Deformed dike swarms as an implication of transpression deformation in Western Arabian shield, Wadi Fatima, Saudi Arabia

Abstract

Utilization of satellite images and field observations of dike swarms in pre-Fatima basement show that these dikes are older than the overlaying Fatima Formation. Dikes digitization and orientation analysis on satellite images show that the prevailing trend of the dikes is ENE-WSW. The granitic rocks of pre-Fatima basement and its hosted dikes expose evidences of completely a different deformation regime from the overlaying Fatima Formation. These evidences include shearing, dextral shear indicators, isoclinal folds, deflection and rotation of crystals, mineral elongation, and mylonitic and gneissose textures. Strain analysis results of using Fry method on quartz and feldspar grains support the presence of deformation in these ENE-WSW dikes. These results gave a strain ratio of 2.1:1.3:1, which suggest an amount of 40% stretching in the ENE-WSW direction parallel to the dike walls, and an amount of 30% shortening in the NNW-SSE direction. Mesoscopic and microscopic scale structures confirm the existence of dextral ductile-brittle shearing followed the emplacement of the dikes and before the pure shear deformation that caused the thrusting and folding of Fatima Formation. This ductile-brittle deformation is correlated with the dextral transpression that formed the Fatima Shear Zone (FSZ).