Constraining the far-field maximum horizontal stress in the plate tectonic area

Abstract

In orogenic belts, the far-field horizontal stresses of tectonic origin often control the stress regime in the nearby regions. However, due to the non-uniform convergence of the tectonic plates, the far-field horizontal stresses show local variation. The syntaxial bends in the orogenic belts are areas where the orientation of far-field maximum horizontal stress shows marked deviation from the general trend of the plate movement. The Hazara Kashmir Syntaxis (HKS), located in the western Himalayas, is one such structure where the major crustal-scale faults are making a loop. This looping of the thrust faults makes it difficult to constrain the orientation of the far-field horizontal stresses in the core of the HKS. The Neelum Jhelum Hydropower Project’s (NJHP) headrace tunnel traversing the core of the HKS revealed important information regarding the bedrock geology and in-situ stress state in the syntaxis. This information has not been previously used for constraining the far-field horizontal stresses in the area. The purpose of this study is to constrain the far-field horizontal stress in the HKS based on field observations and the geological and geotechnical data collected during the excavation of the headrace tunnel. The study utilised 3D finite element modelling approach to examine the complex interaction among the gravitational stresses due to current topography, exhumation-induced remnant stresses, excavation-induced perturbations, and far-field horizontal stresses. The simulated results were compared with the measured in-situ stresses for model validation. The simulation results demonstrated that the orientation and magnitude of gravitational principal and horizontal stresses at shallow depths are largely controlled by the current topography. The addition of the exhumation-induced gravitational remnant stresses caused changes in the orientation and magnitude of the principal stresses. However, the orientation of the maximum horizontal stress (SH) was less affected. The SH was also found to be less perturbed by the tunnel excavation. In the subsequent analysis, the models were compressed using horizontal straining from different directions to get SH magnitude and orientation similar to the measured SH. The results showed that the modelled SH orientation at the different monitoring points could be achieved by applying different magnitudes of horizontal straining. These results suggested that the 0° Model with east-west directed maximum straining shows SH trends consistent with the measured SH trends. The east-west directed far-field horizontal stress derived during this study is consistent with the local movement direction of the Main Boundary Thrust (MBT) fault in the study area. The study revealed that the SH orientation is a better candidate for constraining the far-field horizonal stresses. Moreover, the study revealed that the local variation in the strike of MBT causes local variations in the orientation of far-field horizontal stresses in the HKS.