• Malay Mukul

Articles written in Journal of Earth System Science

• Strain variation in fold-and-thrust belts: Implications for construction of retrodeformable models

Deformation in fold-and-thrust belts such as the Himalayas can be represented by the displacement vector field. The strain component of the displacement vector field across the fold-and-thrust belt varies from near zero in external thrust sheets to a significant part of the field in internal thrust sheets. In addition, strain exhibits three-dimensional patterns in parts of internal sheets, near fault zones, and in the overturned limbs of fault-related folds due to superposition of penetrative-strain producing deformation events. This paper examines superposition of these strain producing deformation events in some detail and points out situations in fold-and-thrust belts wherein the finite strain becomes three-dimensional. This suggests that the plane-strain assumption used in the construction of retrodeformable models of fold-and-thrust belt evolution breaks down in these situations and the models lose their validity. Therefore, current techniques used for construction of retrodeformable models in fold-and-thrust belts need to be modified and three-dimensional models which include three-dimensional finite and incremental strain data need to be constructed for an accurate study of the evolution of geometry and kinematics in fold-and-thrust belts.

• Depositional environment and provenance of Middle Siwalik sediments in Tista valley, Darjiling District, Eastern Himalaya, India

The frontal part of the active, wedge-shaped Indo-Eurasian collision boundary is defined by the Himalayan fold-and-thrust belt whose foreland basin accumulated sediments that eventually became part of the thrust belt and is presently exposed as the sedimentary rocks of the Siwalik Group. The rocks of the Siwalik Group have been extensively studied in the western and Nepal Himalaya and have been divided into the Lower, Middle and Upper Subgroups. In the Darjiling–Sikkim Himalaya, the Upper Siwalik sequence is not exposed and the Middle Siwalik Subgroup exposed in the Tista river valley of Darjiling Himalaya preserves a ∼325 m thick sequence of sandstone, conglomerate and shale. The Middle Siwalik section has been repeated by a number of north dipping thrusts. The sedimentary facies and facies associations within the lithostratigraphic column of the Middle Siwalik rocks show temporal repetition of sedimentary facies associations suggesting oscillation between proximal-, mid- and distal fan setups within a palaeo-alluvial fan depositional environment similar to the depositional setup of the Siwalik sediments in other parts of the Himalaya. These oscillations are probably due to a combination of foreland-ward movement of Himalayan thrusts, climatic variations and mountain-ward shift of fanapex due to erosion. The Middle Siwalik sediments were derived from Higher- and Lesser Himalayan rocks. Mineral characteristics and modal analysis suggest that sedimentation occurred in humid climatic conditions similar to the moist humid climate of the present day Eastern Himalaya.

• Analysis of the accuracy of Shuttle Radar Topography Mission (SRTM) height models using International Global Navigation Satellite System Service (IGS) Network

The Shuttle Radar Topography Mission (SRTM) carried out in February 2000 has provided near global topographic data that has been widely used in many fields of earth sciences. The mission goal of an absolute vertical accuracy within 16 m (with 90% confidence)/RMSE $\sim$10 m was achieved based on ground validation of SRTM data through various studies using global positioning system (GPS). We present a new and independent assessment of the vertical accuracy of both the X- and C-band SRTM datasets using data from the International GNSS Service (IGS) network of high-precision static GPS stations. These stations exist worldwide, have better spatial distribution than previous studies, have a vertical accuracy of 6 mm and constitute the most accurate ground control points (GCPs) possible on earth; these stations are used as fiducial stations to define the International Terrestrial Reference Frame (ITRF). Globally, for outlier-filtered data (135 X-band stations and 290 C-band stations), the error or difference between IGS and SRTM heights exhibits a non-normal distribution with a mean and standard error of 8.2 ± 0.7 and 6.9 ± 0.5 m for X- and C-band data, respectively. Continent-wise, Africa, Australia and North America comply with the SRTM mission absolute vertical accuracy of 16 m (with 90% confidence)/RMSE $\sim$10 m. However, Asia, Europe and South America have vertical errors higher than the SRTM mission goal. At stations where both the X- and C-band SRTM data were present, the root mean square error (RMSE) of both the X- and C-bands was identical at 11.5 m, indicating similar quality of both the X- and C-band SRTM data.

• Accuracy analysis of the 2014–2015 Global Shuttle Radar Topography Mission (SRTM) 1 arc-sec C-Band height model using International Global Navigation Satellite System Service (IGS) Network

Global Shuttle Radar Topography Mission (SRTM) data products have been widely used in EarthSciences without an estimation of their accuracy and reliability even though large outliers exist in them.The global 1 arc-sec, 30 m resolution, SRTM C-Band (C-30) data collected in February 2000 has beenrecently released (2014–2015) outside North America. We present the first global assessment of thevertical accuracy of C-30 data using Ground Control Points (GCPs) from the International GNSS Service(IGS) Network of high-precision static fiducial stations that define the International Terrestrial ReferenceFrame (ITRF). Large outliers (height error ranging from –1285 to 2306 m) were present in the C-30dataset and 14% of the data were removed to reduce the root mean square error (RMSE) of the datasetfrom ∼187 to 10.3 m which is close to the SRTM goal of an absolute vertical accuracy of RMSE ∼10 m.Globally, for outlier-filtered data from 287 GCPs, the error or difference between IGS and SRTM heightsexhibited a non-normal distribution with a mean and standard error of 6.5 ± 0.5 m. Continent-wise,only Australia, North and South America complied with the SRTM goal. At stations where all the XandC-Band SRTM data were present, the RMSE of the outlier-filtered C-30 data was 11.7 m. However,the RMSE of outlier-included dataset where C- and X-Band data were present was ∼233 m. The resultssuggest that the SRTM data must only be used after regional accuracy analysis and removal of outliers.If used raw, they may produce results that are statistically insignificant with RMSE in 100s of meters.

• # Journal of Earth System Science

Volume 129, 2020
All articles
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