Exhumation processes of the Najd Fault System

Towards the end of the Pan-African tectonic evolution, one of the largest Proterozoic shear zone systems on Earth (Stern 1985) exhumed a series of basement domes within the Arabian-Nubian Shield of northern Africa. This fault system - called the Najd Fault System - is about 2000 km long and 400 km wide and strikes NW-SE from Egypt across Sinai into Saudi Arabia. Each of the about 20 basement complexes exhumed along its length is several tens of km long. Curiously the exhumation processes vary between the few basement domes that have been studied so far: Those in the Eastern Desert of Egypt exhumed as extensional core complexes, while the Feiran complex of Sinai exhumed in a transpressive setting. In this project we will study a representative suite of basement complexes from the entire NFS in order to understand the overall process of the Najd Fault System and its control on exhuming the basement. We shall test three hypothesis that can explain the different exhumation mechanisms in different regions: (a) The basement complexes may be in different orientations relative to the principal stresses of the Najd Fault System. This will be tested using structural mapping in the field. (b) Different basement complexes may be exhumed from different depths so that different vertical normal stresses cause a reversal of the principal stress. This will be tested by determining metamorphic formation pressures and constructing metamorphic P-T paths. (c) Different complexes may have been exhumed at slightly different times reflecting a change in the stress regime towards the end of the Pan-African. This will be tested using geochronology. Particular focus will be on the final exhumation of all complexes to the surface as earlier studies have shown that the principal phase of exhumation terminated at 15 km depth. This part of the evolution and potential activity of the Najd Fault System is completely unknown and temporally unconstrained. In order to test these three hypotheses we will study three basement complexes in Saudi Arabia which have not been studied: The Qazaz, the Hamadat and the Wajiyah metamorphic complex. Our choice of complexes is based on geological features allowing the testing of our working hypotheses and on their logistical accessibility. We will also rely on our experience and existing sample collections from Sinai (Feiran complex) and the Eastern Desert of Egypt (Meatiq dome) as well as the paralleling projects of our Middle Eastern project partners.

The Najd Fault System of the Arabian-Nubian Shield is considered to be the largest Proterozoic deformation zone system on Earth and is predominantly exposed in the shield areas of the Eastern desert of Egypt and in Western Saudi Arabia. It is known that the shear zone was responsible for the exhumation of high grade metamorphic rocks in a number of locations across the shield. This project focused on structural mapping, petrology and dating of the shear zone system in order to constrain age and mechanisms how the shear zone brought the metamorphic rocks to the surface. In the project, we managed to identify important differences between processes on the Egyptian and the Saudi Arabian side of the shear zone. In particular, in contrast to Egypt Saudi Arabia we showed that high grade metamorphic rocks in Saudi Arabia are not exclusively confined to basement domes surrounded by shear zones, but also occur within shear zones themselves. Moreover, we recognize both exhumation in extensional and in transpressive regimes to be responsible for exhumation of high grade metamorphic rocks in different parts of the shield. We suggest that these apparent structural differences between different sub-regions of the shield largely reflect different timing of activity of various branches of the Najd Fault System. In order to tackle the ill-resolved timing of the Najd Fault System, zircon geochronology was performed on intrusive rocks with different cross cutting relationships to the shear zone. We are able to constrain an age between 580 Ma and 605 Ma for one of the major branches of the shear zone. The project involved one Postdoctoral fellow, two PhD students (one with external funding) and two MSc students and a series of publications in international journals of the highest standing have already been published.