Tectonic Control on Mineralogical and Microfabric Modifications and Their Impact on Rock Strength: Evidence from the Fault Damage Zone, Tethyan Himalayas, Pakistan

This study examines the influence of tectonically induced mineralogical and microfabric changes on the strength of different rocks within the Hanzel Fault Damage Zone (FDZ) in the Tethyan Himalayas, Pakistan. Integrating field observations, petrographic analysis, and laboratory experiments (uniaxial compressive strength (UCS), Brazilian tensile strength (BTS), ultrasonic pulse-wave velocity (UPV), and porosity), this study systematically characterizes the spatial variations in intact rock strength across horizontal distance from the fault core to the outer limit of the FDZ. Seven rock units—granites (biotite granite, leucogranite schist, granodiorite schist, and diorite) and amphibolites (foliated amphibolite, amphibolite, and plagioclase amphibolite)—were sampled at varying distances (−500 to +4035 m) from the fault core. Results reveal that proximity to the fault core correlates with significant strength reductions (40%–70%): granitic rocks exhibit lower UCS (41–59 MPa) and BTS (4.8–6.7 MPa) compared to distal amphibolites and diorites UCS (75–107 MPa) and BTS (10–13.67 MPa). Petrographic analysis identifies key factors that reduce strength, including high mica content (up to 33%), pervasive micro-fracturing, S-C fabrics, and mineral alteration. These features increase porosity (up to 1.21%) and reduce UPV (2867–3315 m/s) in fault-proximal rocks. Moderate inverse relationships (R2 = 0.68–0.72) between mica percentage and UCS/UPV confirm phyllosilicates as primary strength controls. The spatial variation in rock strength is attributed to ductile–brittle deformation processes, with foliated or schistose textures increasing in proximity to the fault core. This study demonstrates that tectonic processes significantly influence the mineralogy and microfabric within FDZs, leading to variations in rock strength with direct implications for stability in tectonically active regions.