The Krol-Berinag Nappes
The high-angled thrust recognized as the Main Boundary Fault separates the Subathu (Eocene) from the Late Tertiary Siwalik molasse in the Tons-Yamuna valleys in the west. A few kilometres southeast of Haripur on the Yamuna, the MBF is overlapped and hidden by the higher Krol Thrust so that throughout the rest of Kumaon it is the Krol Thrust that defines the boundary between the Siwalik and the various lithological units of the Krol Nappe. In the Yamuna-Tons valleys the Subathu occurs as a narrow, attenuated, severely deformed parautochthonous unit sandwiched between two thrusts. West of Tons the MBF joins up with the Nahan Thrust of southeastern Himachal Pradesh. The inclination of the Krol Thrust is variable, usually quite high at the surface, but declines progressively in depth, as revealed in the hydel tunnel driven in the Tons valley. Wide, gravel-laden valleys with walls scarred with multitudes of landslides and their floors covered with talus fans and cones commonly follow the plane of the Krol Thrust, as clearly seen in the Mandal (southeastern Puri) and Balia (Nainital) rivers.
Subrecent tectonic activity along the thrust plane has brought the Chandpur phyllites of the Krol Nappe over the subrecent gravels of the Rajpur-Rishikesh belt. The Krol Thrust has brought up a huge succession of sediments divisible into two groups, the lower Jaunsar group comprising Mandhali, Chandpur and Nagthat formations of unknown ages, and the upper Masuri group of Blaini, Krol and Tal formations belonging to upper Palaeozoic. In the Mussoorie-Lansdowne hills the Permian Tal is capped by the Subathu of Eocene age. Significantly, the Masuri group is confined in the southern part to the narrow elongate belt stretching from Yamuna to Nainital and in the Nandhaur valley to the southeast where there is a small outlier. The larger northern part is made up of a great thickness of the Jaunsar group.
The northern flank of the Krol Thrust is known as the Tons Thrust in the Tons-Yamuna valley, the Dharkot Thrust in the Bhagirathi valley and the Shrinagar Fault in the Alaknanda valley. It separates the Chandpur of the synclinorially folded Krol Nappe, from the autochthonous sedimentary group (Damtha) of the inner Lesser Himalayas. Later folding of the Krol Nappe (after its emplacement) has not only steepened the inclination of the North Krol Thrust but rendered it almost vertical or locally even reversed the direction to make it look like a fault, but has also caused faulting along the crestal planes of the folds within the nappe so that the thrust plane coincides with these later faults. The Nayar and Aglar Faults of the southern Pauri and southern Tehri, res ectively are examples of this phenomenon.
In the inner Lesser Himalayas the autochthonous sedimentary groups are covered with by and large conformable succession of orthoquartzites and basic volcanics of the Berinag Formation, correlatable with the Nagthat of the Jaunsar group forrring the lower part of the Krol Nappe. As a matter of fact, the Berinag of the Purola-Sandra area in the northwest extends westward, and skirting around the eastern slope of the Chaur massif joins up with the Nagthat formation of the Krol Nappe in the Chandpur mountain. The DamthaDeoban-Mandhali-Berinag succession is apparently conformable in many places, but recent studies clearly demonstrate that a thrust plane separates the Berinag from the underlying formations. Apart from the strong deformations suffered by the rocks involved in thrusting, and local discordance, the considerable attenuation or even elimination of the Mandhali, the complete absence of the Chandpur, the striking thinning of the Berinag at the foot of the Great Himalaya, and the local absence of Deoban between the Berinag and Damtha, provide indisputable proof of the existence of the Berinag Thrust. The Berinag Nappe is thus a much attenuated basecut and beheaded extension of the Krol Nappe of the outer Lesser Himalaya. The Berinag is possibly the southeasterly analogue of the Jaunsar sheet of Himachal Pradesh.
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