US EPA

1773285 

Journal Article 

Geochemical evidence of lithospheric thinning in the southern Main Ethiopian Rift 

Rooney, TO 

2010 

1 

Lithos
ISSN: 0024-4937 

117 

1-4 

33-48 

10.1016/j.lithos.2010.02.002 

WOS:000278731800003 

Lithosphenc thinning is a fundamental process associated
with the transition from continental to oceanic regimes during continental rifting Precisely how
and when this lithospheric thinning proceeds are first order controls on rift basin evolution The
Main Ethiopian Rift, part of the similar to 2000 km long East African Rift System, is the
archetypical modern example of continental rifting. and a key location in which to study the
evolution of the lithosphere during extension. This study explores lithospheric modification in
the interface region between the Main Ethiopian rift and northward propagating rifting from Kenya
through a major and trace element study of rift initiation and maturation using the 19 myr
magmatic record preserved in this region Initial rifting in southern Ethiopia is coincident with
the eruption of basalts along the rift shoulders that are characterized by deep fractionation
trends (0.5 GPa) and poorly developed magmatic pathways The earliest of these basalts are derived
from melting columns where the aluminum phase is garnet-dominated (Tb-N/Yb-N similar to 1 8-2)
and has geochemical characteristics interpreted as melting of the lithospheric mantle. The
transition from initial rift shoulder magmatism to Quaternary magmatic-tectonic fault belts on
the modern rift floor at Arba Minch (6 degrees N) is coincident with a shallowing of the melting
column (Tb-N/Yb-N similar to 1 3-1 7). less significant contributions from the lithospheric
mantle, and the establishment of a shallow fractionation regime (0.1 GPa). At Chencha (similar to
6.3 degrees N) newly dated (12 32 +/- 0 17 Ma) magmatism on the rift shoulder has similar
fractionation paths to contemporaneous magmatism to the south (0 5 GPa), but is derived from a
different, shallower mantle source (Tb-N/Yb-N similar to 1 3-1.5) that we interpret results from
lithospheric thinning associated with the now-inactive Chow Bahir rift. Between 65 and 8 degrees
N, significant surface faulting and shallow magmatic fractionation paths (0 1 GPa) in the
dominant Quaternary structure of the Main Ethiopian Rift (the Wonji Fault Belt and Silti-Debre
Zeyit Fault Zone), highlights the strong connection between magmatism and extensional tectonics
in these structures. Along the eastern rift margin, Wonji Fault Belt magmas are derived from a
dominantly shallow melting column (Tb-N/Yb-N similar to 1.4-1 7) that is similar in composition
to the older rift shoulder lavas at Chencha Adjacent to the western rift margin, magmas erupted
in the Silti-Debre Zeyit Fault Zone are interpreted to have erupted through a thicker lithosphere
as these magmas are derived from a deeper melting column (Tb-N/Yb-N similar to 1 7-2 1) that
contains some minor apatite The inferred variations in lithospheric thickness in southern
Ethiopia outlined in this study illustrate the interaction between northward and southward rift
propagation in addition to lateral variations across the rift floor as extension migrates into
zones of focused magmatic intrusion The results of this investigation show that geochemical
techniques can be applied to probe the history of lithospheric modification during rifting and
provide new constraints for models of rift development (C) 2010 Elsevier B.V All rights reserved 

Ethiopian Rift; Lithosphere; Wonji Fault Belt; Rifting; Geochemistry