Rapid Versus Delayed Linkage and Coalescence of Propagating Rift Tips

Abstract

The tectonic interaction, linkage, and coalescence of propagating isolated rift segments create a through-going along-axis rift floor without which a continental break-up axis cannot develop.However, prior to linkage, interacting rift segments are separated by a topographic basement-high (rift interaction zone, RIZ) which is progressively dismembered and down-thrown by the lateral propagation of rift tip faulting and their hanging wall subsidence. Here, we explore the evolution of the Middle-Shire and Nsanje RIZs located along three slowly extending (~2.2 mm/yr) contiguous propagating rift segments: the southern Malawi Rift, Lower-Shire Graben, and the Nsanje Graben, East Africa. The Middle-Shire RIZ is an overlapping-oblique RIZ in which the NNE/N-trending Malawi Rift is propagating into the shoulder of the NW-trending Lower-Shire Graben, whereas the Nsanje RIZ is a tip-to-tip oblique RIZ in which the Lower-Shire Graben has propagated into the tip of the N-trending Nsanje Graben. These rift zones provide non-volcanic natural examples to explore tectonic strain localization on rift tips and syn-rift RIZ surface processes. We use a landscape evolution model with implemented fault displacement fields of two contiguous RIZs with contrasting geometries, to simulate their geomorphic evolution, and apply a static stress model to evaluate the stress transfer patterns during RIZ evolution. The model results successfully reproduce natural observations in the study area, in which, with progressive extension and tip growth, the Middle-Shire RIZ maintains minor basement down-throw and an unequilibrated axial stream profile, which contrasts the widespread basement burial and equilibrated axial stream profile across the Nsanje RIZ.Modeled static stress distribution predict compounding stress concentrations at tip-to-tip RIZs, favoring brittle strain localization and rift coalescence, and stress relaxation at overlapping oblique RIZs, favoring stalled rift coalescence. We argue that RIZ geometry strongly influences the pace of rift coalescence by modulating the spatial distribution of tectonic stresses necessary to promote riftlinking deformation and the associated surface processes.

Crustal softening at propagating rift tips, East Africa

“…Localization of mechanically weakened crust at active rift tips reflects a critical process facilitating the growth of continental rift systems analogous to microfracture propagation driven by high-stress concentrations at the crack tips (e.g., Kranz, 1979;Olson, 2004). Similarly, relatively large stress concentrations between interacting microcrack tips (Kranz, 1979) agrees with interpretation of stress concentrations within rift interaction zones that separate propagating rift tips (Kolawole et al, 2023).…”

“…Published models for rift linkage demonstrate that rift basins can propagate laterally and interact when in proximity (e.g., Allken et al, 2012;Corti, 2012;Molnar et al, 2019;Nelson et al, 1992;Zwaan et al, 2016;Zwaan and Schreurs, 2020;Neuharth et al, 2021;Kolawole et al, 2021a). Studies of natural rifts also show that inherited crustal mechanical barriers, nonoptimal rift interaction zone geometry, or unfavorable tectonic stress distribution can stall lateral rift propagation (e.g., van Wijk and Blackman, 2005;Kolawole et al, 2022Kolawole et al, , 2023Shaban et al, 2023). Numerical models show that the stalling of a laterally propagating rift tip creates a local stress concentration zone (van Wijk and Blackman, 2005;Le Pourhiet et al, 2018).…”