Large-to Local-Scale Control of Pre-Existing Structures on Continental Rifting: Examples From the Main Ethiopian Rift, East Africa

Abstract

In the Main Ethiopian Rift (East Africa) a complex tectonic history preceded Tertiary rifting creating pre-existing discontinuities that influenced extension-related deformation. Therefore, this area offers the opportunity to analyze the control exerted by pre-existing structures on continental rifting at different scales. In this paper we present an overview of such an influence. We show that at a large scale (up to ∼800–1,000 km) rift localization has been controlled by a lithospheric-scale inherited heterogeneity corresponding to a Precambrian suture zone, separating two different lithospheric domains beneath the plateaus surrounding the rift. The inherited rheological differences between these two lithospheric domains, as well as the presence of pre-existing lithospheric-scale transversal structures, largely controlled the along-axis segmentation and symmetry/asymmetry of different, ∼80–100 km-long rift segments. Inherited transversal structures also controlled the development of off-axis volcano tectonic activity in the plateaus surrounding the rift. At a more local scale (<80 km), inherited fabrics controlled the geometry of normal faults and the distribution and characteristics of rift-related volcanism. These observations document a strong control exerted by pre-existing structures on continental rifting at all different scales.

Analog and Numerical Modeling of Rift‐Rift‐Rift Triple Junctions

“…(2020) suggested that triple junction dynamics in the Afar region likely did not cause the formation of the MER, this latter being related to a later and independent rifting event. This sequence is broadly supported by the majority of geological studies which suggest initiation of the Red Sea‐ Gulf of Aden rifting since 34 Ma, and the MER initiating at 10–20 Ma (e.g., Corti et al., 2022; Girdler, 1984; Watchorn et al., 1998; Wolfenden et al., 2004; Pik et al., 2013, and reference therein). Therefore, both our analog and numerical models, in agreement with other Authors (e.g., Almalki & Betts, 2021; Khalil et al., 2020, 2021), indicate that a simple single‐stage, symmetric dynamic does not explain the structural pattern nowadays observed at the Afar triple junction, and its migration through time.…”

A review of structural inheritance in rift basin formation

“…imparted by Proterozoic orogenic events in East Africa, as observed, for example, in the Malawi (Ring, 1994;Laó-Dávila et al, 2015;Dawson et al, 2018;Hodge et al, 2018;Kolawole et al, 2018;Williams et al, 2019;Scholz et al, 2020;Shillington et al, 2020;Wedmore et al, 2020b), Albertine (Katumwehe et al, 2015), Kivu (Smets et al, 2016), Kenya (Smith and Mosley, 1993;Robertson et al, 2016;Muirhead and Kattenhorn, 2018), the Main Ethiopian (Corti et al, 2022),…”

“…The East African Rift System (EARS) is the pre-eminent example of an active continental rift, exhibiting all stages of continental rift evolution from nascent seafloor spreading in Afar to incipient faulting in the Okavango Rift of Botswana (Figure 10a;McConnell, 1972;Chorowicz, 2005;Ebinger, 2005;Macgregor, 2015;Daly et al, 2020). Hard-and soft-linked structural inheritance has influenced the evolution of the EARS at many spatial scales (and also magma emplacement mechanisms and volcanic activity; see Corti et al, 2022 for examples from the Main Ethiopian Rift). A famous example of plate-scale inheritance is present south of the Main Ethiopian Rift, where the Eastern and Western Branches of the EARS wrap around the relatively rigid Archean Tanzanian craton and spatially co-locate with orogenic belts that formed during the progressive Proterozoic amalgamation of the African Continent (Figure 10a; Daly et al, 1989;Versfelt and Rosendahl, 1989;Nyblade and Brazier, 2002;Corti et al, 2007).…”

“…At the ~10-100 km scale, the EARS can be divided along strike into distinct half-graben or graben basins. It is debated whether the along-strike extent of these basins is guided by the intrinsic strength of the lithosphere or pre-existing rift-perpendicular structures (Ebinger et al, 1987;Ebinger, 1989;Upcott et al, 1996;Katumwehe et al, 2015;Laó-Dávila et al, 2015;Heilman et al, 2019;Scholz et al, 2020;Corti et al, 2022); nevertheless, in either case the patterns of faulting within these basins can be linked to structural inheritance. The most commonly described examples are fault traces that are subparallel to surface foliations (i.e., 'geometric similarity')…”

“…For example, faults in the Tanganyika Rift and central Malawi Rift are not parallel to surrounding crustal fabrics (Muirhead et al, 2019;Scholz et al, 2020), and the degree to which metamorphic fabrics influence fault orientations can change as rift extension progresses (Muirhead and Kattenhorn, 2018;Nutz et al, 2022). Furthermore, local variations in fabric orientation may disrupt geometric similarity at the scale of an individual fault, resulting in faults that locally cross-cut fabrics, along-strike fault segmentation, and/or faults that are Z-shaped in plan-view due to scarps that are continuous across perpendicular bends (Figure 11; Hodge et al, 2018;Corti et al, 2022).…”