Santanu Bose

Presidency University, Kolkata · Department of Geology

This study recognizes a transition in the modes of ductile deformation across the Himalayan Crystalline Complex (HCC) from field and microstructural evidence. This transition leads to the Main Central Thrust (MCT) zone localization in the Darjeeling-Sikkim Himalaya. In the initial stage of crustal shortening (prior to ~16 Ma), the HCC experienced t...

The Himalaya and the Tibetan plateau, the highest mountain range on Earth, have been growing continuously for the last 55 Myr since India collided with Eurasia. The forces driving this protracted mountain building process are still not fully understood. Although subduction zones are considered the main driving force for plate tectonics, mantle flow...

The outer wedge of the Indo-Burma wedge (IBW) has resulted due to oblique subduction of the Indian Plate below the Burma. In this study, we will use the analysis of outcrop-scale structures from Tripura-Mizoram fold belt (TMFB) to evaluate the structural evolution of the outer wedge of IBW. TMFB belongs to the widest section of the outer wedge that...

The Tripura Mizoram fold belt (TMFB) belongs to the widest segment of the outer wedge of Indo-Burma Wedge (IBW). The structure of the outer wedge is characterized by a series of north-south trending anticlines-and synclines of varying tightness, which progressively decreases towards the foreland direction. To explain their development, earlier work...

Tripura Mizoram fold belt (TMFB) belongs to the outer wedge of the Indo-Burma wedge (IBW), resulting from the accretion of a thick pile of Neogene sediments lying over the subducting Indian Plate foundering below the Burmese Plate in the east. TMFB covers an east-west stretch of around 270 km (along 23.5° N latitude), lying over a décollement at sh...

Tripura Mizoram fold belt (TMFB) belongs to the outer wedge of the Indo-Burma wedge (IBW), resulting from the accretion of a thick pile of Neogene sediments lying over the subducting Indian Plate foundering below the Burmese Plate in the east. TMFB covers an east-west stretch of around 270 km width (along 23.5° N latitude), lying over a decollement...

Tripura Mizoram fold belt (TMFB) in northeast India has been evolving by accreting accumulated Surma basin sediments, overlying Indian lithosphere that subducts eastward beneath the Burma Plate. Deformation in the entire stretch of the outer belt is characterized by developing a series of anticlinal ridges and synclinal valleys of variable amplitud...

The mid-crustal ramp on the Main Himalayan Thrust (MHT) in the Lesser Himalaya Sequence (LHS) is the most critical geometrical asperity to trigger major seismic events in the Himalaya, including the 2015 Mw 7.8 Gorkha, Nepal earthquake. However, it is still not well understood what caused the MHT to ramp up at laterally varying locations in central...

Integration of spatial strain localization and folding in 2-layer viscous models to show how folds of varying morphology develop in a single progressive deformation episode

It has been widely shown that the geometry of the basal Main Himalayan Thrust (MHT) below the Himalayan mountain belts provides first-order control on the along-strike variations in the structure, topography, low-temperature thermochronological ages, precipitation, exhumation and erosion patterns. The mid-crustal ramp on the MHT, below the lesser H...

The Tripura-Mizoram Fold Belt (TMFB) represents the outer wedge part of the Indo-Burma wedge. The depositional age of fluvial facies of the fold belt has been estimated to be around early Miocene. TMFB is a west directed fold-and-thrust belts, showing tapering of its width towards the south. Based on prevailing knowledge on the relative movement on...

Fold and thrust belts (FTBs) accommodate tectonic convergence through folding and faulting of crustal rocks during a collisional event between two continental plates. Although evidence of distributed deformation is common in FTBs that usually leads to continuous foliations and regionally occurring ductile structures of multiple orders, it has rarel...

Occurrence of any conspicuous ramp on the low angle basal detachment causes hindrance on the overall wedge progression and behaves as an asperity by localizing stress in the inter-seismic gaps. The mid-crustal ramp on the Main Himalayan Thrust (MHT) results in strain partitioning by focusing seismicity, including the 2015 magnitude 7.8 Gorkha earth...

Collapse of fold-thrust belts and modification of folded structures under the effect of gravity (insights from scaled laboratory analogue models)

The Himalayas are the archetype of continental collision, where a number of long-standing fundamental problems persist in the Greater Himalayan Sequence (GHS): (1) contemporaneous reverse and normal faulting, (2) inversion of metamorphic grade, (3) origin of high- (HP) and ultrahigh-pressure (UHP) rocks, (4) mode of ductile extrusion and exhumation...

The Himalayas are the archetype of continental collision, where a number of long-standing fundamental problems persist in the Greater Himalayan Sequence (GHS): (1) contemporaneous reverse and normal faulting; (2) inversion of metamorphic grade; (3) origin of high- (HP) and ultra-high (UHP) pressure rocks; (4) mode of ductile extrusion and exhumatio...

The Lower Lesser Himalayan Sequence (L-LHS) in Darjeeling- Sikkim Himalaya (DSH) displays intensely deformed, low-grade metasedimentary rocks, frequently intervened by granite intrusives of varied scales. The principal motivation of our present study is to constrain the timing of this granitic event. Using 40Ar/39Ar geochronology, we dated muscovit...

Orogenic wedge is a complex geological structure, which usually evolve in a long time scale in a convergent system. Although a large number of experimental and theoretical studies have been carried out to understand the evolution of thrust wedges, majority of them focused on the influence of frictional resistance at the basal decollement (mb) and t...

Folding in viscous wedge depicting changes in fold initiation, fold propagation, changes in fold morphology and preferential localization of strain during progressive deformation (insights from scaled laboratory analogue models)

A moment magnitude (Mw) 7.8 earthquake associated with a Mw 7.3 aftershock hit the Gorkha region near Kathmandu, Nepal on April 25, 2015. The rupture propagated eastward for about 140 km and caused thousands of deaths. The focal mechanism of the Gorkha earthquake shows thrust sense over the mid-crustal steeply dipping ramp on the basal décollement...

Status of Research in Structural Geology; the Indian Scene During the Last Five Years

Many fold-and-thrust belts display multi-storied thrust sequences, characterizing a composite architecture of the thrust wedges. Despite dramatic progress in sandbox modelling over the last three decades, our understanding of such composite thrust-wedge mechanics is limited and demands a re-visit to the problem of sequential thrusting in mechanical...

We documented centimeter to meter scale, isolated ductile shear zones in the Chotanagpur Granite Gneiss Complex, Eastern India. They display distortion patterns of mylonitic foliation, indicating qualitatively little flattening strain in them. Using analog and finite element (FE) model experiments we evaluated the degree of transpression mechanical...

This study investigates the first-order Himalayan mountain topography from the perspective of deep-crustal flow patterns in the Indo-Asia collision zone. Using a thin-viscous-sheet model we theoretically predict that flat hinterland topography with a stable elevation (Type I) can develop only when the lithospheric slab underthrusts with a threshold...

The 2011 Mw9.0 Tohoku-oki earthquake ruptured to the trench with maximum coseismic slip located on the shallow portion of the plate boundary fault. To investigate the conditions and physical processes that promoted slip to the trench, Integrated Ocean Drilling Program Expedition 343/343 T sailed one year after the earthquake and drilled into the pl...

In long-time scale, viscous wedge models can better explain the orogenic wedge deformation considering the occurrence of dominant ductile deformation in fold-and-thrust belts (FTBs). We present here an analogue model to understand the development of lithospheric scale wedge in convergent belts using Newtonian viscous material, polydimethylsiloxane...

Deformation in an experimental viscous wedge (insights from an analogue model)

The Eastern Lesser Himalayan fold-thrust belt is punctuated by a row of orogen-transverse domal tectonic windows. To evaluate their origin, a variety of thrust-stack models have been proposed, assuming that the crustal shortening occurred dominantly by brittle deformations. However, the Rangit Window (RW) in the Darjeeling-Sikkim Himalaya (DSH) sho...

Many are the situations in Geology in which non-deformable and deformable inclusions are carried about in suspension by the motion of a fluid, or a rock behaving like a fluid. Therefore, it is of crucial importance to Geosciences to understand the rotational behaviour of inclusions in viscous flow, and the effects in the matrix deformation. A major...

Deep Drilling for Earthquake Clues The 2011 M w 9.0 Tohoku-Oki earthquake and tsunami were remarkable in many regards, including the rupturing of shallow trench sediments with huge associated slip (see the Perspective by Wang and Kinoshita ). The Japan Trench Fast Drilling Project rapid response drilling expedition sought to sample and monitor the...

Sandbox experiments are used to study frontal thrust fault spacing, which is a function of physical properties within the thrust wedge. We consider three styles of thrust progression in mono-vergent wedges: Style I, II and III. In Style I, frontal thrusts progress forelandward, maintaining a constant spacing, whereas Style II and Style III progress...

We review the recent developments of structural geology in India, emphasizing the studies on brittle and ductile deformations in rocks. The Himalayan-Tibetan Mountain system has now become a focal point of tectonic studies, often leading to debatable, but exciting ideas and hypotheses. We provide a glimpse of the Indian contributions towards unders...

This paper investigates the evolution of thrust wedges with concomitant surface erosion, and its bearing on the exhumation processes in orogenic belts. We performed sandbox experiments, simulating syn-orogenic erosion on forelandward sloping surfaces (∼4°). Experiments show that the erosion process has a significant control on the progression of fr...

The frontal tectonic wedge in the NW Himalayas shows a sequence of imbricate thrusts with increasing spacing in the foreland direction. We used analogue model experiments to analyze the thrust pattern in relation to the kinematics of wedge evolution. Experimental findings reveal two kinematic states of the foreland-ward propagation of a wedge: unst...

With advent of the plate tectonic theory geoscientists have taken a new turn in order to interpret the evolution of orogenic belts. As a consequence, a large volume of geodynamics models have emanated in recent times. In this paper we review some of the important models in context of the Himalayan-Tibetan system, which is believed to be the most sp...

We use analogue and numerical modelling to show that the flow of a Newtonian viscous fluid around a rigid body, in simple shear, depends strongly on the degree of confinement, i.e. the ratio between the shear zone width (H) and the rigid inclusion's least axis (e2) (S=H/e2). It also depends on how closely we look at the inclusion, which leads to th...

The rotation behavior of rigid inclusions in simple shear flow has received a great deal of attention from geologists in the last decades, as it is a key to the understanding of the kinematics and mechanics within ductile shear zones. Theoretical and experimental studies on the rotation of rigid inclusions under simple shear flow have typically con...

We used analogue modelling to investigate the factors controlling tail geometry in porphyroclast systems. Results show that: (1) σ inclusions can develop in both slipping and non-slipping modes, but δ-inclusions only form in the latter. (2) σ inclusions develop when the mantle production rate is constant and the mantle is transected by the separatr...

Natural occurrences and recent experimental work show that a low-friction inclusion/matrix boundary can be responsible for antithetical rotation and development of stable shape preferred orientations in simple shear. The flow of a viscous matrix around a rigid inclusion to which it may or may not be adherent is still not well studied, but it is rel...

In the Schirmacher Hills, most of the ductile shearing took place under high to medium grade amphibolite facies metamorphism. The microstructure of the mylonites shows characteristic features of high temperature deformation and thus gives us an idea of deformation mechanisms of the constituent minerals at great crustal depth. The variation in micro...

The Precambrian basement of the Schirmacher Hills, Queen Maud Land, East Antarctica has evolved through multiple episodes of deformation and metamorphism. The rocks have suffered at least five phases of deformation. The imprint of the early deformation, D1, is preserved in some mafic isolated enclaves. The second and the third deformations (D2 and...

The central part of the Schumacher Hills have suffered repeated phases of deformations. Five generations of deformations (D 1-D 5) have been delineated. D 1 and D 2 deformations took place under granulite facies conditions. The dominant deformation, D, took place under amphibolite facies conditions. D 3 foliations are further deformed by two later...

In the Precambrian basement of the Schirmacher Hills, ductile shear zones have developed at several stages over a wide time-span of tectonothermal history. The earliest shear zones were formed under granulite facies conditions producing well-foliated gneiss. The majority of the ductile shear zones developed under amphibolite facies conditions. The...

With the help of model experiments and theoretical analyses we evaluate the relationships of imbricate thrust spacing (a) with the bed thickness (H), basal friction (μb), initial taper (mw), and the magnitude (normalized to bed-weight per unit area) of horizontal stress (n). Imbricate thrust spacing increases linearly with bed thickness when mw = 0...

The Western part of the Schirmacher Hills is mapped in detail. The rock types found in the area are interlayered calc-khondalite-phroxene granulite unit, augen gnesis and streaky gneiss. In this area, the early Dl and D2 deformations have been oblitered by dominant D3, deformation which produced isoclinal, in- clined to reclined fold with an axial...