Kinematics of the Ethiopian Rift and motion of Africa and Somalia Plates relative to the mantle 1 2 1 Ameha A. Muluneh (
[email protected] ) , Marco Cuffaro , Carlo Doglioni
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2
Sapienza-University of Rome, Italy; CNR-IGAG, Rome,Italy
[AGU F allM eeting, SanF rancisco, Calif ornia, 2013.P osterN o.T21A-2516]
Abstract
2-Kinematics and Deformation
In the Ethiopian Rift, GPS velocities in the ITRF and regional studies show an eastward magnitude increase in the ENE direction. In the same region, extensional directions derived from earthquake focal mechanisms and fault kinematic studies rather show ◦ N100 E orientation. The deviation between extension direction and velocity field can be explained by the left-lateral transtensional tectonics. Transferring the plate kinematics relative to deep or shallow hotspot reference frames, the rift can be generated either by the WNW-ward or WSW-ward faster motion of Africa relative to Somalia. The second option is more consistent with the left-lateral transtension and several geophysical signatures in the region. The rift and the faster motion of Africa to the "west" can possibly be explained by the occurrence of an underlying lower viscosity low velocity zone (LVZ) beneath Africa relative to the LVZ at the base of the Somalia plate.
Kinematic and modeling studies show that Ethiopian Rift is characterized by oblique kinematics. Polyphase deformation model indicates a rotation of stress field in the region. Plate kinematics and analogue modeling studies argue that the stress field remained constant and all the observed kinematics is a result of oblique reactivation of pre-existing weak zone.
1-Introduction The Ethiopian Rift (ER), in the northern part of East African Rift, accommodates differential motion between Africa and Somalia plates.
Extension directions
Deformation type
Strain accommodation
Deviation between GPS velocity field and extension direction indicates that the rift is characterized by left-lateral transtensional deformation.
Recent GPS study shows strain in the Ethiopian Rift is widely distributed, outside the structural rift valley. 39˚
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T-axes orientations (left) and GPS velocity (right)
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3-Mantle reference frames Depending on the source depth of the hotspot, mantle referenced plate motion can be classified into Deep and Shallow.
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Extension directions are oriented N100 E, in the same region GPS velocities are oriented ENE direction with eastward magnitude increase. Extension axis trajectories diverge by about 40◦ from the velocity field and has a component of boundary normal and rift parallel shear.
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GPS focal mechanism structural data
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Deviation between extension directions and GPS velocity fields due to left-lateral transtensional deformation
4-Ethiopian Rift kinematics
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Seismic and geodetic evidence for sinistral shearing
In addition to magmatism and dyke intrusion, strain in the rift is accommodated by left-lateral shearing, 8˚ small amount of vertical axis block rotation. The absence of oblique slip focal mechanisms indicate 39˚ 40˚ that strain is accommodated on separate normal and sinistral faults. The strike-slip faults could be hidden in the basin sediments in the rift floor.
5-Discussion and Conclusions
A-Depth distribution of deep and shallow hotspots; B-Decoupling of the lithosphere from asthenosphere and missing shear recorded in shallow frame Location of the Ethiopian Rift. The inset shows topography of Africa and Somalia plates
Several studies show the influence of inherited Precambrian lithospheric weak zone on the location, geometry of faulting and deformation type along the Ethiopian Rift. Plate kinematic studies indicate that the separation of Africa and Somalia plates is accommodated by N-S oriented rift floor faults or Tectono-Magmatic Segments (TMS) in the Ethiopian Rift latitude. On the other hand, regional geodetic survey proves that deformation is widely distributed in the region outside the structural rift valley. In this study, we present the kinematics and deformation style of the Ethiopian Rift derived from compilation of geodetic velocities, focal mechanism and fault kinematic inversion and structural data analysis. The observed kinematics in the Ethiopian Rift is explained by the motion of Africa and Somalia plates relative to both deep and shallow mantle reference frames.
In both reference frames, Africa and Somalia plates move westward, being Africa faster than Somalia. Mantle reference frame highlights decoupling between the lithosphere and asthenosphere. −20˚
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Arabian Plate 20˚
A-deep; B-shallow reference frames- velocities of ER (blue arrows), Africa and Somalia (black arrows). Motion of ER is calculated by Vr = (Vaf + Vso)/2
The SW-ward motion implies NE-ward counterflow of mantle as shown by geophysical studies (e.g. shear wave splitting analysis).
Africa Plate 10˚
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crust Lithos pheric Lithospmantle Low V heric m elocity antle Z o n e Asthen (LVZ) osphe re
3D-representation of SW-ward motion of Africa and Somalia and its kinematic consequences
The NE-ward flowing asthenosphere obliquely rises beneath the already sheared and weakened lithosphere forming tectono-magmatic segments.
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depth (km)
Deviation between GPS velocity field and extensional strain axes show that Ethiopian Rift is characterized by left-lateral transtensional deformation. This interpretation is supported by en-echelon tensional and transtensional faults in the rift floor. We explain the observed kinematics due to the faster SW-ward motion of Africa than Somalia plates in the Ethiopian Rift latitude. This motion vector is oriented oblique to the strike of Ethiopian Rift, resulting left-lateral transtension.
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References
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[1] Muluneh, A. , Cuffaro, M. & Doglioni, C. submitted, 2013 −20˚
[2] Keir, D., Ebinger, C., Stuart, G.W., Daly, E. & Ayele, A. JGR, 111, 2006.
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[6] Cuffaro M. & Doglioni, C. GSA, 9, 359-374, 2007. P wave
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Motion of Africa and Somalia plates in shallow reference frame. Somalia w.r.t hotspot is calculated SoΩHs =So ωAf +Af ΩHs
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NE flow of mantle (red arrows), SKS-splitting orientation (thin red bars) and P-wave velocity anomaly at 300 km depth.
[7] Kogan, L., Fisseha, S., Bendick, R., Reilinger, R., McClusky, S. King, R. & Solomon, T., JGR, 117, 2012. [8] Gao, S., Liu, K.H. & Abdelselam, M.G., JGR, 115, 2010. [9] Bagley, B. & Nyblade, A.A. GRL, 40,1-6, 2013
