Microstructural and strain analysis in the Tanumah area, Arabian Shield, Saudi Arabia

Abstract

The magnitude and orientation of the finite strain ellipsoid are critical for constraining displacement across ductile deformation. Finite strain was studied in granitic gneiss and metavolcano-sedimentary rocks of the Tanumah area. Finite strain shows its 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. It used the Rf/φ and Fry methods on quartz, feldspar porphyroclasts and mafic grains (such as hornblende and biotite) from two granite, four tonalite and granodiorite, one gneissic quartz diorite, three biotite quartz diorite, six metasedimentary, one meta-andesite and two metavolcanic rocks. The obtained finite strain data show that the granitic gneisses and metavolcano-sedimentary rocks studied were moderately to highly deformed; the X/Z axial ratios range from 1.70 to 4.80 for Rf/φ method and from 1.20 to 4.50 for Fry method. The direction of finite strain for the long axes shows N to WNW trend and plunges shallowly to the WNW in most studied samples. The Z-axes are subvertical. The data indicate oblate strain symmetry (flattening) in the Tanumah area, and the strain magnitudes do not increase towards the tectonic contacts. It is assumed that the granitic gneisses and metavolcano-sedimentary rocks studied have similar deformation behaviour. 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 thrusting and intrusion of granite rocks. It is concluded that the nappe contacts formed during progressive thrusting under semi-brittle to ductile deformation by simple shear and involved a component of vertical shortening, which caused the subhorizontal foliation in the Tanumah area.