Stress regime changes in the Main Boundary Thrust zone, Eastern Himalaya, decoded from fault-slip analysis

• Published in: Journal of structural geology Vol. 120; pp. 29 - 47
• Main Authors: Patra, Abhijit, Saha, Dilip
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-03-2019
• ISSN: 0191-8141; 1873-1201
• DOI: 10.1016/j.jsg.2018.12.010

The Main Boundary Thrust (MBT) Zone in the Darjeeling foothills, Eastern Himalaya exhibits northerly dipping fault splays, named here as MBT-1 and MBT-2, with Proterozoic Daling Group in the MBT-1 hanging wall, Upper Paleozoic Gondwana rocks between the MBT-1 and the MBT-2, and Neogene Siwalik Group in the footwall of the MBT-2. On the basis of reduced stress tensors from fault-slip analysis and their grouping relative to different stratigraphic units in the MBT zone that crops out over a strike length of ∼20 km, the following successive stress regimes have been interpreted. The earliest brittle deformation recorded only in the basal part of the Gondwana succession was controlled by a pre-Himalayan NW-SE extensional regime. The Himalayan deformation under sustained N-S compressive regime developed in three stages: (a) thrust displacement along the MBT-1, juxtaposing folded Daling rocks against the Lower Gondwana rocks; (b) tilting and fracturing of Lower Gondwana succession and thrust transport over the Siwalik rocks along the MBT-2; (c) strike-slip regime with NW- and NE-trending faults affecting both the Lower Gondwana and Siwalik strata, developed possibly due to increased tectonic loading. The tensor solutions under N-S compressional regime are consistent with the modern plate convergence, supporting a foreland propagating thrust system in the Eastern Himalaya. In addition, an E-W extensional regime led to apparent orogen-parallel extension.
•Stress regime changes within a foreland propagating thrust system, Eastern Himalaya.•Pre-Himalayan NW-SE extension during Early Permian opening of lower Gondwana basin.•N-S compressional regime associated with the Main Boundary Thrust (MBT) zone.•Two stages of compression related to episodic thrust propagation on MBT splays.•Switchover to strike-slip regime under N-S compression due to tectonic loading.