The najd fault system revisited; a two-way strike-slip orogen in the saudi arabian shield

Abstract

The Najd strike-slip fault system extends over the northeastern Arabian Shield in a zone >1200 km in length and >300 km wide. Faults trend NW-SE with strike lengths >500 km but small sinistral displacements of <25 km. Cumulative displacement across the zone is >240 km. Najd faults were active in the late Proterozoic and post-date cratonization of the Shield. Associated secondary structures include grabens, thrust faults, folds and dike swarms. In the southwest of the Najd system, near Zalm, initial faulting was dextral and began earlier than formerly thought. Emplacement of a plutonic complex was controlled by Najd fractures of dextral geometry and displacements. The same fractures were active before and after deposition of a group of volcanosedimentary rocks in grabens orientated consistently with development in a dextral strike-slip regime. Graben deformation was controlled by sinistral motion along the same fractures responsible for graben development and also by younger fractures of sinistral geometry and displacement. Dike swarms in the area are also consistent with early dextral and later sinistral shear of Najd trend. Structures in the Zalm area occur throughout the Najd system and the consistent chronology of older dextral structures dislocated and deformed by younger sinistral faults suggests a reversal in the sense of motion of the Najd system as a whole.