Time constraints on deformation of the Ajjaj branch of one of the largest Proterozoic shear zones on Earth: The Najd Fault System

Abstract

In active tectonic regions, shear zones play an important role in re-configuring the structure of the lithosphere. One of the largest shear zones on Earth is the Najd Fault System of the Arabian–Nubian Shield. The main active phase of this shear zone was during the last stages of the Pan-African Orogeny (ca. 630–540Ma). Six samples of intrusive rocks that were emplaced into the shear zone at different stages during its active phase are used to illustrate the progressive evolution of the Ajjaj shear zone. A sample of coarse-grained diorite, with an intercept U–Pb zircon age of 696±6Ma, shows very weak deformation. Two samples from deformed granodiorite–tonalite intrusions at the border of the Ajjaj shear zone show conspicuous degrees of deformation, and define two U–Pb clusters of concordia ages at 747±12Ma–668±8Ma and 742±5Ma–702±12Ma. Two samples of granites show mylonitic foliation with flattened quartz and biotite parallel to the trend of the shear zone. These samples yield U–Pb ages of 601±3Ma–584±3Ma. Another granite sample is undeformed and shows cross-cutting relations with the shear foliation of the Ajjaj shear zone. It yields a concordia age of 581±4Ma. The metamorphic rocks of the Hamadat complex host the Ajjaj shear zone, and have been useful in determining the metamorphic P-T conditions attending the activity of the shear zone. The peak metamorphism of the Hamadat Complex is 505–700°C at two ranges of pressure 8–11 and 14.5±2kbar. New data confine the activation of the Ajjaj shear zone in a limited period of time between 604Ma and 581Ma and the operation at different crustal levels with a maximum depth of 58km.