Authors: Osama M. K. Kassem and Abdel Aziz Al Bassam
Abstract: New structural, metamorphic, finite strain and kinematic vorticity data for granitic gneiss and metavolcano-sedimentary rocks of the Tanumah area in the Arabian shield in Saudi Arabia reveal a history of deformation reflecting different tectonic regimes. Kinematic analysis shows their relationship to the nappe contacts between the granitic gneiss and metavolcano-sedimentary rocks and explains the nature of the subhorizontal foliation typical for the Tanumah area. The vorticity analysis of porphyroclasts was determined in high temperature mylonites. The kinematic vorticity number for the mylonitic granitic and metavolcano-sedimentary samples in the Tanumah area range from 0.60 to 0.90, and together with the strain data suggest deviations from simple shear. The data indicate oblate strain symmetry (flattening) in the Tanumah area and the strain magnitudes do not increase towards the tectonic contacts. It is suggested that the accumulation of finite strain was not associated with any significant volume change. The penetrative subhorizontal foliation is subparallel to the tectonic contacts with the overlying nappes and foliation was formed during nappe thrusting and intrusion of granite rocks under semi-brittle to ductile deformation in the Tanumah area. It is concluded that the accumulation of ductile strain during thrusting was not by simple shear and involved a component of vertical shortening, which caused the subhorizontal foliation in Tanumah area and adjacent units.
Keywords: kinematic analysis, Deformation, Tanumah area, Arabian Shield, Saudi Arabia