Calais E., N. d'Oreye, J. Albaric, A. Deschamps, D. Delvaux, J. Deverchere, C. Ebinger, R. W. Ferdinand, F. Kervyn, A. S. Macheyeki, A. Oyen, J. Perrot, E. Saria, B. Smets, D. S. Stamps, C. Wauthier, Aseismic strain accommodation by slow slip and dyking in a youthful continental rift, East Africa, Nature, Vol 456, 11 December 2008, doi:10.1038/nature07478. [pdf, 0.8 Mb]

Press release (Purdue university)
Communiqué de presse (French Institut National des Sciences de l'Univers)

Continental rifts initiate and develop through repeated episodes of faulting and magmatism, yet 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. 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 prior to significant crustal thinning. In July-August 2007, a seismo-magmatic crisis in the Natron basin was accompanied by the first dyking event ever captured geodetically in a continental rift. The 5 My-old Natron basin lies near the southern termination of the Eastern branch of the East African Rift, the divergent boundary between the Somalia and Nubia plates. Alkaline and basaltic volcanism was coeval with initial surface faulting, yet strain accommodation by magma intrusion is virtually unconstrained. With current far-field extension rates on the order of 3 to 4 mm/yr, the Eastern rift has experienced only a few kilometers of crustal thinning, while lithospheric thickness is ~90 km relative to the more than 125 km thick lithosphere outside the rift. The Natron crisis therefore provides a rare opportunity to quantify how faulting and magma intrusion contribute to strain accommodation within a youthful continental rift.

Regional setting. Black circles in the Natron rift show location of M>3.8 earthquakes for the 7/16/07 to 8/20/07 interval. Red line shows location of the dyke intruded along the southern flank of Gelai volcano. White triangles show the seismic stations of the SEISMO'TANZ'07 temporary deployment. Background colour shows topography from Shuttle Radar Topography Mission (SRTM) data. Red arrow is GPS displacement at a geodetic benchmark at the southern end of Lake Natron, installed and observed for the first time in August 2006 and reobserved on August 4-7, 2007. MG = Magadi, LN = Lake Natron, MN = Lake Manyara, E = Lake Eyasi, G = Gelai, L = Ol Doinyo Lengai, K = Kilimanjaro. N = Ngorongoro crater. Inset shows study area along the East African Rift. Arrows show predicted extension rates (numbers in mm/yr) and directions across the main rift structures.

Field observations in early August and October showed metres to kilometres-long newly formed en echelon tension fractures along the southern flank of Gelai, delineating two main NNE-SSW strands separated by 2-3 km. Vertical displacements are consistently east-down for the western strand and west-down for the eastern strand, resulting in a narrow central graben. Displacements are larger along the eastern strand in its middle segment with an average of ~12 cm opening and ~35 cm vertical offset.

Volcanic activity at the Ol Doinyo Lengai carbonatitic volcano (background) accompanied the seismo-mamatic crisis, while girafes stroll through the central part of the Natron rift (photo B. Smets).

Radar interferogram superimposed on a GoogleEarth view of the study area. Each fringe on the interferogram represents ~3 cm of ground displacement. The interferogram spanns July 17, 2007 to August 21, 2007.

Left: July 17 - August 21 interferogram showing deformation-related fringes covering an area of about 50x0 km centred on the southern flank of Gelai. The two-lobe fringe pattern indicates up to 45 cm of negative range change (ground moving toward the satellite) in the eastern half of the scene and 17 cm of positive range change in the western half. Right: Opening distribution along the dyke and slip distribution on the shallow normal faults bounding the central graben estimated from the inversion of the interferogram and GPS data.