The Main Boundary Fault (MBF)
Separating the Siwalik Formations of the Sub-Himalayas from the older rocks lying to their north, the Main Boundary Fault is a major structural plane discernible throughout the length of the Himalayas. Hitherto regarded as a steep north dipping fault, it is more likely a thrust which flattens with depth. The MBF, originally defined as the tectonic feature separating the Siwalik from the preSiwalik Tertiaries, is exposed only in the extreme western sector in the Kumaon Himalayas, roughly between the Yamuna and Tons valleys. East of the Yamuna the higher Krol Thrust has overlapped the Eocene Subathu and has completely concealed the MBF. The only exception is seen near Durgapipal in the east where a narrow belt of Subathu is exposed between the Siwalik and overthrust Krol rocks. Secondary faults or thrusts branch off the MBF, as for instance in southern Punjab. These secondary fault zones always diverge in a westward direction and merge with the MBF towards the east. The irregularity and sinuosity of the fault trace is evidence of a highly inclined plane. The older rocks of the Lesser Himalayas are thrust over the Siwaliks along a series of more or less parallel thrust planes. The Main Boundary Fault is a thrust fault with large-scale movements and is still very active. Measurements made by some workers demonstrate that the present day movement is of the order of 0.92 cm/yr.
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