Structural Boundaries of the Kumaon Lesser Himalayas
The Lesser Himalaya is demarcated against the Siwalik subprovince by what is commonly described as the "Main Boundary Fault". But in actuality the MBF, originally defined as the tectonic feature separating the Siwalik from the preSiwalik Tertiaries, is exposed only in the extreme western sector 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. The Krol Thrust, which has brought various lithological units of the Krol Nappe over the Siwalik, constitutes the real southern boundary of the Lesser Himalaya.
Valdiya (1978) takes a somewhat different stance on the northern limit of the Lesser Hirralaya. He does not consider the Main Central Thrust, which separates the graniteintruded low-to medium-grade crystallines of the base of the Great Himalaya from the sedirnentaries below, as the boundary thrust. He believes that these cataclastically deformed and retrogressively transformed crystallines are essentially Lesser Himalayan lithological elements, which cover a vast region of the Lesser Himalaya as thrust sheets. The real boundary in his opinion is the plane that divides these Precambrian granite-injected metamorphics of medium grade at the base (Munsiari formation) from the katazonal, very high-grade metamorphics (intruded by Tertiary granites), making the bulk of the Great Himalaya (Vaikrita group). There is no discordance of dip, and no geomorphic expression of the existence of a thrust is discernible. There is only an abrupt and dramatic change in the grade of metamorphism the finer grained sericite-chlorite schist and micaceous, garnetiferous quartzites abruptly give way to very coarse-grained kyanite- and/or sillimanite-bearing garnetiferous two-mica psammatic gneiss and schist. This abrupt change reflects the presence of the thrust plane which Valdiya has designated as the VaikritaThrust, defining the base of the Vaikrita group of the Higher Himalaya (Fig 1)
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