Tapos Kumar Goswami

Structures and morphotectonic evolution of the frontal fold–thrust belt, Kameng river section, Arunachal Himalaya, India

The Neogene–Quaternary Siwalik foreland fold and thrust belt is studied for better understanding of tectonics along the Kameng river section of Arunachal Pradesh, India. The Kimi, Dafla, Subansiri, and the Kimin Formation correspond to Lower, Middle and Upper Siwaliks, respectively. The lithology in the foreland basin is dominantly sandstones, siltstones, claystones, carbonaceous shales, and boulder beds in the upper part. The structural style of the sedimentary sequence from the Main Boundary Thrust southward shows first order ramp-flat geometry. The brittle shear transfers slip across glide horizons to shallower depth. Repeated splay generations from a major regional-scale floor transfers slip from one glide horizon to another that shortens and thickens the crust. In the micro-scale, the lithological response in the structural development is well documented as pressure solution seams and other diagenetic deformation signatures. The basement asperity plays a significant role as the moving thrust front produced a major lateral ramp. The differential movement of the mountain front on both sides of the ramp is decipherable. This is especially true at the western part of the SE flowing Kameng river. The tectonic evolution of the area initiated with slip along the MBT

Mylonitic volcanics near Puging, Upper Siang district, Arunachal Pradesh: Evidence of oblique-slip thrusting

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Deformation of The Basement Gneissic Complex of The Assam Meghalaya Craton in and Around Sumer -Nayabunglow Area, Meghalaya, India

∼11 Ma ago along with the deposition of the Siwalik sediments. With southward propagation of the mountain front, the foreland basin shifted towards S, produced splay thrusts from the Himalayan Frontal Thrust-1 (HFT-1), which has been uplifting the Kimin and the older terraces.

Meteorite Fall at Komargaon, Assam, India

Analyses of Non-Uniform Rational B-spline (NURB) curve by varying weights at its nodal points and projection ratio produce several kinetically plausible symmetric and asymmetric fold morphologies in 2D promptly and efficiently with varied overall geometries, curvature of limbs, sharpness/bluntness of hinges, extent of hinge zone, tightness/interlimb angles, etc. Some of these folds are new geometries what other approaches, such as those with Bézier curve, did not produce so far. Natural fold profiles can be matched with NURB curves from photographs.

Subduction Related Magmatism: Constrains from the REE Pattern in the Lohit Batholith, Arunachal Pradesh, India

The Abor volcanics of the continental flood basalt affinity are extensively exposed in different parts of the Siang valley. These are associated with Yinkiong Group of rocks of Paleocene–Eocene age and represent syn-sedimentary volcanism in a rift setting. Subsequent folding and thrusting of the Siyom and Rikor sequences above the Yinkiong Group of rocks represent changes from syn-to-post collisional brittle-ductile tectonic episodes. Mylonitic Abor volcanics in the thrust contacts are studied at several locations in the north and south of Puging in the Siang valley. Both the Abor volcanics and associated Rikor and Yinkiong Group of rocks preserve meso to micro-scale fabric asymmetries indicating that the thrust contacts are shear zones of brittle-ductile nature containing mylonitic textures of high shear strain. Two distinct hitherto unrecognised shear zones in the north and south of Puging are named as North Puging Shear Zone (NPSZ) and South Puging Shear Zone (SPSZ). The kinematic indicators along the thrust contact indicate oblique slip thrusting of the Rikor and Siyom thrust sheets above the Yinkiong Group of rocks. This paper provides field evidence proving that the compression due the Burmese plate made oblique slip thrusting and zones of mylonitised volcanics possible and associated metasediments were formed. The kinematic indicators in the NPSZ and SPSZ respectively indicate top-to-SSE and top-to-NNW sense of shears.

Detrital zircon U-Pb geochronology of a Cenozoic foreland basin in Northeast India: Implications for zircon provenance during the collision of the Indian and Asian plates

The Neogene Siwalik sequence of western Arunachal Pradesh comprises northward dipping thrust sheets structurally below the Main Boundary Fault (MBT) and above the Main Frontal Thrust (MFT). In the foreland propagating thrust package, the compact and micaceous Dafla sandstone –shale-clay stone sequence is thrusted over the semi consolidated Subansiri sandstone –clay sequence. Near the zone of cataclasis the micaceous Dafla sandstones show microscale kink folds. These kink bands verge in the same direction of the vergence of the mesoscale folds in the Dafla sandstones. Both mobile and fixed hinge kink bands are observed showing lot of variation in the angles. At the initial stage the high water pressure under elevated confining pressure may play a vital role in the development of these kink bands. At the initial diagenetic temperature where mica shows considerable ductility the quartz and feldspar grains show brittle fracturing in response to the compression parallel to the (00)1 basal plane of mica. Kink band microstructures in Mica in the Dafla Formation of the Siwalik Group of rocks, West Kameng District, Arunachal Pradesh

Flanking structures as shear sense indicators in the Higher Himalayan gneisses near Tato, West Siang District, Arunachal Pradesh, India

The Basement Gneissic and Migmatitic Complex of the Assam Meghalaya craton near Sumer and Nayanbunglow area of Meghalaya underwent four phases of deformation. The tight isoclinal F1 folds with S1 axial planar cleavage and L1 fold axis lineation represents the earliest phase of intense compression. The F1 folds exhibits Class 2 and Class 1c geometry and refolded to open asymmetrical F2 folds with S2 axial planar cleavage and L2 fold axis lineations. The F1 and F2 are coaxial and F2 axial planar cleavage of S2 dips to the north. The third phase of deformation is weak and F3 folds with large wavelength and low amplitudes have axial planar cleavage dips to the southwesterly direction. All the three phases of deformation exhibit ductile phases of deformation and fold shapes reflect the competency contrast in the folded layer and the embedding medium. The fourth phase of deformation is brittle and the contarctional and extensional joints with small scale shear zones are ubiquitous. Introduction The Sumer-Nayabunglow area of Ri-Bhoi District of Assam Meghalaya Plateau (Sarma and Dey,1996; Sarma et al, 1998) covers a part of the Assam Meghalaya craton (Fig.1). The Assam Meghalaya plateau is an ancient landmass which was separated from the Indian Peninsular shield by the Garo-Rajmahal trough fault. (Pascoe, 1950; Ermenco et al, 1969). An intra-cratonic depression or an extension basin was created during the Precambrian time where thick pile of sediments deposited and got metamorphosed on a regional scale. The metasedimentaries constituted what is known as Shillong Group of rocks which unconformably overlies the Basement Gneissic and Migmatitic Complex (BGMC). The Shillong Group of rocks are first intruded by the epidiorites or metadolerites and further intruded by the granitic magma forming some plutons towards the waning phase of the Proterozoic period. The investigated area is bounded by the Survey of India toposheets 78o/13 and 78o/14 of 1:50,000 scale and latitudes 25o39 N to 25o47 N and longitudes 91o50E to 92oE longitudes. Fig.1. Position of Meghalaya and in NE India (shaded); location map of the study area (stippled) and Lineament map of shilling basin (schematic) Fig.2a.Geological map of the Sumer-Nayabunglow area of Meghalaya The gneissic rocks form he basement (BGMC) of the studied area is a complex structural amalgam showing evidences of superposed deformation and associated polymetamorphism (Fig.2). On the small scale the region may be characterized as a mosaic of blocks each having a specific tectonic style. The structural complexity of the area can be systematically studied and referred as deformation phases where the planar and linear structural elements provide excellent scope for undertaking a regional structural analysis of the area. In the present paper, we are reporting the dominant ductile and brittle phases that have affected the rocks and kinematics involved in the formation of these structures. Fig.2b.Structural map Sequences of deformation The rocks of the BGMC have undergone a total of four phases of deformation. However, the different deformation episodes are characterized by style of folding in the marker layers rather than the other planar and linear structures. First phase deformation Folds (F1) Folds of the first phase deformation are preserved in the competent layers embedded in the quartzofeldspathic gneissic host. They are the earliest representation of the folding episode in the form of tight isoclinal to tight appressed F1 folds tectonically attenuated and sheared out in the form of boudins (Fig.3d) and have been floated in the form of tectonic fish within he foliated matrix. They are thicker in the hinge part and thinner in the limbs and belong to Class 2 or more close to Class 1c type of folds (Ramsay, 1967). F1 folds show both dextral and sinistral vergences, inter-limb angle ranging between 5o to 20o. They are associated with highly penetrative axial planar cleavages (S1) which is parallel or sub parallel to the compositional layering (So) except the fold hinges where So and S1 intersect at an angle of 40o to 70o. Large limb of the F1 folds showthin limbs and thickened limbs and high amplitude wavelength ratio. Sometimes they also show pinch and swell structure (Figs.4a and b) and the boudinaged layer indicate pre to post boudi-