Structural and thermochronological studies of the Almora klippe, Kumaun, NW India: implications for crustal thickening and exhumation of the NW Himalaya

Abstract

Crystalline klippen over the Lesser Himalayan Metasedimentary Sequence (LHMS) zone in the NW Himalaya have specific syn- and post-emplacement histories. These tectonics also provide a means to understand the driving factors responsible for the exhumation of the rocks of crystalline klippen during the Himalayan Orogeny. New meso- and microscale structural analyses, and thermochronological studies across the LHMS zone, Ramgarh Thrust (RT) sheet and Almora klippe in the eastern Kumaun region, NW Himalaya, indicate that the RT sheet and Almora klippe were a part of the Higher Himalayan Crystalline (HHC) of the Indian Plate which underwent at least one episode of pre-Himalayan deformation and polyepisodic Himalayan deformation in ductile and brittle–ductile regimes. The deformation temperature pattern within the Almora klippe records a normal thermal profile from its base to top but an inverted thermal profile from the base of Almora klippe down towards the LHMS zone. New fission-track data collected across the RT sheet and Almora klippe along Chalthi–Champawat–Pithoragarh traverse in the east Kumaun region document the exhumation of both units since Eocene times. Zircon fission-track (ZFT) ages from the Almora klippe range between 28.7 ± 2.4 and 17.6 ± 1.1 Ma, and from the RT sheet between 29.8 ± 1.6 and 22.6 ± 1.9 Ma; and the apatite fission-track (AFT) ages from the Almora klippe range between 15.1 ± 1.7 and 3.4 ± 0.5 Ma, and from the RT sheet between 8.7 ± 1.2 and 4.6 ± 0.6 Ma. The age pattern and diverse patterns of the exhumation rates reflect a clear tectonic signal in the RT sheet and the Almora klippe which acknowledge that the Cenozoic tectonics influenced the exhumation pattern in the Himalaya.

Neogene Kinematic Evolution and Exhumation of the NW India Himalaya: Zircon Geo‐ and Thermochronometric Insights From the Fold‐Thrust Belt and Foreland Basin

“…This is likely due to the structural emplacement of the MCT hanging wall above the oLH, which buried the oLH to higher temperatures before these rocks were exhumed from shallow depths (<230 °C) to the surface. A similar relationship is observed in zircon fission track ages from Lansdowne where oLH rocks beneath the Lansdowne klippe yield pre‐Himalayan cooling ages, whereas the MCT klippe rocks yield late Oligocene zircon fission track ages (Yu, )—broadly similar to zircon fission track ages from across the Almora klippe to the east (Puniya et al, ). This indicates that rocks from these MCT klippen experienced a higher‐temperature thermal history prior to late Oligocene time, resetting the zircon fission track system before it was structurally emplaced above the oLH, which was not reset for fission track ages.…”

“…Evidence for unreset zircon fission track ages (Yu, ), yet reset ZHe ages from oLH rocks indicate that the oLH reached peak temperatures of ~180–230 °C due to MCT activity and overburden; thus, the oldest ZHe ages from these rocks record a minimal age for the onset of oLH exhumation after MCT emplacement. It is also possible that zircon fission track relationships and the ZHe age gap of >7 Myr observed across the structural boundary between the oLH and MCT klippen may be explained by normal‐sense reactivation of the MCT (e.g., Patel et al, ; Puniya et al, ; Vannay et al, ) above the active Tons thrust during middle Miocene time. This scenario also requires movement and emplacement of the MCT over the oLH before Tons thrust activation.…”

Mio‐Pliocene exhumation of the hinterland and sediments provenance of the Neogene strata from Kumaun Himalaya of northwest India: Insights from detrital fission‐track thermochronology

“…(Table 3). The AFT/ZFT central ages with 1σ error are given in Chiplakot Crystalline Belt (Patel et al, 2007) 7.6 ± 0.6 to 17.9 ± 0.9 -Almora Klippe (Patel et al, 2015;Singh and Patel., 2017;Puniya et al, 2019) 3.4 ± 0.5 to 15.1 ± 1.7 13.4 ± 0.6 to 28.7 ± 2.4…”

“…Such klippen are the Askot, Baijnath, Lansdown and Almora. Out of these, the Almora klippe is the largest and lies in close proximity to the MBT (Joshi & Tiwari, 2009;Puniya et al, 2019;Valdiya, 1980).…”

Penetrative Strain and Partitioning of Convergence‐Related Shallow Crustal Shortening, Across Scales, in the Lesser‐ and Sub‐Himalayan Thrusts: Insights From the Eastern Himalaya, Sikkim

“…They estimated ∼0.2–3 km of grain‐scale shortening from the MFT. Several studies have documented strain to address the internal deformation of different tectonostratigraphic units (GHS and LHS) in western Himalaya (N. Bose et al., 2018; Joshi et al., 2017, 2019; Puniya et al., 2019; Singh & Thakur, 2001; Tripathy et al., 2009), Nepal Himalaya (Goscombe et al., 2006; Law et al., 2004), and Arunachal Himalaya (Joshi et al., 2021). However, to the best of our knowledge, this is the first study that addresses how shortening gets partitioned across scales (thrust‐sheet scale vs. grain‐scale), strain geometry variation among all the exposed thrusts, and their contribution to the total shortening budget across an orogen.…”