TECTONOSTRATIGRAPHIC ARCHITECTURE OF THE HIMALAYAN FOLD-THRUST BELT IN KUMAON, NW INDIA, AND THE CORRELATION WITH WESTERN NEPAL

The Himalayan fold-thrust belt (FTB), formed as the result of a continent-continent collision between the Indian and Asian plates, consists of four major tectonostratigraphic zones: Subhimalaya, Lesser Himalaya, Greater Himalaya and Tethyan Himalaya. Improper identification of various formations within each zone has led to erroneous structural and kinematic interpretations of the evolution of the FTB in Kumaon, NW India. New mapping, coupled with U-Pb zircon geochronologic data and whole rock major and trace element geochemical data, defines the tectonostratigraphic architecture.

The Paleogene rocks, Subathu and Dagshai Formations, are not present in either the footwall or hanging wall of the MBT. However, these Paleogene rocks have been reported in the hanging wall of the MBT in western Nepal, immediate east of the study area. U-Pb dating of detrital zircons suggests the presence of both inner and outer Lesser Himalayan (LH) rocks in the hanging wall of the MBT, whereas only inner LH rocks (Sangram and Syangia Formations) are present in the hanging wall of the MBT in western Nepal. Field mapping and U-Pb dating of igneous zircons indicates Paleoproterozoic augen gneiss, locally known as Debguru porphyry, in association with the Ramgarh Group in the hanging wall of the Ramgarh thrust. These igneous bodies and the Ramgarh Group are the along strike continuation of the Kushma and Ranimata Formations and associated ca. 1850 Ma augen gneiss in western Nepal. However, U-Pb dating of detrital zircons indicates presence of outer LHS (<1000 Ma) on the hanging wall of the Ramgarh thrust south of the Almora Klippe. This suggests either the Ramgarh thrust has cut stratigraphic up-section to the west, its hanging wall structure been breached by another thrust, or the RT contains the outer LH in stratigraphic continuation with the inner LH rock.

U-Pb dating of igneous zircons from the Askot and Chiplakot Klippen, mapped by many workers as Greater Himalayan rocks, yielded ca. 1850 Ma crystallization ages, suggesting the klippen are inner LH rocks. Occurrence of ca. 1850 Ma augen gneiss with the Ramgarh, Munsiari, Berinag and Jutogh Formations lead us to conclude that these formations have the same origin; however, the metamorphic grade varies as result of differential burial depth during the evolution of the Himalaya.