• SANDEEP SINGH

Articles written in Journal of Earth System Science

• Vorticity patterns along the Main Central Thrust Zone, Alaknanda–Dhauli Ganga Valleys (Garhwal), Uttarakhand Himalaya

The Greater Himalayan Sequence (GHS), constituting the anatectic core of the Himalaya, is generally modelled as a mid-crustal southward extruding channel or wedge. Movements along the Main Central Thrust (MCT) in the south and the South Tibetan Detachment System (STDS) in the north and exhumation along the Himalayan front played an important role in the extrusion of the GHS from beneath the Tibetan plateau during the Miocene. To understand the kinematics of these orogen-scale shear zones, it is important to constrain the percentage of pure shear associated with them. In this paper, we present the kinematic vorticity data from the Main Central Thrust Zone (MCTZ), Alaknanda and Dhauli Ganga Valleys (Garhwal), Uttarakhand Himalaya. The mean kinematic vorticity number (W$_{m}$), which can be used to calculate the percentage of pure shear, has been estimated by analysing the rotational behaviour of rigid grains in a ductile matrix. The analysis reveals that pure shear provides significant contribution (30–52%) to the deformation associated with southward ductile shearing along the MCT, with the highest mean kinematic vorticity number (W$_{m}$) values close to the MCT. The results provide important quantitative constraints for the boundary conditions in the extrusion models. The Wm values from within the anatectic core have not been reported as most of the vorticity gauges fail due to increased deformation temperatures in this region.

$\bf{Highlights}$

$\bullet$ Orogen-scale mid-crustal southward extruding channel or wedge models deformation of the Great Himalayan Sequence (GHS) of the anatectic core, whose kinematics is to be understood by constraining the percentage of pure shear.

$\bullet$ Vorticity estimation near the Main Central Thrust Zone (MCTZ) is performed along the Alaknanda–Dhauli Ganga Valleys, Uttarakhand Himalaya along with critical analysis of published vorticity data from the other areas.

$\bullet$ Mean kinematic vorticity number (Wm), a quantitative estimator of pure shear percentage during non-coaxial deformation in a shear zone, varies between 0.675 and 0.875 within the MCTZ, corresponding to a pure shear percentage between 30% and 52%.

$\bullet$ A general trend of decreasing pure shear component towards the channel boundaries is explained by velocity profile within an extruding channel of hot and low-viscosity mid-crustal rocks and observed from the compiled vorticity data from other Himalayan traverses.

$\bullet$ Our results agree with the channel flow conceptual model and provide quantitative constraints on the percentage of pure shear associated with deformation within the GHS.

• Rb–Sr and Sm–Nd ages of the basement from Cauvery Basin: Crustal linkage to the Madurai Block, Peninsular India

The Cauvery Basin, formed as a result of the fragmentation of Gondwana during the Late Jurassic/Early Cretaceous period, is located on the eastern continental margin of India. The basin, covered under thick Phanerozoic sediments, has a geochronological lesser understood basement that forms the easternmost extremity of the Madurai Block in the Southern Granulite Terrane. This paper reports Rb–Sr and Sm–Nd ages between 2173–2307 Ma from northern onshore and Rb–Sr ages between 1223–983 Ma from southern offshore parts of the basin. The studied samples include basement core samples of hornblende–gneisses, granites, and metapelites (chlorite–biotite and garnet–biotite schists). These ages represent at least two episodes of tectonothermal events during Early Paleoproterozoic and Late Neoproterozoic, suggesting a polymetamorphic history of the basement and are correlatable with the reported events. The study also yielded Early Paleozoic Cambro–Ordovician whole-rock-biotite mineral Rb–Sr ages of 443–487 Ma, coinciding with the cooling stages of the Pan-African tectonothermal event post the thermal resetting in the studied basement of the Cauvery Basin. Further, the Sm–Nd systematics yielded two groups of model ages (2.1–3.4 Ga and 1.5 Ga), based on which two distinct crustal domains have been identified in the basement, viz., a Paleoarchean to Early Paleoproterozoic northern domain and an Early Mesoproterozoic southern domain, respectively. This has also been supported by distinct radiometric ages obtained fromthese domains.

• Three-dimensional attenuation tomography of Garhwal Himalaya, India obtained from strong motion data

This paper investigates the three-dimensional frequency-dependent attenuation structure of the Garhwal Himalaya in the Indian subcontinent. Based on the distribution of earthquakes and recording stations in the Garhwal Himalaya, the entire region of 152 ${\times}$ 94 km$^2$ is divided into 108 three-dimensional uniform rectangular blocks. These blocks are assumed to be of thickness 5 km that extends to 15 km depth. Each block represents the rock of different attenuation coefficients. The S-phase of strong motion records has been used to estimate the shear wave quality factor in each block by the inversion of spectral acceleration data. The inversion of spectral acceleration data is based on the modified technique of Joshi (2007) and Joshi et al. (2010) which was initially given by Hashida and Shimazaki (1984). The earthquake data of 19 events digitally recorded by 33 stations of the strong motion network between 2005 and 2017 have been used in this paper. The outcome of the inversion process is the shear wave quality factor at all frequencies present in the records. The three-dimensional attenuation structure at various frequencies is presented in this paper and is correlated with the regional tectonics of the Garhwal Himalaya. The correlation of attenuation structure at 10, 12 and 15 Hz with the tectonics of the region indicates that the shear wave quality factor has a strong relationship with the tectonics of the region. The values of the shear wave quality factor at different frequencies obtained from inversion have been used to obtain a relation of shear wave quality factor Q$_{\beta}$(f)=107f$^{0:82}$ for the region of Garhwal Himalaya for frequencies 10–16 Hz. The comparison of obtained shear wave quality factor with other studied relations clearly indicates that the obtained relation is close to what has been obtained in earlier studies and thereby indicates the reliability of obtained three-dimensional shear wave attenuation structure from inversion of spectral data.

• # Journal of Earth System Science

Volume 132, 2023
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Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019