The forces required to initiate rifting in cratonic plates far exceed the available tectonic forces. High temperatures and resultant melts can weaken the lithosphere, but these factors do not readily explain the extension of old and strong lithosphere in magma-poor rifts, such as the Malawi Rift. Here, new seismic converted-wave imaging shows that even in this magma-poor rift, upper-crustal rift basins are associated with localized preferential thinning of the lithospheric mantle. We calculated the beta factor, the ratio between current and prerift thickness, beneath the rift axis and found crustal beta of 1.7 ± 0.3 and lithospheric-mantle beta of 3.8 ± 1.7. Purely mechanical stretching cannot explain the preferential lithospheric mantle thinning—instead, thinning of the rheological lithosphere was probably augmented by thermochemical rejuvenation and erosion. Although local surface-wave-derived shear-wave velocities preclude a substantially elevated temperature and partial melt today, fusible materials preserved in the lower lithosphere that underlie the Ubendian Belt and its bounding subduction-related sutures in which the Malawi Rift nucleated may have provided an early supply of melt that enabled localized lithospheric alteration and/or removal. A plume-related or other asthenospheric perturbation would preferentially melt the more fusible lithospheric materials and the rising melts would heat and weaken progressively shallower parts of the lithosphere, which spatially localizes weakening (hence the lithospheric-mantle thinning) and enables the onset of rifting.
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We thank R. Marzen for calculating partial melt fractions and C. Ebinger for her comments on this manuscript and her contributions to the SEGMeNT project in general. Funding for this work was provided by the NSF Continental Dynamics Program, with grants EAR-1110921, 1109293 and 1110882.
The authors declare no competing interests.
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Cross sections are through the mean bootstrapped 1–100 s Ps common-conversion-point stack, masked by standard deviation between bootstraps (second colour bar) or fewer than 3 contributing waveforms (dark grey). 2x vertical exaggeration. Dashed black lines: Moho, picked from this Ps stack. Grey dashed lines: uncertainty on Moho (± 3 km). Black open symbols correspond to crustal estimates from other studies: circles27; diamonds55; triangle53; inverted triangle29. Where symbol is missing error bars, the reported uncertainty was smaller than the symbol. Red circles correspond to those on the location map (Fig. 1b). Shaded boxes correspond to terranes (Fig. 1a). Solid black line: topography at 10x v.e., blue across Lake Malawi.
Opacity scaled by standard deviation (as in Figs. 2, 3, S1). Plotted thicknesses are within 0.25° of a converted-wave pick, save crustal thickness beneath Lake Malawi. Grey lines: cross section locations (Fig. 2, S1). Blue line: Lake Malawi. a) Crustal thickness: difference between the topography (incorporating sedimentary basins in Lake Malawi25) and the Moho PVG. Crustal thickness beneath Lake Malawi is not masked as extrapolated crustal thicknesses are compatible with active source inversions55. b) Lithospheric thickness: difference between the topography and the LAB NVG. c) Lithospheric mantle thickness: difference between the Moho and the LAB. The mask is the maximum of the previous two panels.
Extended Data Fig. 3 Shear velocity, geotherm and yield strength envelope demonstrating the effect of conductive heating alone.
a) Geotherm reflects conditions soon after a large (500 °C) asthenospheric temperature anomaly perturbs the previously equilibrated geotherm. b) Same panels after 60 Ma of thermal evolution. This timescale is longer than the total age of the East African Rift System. Despite the large temperature anomaly and long timescale allowed for conduction, the lithosphere does not substantially weaken due to conductive heating alone.
Integrated partial melt is displayed as total kilometres of igneous material added to the crust. Dependence on asthenospheric potential temperature and crustal beta factor are shown. In this calculation, lithospheric mantle thinning is assumed to be twice the magnitude of crustal thinning, based on our observations of present-day thinning across the Malawi Rift. Accardo et al. 14 estimate uppermost mantle temperatures most consistent with the cooler end of the range of potential temperatures shown here.
(a) Sp stack. (b) Ps stack. Solid black lines with red dots correspond to cross section locations (Figs. 2, S1). Blue line: Lake Malawi.
LAB depths are calculated from the offset in seconds between the Sp and the primary S phases. The effect of Moho depth and vP/vS ratio are shown here across substantially larger ranges than the expected difference from the values used. These calculations are for a relatively short offset (8.5 s), relevant to the areas of thinnest lithosphere upon which our conceptual model is based. Given a longer observed offset, the changes due to vP/vS ratio will be slightly larger (e.g. for 16 s, varying vP/vS between 1.7-1.85 for a 34 km Moho is 116-129 km). (a) Depth variation assuming crustal vP/vS constrained by Borrego et al.27 is correct, so vP/vS is varied only in the mantle. (b) Depth variation changing the vP/vS used across the whole depth range of the velocity model.
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Hopper, E., Gaherty, J.B., Shillington, D.J. et al. Preferential localized thinning of lithospheric mantle in the melt-poor Malawi Rift. Nat. Geosci. 13, 584–589 (2020). https://doi.org/10.1038/s41561-020-0609-y
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