The Bi’r Umq-Nakasib Shear Zone: Geology and Structure of a Neoproterozoic Suture in the Northern East African Orogen, Saudi Arabia and Sudan

SGS-TR-2002-1 Peter R. Johnson, Mohammed Abdelsalam, and Robert J. Stern
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ABSTRACT

The Bi’r Umq-Nakasib shear zone, 5-65 km wide and more than 600 km long, is a belt of highly strained rocks in the Neoproterozoic of western Saudi Arabia and northeastern Sudan that has a unifying structural, geochronologic, and lithologic character and forms a through-going structure on either side of the Red Sea.
Dismembered ophiolites preserved in nappes that appear to root in the shear zone are evidence that the shear zone is the site of closure of an oceanic basin, and differences in ages, isotopic characteristics, and geologic relations indicate that the crustal units flanking the shear zone constitute separate tectonostratigraphic terranes.
The shear zone developed by progressive deformation involving crustal shortening and dextral shear indicative of oblique dextral transpression and represents a zone of high strain coincident with the site of collapse of an ocean basin.
By comparison with standard criteria, the shear zone is clearly a suture.
Available geochronology brackets shearing at 780-760 Ma and implies that the shear zone is the oldest known suture in the northern part of the East African orogen.
The shear zone is arguably one of the best-preserved Neoproterozoic sutures worldwide and provides unambiguous evidence of platetectonic processes in the Precambrian.
The Arabian and Sudanese segments of the suture constitute tie points for the Arabian and Nubian sectors of the East African orogen across the Cenozoic Red Sea, and the relative positions of the tie points are evidence that opening of the Red Sea involved mostly orthogonal divergence with less than 50 km offset parallel to the Red Sea axis. Johnson, P.R., Abdelsalam, M., and Stern, R.J., 2002, The Bi’r Umq-Nakasib Shear zone: Geology and structure of a Neoproterozoic suture in the northern East African Orogen, Saudi Arabia and Sudan: Saudi Geological Survey Technical Report SGS-TR-2002-1, 33 p., 13 figs., 1 table