Microstructural analyses of the Najd Fault System in Midyan Terrane, NW Arabian Shield, Saudi Arabia

Abstract

This article discusses the microstructural indicators exposed within the Precambrian Ajjaj–Qazaz–Hanabiq (AQH) shear zones in order to determine the shear sense. The AQH shear zones are located in the southern sector of the Midyan Terrane in the northwestern part of the Arabian Shield. Structural and microfabric analyses along these shear zones indicate a top-to-the-NW sense of shear. Exposures in this area comprise elongated ridges of ophiolitic rocks, arc metavolcanics and sporadic arc-related Neoproterozoic intrusions. They are unconformably overlain by post-amalgamation molasses-type sediments and were finally intruded by late to post-tectonic intrusions. Structural field relations along with microfabric analyses and overprinting relationships provide evidence of the sense of movements and phases of deformation. There were at least four phases of deformation. The oldest (Hijaz) D1 structures are rare and are represented by foliation striking WNW to E–W and by F1 interfolial folding with an axial plane striking ENE–WSW and moderately plunging fold axes trending WSW. D2 microstructures are also scarce and represented by foliation and (F2) folding directed in NNW to N–S trends parallel to the Nabitah suture. However, they are commonly deformed and overprinted by the Najd Fault System that formed during the D3 deformational phase. The D3 deformational phase is prominent, and it developed during the propagation of the NW transpressional shearing of the Najd Fault System. These shear zones could be identified in this region with the help of the following microstructures: sigmoidal structures, mantled porphyroclastic structures, asymmetric boudins, tension gashes, fish-shaped structures, oblique foliation, bookshelf sliding structures, slickenlines, C–S tectonites and fracturing along the AQH shear zones. Shear sense analysis of microstructural markers at the microscopic scale, supported by field observations and map-scale structures, implies that both the Ajjaj and Qazaz shear zones display top-to-the-NW or top-to-the-WNW slip. The shear sense indicators along the Hanabiq shear zone show top-to-the-NNE slip. These inconsistencies in their shear sense and their orientations were caused by the exhumation of high-grade gneisses, which distorted the shear trend to the north in the Hanabiq shear zone. All the previous structures are overprinted by the brittle D4 deformational phase in the form of microfaulting and fracturing.