1. Purdue University, Department of Earth and Atmospheric Sciences, West Lafayette, Indiana 47906, USA
2. National Museum of Natural History, Department of Geophysics/Astrophysics, L-2160 Luxembourg
3. Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, Plouzané 29280, France
4. Géosciences Azur, UNS/CNRS, Valbonne 06560, France
5. Royal Museum for Central Africa, Tervuren 3080, Belgium
6. University of Rochester, Department of Earth and Environmental Sciences, New York, New York 14627, USA
7. University of Dar es Salaam, Department of Geology, PO Box 35091, Dar es Salaam, Tanzania
8. Renard Centre of Marine Geology, Department of Geology and Soil Science, Universteit Gent, Gent 9000, Belgium
9. Mineral Resources (Madini) Institute, PO Box 903, Dodoma, Tanzania
10. Delft University of Technology, Delft 2600, The Netherlands
11. Ardhi University, Department of Geomatics, PO Box 35176, Dar es Salaam, Tanzania
12. University of Liège, Liège 4000, Belgium

Correspondence to: Eric Calais¹ Correspondence and requests for materials should be addressed to E.C. (Email: ecalais@purdue.edu).

Abstract

Continental rifts begin and develop through repeated episodes of faulting and magmatism, but strain partitioning between faulting and magmatism during discrete rifting episodes remains poorly documented. In highly evolved rifts, tensile stresses from far-field plate motions accumulate over decades before being released during relatively short time intervals by faulting and magmatic intrusions^{1, 2, 3}. These rifting crises are rarely observed in thick lithosphere during the initial stages of rifting. Here we show that most of the strain during the July–August 2007 seismic crisis in the weakly extended Natron rift, Tanzania, was released aseismically. Deformation was achieved by slow slip on a normal fault that promoted subsequent dyke intrusion by stress unclamping. This event provides compelling evidence for strain accommodation by magma intrusion, in addition to slip along normal faults, during the initial stages of continental rifting and before significant crustal thinning.

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