Neotectonic Control on the Geomorphic Evolution of the Gangotri Glacier Valley, Garhwal Himalaya

The paper records evidences of neotectonic activities in the Gangotri glacier valley that are found to be responsible for the present-day geomorphic set-up of the area since the last phase of major glaciation. Geomorphological features indicate the presence of a large glacier in the valley in the geological past. Prominent planar structures present in the rocks were later on modified into sets of normal faults in the present-day Himalayan tectonic set-up giving rise to graben structures. The block nearest the snout is traversed by the NW-SE trending Gaumukh fault. A number of terraces mark the entrenchment of Bhagirathi River in this part. The contrasting drainage morphometric parameters of two sides of the valley and asymmetric recessional patterns of the tributary glaciers further document movement along the fault. The distribution and orientation of debris fans also seem to be controlled by neotectonic activity. The neotectonic activity that followed the process of deglaciation has brought the glacially carved, wide U- shaped valley in contact with the present-day fluvially incised narrow and relatively deep valley. The wider segments have become sites of active deposition of glacially eroded debris. The low gradient and excessive filling has resulted in the river attaining a braided nature in these segments.     

Applicability of Critical State Concept in Determination of Triaxial Strength of Non-Persistent Jointed Rock

Geotechnical and Geological Engineering - Rocks encountered in civil and mining engineering structures are generally intersected by discontinuities. The triaxial strength behaviour of a jointed...     

Estimating Solar Flux Density at Low Radio Frequencies Using a Sky Brightness Model

Sky models have been used in the past to calibrate individual low radio frequency telescopes. In this article we generalize this approach from a single antenna to a two element interferometer, and formulate the problem in a way that allows us to estimate the flux density of the Sun using the normalized cross-correlations (visibilities) measured on a low resolution interferometric baseline. For wide field-of-view instruments, typically the case at low radio frequencies, this approach can provide robust absolute solar flux calibration for well characterized antennas and receiver systems. It can provide a reliable and computationally lean method for extracting parameters of physical interest using a small fraction of the voluminous interferometric data, which can be computationally prohibitively expensive to calibrate and image using conventional approaches. We demonstrate this technique by applying it to data from the Murchison Widefield Array and assess its reliability.     

Observations of mid-troposhheric circulation from water vapour radiances of polar orbiting satellites

Water vapour tracers can provide useful information on winds at ≈ 500mb by observing the 6·7μ radiances. This fills the data gap in the cloud motion winds provided by conventional meteorological geostationary satellites. There is no geostationary satellite at present over the Indian Ocean with 6·7μ imaging capability to provide mid-tropospheric winds. The potentials of 6·7μ radiances, available from polar orbiting satellites, for mid-tropospheric circulation features have been examined in this study. Tiros-N satellite data of May 1979 and ECMWF level-IIIb wind data were analysed to relate the radiances with the streamlines. We find that the radiances of 6·7μ from orbiting satellites agree well with the wind field.     

Equatorial Indian Ocean evaporation estimates from operational meteorological satellites and some inferences in the context of monsoon onset and activity

The estimation of evaporation from the sea surface is not yet achieved adequately by remote sensing techniques, in general. However, for approximate averaged estimates over moderate space and time scales over a specific tropical region, e.g., weekly values over the Indian Ocean as needed in monsoon moisture diagnosis, it may be possible to extrapolate satellite wind and humidity data to the ocean surface and then use bulk aerodynamic parameterization for estimating evaporation. In the present investigation, GOES low-level cloud winds and TIROS-N moisture profiles over the Indian Ocean are extrapolated to the ocean surface. The planetary boundary layer (PBL) wind shear is obtained over different sub-regions and periods during the monsoon season, by reference to objectively analysed fields. These shear values are applied to GOES satellite winds to obtain sea-surface winds. The humidity extrapolation was based on (i) an exponential fit for water vapour density and (ii) a vertical distribution of relative humidity approximately proportional to atmospheric pressure. The exchange coefficient is varied slightly depending on wind speed and boundary-layer stability inferred approximately from TIROS-N sea surface temperatures and temperature profiles. The evaporation estimate as based on these satellite parameters is assessed by comparison with ships' surface observations. Sensible heat exchange is also estimated and assessed. Some inferences based on these estimates are also presented, in relation to monsoon onset and activity.     

A study on the hyperspectral signatures of sandy soils with varying texture and water content

This paper examines the hyperspectral signatures (in the visible near-infrared (VNIR)–shortwave infrared (SWIR) regions) of different samples of sandy soils possessing varying grain size and water content. Ten samples of sandy soils with differing textures and water contents were examined using a hyperspectral radiometer operating in the 350–2,500-nm range, and the spectral curves were obtained. Analyses of the curves indicate that grain size of sand has a considerable influence on the spectral response in the visible, NIR and SWIR regions, i.e. there is a decrease in the overall reflectance with increase in grain size. While the 350–575-nm region exhibits maximum overlap of spectral reflectance and, hence, least spectral separability between the soil types, the 2,000–2,100-nm region exhibits highest spectral separability between the sand samples of different textures. As regards the water content, it is seen that overall reflectance increases with decreasing water content in the sample, especially in the 1,400–1,800-nm regions. Further, the slope of the curves in the 1,890–2,100-nm region shows a well-defined relationship with the water content. Based on these well-defined relations, it is inferred that hyperspectral radiometry in the VNIR and SWIR regions can be used to estimate the texture and water content of sandy soils.     

Stability assessment of slide zones in Lesser Himalayan part of Yamunotri pilgrimage route,Uttarakhand, India

The present study deals with the slope stability analysis and geotechnical assessment of a part of pilgrimage route to one of the holy shrines of India, i.e. Yamunotri. The route also embraces a proposed site for 204 m high concrete gravity dam across River Yamuna near Lakhwar village with the aim of generation of 300 MW power. Several slide zones were identified and based on the discontinuity orientation, structural features and debris materials, they are recognised as planar, wedge or circular failure types. The morphological dimensions, structural data, orientations and geotechnical parameters of circular failure slides within weathered quartzites, phyllites and shales were evaluated by extensive field work and by laboratory tests for their stability analysis. Internal mechanisms, cohesion and angle of internal friction that resist shear stress in slope materials, obtained from direct shear test are showing minor variation due to relatively consistent grain size distribution and mineralogical composition. General slope is about 40°–43° with sparse vegetation. Materials in and around slide zones are sands with appreciable amount of fines falling in SP–SM category as per Unified Soil Classification System (USCS), except Niste B slide which has clean sands lying in SP group. Factor of safety, computed by requisite parameters of strength, soil and slope properties in circular failure charts, varies from 1.02 to 1.23 in dry conditions while it reduces below unity with increasing saturation, representing stable conditions in dry conditions but with seepage and saturation along the cracks and discontinuities during rainfall make them unstable. Presence of steep slopes, proximity to stream channels and significant weathered and jointed area are causative factors in the route with rainfall and road widening as major triggers initiating the failure.     

Direct dating of thick‐ and thin‐skin thrusts in the Peruvian Subandean zone through apatite (U–Th)/He and fission track thermochronometry

Although the structure of the central Peruvian Subandean zone is well defined, the timing of thrust‐related exhumation and Cenozoic sedimentation remain poorly constrained. In this study, we report new apatite (U–Th)/He (AHe) and fission track (AFT) ages from thrust‐belt and foreland strata along three published balanced cross sections. AHe data from the northern, thick‐skinned domain (i.e. Shira Mountain, Otishi Cordillera and Ucayali Basin) show young AHe ages (ranging from 2.6 ± 0.2 to 13.1 ± 0.8 Ma) compared with AFT ages (ranging from 101 ± 5 to 133 ± 11 Ma). In the southern Camisea Basin, where deformation is mainly thin‐skinned, AHe and AFT ages have been both reset and show young cooling ages (3.7 ± 0.8 Ma and 8 ± 2 Ma respectively). Using low‐temperature thermochronology data and the latest fission track annealing and He diffusion codes, the thermal history of the study area has been reconstructed using inverse modelling. This history includes two steps of erosion: Early Cretaceous and late Neogene, but only Neogene sedimentation and exhumation varies in the different sectors of the study area. From a methodological point of view, large AHe data dispersion point to the need for refinement of AHe damage and annealing models. The influence of grain chemistry on damage annealing, multiple age components and the possibility of fission tracks as traps for He need further consideration. For the central Peruvian Subandes, AHe and AFT ages combined with balanced cross sections emphasize the dominant control of Paleozoic inheritance rather than climate on Cenozoic infilling and exhumation histories. Finally, our data provide the first field example of how thick‐skinned thrust‐related deformation and exhumation in the Subandes can be directly dated through AHe thermochronology.     

Seasonal extreme rainfall variability over India and its association with surface air temperature

Theoretical and Applied Climatology - Spatial variability in catchment processes is crucial for hydrologic and water resources planning and management. The spatial density of ground-based rain...