Examining the Tectono-Stratigraphic Architecture, Structural Geometry, and Kinematic Evolution of the Himalayan Fold-Thrust Belt, Kumaun, Northwest India

Existing structural models of the Himalayan fold-thrust belt in Kumaun, northwest India, are based on a tectono-stratigraphy that assigns different stratigraphy to the Ramgarh, Berinag, Askot, and Munsiari thrusts and treats the thrusts as separate structures. We reassess the tectono-stratigraphy of Kumaun, based on new and existing U-Pb zircon ages and whole-rock Nd isotopic values, and present a new structural model and deformation history through kinematic analysis using a balanced cross section. This study reveals that the rocks that currently crop out as the Ramgarh, Berinag, Askot, and Munsiari thrust sheets were part of the same, once laterally continuous stratigraphic unit, consisting of Lesser Himalayan Paleoproterozoic granitoids (ca. 1850 Ma) and metasedimentary rocks. These Paleoproterozoic rocks were shortened and duplexed into the Ramgarh-Munsiari thrust sheet and other Paleoproterozoic thrust sheets during Himalayan orogenesis. Our structural model contains a hinterland-dipping duplex that accommodates ~541–575 km or 79%–80% of minimum shortening between the Main Frontal thrust and South Tibetan Detachment system. By adding in minimum shortening from the Tethyan Himalaya, we estimate a total minimum shortening of ~674–751 km in the Himalayan fold-thrust belt. The Ramgarh-Munsiari thrust sheet and the Lesser Himalayan duplex are breached by erosion, separating the Paleoproterozoic Lesser Himalayan rocks of the Ramgarh-Munsiari thrust into the isolated, synclinal Almora, Askot, and Chiplakot klippen, where folding of the Ramgarh-Munsiari thrust sheet by the Lesser Himalayan duplex controls preservation of these klippen. The Ramgarh-Munsiari thrust carries the Paleoproterozoic Lesser Himalayan rocks ~120 km southward from the footwall of the Main Central thrust and exposed them in the hanging wall of the Main Boundary thrust. Our kinematic model demonstrates that propagation of the thrust belt occurred from north to south with minor out-of-sequence thrusting and is consistent with a critical taper model for growth of the Himalayan thrust belt, following emplacement of midcrustal Greater Himalayan rocks. Our revised stratigraphy-based balanced cross section contains ~120–200 km greater shortening than previously estimated through the Greater, Lesser, and Subhimalayan rocks

Zircon U-Pb Ages and Hf Isotopes of the Askot Klippe, Kumaun, Northwest India: Implications for Paleoproterozoic Tectonics, Basin Evolution and Associated Metallogeny of the Northern Indian Cratonic Margin

Throughout the Himalayan thrust belt, klippen of questionable tectonostratigraphic affinity occur atop Lesser Himalayan rocks. Integrated U-Pb ages, Hf isotopic, and whole rock trace element data establish that the Askot klippe, in northwest India, is composed of Paleoproterozoic lower Lesser Himalayan rocks, not Greater Himalayan rocks, as previously interpreted. The Askot klippe consists of 1857 ± 19 Ma granite-granodiorite gneiss, coeval 1878 ± 19 Ma felsic volcanic rock, and circa 1800 Ma Berinag quartzite, representing a small vestige of a Paleoproterozoic (circa 1850 Ma) continental arc, formed on northern margin of the north Indian cratonic block. Detrital zircon from Berinag quartzite shows εHf 1850 Ma values between —9.6 and —1.1 (an average of —4.5) and overlaps with εHf 1850 Ma values of the Askot klippe granite-granodiorite gneiss (—5.5 to —1.2, with an average of —2.7) and other Paleoproterozoic arc-related Lesser Himalayan granite gneisses ( —4.8 to —2.2, with an average of —4.0). These overlapping data suggest a proximal arc source for the metasedimentary rocks. Subchondritic εHf 1850 Ma values (—5.5 to —1.2) of granite-granodiorite gneiss indicate existence of a preexisting older crust that underwent crustal reworking at circa 1850 Ma. A wide range of εHf 1850 Ma values in detrital zircon (—15.0 to —1.1) suggests that a heterogeneous crustal source supplied detritus to the northern margin of India. These data, as well as the presence of a volcanogenic massive sulphide deposit within the Askot klippe, are consistent with a circa 1800 Ma intra-arc extensional environment

(U-Th)/He ages from zircons collected from the upper Marsyangdi valley of central Nepal: summary

This dataset contains the results of three individual (U-Th)/He analyses using Zircons extracted from bedrock samples. The bedrock samples were collected from medium-high grade metamorphic rocks exposed along the upper Marsyangdi river valley. They were collected in order to add to the already available thermochronometric datasets and better constrain the exhumation history of the central Himalayas. The measurements of the ages follow the general procedures outlined in Reiners (2005) and Reiners et al. (2004)

(U-Th)/He ages from zircons collected from the upper Marsyangdi valley of central Nepal: raw data

This dataset contains the results of three individual (U-Th)/He analyses using Zircons extracted from bedrock samples. The bedrock samples were collected from medium-high grade metamorphic rocks exposed along the upper Marsyangdi river valley. They were collected in order to add to the already available thermochronometric datasets and better constrain the exhumation history of the central Himalayas. The measurements of the ages follow the general procedures outlined in Reiners (2005) and Reiners et al. (2004)

(U-Th)/He ages from zircons collected from the upper Marsyangdi valley of central Nepal

This dataset contains the results of three individual (U-Th)/He analyses using Zircons extracted from bedrock samples. The bedrock samples were collected from medium-high grade metamorphic rocks exposed along the upper Marsyangdi river valley. They were collected in order to add to the already available thermochronometric datasets and better constrain the exhumation history of the central Himalayas. The measurements of the ages follow the general procedures outlined in Reiners (2005) and Reiners et al. (2004)

40Ar/39Ar ages from muscovite collected in the far western Nepal Himalaya

This dataset contains the cooling age results of 3 individual 40Ar/39Ar analyses using muscovite extracted from bedrock samples collected from medium-high grade metamorphic rocks along the Api transect in far western Nepal. Bedrock samples were collected for thermochronologic cooling ages to better constrain the exhumation history of the Nepal Himalaya. 40Ar/39Ar analyses were performed at the University of Vermont Noble Gas Geochronology Laboratory. The summary results include the weighted mean age and 2σ error calculated from the 40Ar/39Ar age spectra. Where a mean age and error could not be calculated, the minimum and maximum age of individual steps in the 40Ar/39Ar age spectra were used to delimit the time window the samples were exhumed through the closure temperature