Projet DORA : Dynamics of Rifting in Afar

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Coordinator : Cécile Doubre (EOST)

ISTerre Research teams involved :
– Seismic cycle and transient deformation

ISTerre contact  : A. Socquet

Collaborations :
– EOST, Strasbourg (C. Doubre, F. Masson, A. Deprez...)
– IPG Paris (R. Grandin, S. Dumont, E. Jacques, A. Nercessian, G. King)
– CRPG Nancy (R. Pik)
– Geosciences Montpellier (R. Cattin)
– Observatoire Géophysique d’Arta et le Centre de Recherches et d’Etudes Djiboutiennes, R. Djibouti (M. Jalludin)
– Institute of Geophysics, Space Sciences and Astronomy, Addis Abbeba, Ethiopia (A. Ayele, E. Lewi)
– Univ. Leeds (T. Wright)
– Afar Consortium
– UCLA-JPL, Los angeles, California (Gilles Peltzer)
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Abstract :

The Afar Depression is one of a few places in the world that offer the opportunity to study on land the dynamics of divergent plate boundaries. The DoRA project aims to conduct complementary studies in two volcano-tectonic rifts : the Wal’is Dabbahu Rift (WD, Ethiopia) and the Asal-Ghoubbet Rift (AG, Djibouti). In Northern Afar, the WD Rift is currently undergoing a major rifting episode. This event started in September 2005 with a significant seismic activity. InSAR data revealed the injection of a 65 km-long mega-dyke that opened by up to 8 m, the slip of numerous normal faults and opening of fissures, and a rhyolitic eruption. Similarly, the AG Rift was affected in 1978 by a smaller episode of rifting associated with the intrusion of a 2 m wide dyke into the crust. Since then, a large catalog of geodetic data that includes recent InSAR time series reveals the importance of non-steady deformation controlling the rift dynamics. Whereas deep, slow and aseismic fault slips are gaining the attention of a large number of scientists working mainly on subduction zones, we propose to study similar transients in great detail in these active rifts, where the crustal layer is thin.

Our goal is to gain an understanding of such volcano-tectonic segments on several time scales, including the dyking period itself and the post-event period. Essentially, the study of the behavior of the AG Rift during its whole post-rifting period offers an image at t+30 years of the WD segment, while keeping in mind important structural and scale differences.

To do so, this proposal is based on 4 tasks. First, we propose to build a complete and accurate set of geodetic data, including InSAR and field measurements (cGPS, GPS), covering the period under study. With a narrow temporal sample window, we plan to precisely describe the aseismic slip affecting the normal faults of these rifts, the periods of sudden slip and/or slip acceleration but also measure the deformation associated with probable future dyke intrusion in the WD Rift. Second, we aim to constrain the origin of these displacements and their relation with mass transfers within the crust. Series of gravity measurements will be pursue or initiated in both rifts. The combination of both gravity and geodetic data will help to discriminate density variations and vertical displacements. Third, the recording of seismic activity is essential to constrain the relative importance of seismic and aseismic deformation. This will also help to evaluate the thickness of the seismogenic layer. Together with structural data collected during a seismic survey in the AG Rift (complementary project), the results of these three first tasks will offer crucial constraints on modeling to shed light on the rifting dynamics, which corresponds to the fourth tack. Three kinds of mechanical analysis of the observed displacements will be conducted taking into account the knowledge of the structure to test the relative influences of the rheology, the fault/dyke geometry and fluids on the rupture mechanics. With a thermo-mechanical approach, we investigate the respective role of the processes controlling the current 3D deformation field of these rifts, including viscous relaxation, dyke intrusion/inflation and aseismic slip. These models will be completed by Stress Coulomb analysis in order to examine the interactions between dyke intrusions within the elastic crust and the creation and development of surface faulting. Finally, we plan to compare the dynamics and morphology of the slip distribution profile along normal faults with rupture of small-scale transparent disordered materials and with numerical models of faults to investigate the role asperities on the fault dynamics.

Our multidisciplinary approach should provide important new constraints on the dynamics of rifting along divergent plate boundaries, and ultimately, in other geodynamical contexts affected by aseismic fault slip transients.