• SANTOSH KUMAR

      Articles written in Journal of Earth System Science

    • Characteristics of spectral aerosol optical depths over India during ICARB

      S Naseema Beegum K Krishna Moorthy Vijayakumar S Nair S Suresh Babu S K Satheesh V Vinoj R Ramakrishna Reddy K Rama Gopal K V S Badarinath K Niranjan Santosh Kumar Pandey M Behera A Jeyaram P K Bhuyan M M Gogoi Sacchidanand Singh P Pant U C Dumka Yogesh Kant J C Kuniyal Darshan Singh

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      Spectral aerosol optical depth (AOD) measurements, carried out regularly from a network of observatories spread over the Indian mainland and adjoining islands in the Bay of Bengal and Arabian Sea, are used to examine the spatio-temporal and spectral variations during the period of ICARB (March to May 2006). The AODs and the derived ˚Angström parameters showed considerable variations across India during the above period. While at the southern peninsular stations the AODs decreased towards May after a peak in April, in the north Indian regions they increased continuously from March to May. The ˚Angström coefficients suggested enhanced coarse mode loading in the north Indian regions, compared to southern India. Nevertheless, as months progressed from March to May, the dominance of coarse mode aerosols increased in the columnar aerosol size spectrum over the entire Indian mainland, maintaining the regional distinctiveness. Compared to the above, the island stations showed considerably low AODs, so too the northeastern station Dibrugarh, indicating the prevalence of cleaner environment. Long-range transport of aerosols from the adjoining regions leads to remarkable changes in the magnitude of the AODs and their wavelength dependencies during March to May. HYSPLIT back-trajectory analysis shows that enhanced long-range transport of aerosols, particularly from the west Asia and northwest coastal India, contributed significantly to the enhancement of AOD and in the flattening of the spectra over entire regions; if it is the peninsular regions and the island Minicoy are more impacted in April, the north Indian regions including the Indo Gangetic Plain get affected the most during May, with the AODs soaring as high as 1.0 at 500 nm. Over the islands, the ˚Angström exponent (𝛼) remained significantly lower (∼1) over the Arabian Sea compared to Bay of Bengal (BoB) (∼1.4) as revealed by the data respectively from Minicoy and Port Blair. Occurrences of higher values of 𝛼, showing dominance of accumulation mode aerosols, over BoB are associated well with the advection, above the boundary layer, of fine particles from the east Asian region during March and April. The change in the airmass to marine in May results in a rapid decrease in 𝛼 over the BoB.

    • Spatial variation of the aftershock activity across the Kachchh Rift Basin and its seismotectonic implications

      A P Singh O P Mishra Dinesh Kumar Santosh Kumar R B S Yadav

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      We analyzed 3365 relocated aftershocks with magnitude of completeness (Mc) ≥ 1.7 that occurred in the Kachchh Rift Basin (KRB) between August 2006 and December 2010. The analysis of the new aftershock catalogue has led to improved understanding of the subsurface structure and of the aftershock behaviour. We characterized aftershock behaviour in terms of 𝑎-value, 𝑏-value, spatial fractal dimension ($D_s$), and slip ratio (ratio of the slip that occurred on the primary fault and that of the total slip). The estimated 𝑏-value is 1.05, which indicates that the earthquake occurred due to active tectonics in the region. The three dimensional 𝑏-value mapping shows that a high 𝑏-value region is sandwiched around the 2001 Bhuj mainshock hypocenter at depths of 20–25 km between two low 𝑏-value zones above and below this depth range. The $D_s$-value was estimated from the double-logarithmic plot of the correlation integral and distance between hypocenters, and is found to be 2.64 ± 0.01, which indicates random spatial distribution beneath the source zone in a two-dimensional plane associated with fluid-filled fractures. A slip ratio of about 0.23 reveals that more slip occurred on secondary fault systems in and around the 2001 Bhuj earhquake (Mw 7.6) source zone in KRB.

    • Mineralogy and geochemistry of granitoids from Kinnaur region, Himachal Higher Himalaya, India: Implication on the nature of felsic magmatism in the collision tectonics

      Brajesh Singh Santosh Kumar Masao Ban Kazuo Nakashima

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      Felsic magmatism in the southern part of Himachal Higher Himalaya is constituted by Neoproterozoic granite gneiss (GGn), Early Palaeozoic granitoids (EPG) and Tertiary tourmaline-bearing leucogranite (TLg). Magnetic susceptibility values ($\lt$3 ×10$^{−3}$ SI), molar Al$_2$O$^3$/(CaO+Na$_2$O+K$_2$O) ($\geq$1.1), mineral assemblage (bt–ms–pl–kf–qtz ± tur ± ap), and the presence of normative corundum relate these granitoids to peraluminous S-type, ilmenite series (reduced type) granites formed in a syncollisional tectonic setting. Plagioclase from GGn (An$_{10}$–An$_{31}$) and EPG (An$_{15}$–An$_{33}$) represents oligoclase to andesine and TLg (An$_2$–An$_{15}$) represents albite to oligoclase, whereas compositional ranges of K-feldspar are more or less similar (Or$_{88}$ to Or$_{95}$ in GGn, Or$_{86}$ to Or$_{97}$ in EPG and Or$_{87}$ to Or$_{94}$ in TLg). Biotites in GGn (Mg/Mg+Fe$^t$ = 0.34–0.45), EPG (Mg/Mg+Fe$^t$ = 0.27–0.47), and TLg (Mg/Mg+Fe$^t$ = 0.25–0.30) are ferribiotites enriched in siderophyllite, which stabilised between FMQ and HM buffers and are characterised by dominant 3Fe$\rightleftarrows$2Al, 3Mg$\rightleftarrows$2Al substitutions typical of peraluminous (S-type), reducing felsic melts. Muscovite in GGn (Mg/Mg+Fe$^t$ = 0.58–0.66), EPG (Mg/Mg+Fe$^t$ = 0.31−0.59), and TLg (Mg/Mg+Fet = 0.29–0.42) represent celadonite and paragonite solid solutions, and the tourmaline fromEPG and TLg belongs to the schorl-elbaite series, which are characteristics of peraluminous, Li-poor, biotite-tourmaline granites. Geochemical features reveal that the GGn and EPG precursor melts were most likely derived from melting of biotite-rich metapelite and metagraywacke sources, whereas TLg melt appears to have formed from biotite-muscovite rich metapelite and metagraywacke sources. Major and trace elements modelling suggest that the GGn, EPG and TLg parental melts have experienced low degrees (∼13, ∼17 and ∼13%, respectively) of kf–pl–bt fractionation, respectively, subsequent to partial melting. The GGn and EPG melts are the results of a pre-Himalayan, syn-collisional Pan-African felsicmagmatic event, whereas the TLg is a magmatic product of Himalayan collision tectonics.

    • A local magnitude scale $M_{L}$ for the Saurashtra horst: An active intraplate region, Gujarat, India

      VISHWA JOSHI SUMER CHOPRA SANTOSH KUMAR

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      The calibration of local magnitude scale to local tectonics is essential for seismic hazard assessment and quantifying the seismicity in active regions. In the present study, we have developed a local magnitude scale $M_{L}$, for the Saurashtra region, which is a horst, located in the western continental margin of India. The local magnitude scale is developed using 1968 amplitude measurements from horizontal component recordings of 319 earthquakes, obtained from sites in the Saurashtra region, with hypocentral distances ranging from 3 to 298 km. All the 1968 amplitude measurements were inverted simultaneously to determine attenuation curve, magnitudes and station corrections for the studied region. The resultant distance correction term for the Saurashtra is $-log(A_{0}) = 1.31 log(r/100) + 0.0002 (r - 100) + 3$ for 100 km normalization, where $A_{0}$ is the distance correction and $r$ is the hypocentral distance. The distance correction term ($-log A_{0}$) suggests that the attenuation in the Saurashtra region is lower as compared to neighbouring Kachchh region. The station corrections obtained in the present study varies from $-0.31$ to $+0.24$. Overall, standard deviation of the magnitude residuals without station correction is 0.28, while with station correction, it is 0.23, which indicates that applying station correction reduces the variance by 31% and brings the average residual closer to zero.

    • Geochemistry of Neoproterozoic Chhaosa shales, Simla Group, Lesser Himalaya: Its implications on provenance and tectonics

      PRIYANKA MAZUMDAR ANANYA MUKHOPADHYAY ALONO THORIE TITHI BANERJEE SANTOSH KUMAR RAI

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      Neoproterozoic Chhaosa shales of Simla Group, Lesser Himalaya, were considered for major and trace element analysis to delineate palaeo-weathering, palaeo-oxygenation, tectonic setting, sediment maturity, and provenance. The shales exhibit a significant proportion of SiO$_{2}$, Al$_{2}$O$_{3}$, K$_{2}$O, MgO, Fe$_{2}$O$_{3}$, Zr, Zn, Rb, V, Cr, Sr, Y, Co, Ga, Th, Nb, Sc, and U. Source rocks are primarily of granitic (acidic) origin, as indicated by Al$_{2}$O$_{3}$ wt.% vs. TiO$_{2}$ wt.% and Cr (ppm) vs. Ni (ppm) bivariate plots. Th/Cr, Cr/Th, Th/Sc, and Th/Co values of studied samples have been correlated with post-Archaean Australian average shale (PAAS) and upper continental crust (UCC) values, which indicate that the source rock is felsic. Palaeo-weathering condition of the source is delineated by average CIA, chemical index of weathering (CIW), and A–CN–K triangular plot values. The shales depict intermediate to strong chemical weathering and semi-arid to humid climatic conditions. V/Cr, Cu/Zn, Ni/Co, and U/Th values reveal sedimentation of Chhaosa Formation under oxic atmospheric conditions, which corroborates with the inferred oxic conditions of the Neoproterozoic. SiO$_{2}$ vs. K$_{2}$O/Na$_{2}$O discriminant diagram exhibits deposition along a passive margin field. ICV values suggest deposition of compositionally mature sediments under tectonically stable conditions. Deposition of Chhaosa delta system occurred during a relatively quiescent stage of tectonism between intermittent phases of rifting related to the Rodinia Supercontinent.

      $\bf{Highlights}$

      $\bullet$ This is the first geochemical work ever done for the shales of Neoproterozoic Chhaosa Formation of Simla Group, Lesser Himalaya.

      $\bullet$K$_{2}$O/Na$_{2}$O vs. SiO$_{2}$/Al$_{2}$O$_{3}$, TiO$_{2}$ vs. Al$_{2}$O$_{3}$, and Zr vs. TiO$_{2}$ binary diagrams of the studied shales indicate felsic source rock.

      $\bullet$ High CIA, CIW, A–CN–K and Al$_{2}$O$_{3}$ values of Chhaosa shales support intermediate to strong weathering of source rock.

      $\bullet$ K$_{2}$O/Na$_{2}$O vs. SiO$_{2}$ plot, K$_{2}$O vs. Na$_{2}$O, plot and CaO–K$_{2}$O–Na$_{2}$O ternary plot represent deposition of Chhaosa deposits in a passive margin setting.

      $\bullet$ Values of Ni/Co, Cu/Zn, V/Cr, authigenic U and U/Th, represent a well-oxygenated condition during the deposition of the Chhaosa delta system. This is consistent with the concept of an increase in oxygen levels during the Neoproterozoic era is favoured by many workers.

    • Protracted Paleozoic–early Triassic thermal events in the Almora nappe, Kumaun Lesser Himalaya, India: Evidence from zircon U–Pb geochronology of Almora paragneiss

      SAURABH GUPTA SANTOSH KUMAR

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      The Almora nappe in the Kumaun Lesser Himalaya (KLH) is mainly composed of Saryu Formation, Gumalikhet Formation and Cambrian felsic intrusives. Zircons from a paragneiss, exposed near Kwarab along Sualbari–Almora transect, of Saryu Formation, are subjected to U–Pb geochronology in order to delineate the timing of tectono-thermal events and its implication on pre-Himalayan orogenic cycle. Zircon rims developed over Neoarchean–Paleoproterozoic zircon cores record U–Pb ages which broadly correspond to three major pre-Himalayan tectono-thermal (metamorphic) events, early Paleozoic, late Paleozoic and early Triassic. These protracted thermal imprints likely represent the metamorphic episodes experienced by the rocks of Almora nappe. The observed early Triassic rim ages of zircons from paragneiss are very well correlatable with the opening-closure of the Palaeo-Tethys Ocean.

    • Estimation of Coda-Q for Mainland Gujarat region of Western Deccan Volcanic Province, India

      ARPAN SHASTRI SANTOSH KUMAR

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      In the present study, we have estimated the quality factor Coda-Q (Q$_c$) using the single backscattering model for Mainland Gujarat region of Western Deccan Volcanic Province (DVP) India. The said region comes under seismic zone III as per seismic zonation map of India and is vulnerable for earthquakes of magnitude up to 6.0. Mainland Gujarat comprises of the eastern rocky highlands and western alluvial plains. The broadband waveforms of 80 local earthquakes (Mw 2.5–4.3) having depths in the range (2.0–34.0 km) recorded at eight stations of Mainland Gujarat have been used for the analysis. The average values of Q$_c$ for the lapse time windows of 20–90 s with standard error varies respectively from 113 ± 12 to 411 ± 37 at 2 Hz and 1008 ± 150 to 3600 ± 400 at 16 Hz, respectively. This suggests that the region is more attenuative and heterogeneous than the Kachchh and Saurashtra of DVP. The increase in ‘Q$_c$’ values with lapse time displays the depth dependence of ‘Q$_c$’ because of the fact that longer lapse time windows will sample larger area. We have compared the frequency-dependent relations for ‘Q$_c$’ derived here with those of other parts of India and the World. The study is inevitable for the estimation of source parameters, thereby, evaluation of seismic hazard of the region.

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