Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

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

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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

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.

References

  1. 1.

    Forsyth, D. & Uyeda, S. On the relative importance of the driving forces of plate motion. Geophys. J. Int. 43, 163–200 (1975).

    Google Scholar 

  2. 2.

    Buck, W. R. in Rheology and Deformation of the Lithosphere at Continental Margins (eds Karner, G. D. et al.) 1–30 (Columbia Univ. Press, 2004).

  3. 3.

    Versfelt, J. & Rosendahl, B. R. Relationships between pre-rift structure and rift architecture in Lakes Tanganyika and Malawi, East Africa. Nature 337, 354–357 (1989).

    Google Scholar 

  4. 4.

    Celli, N. L., Lebedev, S., Schaeffer, A. J. & Gaina, C. African cratonic lithosphere carved by mantle plumes. Nat. Commun. 11, 92 (2020).

    Google Scholar 

  5. 5.

    Wang, H., van Hunen, J. & Pearson, D. G. The thinning of subcontinental lithosphere: the roles of plume impact and metasomatic weakening. Geochem. Geophys. Geosyst. 16, 1156–1171 (2015).

    Google Scholar 

  6. 6.

    Rooney, T. O. The Cenozoic magmatism of East Africa: part V—magma sources and processes in the East African Rift. Lithos 360–361, 105296 (2020).

    Google Scholar 

  7. 7.

    Furman, T. Geochemistry of East African Rift basalts: an overview. J. Afr. Earth Sci. 48, 147–160 (2007).

    Google Scholar 

  8. 8.

    Bastow, I. D., Nyblade, A. A., Stuart, G. W. & Rooney, T. O. Upper mantle seismic structure beneath the Ethiopian hot spot: rifting at the edge of the African low-velocity anomaly. Geochem. Geophys. Geosyst. 9, Q12022 (2008).

    Google Scholar 

  9. 9.

    Chorowicz, J. The East African rift system. J. Afr. Earth Sci. 43, 379–410 (2005).

    Google Scholar 

  10. 10.

    Hanson, R. E. in Proterozoic East Gondwana: Supercontinent Assembly and Breakup Special Publications 206 (eds Yoshida, M. et al.) 427–463 (The Geological Society, 2003).

  11. 11.

    Ebinger, C. J., Jackson, J. A., Foster, A. N. & Hayward, N. J. Extensional basin geometry and the elastic lithosphere. Philos. Trans. R. Soc. Lond. A 357, 741–765 (1999).

    Google Scholar 

  12. 12.

    Craig, T. J., Jackson, J. A., Priestley, K. & Mckenzie, D. Earthquake distribution patterns in Africa: their relationship to variations in lithospheric and geological structure, and their rheological implications. Geophys. J. Int. 185, 403–434 (2011).

    Google Scholar 

  13. 13.

    O’Donnell, J. P., Adams, A., Nyblade, A. A., Mulibo, G. D. & Tugume, F. The uppermost mantle shear wave velocity structure of eastern Africa from Rayleigh wave tomography: constraints on rift evolution. Geophys. J. Int. 194, 961–978 (2013).

    Google Scholar 

  14. 14.

    Accardo, N. J. et al. Thermo-chemical modification of the upper mantle beneath the Northern Malawi Rift constrained from shear velocity imaging. Geochem. Geophys. Geosyst. 21, e2019GC008843 (2020).

    Google Scholar 

  15. 15.

    Grijalva, A. et al. Seismic evidence for plume- and craton-influenced upper mantle structure beneath the Northern Malawi Rift and the Rungwe Volcanic Province, East Africa. Geochem. Geophys. Geosyst. 19, 3980–3994 (2018).

    Google Scholar 

  16. 16.

    Sarafian, E. et al. Imaging Precambrian lithospheric structure in Zambia using electromagnetic methods. Gondwana Res. 54, 38–49 (2018).

    Google Scholar 

  17. 17.

    Lavayssiere, A. et al. Imaging lithospheric discontinuities beneath the northern East African Rift using S-to-P receiver functions. Geochem. Geophys. Geosyst. 19, 4048–4062 (2018).

    Google Scholar 

  18. 18.

    Ebinger, C. J. & Scholz, C. A. in Tectonics of Sedimentary Basins: Recent Advances (eds Busby, C. & Azor, A.) 185–208 (Wiley-Blackwell, 2012).

  19. 19.

    Roberts, E. M. et al. Initiation of the Western Branch of the East African Rift coeval with the eastern branch. Nat. Geosci. 5, 289–294 (2012).

    Google Scholar 

  20. 20.

    Stamps, D. S., Saria, E. & Kreemer, C. A geodetic strain rate model for the East African Rift system. Sci. Rep. 8, 732 (2018).

    Google Scholar 

  21. 21.

    Scholz, C. A. et al. Intrarift fault fabric, segmentation, and basin evolution of the Lake Malawi (Nyasa) Rift, East Africa. Geosphere (in the press).

  22. 22.

    Shillington, D. J. et al. Acquisition of a unique onshore/offshore geophysical and geochemical dataset in the Northern Malawi (Nyasa) Rift. Seismol. Res. Lett. 87, 1406–1416 (2016).

    Google Scholar 

  23. 23.

    Fischer, K. M. in Treatise on Geophysics 2nd edn, Vol. 1 (ed. Schubert, G.) 587–612 (Elsevier, 2015).

  24. 24.

    Wölbern, I., Rümpker, G., Link, K. & Sodoudi, F. Melt infiltration of the lower lithosphere beneath the Tanzania craton and the Albertine rift inferred from S receiver functions. Geochem. Geophys. Geosyst. 13, Q0AK08 (2012).

    Google Scholar 

  25. 25.

    Accardo, N. J. et al. Constraints on rift basin structure and border fault growth in the Northern Malawi Rift from 3D seismic refraction imaging. J. Geophys. Res. Solid Earth 123, 10003–10025 (2018).

    Google Scholar 

  26. 26.

    Hopper, E. & Fischer, K. M. The changing face of the lithosphere–asthenosphere boundary: imaging continental scale patterns in upper mantle structure across the contiguous U.S. with Sp converted waves. Geochem. Geophys. Geosyst. 19, 2593–2614 (2018).

    Google Scholar 

  27. 27.

    Borrego, D. J. et al. Crustal structure surrounding the northern Malawi Rift and beneath the Rungwe Volcanic Province, East Africa. Geophys. J. Int. 215, 14140–1426 (2018).

    Google Scholar 

  28. 28.

    Njinju, E. A. et al. Lithospheric structure of the Malawi Rift: implications for magma-poor rifting processes. Tectonics 38, 3835–3853 (2019).

    Google Scholar 

  29. 29.

    Kachingwe, M., Nyblade, A. & Julià, J. Crustal structure of Precambrian terranes in the southern African subcontinent with implications for secular variation in crustal genesis. Geophys. J. Int. 202, 533–547 (2015).

    Google Scholar 

  30. 30.

    Hodgson, I. et al. Crustal structure at a young continental rift: a receiver function study from the Tanganyika Rift. Tectonics 36, 2806–2822 (2017).

    Google Scholar 

  31. 31.

    Davis, M. & Kusznir, N. in Rheology and Deformation of the Lithosphere (eds Karner, G. D. et al.) 92–137 (Columbia Univ. Press, 2004); https://doi.org/10.7312/karn12738-005

  32. 32.

    McKenzie, D. Some remarks on the development of sedimentary basins. Earth Planet. Sci. Lett. 40, 25–32 (1978).

    Google Scholar 

  33. 33.

    Stamps, D. S., Flesch, L. M., Calais, E. & Ghosh, A. Current kinematics and dynamics of Africa and the East African Rift System. J. Geophys. Res. Solid Earth 119, 5161–5186 (2014).

    Google Scholar 

  34. 34.

    Holtzman, B. K. Questions on the existence, persistence, and mechanical effects of a very small melt fraction in the asthenosphere. Geochem. Geophys. Geosyst. 17, 470–484 (2016).

    Google Scholar 

  35. 35.

    Takei, Y. Effects of partial melting on seismic velocity and attenuation: a new insight from experiments. Annu. Rev. Earth Planet. Sci. 45, 447–470 (2017).

    Google Scholar 

  36. 36.

    Schmeling, H. & Wallner, H. Magmatic lithospheric heating and weakening during continental rifting: a simple scaling law, a 2D thermomechanical rifting model and the East African Rift System. Geochem. Geophys. Geosyst. 13, Q08001 (2012).

    Google Scholar 

  37. 37.

    Phipps Morgan, J. Thermodynamics of pressure release melting of a veined plum pudding mantle. Geochem. Geophys. Geosyst. 2, 2000GC000049 (2001).

    Google Scholar 

  38. 38.

    Furman, T., Nelson, W. R. & Elkins-Tanton, L. T. Evolution of the East African rift: drip magmatism, lithospheric thinning and mafic volcanism. Geochim. Cosmochim. Acta 185, 418–434 (2016).

    Google Scholar 

  39. 39.

    Roy, M. et al. Macroscopic coupling of deformation and melt migration at continental interiors, with applications to the Colorado Plateau. J. Geophys. Res. Solid Earth 121, 3762–3781 (2016).

    Google Scholar 

  40. 40.

    Lenoir, J. L., Liegeois, J.-P., Theunissen, K. & Klerkx, J. The Palaeoproterozoic Ubendian shear belt in Tanzania: geochronology and structure. J. Afr. Earth Sci. 19, 169–184 (1995).

    Google Scholar 

  41. 41.

    Boniface, N. & Schenk, V. Neoproterozoic eclogites in the Paleoproterozoic Ubendian Belt of Tanzania: evidence for a pan-African suture between the Bangweulu Block and the Tanzania Craton. Precambrian Res. 208–211, 72–89 (2012).

    Google Scholar 

  42. 42.

    Yaxley, G. M. Experimental study of the phase and melting relations of homogeneous basalt + peridotite mixtures and implications for the petrogenesis of flood basalts. Contrib. Mineral. Petrol. 139, 326–338 (2000).

    Google Scholar 

  43. 43.

    Hacker, B. R. & Abers, G. A. Subduction Factory 3: an Excel worksheet and macro for calculating the densities, seismic wave speeds, and H2O contents of minerals and rocks at pressure and temperature. Geochem. Geophys. Geosyst. 5, Q01005 (2004).

    Google Scholar 

  44. 44.

    Wheeler, W. H. & Karson, J. A. Structure and kinematics of the Livingstone Mountains border fault zone, Nyasa (Malawi) Rift, southwestern Tanzania. J. Afr. Earth Sci. 8, 393–413 (1989).

    Google Scholar 

  45. 45.

    Mana, S., Furman, T., Turrin, B. D., Feigenson, M. D. & Swisher, C. C. III Magmatic activity across the East African North Tanzanian divergence zone. J. Geol. Soc. Lond. 172, 368–389 (2015).

    Google Scholar 

  46. 46.

    Hilton, D. R. et al. Helium isotopes at Rungwe Volcanic Province, Tanzania, and the origin of East African Plateaux. Geophys. Res. Lett. 38, L21304 (2011).

    Google Scholar 

  47. 47.

    Liao, J., Wang, Q., Gerya, T. & Ballmer, M. D. Modeling craton destruction by hydration-induced weakening of the upper mantle. J. Geophys. Res. Solid Earth 122, 7449–7466 (2017).

    Google Scholar 

  48. 48.

    Holtzman, B. K. & Kendall, J. M. Organized melt, seismic anisotropy, and plate boundary lubrication. Geochem. Geophys. Geosyst. 11, Q0AB06 (2010).

    Google Scholar 

  49. 49.

    Heron, P. J., Pysklywec, R. N. & Stephenson, R. in Fifty Years of the Wilson Cycle Concept in Plate Tectonics Special Publications 470 (eds Wilson, R. W. et al.), 137–155 (The Geological Society, 2018).

  50. 50.

    Holtzman, B. K. & Havlin, C. The Very Broadband Rheology Calculator (2020); https://vbr-calc.github.io/vbr/

  51. 51.

    Ford, H. A., Fischer, K. M., Abt, D. L., Rychert, C. A. & Elkins-Tanton, L. T. The lithosphere–asthenosphere boundary and cratonic lithospheric layering beneath Australia from Sp wave imaging. Earth Planet. Sci. Lett. 300, 299–310 (2010).

    Google Scholar 

  52. 52.

    Mancinelli, N. J. & Fischer, K. M. The spatial sensitivity of Sp converted waves–scattered wave kernels and their applications to receiver-function migration and inversion. Geophys. J. Int. 212, 1722–1735 (2017).

    Google Scholar 

  53. 53.

    Tugume, F., Nyblade, A. & Julia, J. Moho depths and Poisson’s ratios of Precambrian crust in East Africa: evidence for similarities in Archean and Proterozoic crustal structure. Earth Planet. Sci. Lett. 356, 73–81 (2012).

    Google Scholar 

  54. 54.

    Last, R. J., Nyblade, A. A., Langston, C. A. & Owens, T. J. Crustal structure of the East African Plateau from receiver functions and Rayleigh wave phase velocities. J. Geophys. Res. Solid Earth 102, 24469–24483 (1997).

    Google Scholar 

  55. 55.

    Shillington, D. J. et al. Intrabasin faults accommodate significant cumulative and recent extension in the early stage Malawi Rift, East Africa. In 2018 AGU Fall Meeting Abstract T13F-0267 (American Geophysical Union, 2018).

  56. 56.

    Kennett, B. L. N. The removal of free surface interactions from three-component seismograms. Geophys. J. Int. 104, 153–163 (1991).

    Google Scholar 

  57. 57.

    Bostock, M. G. Mantle stratigraphy and evolution of the Slave province. J. Geophys. Res. Solid Earth 103, 21183–21200 (1998).

    Google Scholar 

  58. 58.

    Hopper, E., Ford, H. A., Fischer, K. M., Lekic, V. & Fouch, M. J. The lithosphere-asthenosphere boundary and the tectonic and magmatic history of the northwestern United States. Earth Planet. Sci. Lett. 402, 69–81 (2014).

    Google Scholar 

  59. 59.

    Bostock, M. G. Seismic imaging of lithospheric discontinuities and continental evolution. Lithos 48, 1–16 (1999).

    Google Scholar 

  60. 60.

    Rychert, C. A., Rondenay, S. & Fischer, K. M. P-to-S and S-to-P imaging of a sharp lithosphere–asthenosphere boundary beneath eastern North America. J. Geophys. Res. 112, B08314 (2007).

    Google Scholar 

  61. 61.

    Helffrich, G. Extended-time multitaper frequency domain cross-correlation receiver-function estimation. Bull. Seismol. Soc. Am. 96, 344–347 (2006).

    Google Scholar 

  62. 62.

    Lekić, V., French, S. W. & Fischer, K. M. Lithospheric thinning beneath rifted regions of southern California. Science 334, 783–787 (2011).

    Google Scholar 

  63. 63.

    Zhu, L. & Kanamori, H. Moho depth variation in southern California. J. Geophys. Res. 105, 2969–2980 (2000).

    Google Scholar 

  64. 64.

    Tedla, G. E., van der Meijde, M., Nyblade, A. A. & Van der Meer, F. D. A crustal thickness map of Africa derived from a global gravity field model using Euler deconvolution. Geophys. J. Int. 187, 1–9 (2011).

    Google Scholar 

  65. 65.

    Priestley, K. & McKenzie, D. The thermal structure of the lithosphere from shear wave velocities. Earth Planet. Sci. Lett. 244, 285–301 (2006).

    Google Scholar 

  66. 66.

    Laske, G., Ma, Z., Masters, G. & Pasyanos, M. E. Crust 1.0: A New Global Crustal Model at 1×1 Degrees (2020); https://igppweb.ucsd.edu/~gabi/crust1.html

  67. 67.

    Havlin, C., Parmentier, E. M. & Hirth, G. Dike propagation driven by melt accumulation at the lithosphere–asthenosphere boundary. Earth Planet. Sci. Lett. 376, 20–28 (2013).

    Google Scholar 

  68. 68.

    Bellis, C. & Holtzman, B. K. Sensitivity of seismic measurements to frequency-dependent attenuation and upper mantle structure: an initial approach. J. Geophys. Res. Solid Earth 119, 5497–5517 (2014).

    Google Scholar 

  69. 69.

    Tokle, L., Hirth, G. & Behr, W. M. Flow laws and fabric transitions in wet quartzite. Earth Planet. Sci. Lett. 505, 152–161 (2019).

    Google Scholar 

  70. 70.

    Rybacki, E., Gottschalk, M., Wirth, R. & Dresen, G. Influence of water fugacity and activation volume on the flow properties of fine-grained anorthite aggregates. J. Geophys. Res. Solid Earth 111, B03203 (2006).

    Google Scholar 

  71. 71.

    Hirth, G. & Kohlstedt, D. L. Rheology of the upper mantle and the mantle wedge: a view from the experimentalists. Geophys. Monogr. 138, 83–105 (2003).

    Google Scholar 

  72. 72.

    Katz, R. F., Spiegelman, M. & Langmuir, C. H. A new parameterization of hydrous mantle melting. Geochem. Geophys. Geosyst. 4, 1073 (2003).

    Google Scholar 

  73. 73.

    Davis, J. K. & Lavier, L. L. Influences on the development of volcanic and magma-poor morphologies during passive continental rifting. Geosphere 13, 1524–1540 (2017).

    Google Scholar 

Download references

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

Affiliations

Authors

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.

Corresponding author

Correspondence to Emily Hopper.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Peer review information Primary Handling Editor: Stefan Lachowycz.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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

Download citation

Further reading

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing