The Najd Fault System, Saudi Arabia and Egypt: A Late Precambrian rift‐related transform system?

Abstract

The Najd Fault System is a complex set of left‐lateral strike‐slip faults and ductile shear zones that strike NW‐SE across the Precambrian of Arabia and Egypt. This system was developed during the interval 540–620 Ma. It is up to 400 km wide with an exposed length of 1100 km; inferred buried extensions of the Najd give it a total length of 2000 km. It is the best exposed and may be the largest pre‐Mesozoic zone of transcurrent faulting on earth. Previous models for the Najd Fault System suggest it formed as a result of a major Late Precambrian continent‐continent collision. This model is not preferred here because (1) the lack of evidence for a pre‐Late Precambrian continent to the east of the Najd Fault System; (2) the difference between the orientation of the Najd Fault System and that predicted by slip‐line theory; (3) the younger age of Najd movements compared with that of collisional sutures in the Arabian Shield; and (4) lack of evidence for wide‐spread crustal uplift that would be expected to accompany collision. A new model for the origin of the Najd Fault System accounts for each of these objections: The Najd Fault System formed in response to a broad zone of NW‐SE directed crustal extension that accompanied juvenile continental crustal formation in northernmost Afro‐Arabia. This model also accounts for the following observations: (1) Strands of the Najd parallel the direction of extension in the North Eastern Desert of Egypt and Sinai; (2) the timing of the principal rifting movements (ca. 575–600 Ma) overlap with those of the Najd (ca. 560–620 Ma); (3) in spite of observation (2), the Najd Fault System is not recognized in northernmost Afro‐Arabia; instead the Najd deformation becomes increasingly ductile and these zones are more commonly intruded by sheared and foliated granites as the principal zone of extension is approached. The Najd Fault System thus represents a set of continental transforms developed in response to a major episode of Late Precambrian extensional continental crust formation in northernmost Afro‐Arabia.