• Soumyajit Mukherjee

Articles written in Journal of Earth System Science

• Shear heating by translational brittle reverse faulting along a single, sharp and straight fault plane

Shear heating by reverse faulting on a sharp straight fault plane is modelled. Increase in temperature (Ti) of faulted hangingwall and footwall blocks by frictional/shear heating for planar rough reverse faults is proportional to the coefficient of friction (μ), density and thickness of the hangingwall block (ρ). Ti increases as movement progresses with time. Thermal conductivity (Ki) and thermal diffusivity (k'i) of faulted blocks govern Ti but they do not bear simple relation. Ti is significant only near the fault plane. If the lithology is dry and faulting brings adjacent hangingwall and footwall blocks of the same lithology in contact, those blocks undergo the same rate of increase in shear heating per unit area per unit time.

• E–W strike slip shearing of Kinwat granitoid at South East Deccan Volcanic Province, Kinwat, Maharashtra, India

We study the margin of South East Deccan Volcanic Province around Kinwat lineament, Maharashtra, India, which is NW extension of the Kaddam Fault. Structural field studies document ∼E–W strike-slip mostly brittle faults from the basement granite. We designate this as ‘Western boundary East Dharwar Craton Strike-slip Zone’ (WBEDCSZ). At local level, the deformation regime from Kinwat, Kaddam Fault, micro-seismically active Nanded and seismically active Killari corroborate with the nearby lineaments. Morphometric analyses suggest that the region is moderately tectonically active. The region of intense strike-slip deformation lies between seismically active fault along Tapi in NW and Bhadrachalam in the SE part of the Kaddam Fault/lineament. The WBEDCSZ with the surface evidences of faulting, presence of a major lineaments and intersection of faults could be a zone of intraplate earthquake.

• Analyses of fold profiles by changing weight parameters of NURB curves

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.

• Moment of inertia for rock blocks subject to bookshelf faulting with geologically plausible density distributions

Moment of Inertia (MOI) for rock blocks that glided smoothly into book-shelf dispositions are deduced considering realistic linear and exponential 3D variations in density along specific axes/directions. Knowing (empirical) algebraic relations of density with depth, which could also be anything other than the exponential and linear variations considered in this work, geoscientists can deduce the MOI by following the same process. MOI for a homogeneous parallelepiped block along any direction is proportional to the length of the block in that direction. However, this simple relation does not hold true for rock blocks with variable densities. Nevertheless, as the block length increases, the MOI along that direction would also increase.

• 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 glidehorizon 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 majorlateral 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 $\sim11 \rm{Ma}$ 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.

• Arc-parallel compression in the NW Himalaya: Evidence from structural and palaeostress studies of brittle deformation from the clasts of the Upper Siwalik, Uttarakhand, India

The sub-Himalayan Upper Siwalik rocks, between the Main Boundary Thrust (MBT) to the north and the Main Frontal Thrust (MFT) to the south, are intensely brittle sheared and jointed. Our field studies around Dehradun (India) furnished at least eight small-scale brittle slip directions, viz., $\sim$top-to-SW/SSW (up), top-to-SW/SSW (down), top-to-NE/NNE (up), top-to-NE/ENE (down), top-to-NW (down), top-to-SE/SSE (up), top-to-SE/SSE (down) and top-to-NW/NNW (up). Additionally, we report near-vertical faults, four sets of joints (inclined: J$_{1}$ and J$_{2}$; near-vertical: J$_{\rm{1V}}$ and J$_{\rm{2V}}$). Palaeostress analyses using T-TECTO Studio X5 with all joint sets reveal two compression directions $\sim$ENE–WSW and $\sim$NNW–SSE. We propose two possible temporal relations between the joint sets: (i) J$_{1}$, J$_{2}$, J$_{\rm{1V}}$ and J$_{\rm{2V}}$ are coeval ($\sim$ENE–WSW compression) and (ii) J$_{\rm{1V}}$ and J$_{\rm{2V}}$ developed coevally ($\sim$ENE–WSW compression) followed by J$_{1}$ and J$_{2}$ ($\sim$NNW–SSE compression), because arc-parallel compression (if any) occurs later than arc-perpendicular compression. The presence of already well-known strike-slip faults, viz., the Yamuna tear fault and the Ganga tear fault, at high angles, $\sim$55$^{\circ}$ and $\sim$85$^{\circ}$ to the orogenic trend, implies a possible arc-parallel compression in the Siwalik Himalaya in the study area. This $\sim$NNW–SSE compression could also indicate a localised stress reorientation due to the curvature of the Thrust planes, viz., the MFT and the Asan Thrust (as observed in plan view) close to the study area. This study further shows that arc-parallel compression need not be restricted to the inner arc of an orogen, and/or, as in the case of the Himalaya, near the syntaxes.

• Kinematics of ductile shear zones with deformable or mobile walls

Shear zones are important phenomena in the Earth’s middle and lower crust and are of great interest to structural geologists. Models involving rigid boundaries moving parallel to themselves are extended here to include the case where (i) walls are deformable and (ii) mobile rigid walls approach each other. These models are combined with Couette and Poiseuille flow to define a broad range of kinematic possibilities. Deformable wall models lead to smooth transitions from deformed to undeformed materials as well as with the zone transitions to gentler and more spread out deformation. Mobile walls, on the other hand, lead to shear zones where shear sense can change along a shear zone boundary.

• Fuzzy set concept in structural geology: Example of ductile simple shear

Much of the time, geological data are not accurately known. Instead, geoscientists work with a range of possible data. These data can have a range since they can vary spatially and temporally. In this context, we demonstrate how a well-established structural geological model of velocity profile of ductile simple shear, expressed as an equation, can be furthered by fuzzy set concept. The implementation of fuzzy set concepts in terms of fuzzification in (geo)scientific models takes into consideration the uncertainties of magnitudes of the parameters that define the model. The present study shows that the fuzzified velocity profile of a ductile simple shear zone with parallel boundaries is effectively independent of the dip of the shear zone. This article demonstrates how models of (structural) geological processes can be modelled by fuzzy set approach capable of incorporating a range of magnitudes of parameters.

• Tectonic instability of the petroliferous upper Assam valley (NE India): A geomorphic approach

Tectonic (in)stability of terrains is of major concern in financial investment for hydrocarbon exploration from such terrains. Morphometric studies of a river can explain neotectonics of the terrain. This study presents the southeast (SE) frontier of the Brahmaputra river valley of the upper part of the state of Assam, northeast India, where geomorphic changes are observed through remote sensing studies. Two primary geomorphic observations of the study are: (1) formation of palaeostream at the valley due to cut off of headwater discharge due to the activation of frontal thrust (Naga thrust) along the SE boundary of the valley and the Naga Hills (Assam–Arakan fold-thrust belt), and (2) shifting of confluence points Lohit river (tributary of the Brahmaputra river) causing avulsion and formation of a river island (Dibru Saikhowa). The first event is related to the recent tectonic activities, i.e., uplift of a fault-propagation fold (Digboi anticline). The later event of migration of the Lohit river might be associated with tilting of the SE side of the basin due to activities along the blind thrust systems of the Himalayan foot-hills and the basement high over the last 25 years. This gradually elongated its drainage basins. The southern propagation of the frontal thrust systems of Arunachal Himalaya and Mishmi hills has already shifted the course of the Lohit river towards the south leading to significant changes in geomorphology over the last few decades. N–S-oriented compressional stress axis apparently indicates possible movements along the fault systems bounding the inter-drainage basin facilitating the fluvio-morphometric changes.

• # Journal of Earth System Science

Volume 131, 2022
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019