Evolution of Quaternary alluvial fans and terraces in the intramontane Pinjaur Dun, Sub-Himalaya, NW India: interaction between tectonics and climate change

Quaternary alluvial fans in the tectonically active Pinjaur Dun, an intramontane valley in the Sub‐Himalaya, were deposited in front of the Nalagarh Thrust and were influenced both by tectonics and glacial climate fluctuations. The surface morphology indicates that an earlier set of first‐order fans (Qf1) became entrenched and onlapped by a series of second‐order fans (Qf2). The younger fan segments were then cut by a pair of terraces (T1 and T2). Quartz optically stimulated luminescence dating establishes that the Qf1 aggradational phase was initiated before 96·5 ± 25·3 ka and terminated after 83·7 ± 16·3 ka. This was followed by a period of incision, before Qf2 fan deposition started at 72·4 ± 13·4 ka and continued until 24·5 ± 4·9 ka. Sediment was deposited on the T1 (upper) and T2 (lower) terraces at 16·3 ± 2·1 and 4·5 ka, respectively, recording a return to overall degradation punctuated by minor deposition on terraces. The period of incision separating the younger and older fan deposits coincided with enhanced SW monsoon precipitation. The subsequent development of the Qf2 fans and their progradation until 20 ka suggest erosional unloading of the thrust hangingwall during a tectonically quiescent phase. Toe cutting, deposition of axial river and lacustrine facies, and retreat of Qf2 around 45 ka, indicate fanward shift of the axial river due to tilting of the valley towards the NE in response to reactivation of the Nalagarh Thrust. The cessation of Qf2 deposition around 20 ka and the onset of through‐fan entrenchment suggest reduced sediment supply but relatively high stream power during the last glacial maxima (LGM). The prolonged stream incision since the cessation of Qf2 deposition, with only minor depositional phases at 16·3 ± 2·1 and 4·5 ka, resulted from high water discharge and low sediment input during intensification of the SW monsoon and vegetation changes in the hinterland.     

Anthropogenic and impact spherules: Morphological similarity and chemical distinction – A case study from India and its implications

This paper provides first report of silica-rich anthropogenic spherules of varying colour, shape, size, surface texture and chemical composition found in road-deposited sediments (RDS) of Allahabad city, Uttar Pradesh, India. Morphological details and lithophile elemental composition of the silica-rich spherules are compared to microtektites and impact spherules from India to demonstrate their striking morphological similarities and chemical variability. This study suggests the need to use spherule data carefully while assigning an impact origin to spherule-finds or spherule-bearing lithological horizons.     

Partial drought conditions in Nepal / Les conditions de sécheresse partielle en Nepal

Abstract

In a mountainous country like Nepal, where precipitation from the monsoon (July) and the westerlies (January) occurs yearly, it is surprising to find partial drought conditions in space and time. This paper deals with the partial drought conditions and also the measures to combat them.     

Rb–Sr and Sm–Nd study of granite–charnockite association in the Pudukkottai region and the link between metamorphism and magmatism in the Madurai Block

Pudukkottai region in the northeastern part of the Madurai Block exposes the garnetiferous pink granite that intruded the biotite gneiss. Charnockite patches are associated with both the rock types. Rb–Sr biotite and Sm–Nd whole-rock isochron ages indicate a regional uplift and cooling at ～550 Ma. The initial Nd isotope ratios (\(\varepsilon _{\text {Nd}}^{\mathrm {t}}=-20\) to ?22) and Nd depleted-mantle model ages (T_DM = 2.25 to 2.79 Ga) indicate a common crustal source for the pink-granite and associated charnockite, while the biotite gneiss and the charnockite within it represent an older crustal source (\(\varepsilon _{\text {Nd}}^{\mathrm {t}}= -29\) and T_DM = > 3.2 Ga). The Rb–Sr whole-rock data and initial Sr–Nd isotope ratios also help demonstrate the partial but systematic equilibration of Sr isotope and Rb/Sr ratios during metamorphic mineral-reactions resulting in an ‘apparent whole-rock isochron’. The available geochronological results from the Madurai Block indicate four major periods of magmatism and metamorphism: Neoarchaean–Paleoproterozoic, Mesoproterozoic, mid-Neoproterozoic and late-Neoproterozoic. We suggest that the high-grade and ultrahigh-temperature metamorphism was preceded by magmatism which ‘prepared’ the residual crust to sustain the high P–T conditions. There also appears to be cyclicity in the tectono-magmatic events and an evolutionary model for the Madurai Block should account for the cyclicity in the preserved records.     

Elastic thickness estimates at northeast passive margin of North America and its implications

Global estimates of the elastic thickness (Te) of the structure of passive continental margins show wide and varying results owing to the use of different methodologies. Earlier estimates of the elastic thickness of the North Atlantic passive continental margins that used flexural modelling yielded a Te value of ~20–100 km. Here, we compare these estimates with the Te value obtained using orthonormalized Hermite multitaper recovered isostatic coherence functions. We discuss how Te is correlated with heat flow distribution and depth of necking. The E–W segment in the southern study region comprising Nova Scotia and the Southern Grand Banks show low Te values, while the zones comprising the NE–SW zones, viz., Western Greenland, Labrador, Orphan Basin and the Northern Grand Bank show comparatively high Te values. As expected, Te broadly reflects the depth of the 200–400°C isotherm below the weak surface sediment layer at the time of loading, and at the margins most of the loading occurred during rifting. We infer that these low Te measurements indicate Te frozen into the lithosphere. This could be due to the passive nature of the margin when the loads were emplaced during the continental break-up process at high temperature gradients.