Fluvial Geomorphic Evidence for Late Quaternary Reactivation of a Synclinally Folded Nappe in Kumaun Lesser Himalaya

In parts of the Kumaun Lesser Himalayan terrane that is defined by the very active Main Boundary Thrust and Main Central Thrust, tectonic strain is progressively building up due to horizontal compression with resultant crustal shortening. Consequently, the thrust that bounds a synclinal nappe of Precambrian crystalline rocks has been reactivated in the Late Pleistocene and Holocene times. The neotectonic movements along the nappe-bounding thrust and associated faults within the nappe and in the adjacent autochthonous terrane are manifest in the pronounced geomorphic rejuvenation of the landscape, the development of rugged mountain barriers along the active thrusts and the dismemberment and dislocation of Late Quaternary fluvial terraces, lacustrine flats and colluvial cones. The resultant stream ponding culminated in the formation of lakes behind recently uplifted blocks upstream of active faults. Formed within the fluvial regimes and stretching 7 to 11 km upstream of the neotectonic impediments, the lakes are represented by 10 to 13 m thick succession of clays and muds. Radiocarbon dates of the lacustrine clay and mud in eastern Kumaun indicate the timings of the reactivation of these faults of the synclinal Almora Nappe at >35 ka and 9 ka. Several other palaeolakes in the Himalayan province originated in the temporal interval of 35 to 40 ka.