Effects of tectonically induced fabrics on geomechanical properties of rocks, NW Pakistan

Abstract

The Chakdara Granitic Gneisses (CGG) of the Indian plate and Kamila Amphibolite of the Kohistan Island Arc (KIA) along the Main Mantle Thrust (MMT) in Shigo Kas, Talash Dir Lower, indicate that tectonically induced foliations and lineations strongly affected the geomechanical properties of these rocks. The earlier S1 crenulated cleavages are well preserved in the microlithon of a well-developed ENE-WSW trending S2 crenulation cleavage. The pervasive S2 foliations, D2 fold axes, and L22 lineations are induced by NNW-SSE horizontal bulk shortening. The core samples obtained parallel and perpendicular to the main ENE-WSW trending S2 and L22 have higher and lower uniaxial compressive strength (UCS) values, respectively. The UCS and uniaxial tensile strength (UTS) average values of four core samples obtained parallel and perpendicular to the main S2 are 51.8 MPa and 12.21 MPa versus 45.65 MPa and 12.45 MPa, respectively. Core samples from the weakly foliated S-2 specimen shows little variation in the UCS and UTS values. The variation in the UCS values in the core samples cut perpendicular and parallel to the main tectonic fabric has been controlled by micro-shear zones at the contact zones of crenulated and crenulation cleavages and sigmoidal mica fish. The UCS values are higher in the core samples parallel to the pervasive S2 and L22 because the parallel shear on the sigmoidal crenulated cleavages in microlithon of the S2 and S2 mica fish counterbalance the parallel external applied load. However, the UCS values decrease in the core samples that were cut perpendicular to the pervasive S2 and L22 because the perpendicular shear on the sigmoidal crenulated cleavages in microlithon of the S2 and S2 mica fish enhances the external applied load, which lead to the failure of core samples.