Stephen S. Gao, PhD , Missouri University of Science and Technology
Mohamed G. Abdelsalam, PhD , Missouri University of Science and Technology
Crustal deformation of East Central Block in the Afar Depression of Ethiopia and Djibouti was measured using the Envisat satellite, Advanced Synthetic Apeture Radar (ASAR), C-Band data to create ascending and descending interferograms. This study examined ASAR data collected over different time sequences since 2002-2010 of the horsts and grabens of the East Central Block, which is hypothesized to be the location of the Nubian-Arabian plate boundary in the Afar Depression. Strain localization is expected to be manifested by a high phase change. For the East Central Block, 2 Pass + DEM interferometry was utilized to create 22, cm scale phase displacement maps, from 21 ASAR scences. In addition, mean surface velocity maps in cm/yr was create using 3-Pass + DEM interferometry for ASAR data from 2007, 2008, and 2009. Only master and slave pairs that had suitable perpendicular baselines (>500 m) were processed.
The Afar Depression hosts a rift-rift-rift triple junction where the Arabian, Somalian and Nubian plates are separating from one another (Beyene and Abdelsalam, 2005) The Afar is the only location in the world where two oceanic rifts step onto land (the Red Sea and Gulf of Aden) and meet with a continental rift (the Main Ethiopian Rift). The Red Sea and Gulf of Aden trends within Afar form an overlapping rift system represented by the SE-propagating Manda Hararo-Gobaad, (the Red Sea propagator) and the NW-propagating Asal-Manda Inakir rifts (the Gulf of Aden propagator) (Acocella et al., 2008). The Red Sea and Gulf of Aden propagators enclose an overlap zone, called the East Central Block. The way strain is partioned within the East Central Block is not certain and there are 5 main grabens (Dobe, Data-Yager-Hanle, Der'Ela-Gaggade, North Balho and the Immino) opening within the East Central Block. A bookshelf model is proposed in which the East Central Block is rotating clockwise (14 degrees in the last 3 Ma) due to differential extension in the Red Sea and Gulf of Aden propagators (Kidane et al., 2003 and Tapponnier et al., 1990). Such rotation is accompanied by clockwise rotation of micro-blocks (Gammari-Dakka, Unda-Gammari, Isso-Dedai, Siyarrou and East of GAP) separated by the dominantly NW-trending grabens.
Our preliminary results (Figure 1) show that about 2 cm of vertical Displacement can be seen on the northeastern border fault of the Dobe Graben and the Data Yager Hanle Border fault in an ASAR pair collected from 10/12/2007 and 10/31/2008. In general, high phase change in the vertical displacement maps is concentrated in the Data-Yager Hanle-Dobe, Der Ela' Gaggade, North Bahlo grabens and the southeastern Siyarrou Block. The eastern and southeastern portions of the East Central Block are likely undergoing the most crustal deformation. The Arabian-Nubian plate boundary in the Afar will likely follow a deformation pathway in the southeastern ECB. InSAR measurements from 2007-2010, have a maximum of ~5-6 cm/yr with 2 Pass DInSAR (2005-2010). A mean surface velocity map collected from 3 ASAR scenes from 2007, 2008 and 2009 shows that the vertical deformation in the East Central Block is ranges from -4.2 to 4.2 cm/yr.
Figure 1. Phase Displacement Map: 10/12/2007-10/31/2008 ASAR Scenes with horst & grabens of East Central Block
Acocella, V., Abebe, B., Korme, T., Barberi, F. 2008. Strcture of Tendahao Graben and Manda Hararo Rift: Impliacations for the evolution of the southern Red Sea propagator in Central Afar. Tectonics, 27, doi: 10.1029/2007TC002236.
Beyene, A.M., Abdelsalam, M.G. 2005. Tectonics of the Afar Depression: A review and synthesis. Journal of African Earth Sciences, 41, 41-59.
Kidane, T., Courtillot, V., Manighetti, I., Audin, L., Lahitte, P., Quidelleur, X., Gillot, P-Y., Gallet, Y., Carlut, J., Haile, T. 2003. New paleomagnetic and geochronologic results from Ethiopian Afar: Block rotations linked to rift overlap and propagation and determination of ~ 2 Ma reference pole for stable Africa. Journal of Geophysical Research, 108, 1-32.
Tapponnier, P., Armijo, R., Manighetti, I., Courtillot, V. 1990. Bookshelf faulting and horizontal block rotations between overlapping rifts in southern Afar. Geophysical Research Letters, 17, 1-4.