TECTONIC IMPLICATIONS OF EARLY E-W ORIENTED STRUCTURES IN THE SOUTHERN ARABIAN-NUBIAN SHIELD, NORTHERN ETHIOPIA

Precambrian basement rocks of NE Africa and W Arabia belong to the Arabian Nubian Shield (ANS), which formed by accretion of 800-700 Ma island arc/back arc complexes. The ANS and its southern extension, the Mozambique Belt (MB), have been collectively interpreted as strongly deformed at 750-650 Ma during WNW oriented closure of the Mozambique Ocean (Stern, 1994). Suture zones in the southern ANS trend N to NE, but in the northern ANS sutures are oriented E to NE. Bedrock mapping and structural analysis in the Werri area, northern Ethiopia show that Neoproterozoic greenstone rocks were first deformed into north verging recumbent folds. Early E-W oriented structures indicating N-S transport have also been recognized in the Tigrai region (Alene, 1998). Subsequent D2 deformation produced NNE-SSW oriented tight folds and a steep NW dipping axial planar foliation. Younger D3 and D4 deformations resulted in formation of NE and NW oriented dextral shear planes, respectively; features that are correlatable with post accretion structures documented elsewhere in the ANS. Early E-W oriented faults, folds, and sutures in the ANS are at odds with a model of accretion via WNW directed closure. We propose that the ANS formed during closure of an approximately E-W oriented ocean. In our model, the MB is a N-S trending failed rift; the oceanic basin within which the ANS formed having initiated to the north as a triple junction of rifts over a Pan African plume (Stein, 2003). The closure of this ocean north of the MB resulted in the accretion of island arcs along WNW and ENE axes forming the ANS. Arc accretion was then followed by closure of the failed rift and D2 shortening of the ANS during WNW oriented collision at 750-650 Ma. This younger collision intensely overprinted the failed rift, producing the MB. North of the MB, WNW directed collision strongly overprinted early E-W D1 accretion structures of the southern ANS, but only mildly overprinted similar D1 structures and sutures in the northern ANS. We note that this disparate tectonic evolution proposed for the development of the ANS (accreted Neoproterozoic arcs) and MB (rifted and reworked pre-Neoproterozoic crust) is consistent with Nd model ages (Stern, 2001) and with the absence of unequivocal ophiolites in the MB and their abundance in the ANS.