The 'Great Boundary Fault'.

Geographical Extent and Tectonic Setting

The Great Boundary Fault extends for approximately 2,500 kilometers, stretching from Nanga Parbat in the northwest to Arunachal Pradesh in the northeast. It runs parallel to the Himalayan arc, generally following the southern margin of the Higher Himalayas. The fault is a result of the oblique collision between the Indian and Eurasian plates, which began around 50-55 million years ago. This collision caused the Indian plate to subduct beneath the Eurasian plate, leading to crustal thickening, folding, and faulting.

Formation and Geological Features

The formation of the GBF involved several stages of tectonic activity:

• Initial Subduction: Early stages involved the subduction of the Indian plate.
• Crustal Thickening: As subduction continued, the crust began to thicken, leading to the uplift of the Himalayas.
• Thrust Faulting: The GBF developed as a major thrust fault, accommodating the shortening of the crust.

Key geological features associated with the GBF include:

• Thrust Sheets: Large blocks of rock are thrust over one another along the fault plane.
• Folded Sediments: Sedimentary rocks are intensely folded and deformed due to compressional forces.
• Metamorphic Rocks: High pressure and temperature conditions associated with faulting have led to the formation of metamorphic rocks.
• Duplex Structures: Complex arrangements of thrust sheets and folds, indicating multiple phases of deformation.

Significance in Himalayan Orogeny

The GBF plays a pivotal role in the Himalayan orogeny (mountain-building process). It represents a major zone of crustal shortening and deformation, accommodating the convergence between the Indian and Eurasian plates. The fault has facilitated the uplift of the Higher Himalayas and the formation of the complex geological structures observed in the region. The GBF is also associated with significant seismic activity, as the ongoing movement along the fault generates earthquakes.

Associated Geological Formations

Several geological formations are closely associated with the GBF:

Formation	Description
Tethys Sediments	Marine sediments deposited in the Tethys Sea, now found in the core of the Higher Himalayas.
Proterozoic Crystalline Basement	Ancient crystalline rocks that form the base of the Himalayan sequence.
Siwalik Group	Younger sedimentary rocks deposited in the foreland basin south of the GBF.

Recent Research and Understanding

Recent research utilizing GPS data and seismic studies has revealed that the GBF is still actively moving, with varying rates of slip along different segments. Studies suggest that the fault is locked in some areas, leading to stress accumulation and potential for large earthquakes. Understanding the spatial variations in fault slip and stress distribution is crucial for seismic hazard assessment.