Rift—Evidence for rift initiation. Tectonophysics, 566, 95-104; Lavayssière, A., Drooff, C., Ebinger, C., Gallacher, R., Illsley‐Kemp, F., Oliva, S. J., & Keir, D. (2019). Depth extent and kinematics of faulting in the southern Tanganyika rift, Africa. Tectonics, 38(3), 842-862; Ebinger, C. J., Oliva, S. J., Pham, T. Q., Peterson, K., Chindandali, P., Illsley‐Kemp, F., ... & Mulibo, G. (2019). Kinematics of active deformation in the Malawi rift and Rungwe Volcanic Province, Africa. Geochemistry, Geophysics, Geosystems, 20(8), 3928-3951; Weinstein, A., Oliva, S. J., Ebinger, C. J., Roecker, S., Tiberi, C., Aman, M., ... & Fischer, T. P. (2017). Fault‐magma interactions during early continental rifting: Seismicity of the M agadi‐N atron‐M anyara basins, A frica. Geochemistry, Geophysics, Geosystems, 18(10), 3662-3686; Zheng, W., Oliva, S. J., Ebinger, C., & Pritchard, M. E. (2020). Aseismic deformation during the 2014 M w 5.2 Karonga earthquake, Malawi, from satellite interferometry and earthquake source mechanisms. Geophysical Research Letters, 47(22), e2020GL090930) and geodetic strain from active plate motion and time-averaged strains (e.g., DeMets, C., & Merkouriev, S. (2021). Detailed reconstructions of India–Somalia Plate motion, 60 Ma to present: implications for Somalia Plate absolute motion and India–Eurasia Plate motion. Geophysical Journal International, 227(3), 1730-1767; Stamps, D. S., Kreemer, C., Fernandes, R., Rajaonarison, T. A., & Rambolamanana, G. (2021). Redefining east African rift system kinematics. Geology, 49(2), 150-155; Knappe, E., Bendick, R., Ebinger, C., Birhanu, Y., Lewi, E., Floyd, M., ... & Perry, M. (2020). Accommodation of East African rifting across the Turkana depression. Journal of Geophysical Research: Solid Earth, 125(2), e2019JB018469; Birhanu, Y., Bendick, R., Fisseha, S., Lewi, E., Floyd, M., King, R., & Reilinger, R. (2016). GPS constraints on broad scale extension in the Ethiopian Highlands and Main Ethiopian Rift. Geophysical Research Letters, 43(13), 6844-6851; Viltres, R., Jónsson, S., Ruch, J., Doubre, C., Reilinger, R., Floyd, M., & Ogubazghi, G. (2020). Kinematics and deformation of the southern Red Sea region from GPS observations. Geophysical Journal International, 221(3), 2143-2154.)
The authors cite decades-old papers to quote a N to S younging, inferring a N-S propagation of rifting throughout the EAR. Yet, the only two sectors of the EAR confirming that pattern are the Eastern rift in Kenya and N Tanzania, and the southern Malawi rift. The MER appears to have propagated from S to N, and the very poorly date Western rift may have initiated near the Rungwe volcanic province, and propagated both N and S from there (e.g., Roberts, E. M., Stevens, N. J., O’Connor, P. M., Dirks, P. H. G. M., Gottfried, M. D., Clyde, W. C., ... & Hemming, S. (2012). Initiation of the western branch of the East African Rift coeval with the eastern branch. Nature Geoscience, 5(4), 289-294.). See also Daly MC, Green P, Watts AB, Davies O, Chibesakunda F, and Walker R (2020) Tectonics and Landscape of the Central African Plateau, and their implications for a propagating
Southwestern Rift in Africa. Geochemistry, Geophysics, Geosystems. e2019GC008746. For information on the SW rift zone.
Other major concerns are as follows:
Segments is generally used to describe a single fault-defined basin unit, whose length depends on the strength of the plate.
Line 48 - Strike slip faulting happens in accommodation zones and may reactivate a variety of basement sttructures, but overall, the rift structures form irrespective of basement structures. Please don't list my name there.
Line 53: What do you mean by an inherited weakness? THe 'weaknesses' are highly variable in the cited papers. What about the role of variable rheology linked to plate thickness (and hence geothermal gradient), composition, pre-existing thin zones, availability of melt, not just shear zones: see Muirhead et al., 2020;
The authors have published earlier papers considering rift models, which is in part why I urge them to dig more deeply into assumptions, and to move beyond EAR mythology to factual data constraining modern plate boundary deformation, and to move past lineament analyses.