Structural, kinematic and geochronologic evolution of the Himalayan fold-thrust belt in Kumaun, Uttaranchal, Northwest India

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The Lesser Himalaya is a key tectonostratigraphic zone of the Himalayan fold-thrust belt; however, its poorly defined chronostratigraphy and erroneous stratigraphic correlations hamper the determination of valid structural models. In Kumaun, northwest India, I reassess the tectonostratigraphic and structural architecture using U-Pb zircon geochronology, whole rock neodymium isotopes, and structural data to establish that rock in the Almora klippe and north of the Main Central thrust have Neoproterozoic (~900 Ma) detrital zircon ages and similar ε_Nd(0) (-7.6 to -11.8) values, which suggests that these two units are the same tectonostratigraphic unit, and that the Almora klippe is the southern continuation of the Main Central thrust or another thrust in the Greater Himalayan thrust system. However, north of the Almora klippe, detrital zircon age populations establish the presence of Palaeoproterozoic Lesser Himalayan rock [ca. 1600 Ma youngest zircon, ε_Nd(0)=-22.0] instead of the previous interpretation of Neoproterozoic rocks. South of the klippe, the Ramgarh-Munsiari thrust carries both the less metamorphosed Palaeoproterozoic and Neoproterozoic Lesser Himalayan rocks, in disagreement with the idea that only Neoproterozoic-Cambrian Lesser Himalayan rocks are present south of Almora klippe. U-Pb ages, Hf isotopes and whole rock trace element geochemical data coupled with detail mapping establish that the rocks in the Askot klippe belong to Paleoproterozoic Lesser Himalayan rock, and do not continue to the south as the rock in the Almora klippe. The Askot klippe consists of 1856.6±1.0 Ma granite-granodiorite gneiss, coeval 1869±12 Ma felsic volcanic rocks, and ca. 1820 Ma Berinag Formation quartzite. These rocks represent a small vestige of a Paleoproterozoic continental arc that formed on the northern margin of the northern Indian cratonic block during the Paleo-Mesoproterozoic amalgamation of the supercontinent Columbia. εHf(0) values from ca. 1850 Ma detrital zircons from the Berinag Formation quartzite (-9.6 to -1.1, with an average of -4.5) overlap with the εHf values of the ca. 1850 Ma granite-granodiorite gneiss of the Askot klippe [εHf(0)=-5.5 to -1.2, with an average of -2.7] and other ca. 1850 Ma arc-related granite gneiss [εHf(0)=-4.8 to -2.2, with an average of -4.0] within the Lesser Himalaya. Subchondritic εHf(0) values (1.1 to -4.9) of ca. 1850 Ma granite-granodiorite gneiss indicate pre-existing older crust, which underwent crustal reworking at ca. 1850 Ma. This revised chronostratigraphy along with new structural mapping provide the basis for a new structural model for the evolution of the fold-thrust belt in Kumaun that establishes the thrust system as a forward propagating system and contains a hinterland dipping duplex composed of Lesser Himalayan rocks. A balanced cross-section reveals a total minimum shortening estimate of 541-575 km or 79-80% between Main Frontal thrust and South Tibetan Detachment system, with the MCT accommodating 163-128 km, the RMT accommodating 112 km, the LHD accommodating 270 km, the MBT accommodating 8 km, and the Subhimalayan thrust system accommodating ~22 km. Adding the shortening from the Tethyan Himalaya from published work brings the total minimum shortening estimate for the Himalayan fold-thrust belt along this cross-section line to 674-751 km. The Ramgarh-Munsiari thrust is the roof thrust of the Lesser Himalayan duplex, and its footwall rocks, the Ramgarh, Berinag, Chiplakot and Munsiari Formations, are part of the same, once continuous stratigraphic unit that was dissected and turned into the Ramgarh-Munsiari thrust sheet and other thrust sheets in the Lesser Himalayan duplex.

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