UPPER CRUSTAL STRUCTURE AND SHORTENING IN THE HIMALAYAN THRUST BELT IN KUMAUN, NORTHWEST INDIA

Existing structural models of the Himalayan thrust belt in Kumaun, northwest India, are based on outdated Himalayan tectonostratigraphy. We reassess the tectonostratigraphy of Kumaun based on new and existing U-Pb zircon and whole rock Nd data, and present a new structural model and kinematic history for this part of the thrust belt. The Ramgarh, Berinag, Chiplakot and Munsiari Formations are part of the same, once continuous stratigraphic unit, which was dissected and turned into the Ramgarh-Munsiari thrust sheet and other thrust sheets in the Lesser Himalayan duplex. They are all Paleoproterozoic Lesser Himalayan rocks, contain ca. 1850 Ma granite of continental arc affinity, and have whole rock εNd values from -20 to -25. Our new structural model shows a hinterland dipping duplex system that accommodated 541-575 km or 79-80% of shortening between Main Frontal thrust and South Tibetan Detachment system with each system accommodating the following minimum amount of shortening: the Main Central thrust - 128 to 163 km, Ramgarh-Munsiari thrust - 112 km, Lesser Himalayan duplex - 270 km, Main Boundary thrust - 8 km, Subhimalayan thrust system -~22 km. By adding in shortening from the Tethyan Himalaya, we estimate a total minimum shortening of 674-751 km. The roof thrust, which is the Ramgarh-Munsiari thrust, and the Lesser Himalayan duplex are breached by erosion separating the Paleoproterozoic Lesser Himalayan rocks of the Ramgarh-Munsiari thrust into isolated klippen. This thrust carries Lesser Himalayan rocks 120 km southward from the footwall of the Main Central thrust, folded underneath the Almora klippe, to the hanging wall of the Main Boundary thrust. The Ramgarh-Munsiari thrusts structurally underlies the Main Central thrust and together form a coupled roof thrusts for the Lesser Himalayan duplex best displayed to the north and south of the Almora klippe. The reconstruction demonstrates that propagation of the thrust belt occurred from north to south with minor out of sequence faulting, supporting an in-sequence model for growth of the Himalayan thrust belt.