Structural Evolution for AJJAJ Shear Zone, Northwestern Arabian Shield, Saudi Arabia

Abstract

AbstractThe Ajjaj shear zone is part of the regional-scale Quazaz-Ajjaj-Hamadat (QAH) shear zone that controls the structure of the northwestern Arabian shield. It consists of a sequence of volcanic rocks and sedimentary detrital, which is characterized by a difference in metamorphism facies with intruded granitoid bodies. Our work dealt with the structural characteristics of Ajjaj shear zone, accomplished through field reconnaissance and finite strain analysis, and microstructural investigation for understanding the tectonic evolution for studied area. Rf/φ and Fry techniques were applied to quartz and feldspar with some mafic minerals such as biotite and hornblende. The finite strain data show that the axial ratios average from 2.55 to 4.54 using the Rf/\(\varphi \) method and 1.95 to 4.33 using the Fry method in the XZ sections for the gneisses, metavolcanic, metasedimentary, and schist rocks. The finite strain direction for the long axes displays clustering along a NWN–SES trend with slight to moderate plunging. The Z axes are subvertical and associated with a subhorizontal foliation. The majority of data reveals oblate strain symmetry (flattening) with minor of prolate strain symmetry (constructional) and the strain magnitudes show no considerable increase toward the tectonic contacts. The volume change in the different lithologies was not associated with the accumulation of finite strain in the studied area. Furthermore, the subhorizontal foliation was parallel to the thrusting and illustrated mainly the same attitudes of tectonic contacts with the overlying nappes. It is concluded that the field observations and finite stain data resulted from simple-shear deformation for nappes in the orogeny.KeywordsFinite strainMicrostructural analysisAjjaj shear zoneArabian shieldSaudi Arabia