Geological Society of India

The Karakoram Shear Zone (KSZ) is a northwest-southeast trending dextral ductile shear zone that has mylonitized the Tangste and Darbuk granitoids of the southern margin of the Asian plate. Kinematic vorticity (Wk) has been estimated in 6 mylonitized Tangste granite samples, using Porphyroclast Hyperbolic Distribution (PHD) and Shear Band (SB) Analyses methods on well-developed quartz and feldspar porphyroclasts, and synthetic and antithetic shear bands respectively to visualize the overall deformation of the KSZ. The PHD and SB analyses yield Wk values ranging from Wk=0.29 to 0.43 and 0.45 to 0.93, respectively, thus indicating distinct pure and simple shear dominant regimes during different stages of the evolution of the KSZ. Strain has essentially been pure shear when southern edge of the Asian plate was initially juxtaposed against the Indian plate around 70 Ma, and changed to simple shear, possibly during the reactivation of this shear zone during 21-13 Ma to produce the shear bands.

The Middle Jurassic rocks of the Kaladongar Formation well exposed in the Kaladongar Hill range of the Patcham Island and Kuar Bet of the Northern Kachchh comprises of ∼450 m thick sequence of mixed siliciclasticcarbonate sediments intercalated with shales. These Mixed siliciclastic-carbonate sediments show wide variation in textural and mineralogical composition and represent genetically related six rock types: micritic sandstone, allochemic sandstone, sandy allochemic limestone, micrtic mudrock, sandy micrite and muddy micrite; which are highly bioturbated and show behaviourally diverse groups of trace fossils. Total 34 ichnogenera are identified, which includes, Arenicolites, Asterosoma, Beaconites, Bergaueria, Chondrites, Cochlichnus, Dactylophycus, Daedalus, Didymaulichnus, Diplocraterion, Gordia, Gyrochorte, Gyrolithes, Ichnocumulus, Laevicyclus, Lockeia, Margaritichnus, Monocraterion, Nereites, Ophiomorpha, Palaeophycus, Phoebichnus, Phycodes, Pilichnus, Planolites, Plug Shaped Form, Protovirgularia, Rhizocorallium, Scolicia, Skolithos, Taenidium, Teichichnus, Thalassinoides and Walcottia. These trace fossils are classified into six morphological groups namely, circular and elliptical structures; simple structures; branched structures; rosette structures; spreiten structures; and winding and meandering structures. These trace fossils are further group into eight assemblages which occurred together into mixed siliciclastic-carbonate sediments, include, Asterosoma assemblage, Gyrochorte assemblage, Rhizocorallium assemblage, Thalassinoides assemblage, Planolites-Palaeophycus assemblage, Phycodes assemblage, Ophiomorpha assemblage and Skolithos assemblage. The recurring pattern of these assemblages through the sequence displays the development of Skolithos and Cruziana ichnofacies and at places the mixed Skolithos-Cruziana ichnofacies which suggest a low wave and current energy conditions with intervening period of high wave and current energy conditions and an intermediate period of stressful environments, respectively. Sedimentological and ichnological data suggest that the deposition of the mixed siliciclastic-carbonate sediments of the Kaladongar Formation took place in the foreshore to offshore environment under fluctuating wave and current energy condition.

Partially due to lack of structural and sedimentary records to constrain the Jurassic-to-Cretaceous evolution, there was a missing process here in the eastern margin of Tibetan Plateau as it changed from the Paleo-Tethyan to Neo-Tethyan regime. Based on the analysis of 125 thermochronology ages (U/Pb, 40Ar/39A, 87Rb/86Sr, FT, U-Th/He) of igneous rocks from the eastern margin of Tibet, we propose a multisystem thermochronology approach to restore the cooling and emplacement of granites and decipher the missing process. Our integrated study suggests that a key Late Cretaceous (about 100Ma) tectonic change from the Paleo-Tethyan to Neo-Tethyan regime took place there. In the Late Triassic period, the initial emplacement of granite in the Songpan-Ganzi Fold Belt (SGFB) was characterized by a decrease in emplacement age and depth from west to east, and from north to south. Subsequently, all were followed by a very long period of slow cooling, which was followed by a rapid emplacement of about 100Ma. The intensive granite emplacement took place all over except northeastern SGFB, with a decrease in emplacement depth from west to east, which was linked with the far-field effect of Lhasa-Qiangtang collision. After this episode, the cooling history of granite in SGFB had a rapid emplacement on the subsurface under the control of the Neo-Tethyan regime. This process has control of the post Late Cretaceous regional magmatic activity and tectonics, as well as the sedimentary response in Sichuan and Xichang basin.

Growing human population along the river valleys in hilly terrain particularly on the fluvial sediments poses increasing risk of terrace instability and subsequent failure. Such instability and failure result in frequent loss of settlement, agricultural lands, and often lives. Alaknanda River valley in Uttarakhand comprises many fluvial terrace slopes that accommodate human settlement and hence one such fluvial terrace slope was taken as a case study area. On Feb. 28, 2022, Saari (or Sari) village, situated on the fluvial sequence, witnessed a slope failure collapsing 3 houses but no casualties. The hillslope with a total disturbed area of ∼3889±5.0 m2 and failed material volume of ∼16858±4.3 m3 partially dammed a tributary of Alaknanda River, which passes through the toe of the failed slope. The present study is an attempt to understand the instability that led to such failure because there was no extreme rainfall or earthquake prior to this failure. Pre- and post-failure topography of slope was used to demarcate the detachment and deposition zones. Pre-failure topography was used to perform the Finite Element Method (FEM) based slope stability simulation. In order to evaluate the possible contribution of exposed rockmass in failure, kinematic analysis was also performed. Results revealed the development of displacement pattern, particularly due to anthropogenic loads that must have initiated this failure. Such studies are primary requisites for an effective disaster mitigation in the NW Himalaya where growing human population on fragile hillslopes are at risk.

This study is a comparative investigation of the debris layer and underlying ice of the Koxkar Glacier using multi-frequency GPR with antennae having different frequencies. Together with analysis of the fluctuation of the radar signal amplitude and polarity, the debris layer and underlying ice were analyzed on the basis of high-resolution GPR images. It was found that the optimal average velocity in the shallow layer (0–4 m) is 0.06 m/ns. Images obtained with different frequency antennas have different characteristics; and the performance of the 200 MHz antenna for a debriscovered glacier is the best. The interpretation of typical GPR image is validated by using FDTD numerical model. Combining the debris layer thickness and the underlying ice structure, the effect of debris layer on ablation of glacier ice and forecast of the glacier change in the aspect of thickness-thinning and glacier retreat can be estimated. This study can provide as a reference to the formation mechanisms and estimation of the ice volume of glaciers covered by debris.