Rapid Versus Delayed Linkage and Coalescence of Propagating Rift Tips

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 rift-linking deformation and the associated surface processes.