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doi:10.1029/2003JB0022741, 2004. Yang, Y., and M. Liu, Deformation of convergent plates: evidence from discrepancies between GPS VIGNY ET AL.: ARABIA PLATE MOTION 16 velocities and rigid-plate motions, Geophys. Res. Lett. , 29 (10), 10.1029, 2002.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL ?, 2006, DOI 25 years of geodetic measurements along the Tadjoura-Asal rift system, Djibouti, East Africa. Christophe Vigny 1 , Jean-Bernard de Chabalier 2 , Jean-Claude Ruegg 2 , Philippe Huchon 3 , Kurt L. Feigl 4 , Rodolphe Cattin 1 , Laike Asfaw 5 , Khaled Kanbari 6 . Abstract. Since most of Tadjoura-Asal rift system sits on dry land in the Afar depression near the triple junction between the Arabia, Somalia and Nubia plates, it is an ideal natural laboratory for studying rifting processes. We analyze these processes in light of a time series of geodetic measurements from 1978 through 2003. The surveys used triangulation (1973), trilateration (1973, 1979 and 1981 to 1986), leveling (1973, 1979, 1984-1985, and 2000), and the Global Positioning System (GPS, in 1991, 1993, 1995, 1997, 1999, 2001 and 2003). A network of about 30 GPS sites covers the Republic of Djibouti. Additional points were also measured in Yemen and Ethiopia. Stations lying in the Danakil block have almost the same velocity than Arabian plate indicating that the opening of the Red Sea at its southern tip is almost totally accommodated in the Afar depression. Inside Djibouti, the Asal- Ghoubbet rift system accommodates 16±1 mm/yr of opening perpendicular to the rift axis, and exhibits a pronounced asymmetry with essentially null deformation on its southwestern side and significant deformation on its northeastern side. This rate, slightly higher than the large-scale Arabia-Somalia motion (13±1 mm/yr), suggests transient variations associated to the relaxing processes after the Asal-Ghoubbet seismo-volcanic sequence of 1978. Inside the rift, the deformation pattern exhibits clear 2-D pattern. Along the rift axis, the rate is decreasing to the northwest, suggesting propagation of the system in the same direction. Perpendicular to the rift axis, the localization of the opening is clearly shifted to the northeast, relatively to the topographic rift axis, in the “Petit Rift”, a rift-in-rift structure, where is concentrated most of the active faults and the present seismicity. Vertical motions (comparisons of leveling profiles) also confirm this asymmetry, with a current bulge of the northeastern shoulder. Finally, all sites inside the rift system show varying vertical motions from 0 to 10 mm/yr, but always positive, indicating that although the internal floor is subsiding relative to the rift shoulders, the whole system is moving upward. Introduction The Afar depression, at the triple junction between Arabia, Somalia, and Nubia, is actively deforming by continental stretching, rifting and volcanism. Here the three extensional structures of the Sheba Ridge, the Red Sea Ridge and the East African Rift join in a complicated geometry. Both the Red Sea Ridge and Sheba Ridge have been propagating for the last 30 Myr, toward the south andwest, respectively. Yet they penetrate into the Afar depression, rather than connecting directly through the Straits of Bab el Mandeb. Consequently, the recent tectonic action there focuses around a set of disconnected, but . . 1 Laboratoire de Géologie, Ecole Normale Supérieure (ENS), CNRS, Paris, France 2 Institut de Physique du Globe (IPGP), Paris, France 3 Laboratoire de Tectonique, Université Pierre et Marie Curie-Paris6 & Institut Océanographique, Paris, France 4 Centre National de Recherche Scientifique (CNRS) Toulouse, France 5 Addis Observatory, Department of Geophysics, Addis Ababa University, Addis Ababa, Ethiopia 6 Department of Geosciences, University of Sana’a, Sana'a, Yemen 1 overlapping, propagating rift segments, that have created a complex network of normal faults (e.g., Huchon et al. [1991]; Manighetti et al. [1997]). To better understand the kinematics and the processes taking place in this area, we use geodetic measurements which characterize the deformation at three scales defined by different geophysical objects: the plates (distances ~ 1000 km), tectonic regions (between ~1000 km and ~10 km), and rift segments (~10 km). At the scale of the plates, the existing long-term kinematic models disagree markedly. The conventional NUVEL-1A model considers Africa as a single plate to predict divergence between Africa and Arabia at a rate of about 16mm/yr and N28°E in azimuth at the southern tip of the Red-Sea [Demets et al., 1994]. Separating Africa into two plates, Somalia and Nubia, Jestin et al. [1994] propose a similar relative velocity: 17 mm/yr at azimuth N30°E. Yet the rate of Arabia-Somalia motion varies along the Sheba Ridge from 22 mm/yr at the horn of Africa to 17mm/yr at the entrance of the Gulf of Tadjoura [DeMets et al., 1994]. For the East African Rift (EAR), that splits Africa into Somalia and Nubia, estimates of the divergence rate also vary considerably: 1 mm/yr [Asfaw et al., 1992], 5 mm/yr [Jestin et al., 1994], or 6 mm/yr [Chu and Gordon, 1999]. In contrast, recent GPS measurements in this area clearly differ from plate kinematic predictions. Different
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Thèse en vue d'obtenir le diplôme
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Résumé Depuis une quinzaine d'ann
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écepteur GPS devient un sismograph
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au diamètre de l'enveloppe du nuag
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Figure 3. répétabilités (c'est
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Les domaines d’application a) con
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cours desquels la faille va moins v
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- Y a-t-il un régime permanent ? o
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temps en temps (pour refaire régul
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Conclusion Pour tous les objectifs
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Les vitesses calculées sur la pér
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A cause des progrès des mesures en
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Il est probable que l'on va bientô
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attendre longtemps pour que ce mouv
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Les réseaux GPS en Birmanie et les
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Chargement élastique sur la faille
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à l'échelle de toute la Birmanie
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vitesses observées et modélisées
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Le séisme de Sumatra (Banda Aceh)
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la composante Nord-Sud du mouvement
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Propagation de la rupture et des on
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La subduction Chilienne Contexte :
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La modélisation (dislocation en "b
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Mesures dans la lacune de Coquimbo
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Là encore, nous avons montré qu
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Erosion: GPS cinématique sur un pl
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Christophe VIGNY Né le 02 Mars 196
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d) j’ai été pendant 4 ans charg
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18. Instantaneous and finite kinema
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C] Publications dans des revues à
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Michel, G., Y. Yu, S. Zhu, C. Reigb
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X - 2 SIMONS ET AL.: GPS MEASUREMEN
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1 JOURNAL OF GEOPHYSICAL RESEARCH,
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3 SOCQUET ET AL.: GPS MEASUREMENTS
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