Bookshelf faulting and horizontal block rotations between overlapping rifts in southern Afar

Abstract

Lateral slip on initially rift‐parallel normal faults may be a particularly efficient mechanism to accomodate strain between overlapping oceanic rifts. It occurs in southern Afar, where clockwise block rotations result from distributed dextral shear between the overlapping Ghoubbet Asal‐Manda Inakir and Manda Hararo‐Abhe Bad rifts. Faulting observed during the 1969, Serdo earthquakes and on SPOT images is consistent with the shear being taken up by left‐lateral slip on steep NW‐SE striking faults, which formed as normal faults before extensional strain became localized in the two rifts. This bookshelf faulting accounts quantitatively for the 14.5°± 7.5° rotation documented by paleomagnetism in the 1.8 ± 0.4 Ma old Afar stratoid basalts, given the 17.5 ± 5 mm/yr rate of separation between Arabia and Somalia. Copyright 1990 by the American Geophysical Union.