Strain accommodation by slow slip and dyking in a youthful continental rift, East Africa

Abstract

International audienceContinental 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 intrusions1, 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

Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

“…The intense and persistent seismicity 8-10 km deep organized in an elliptical (map view) zone between Gelai and Oldoinyo Lengai volcanoes has been interpreted as a magma chamber and sill complex feeding the 2007 Lengai eruption (Baer et al 2008). But our new results indicate a much deeper magma body and feeding than was first inferred from models of InSAR (Calais et al 2008;Biggs et al 2009Biggs et al , 2013. The weaker correlation between high Vp/Vs ratio and disturbed Ps phase recorded in the axial rift valley may also result from a number of combined factors.…”

“…Recent seismicity studies (Weinstein et al, submitted), active CO 2 degassing along border faults (Lee et al 2016), and the 2007 dike intrusion (Calais et al 2008;Biggs et al 2009) attest to fluidfilled crustal cracks within the study area, that are likely able to produce anisotropic behaviour recordable by our network (Gerst & Savage 2004;Keir et al 2011;Hammond et al 2014). If some of our stations present backazimuthal dependency clearly imputable to anisotropic crustal structures (e.g.…”

“…1) is an active carbonatitic volcano lying in the rift valley near the tip of the Manyara border fault. A sequence of damaging earthquakes, dike intrusion, and eruption at Ol Doinyo Lengai volcano in 2007 documents ongoing activity (Baer et al 2008;Calais et al 2008). The source of the dike that intruded the flank of Gelai volcano in the central Natron basin was assumed to be from an active magma chamber beneath Gelai, but the youngest lava flows here have been dated at ∼1 My (Mana et al 2015;Muirhead et al 2015).…”

Magma sources involved in the 2002 Nyiragongo eruption, as inferred from an InSAR analysis

“…In contrast to the 2007 Natron basin seismomagmatic event [Baer et al, 2008;Calais et al, 2008;Biggs et al, 2009], field observations performed after the Nyiragongo eruption [Komorowski et al, 2002[Komorowski et al, /2003 showed no significant fault trace, with the exception of the two 2.5 kmlong 1/3 fringe discontinuities visible in the RADARSAT-1 descending interferogram in the Gisenyi area ( Figure 5 and section 6.3). Inversions considering a dike associated with the 2002 eruptive fissure and one to three westward-dipping normal faults located to the west of this dike led to a poor data fit with a RMS error of 3.5 cm (Table 2, column 3).…”

“…This has often been observed during dike intrusions, for instance in Afar in 2006 [Grandin et al, 2009;Keir et al, 2009], at the volcanic island of Izu in 2000 [Nishimura et al, 2001] or during the 2007 Natron faulting and diking event [Baer et al, 2008;Calais et al, 2008;Biggs et al, 2009]. Large earthquakes, such as the 3 February 2002 5.9 M w Bukavu event [d 'Oreye et al, 2011], or the October 24, 2002 6.2 M w south Lake Kivu event ( Figure S8 in the auxiliary material), might remain undetected by interferometry if they were to occur beneath Lake Kivu or in areas of low coherence.…”