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Preferential localized thinning of lithospheric mantle in the melt-poor Malawi Rift

Nature Geoscience volume 13, pages 584–589 (2020)Cite this article

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Abstract

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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Fig. 1: Maps of the study region.
Fig. 2: Lithospheric structure beneath Lake Malawi.
Fig. 3: Stretching factor, beta (original thickness/current thickness).
Fig. 4: Possible evolution of the Malawi Rift.

Data availability

The seismic data were obtained from the Incorporated Research Institutions for Seismology Data Management Center. We used data from the SEGMeNT (https://doi.org/10.7914/SN/YQ_2013), SAFARI (https://doi.org/10.7914/SN/XK_2012), and Africa Array stations (https://doi.org/10.7914/SN/AF) networks.

Code availability

RF codes (written in MATLAB) are freely available from the corresponding author.

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Acknowledgements

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.

Author information

Authors and Affiliations

  1. Lamont-Doherty Earth Observatory, Palisades, NY, USA

    Emily Hopper, Natalie J. Accardo, Benjamin K. Holtzman & Christopher Havlin

  2. Northern Arizona University, Flagstaff, AZ, USA

    James B. Gaherty & Donna J. Shillington

  3. Pennsylvania State University, State College, PA, USA

    Andrew A. Nyblade

  4. Syracuse University, Syracuse, NY, USA

    Christopher A. Scholz

  5. Geological Survey of Malawi, Zomba, Malawi

    Patrick R. N. Chindandali

  6. University of Dar es Salaam, Dar es Salaam, Tanzania

    Richard W. Ferdinand & Gabriel D. Mulibo

  7. Geological Survey of Tanzania, Dodoma, Tanzania

    Gabriel Mbogoni

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  1. Emily Hopper
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Contributions

E.H. processed and analysed the data and wrote the bulk of the manuscript. J.B.G. and D.J.S. oversaw the project and were instrumental in developing the conceptual model. N.J.A. carried out the surface wave analysis integral to understanding these results and contributed to the manuscript. A.A.N. contributed to discussions and editing of the manuscript. B.K.H. and C.H. provided access to the Very Broadband Rheology code and were involved in discussions on its application, as well as contributing to the broader conceptions of the role of melt discussed in the manuscript. J.B.G., D.J.S., N.J.A., A.A.N., C.A.S., P.R.N.C., R.W.F., G.D.M. and G.M. were heavily involved in collecting the SEGMeNT dataset and discussions to place these results in a broader context.

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Correspondence to Emily Hopper.

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Extended data

Extended Data Fig. 1 Crustal structure beneath Lake Malawi.

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.

Extended Data Fig. 2 Layer thicknesses used in calculating beta.

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.

Extended Data Fig. 4 Partial melt generated by lithospheric thinning.

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.

Extended Data Fig. 5 Data coverage maps for common-conversion-point stacks.

(a) Sp stack. (b) Ps stack. Solid black lines with red dots correspond to cross section locations (Figs. 2, S1). Blue line: Lake Malawi.

Extended Data Fig. 6 Dependence of calculated LAB depth on migration model.

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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