The Main Boundary Thrust is a major fault of the Himalayan system that runs for more than 2,000 kilometres along the range, separating the older rocks of the Lesser Himalaya from the younger Siwalik foothill deposits to the south. It formed as deformation migrated southward toward the plains over the last several million years.
This north-dipping thrust fault carries Lesser Himalayan rocks over the Siwalik molasse and links at depth to the same Main Himalayan Thrust decollement that controls the entire mountain belt. It marks a clear topographic and geological step between the mid-hills and the outer foothill ranges.
The Main Boundary Thrust remains active and has been associated with strong historical earthquakes along the Himalayan front. Together with the Main Frontal Thrust to its south, it defines the active deformation front where the crust shortens and the mountains grow, posing a persistent seismic hazard to the densely settled foothill belt.