Devender Kumar

Timing, quantification and tectonic modelling of Pliocene–Quaternary movements in the NW Himalaya: evidence from fission track dating

Abstract Variable exhumation rates, deduced from the Pliocene–Quaternary FT zircon–apatite ages from the Himalayan Metamorphic Belt (HMB) of the NW Himalaya along the Sutlej Valley in Himachal Pradesh, have been modelled in the tectonic framework of fast exhumed Lesser Himalayan windows, which caused lateral extensional sliding of the metamorphic nappe cover along the well-known Main Central Thrust (MCT) and differential movements along thrust zones as well. In the northern belt of the Higher Himalayan Crystallines (HHC), two distinct clusters of the FT apatite ages have been deciphered: apatite ages having a weighted mean of 4.9±0.2 Ma (1 σ) in basal parts on the hanging wall of the MCT, and 1.49±0.07 Ma (1 σ) in the hanging wall of a newly, recognized NE, dipping Chaura thrust further north. Fast exhumation of the Chaura thrust hanging wall has been inferred at a rate of 4.82±0.55 mm/yr from the zircon–apatite cogenetic pairs during 1.54 Ma and 0.97 Ma, and 2.01±0.35 mm/yr since 1.49 Ma. In comparison, its foot wall has been exhumed at a much slower rate of 0.61±0.10 mm/yr since 4.9 Ma. The overlying Vaikrita Thrust zone rocks reveal an exhumation rate of 1.98±0.34 mm/yr from 2.70±0.40 Ma to 1.31±0.22 Ma and 2.29±0.66 mm/yr since 1.31±0.22 Ma. Using these data, a vertical displacement of ca. 2.08±0.68 km has been calculated along the Chaura thrust between 4.9 and 1.50 Ma on an average rate of 0.6 mm/yr. It is of the order of 1.18 km from 2.70 Ma to 1.54 Ma along the Vaikrita Thrust, and 0.78 mm/yr from 1.31 Ma to 0.97 Ma, and has behaved as an extensional normal fault during these periods. Tectonic modelling of the exhumation rates in the NW Himalaya reveals fastest uplifting Himalayan domes and windows like the Nanga Parbat in Pakistan, Suru and Chisoti domes in Zanskar and Kishwar–Kulu–Rampur Window axis in SE Kashmir and Himachal Pradesh during Pliocene–Quaternary. These windows appear to have caused lateral extensional sliding of the Himalayan metamorphic nappes in the lower parts. The middle parts of the HHC belt have witnessed both overthrusting and extensional faulting due to complex and variable exhumation patterns within the hanging and foot walls of the MCT and Vaikrita Thrust along the Sutlej Valley, thus causing movement of upthrust crustal wedge between the extensional ones. Thus, FT zircon–apatite ages provide evidence for the presence of a number of crustal wedges having distinct tectonothermal history within the HHC.

Detrital-zircon fission-track ages from the Lower Cenozoic sediments, NW Himalayan foreland basin: Clues for exhumation and denudation of the Himalaya during the India-Asia collision

Detrital-zircon fission-track (FT) ages from the Lower Cenozoic Sub-Himalayan foreland basin reflect the progressive effects of crustal thickening and exhumation on the Himalayan source rocks as a consequence of the India-Asia collision. The oldest stratum, the transgressive marine Paleocene-Eocene Subathu Formation (57–41.5 Ma) contains ca. 50 Ma detrital-zircon P1 peak, which was derived from the Indus Tsangpo Suture Zone and the Ladakh Batholith of the Asian plate. A dominant 302.4 ± 21.9 Ma peak with a few 520 Ma grains in this formation has been derived by erosion of the zircon partial-annealing zone (ZPAZ) of 240–180 °C. As the first imprint of the collision, this zone affected the Himalayan Proterozoic basement and its Tethyan sedimentary cover. Since the detritus in the Subathu has been derived both from the Indian and Asian plates, the possible suturing of these plates took place during the Subathu sedimentation. A sudden change in the provenance is recorded in the detrital-zircon FT cooling ages in the Oligo-Miocene Dagshai and Kasauli Formations, which have dominant 30 and 25 Ma P1 peaks, respectively. We interpret a distinct unconformity spanning ~10 m.y. between the Subathu and Dagshai Formations. Since ca. 30 Ma, molassic sedimentation coincides with shifting of the source rocks to the Himalayan metamorphic belt. This belt has sequentially undergone three distinct cooling and exhumation pulses after the ultrahigh-pressure–high-pressure (UHP-HP) metamorphism (53–50 Ma) in the extreme north and two widespread M1 and M2 metamorphisms (40–30 and 25–15 Ma) in the middle parts. These events appear to be largely responsible for the deposition of the ca. 30 Ma zircon Himalayan peak and ca. 25 and 15 Ma young Himalayan peaks, respectively; the latter appears within the Lower Siwalik Subgroup (13–11 Ma). During the Lower Siwalik deposition, pre-Himalayan peaks gradually decrease with the intensification of the Himalayan events in source rocks. In spite of uninterrupted fluvial sedimentation in the Dagshai-Kasauli–Lower Siwalik sequences since 30 Ma, breaks of ~5–7 m.y. in the zircon FT ages reveal pulsative cooling and exhumation in the well-identified source areas. Although cooling and exhumation of the Himalayan source rocks remained almost uniform during the Eocene, source heterogeneity is reflected in fluvial sedimentation since 37 Ma from Pakistan to Nepal in response to the India-Asia collision.

Long-term hydrochemical earthquake precursor studies at the Koyna-Warna reservoir site in western India

Koyna-Warna region of western India is an active seismic zone due to the Reservoir Triggered Seismicity (RTS). Earthquake precursor studies are carried out monitoring hydrochemical and stable isotope signatures in the groundwater from 15 bore wells since January 2005, for more than 12 years (January 2005 to February 2017). Depth of these boreholes ranges from 100 to 250 m. Cyclic or temporal variation in hydrochemistry is observed in few sensitive wells in Koyna region. The Govare well in Koyna is found to be most sensitive and the observed hydrochemical cycle is closely associated with local earthquakes of M > 5. The earthquakes M <5 occurring either in Warna cluster or close to the observation wells, did not generate hydrochemical precursory changes. The increase in hydrochemistry is hypothesized as mixing of two aquifer waters with different hydrochemistry. It is noted that a precursory hydrochemical cycle is observed during first quarter of 2015, but no earthquake M > 5.0 occurred till date. The cyclic changes in hydrochemistry, however, indicate on-going earthquake processes and an impending earthquake of M > 5 in the region.