A NUMERICAL APPROACH TO UNDERSTAND THE TECTONIC STRESS FIELD VIS-À-VIS ONGOING DEFORMATION OF CONVERGING LITHOSPHERE IN NORTHEAST INDIA

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1 1 ISET GOLDEN JUBILEE SYMPOSIUM Indian Society of Earthquake Technology Department of Earthquake Engineering Building IIT Roorkee, Roorkee October 20-21, 2012 Paper No. A024 A NUMERICAL APPROACH TO UNDERSTAND THE TECTONIC STRESS FIELD VIS-À-VIS ONGOING DEFORMATION OF CONVERGING LITHOSPHERE IN NORTHEAST INDIA Suparna Chowdhury 1 and Prosanta K. Khan 1 1 Department of Applied Geophysics, Indian School of Mines, Dhanbad, Jharkhand, India suparna.chowdhury@outlook.com; pkkhan_india@yahoo.com ABSTRACT The northeast Indian tectonic zone is demarcated by the Himalayan orogenic belt to the north, Indo-Mayanmar Ranges to the east, crustal-scale Dauki fault to its south and the Yamuna-Brahmaputra lineament to the west. This entire land-mass is converging with a velocity of ~ 5.2 cm/yr towards NNE against the Asian plate. Though, the seismicity is broadly distributed over the area, three main concentrations noted between Mikir Hills and Main Boundary Thrust (MBT), southwestern part of Shillong Plateau and near the Haflong towards the southwestern part of Naga thrust, and might be reflecting a strong influence of local as well as regional tectonics, particularly caused by convergence of the Indian plate. In our present study we perform 2D finite element modeling along a NW-SE profile encompassing MCT, MBT, Assam valley, Oldham fault, Shillong plateau, Dauki fault and some parts of Tripura fold thrust belt to understand the present day stress distributions and the on-going deformation of the lithosphere in these areas. A generalized model of approximately 480 km long and 84 km deep is divided into mainly two boundaries, crust upper mantle interface analogous to Moho discontinuity. We consider upper mantle as a single lithologic unit and the crust is again subdivided laterally into ten lithologic units. Weak rheology is considered around the MBT, MCT, Oldham, and Dauki Faults during modeling. The lithology-dependent physical parameters like density, Young modulus, frictional angle, cohesion are estimated and used for simulation. Initially, a plate driving force of amount N/m is imparted towards NW directions on the converging Indian lithosphere and the specific part of its opposite edge is kept rigidly fixed under the presence of all the discussed crustal heterogeneities. It was noted that the compressive stress regime is more predominant around these heterogeneities, and also conformable with their respective dip. Instead, the compressive stress as well as shearing is more concentrated between heterogeneities or at their shallower segments when the upper crustal part on opposite edge remains rigidly fixed keeping the other initial conditions unaltered. Further, a decoupling layer between upper and lower crusts was evolved under this boundary condition, and similar feature is also not unusual in other parts of the Himalaya. It is thus imperative to state that the segment specific compressive stress regimes are predominant under both conditions, and also conformable with the focal mechanism solutions and other similar studies for the northeast part of India. Keywords: Himalaya, Tectonics, Finite Element Modelling (FEM), Elastic, Stress INTRODUCTION

2 2 The study area (Fig. 1), comprising complex tectonic domain, is located between latitude 24 and 28 N and longitude 89 and 97 E belongs to northeast part of India bounded by Himalayan orogenic belt to the north, Indo-Mayanmar Ranges to the east, crustal-scale Dauki fault to its south and the Yamuna- Brahmaputra lineament to the west. The Himalayan orogen, the largest collisional system on the earth, represents one of the excellent examples of active continent-continent collision zone where the Indian lithosphere is underthrusting beneath the Eurasian crust. The tectonic stress distribution is normally associated directly with plate movement and also varies from place to place. The tectonic setting of the northeast part of India involving Shillong plateau, Tripura fold belt, Indo-Mayanmar ranges, Dauki fault, etc. is highly complicated because of the collision and overthrusting /underthrusting of different plates and microplates. Tectonic understanding through finite element modeling for the Himalaya was attempted by Burg and Podladchikov (1998), Chamlagain and Hayashi (2008), Monslave et al. (2009), Joshi and Hayashi (2010). Burg and Podladchikov (1998) tried to understand the crustal and lithospheric scale folding for the eastern and western syntaxial belts. Chamlagain and Hayashi (2008) aimed at understanding the contemporary tectonic stress along the India-Eurasia collision zone. The effort of Monslave et al. (2009) was mainly associated with the flexure of the continental converging lithosphere. Joshi and Hayashi (2010) studied the development of extensional stresses under compressional settings of the Himalaya. Islam et al. (2011) performed finite element simulation for northeast India in understanding the regional stress distributions and associated faulting patterns under the effect of northwestward convergence of the Indian plate against the Eurasian plate. In the present study, we perform finite element modeling on a 2D vertical section in the converging Indian and overriding Eurasian lithospheres containing the tectonic elements e.g. MCT, MBT, Assam valley, Oldham fault, Shillong plateau, Dauki fault and some parts of Tripura fold thrust belt along a NW-SE profile (i.e., AA, Fig. 1). This generalized model for the Indian and overriding Eurasian lithospheres of 480 km strike-length and 84 km vertical extension is divided into two layers consisting of crust and upper mantle with Moho interface. The numerical modeling is done for evaluating the stress distribution vis-à-vis lithospheric deformation under the presence of ambient ridge push force of the order of N/m. This numerical analysis will offer better understanding of the regional tectonics over the northeast part of India. TECTONIC SETUP The northeast part of India is one of the most attractive regions to Earth scientists in the world in terms of its seismicity and crustal deformation. The tectonics of this region and its adjoining areas including the Eastern Himalaya, Assam-Brahmaputra valley, Shillong - Mikir Hills, Bengal basin, and Tripura fold thrust belt, Indo-Mayanmar ranges, were principally evolved during the late Cenozoic deformation caused by collision between the Indian plate and the Eurasian plate (Molnar and Tapponnier; 1975). The Main Central Thrust (MCT) and Main Boundary Thrust (MBT) are the two foremost Himalayan crustal discontinuities passing all along the northern boundary of northeast India. Our present study area, consist of crystalline rocks that are partly roofed with gently dipping tertiary and younger sediments. Shillong plateau and Mikir hills consist of precambrian gneiss, granites and the Shillong group (sandstones and conglomerate) (Mishra and Sen, 2001) and have an average elevation of ~ 1 km. The Shillong plateau is bounded by E-W trending southerly dipping Brahmaputra river fault and /or Oldham fault to the north and E-W oriented southerly dipping Dauki fault to the south. Eastward motion of the Shillong Plateau along the transcurrent Dauki fault along with the vertical tectonics (Evans, 1964) was responsible for the development of complex tectonic setting in northeast India. The 1897 Shillong earthquake and the 1950 Sadiya earthquake were explained as the consequences of southward movement of the Shillong Plateau at the last stage of Himalayan orogeny, and oblique convergence of the Indian plate against the Burmese micro-plate (Evans, 1964; Ben Menahem et al., 1974). As detached blocks from the Peninsular India, Shillong plateau and Mikir Hills shifted over 200 km towards east through the gap between Rajmahal and Garo Hills (Evans, 1964). Thus horizontal compression stresses, due to resistance at the continentcontinent India-Eurasia collisional zones, are the prime driving forces behind the upliftment of the

3 3 Shillong Plateau. Consequently, the Shillong Plateau was built as an effect of contractional deformation. The distinctiveness of regional stress orientation and deformation pattern is directly related to these fault activities. The Shillong plateau is alienated from the western part of the Surma valley by a narrow strip of southerly dipping beds associated with the Dauki fault, Disang thrust, at the southwest of Naga Hills, passes to a narrow but complex fracture belt near Haflong i.e. the Haflong fault. With vary in orientation from NE-SW to due west (Evans, 1964) the fracture belt continues westward up to the Dauki fault. Tripura fold belt covers the eastern part of Bangladesh (eastern sylhet and Chittagong hill tracts), Tripura the southern part of Assam, Mizoram, Mayanmar (Burma). Adjacent to SE of the Chittagong hill tracts that have a large number of narrow elongated N-S trending folds with tertiary sediments (Alam, 1989). The Chittagong folded belt of north south-trending curvilinear anticlines and synclines, that constitutes the westward extension of Arakan Chin fold system of the Indo Mayanmar Ranges, and the prevailing fold-generating mechanism is the E W-directed compressional force arising from oblique subduction of the Indian plate beneath the Burma plate that resulted in the growth of fault-propagation folds above a detachment or decollement at depth (Sikder and Alam, 2003). Moreover, tectonogenesis of this folded area is intimately linked with geodynamics and nature of subduction of the Indian plate beneath Burmese subplate (Khan, 2005a). HISTORICAL SEISMICITY Northeast India and adjoining areas fall in one of the most intense seismic zone of the world. This area experienced two most important historical great earthquakes in the past; one is the 1897 M 8.7 Shillong earthquake and the other 1950 M 8.6 Sadiya earthquake (Evans, 1964; Ben Menahem et al., 1974), and these earthquakes explain as the fall out of southward movement of the Shillong Plateau in the last phase of the Himalayan orogeny, and northwestward oblique convergence of the Indian plate against the Myanmar-China border region. The 1897 event caused destruction of structures over much of the plateau and surrounding areas and also widespread liquefaction and flooding in the Brahmaputra and Sylhet Floodplains (Ambraseys and Bilham, 2003). While the 1950 event devastated the mountainous region further upstream the Brahmaputra River, and affected a large area in Assam and Tibet. Widespread landslide failure also occurred following the 1950 event far to the east of the 1897 event. Besides these two great earthquakes, 18 large earthquakes (8.0 > M 7.0) occurred in these areas during the last 100 years. Along with the occurrences of the major earthquakes, several smaller to moderate size earthquakes (M < 7.0) have also been recorded from this region (Verma, Mukhopadhyay & Ahuluwalia.1976; Mukhopadhyay 1984; Kayal 1987), and drawn attention of several geoscientists over decades due to its higher vulnerability of earthquake hazards. Regional seismicity data compiled by several workers (Oldham 1899; Tandon, 1954; Le Dain et al., 1984; Kayal, 1987; Molnar, 1987; Chen and Molnar, 1990; Bilham and England, 2001; Khan, 2005b; Angelier and Baruah, 2009) though reveals that most of the deep and intermediate earthquakes are associated with the subduction zone beneath the Indo-Myanmar fold and thrust belt, more frequent shallow earthquakes invariably are also recorded across the region both within the plateau and along the well delineated structural features: northern boundary of the Himalayan thrusts, along the Naga-Mishmi thrust, etc. FINITE ELEMENT MODELING BASIC APPROACH The tectonic stress distribution is directly linked with plate movement and varies place to place over the Earth (Gowd et al., 1992). The present work aims to investigate the northeast region through 2D finite element modeling (Grunthal and Stromeyer, 1992; Burg and Podladchikov, 1998; Liu and Bird, 2002; Hu et al., 2004; Burg and Schmalholz, 2008; Joshi and Hayashi, 2008; Islam, 2009; Islam and Shinjo, 2010; Kumar et al, 2010) for a more generalized elastic layers, separated by crustal discontinuities and/or transitional boundary, in the converging Indian and overriding Eurasian lithospheres. The converging

4 4 Indian and overriding Eurasian lithospheres include the tectonic elements mainly the Himalaya, MCT, MBT, Assam valley, Oldham fault, Shillong plateau, Dauki fault and some parts of Tripura fold thrust belt along a NW-SE profile. This generalized model (Fig. 2) for the Indian and overriding Eurasian lithospheres of 480 km strike-length and 84 km vertical extension is divided into two layers consisting of crust and upper mantle with an interface analogous to Moho. We consider uppermost part of the mantle as a single lithologic unit (layer 1) which is made up of peridotite, dunite, pyrolite, eclogite, gabbro (Keary and Frederick, 1998; Islam et al., 2011) and further the crust is subdivided into ten lithologic units (layers 2-11). The lower crust (layer 2) beneath the Assam Valley area consists of granite, syenite, mafic and ultramafic rocks (Santosh, 1999; Srivastava and Sinha, 2004), whereas the lower crust (layer 10) beneath the Tripura fold belt is partially a part of Burmeses plate. The sheared formations like MBT (layer 3), MCT (layer 4), Oldham (layer 6) and Dauki faults (layer 7) are considered as weak rheology in our present modeling, whereas the Shillong plateau (layer 8) is thought to be of archean gneissic complex, schist (Evans; 1964) and Shillong group of sediments (Mishra and Sen; 2001) and Assam valley (layer 11) is composed of late Tertiary sandstones, siltstone and other loose sediments (Evans, 1964). Tripura fold thrust belt (layer 9) belongs to neogene sedimentation and composed of basic to ultrabasic rocks (Evans, 1964). A mesh is generated with nodes for the entire cross-section. The entire vertical section is divided into small triangular elements or domains. The simulation was performed using the finite element (FE) modeling software ANSYS (version 14). ROCK PROPERTIES Any numerical modelling for understanding the stress-state and displacements mainly lies in the facts of rheology of the constituent rocks. The rock rheology has a significant influence on the occurrences of earthquakes vis-a-vis the source dynamics/kinematics (Watts and Burov, 2003; Khan and Chakraborty, 2009). Rock properties, such as density, Young modulus, Poisson's ratio, angle of internal friction, cohesion are more important physical parameters for elastic medium and were chosen as basic inputs for the modeling (Table 1). The densities range from 2000 kg/m 3 (for layer 3, 4, 6 and 7) to 3300 kg/m3 (for layer 1) were taken from the works of Khan and Hoque (2006) and Rajesekhar and Mishra (2008), Islam et al. (2011). The values of angle of internal friction and cohesion were taken from the works of Clark (1966) and Joshi and Hayshi (2010). Poisson's ratio was considered to be 0.25 (Islam et al., 2011). BOUNDARY CONDITION The relative motion of lithospheric plates is normally associated with number of ambient forces, some of which drive the motion and some of which resist (Lowrie, 1997). The ridge-push force acts at the midoceanic ridges on the edge of the plates and slab-pull force is due to the negative buoyancy of the plate being subducted at a convergent plate boundary and this arises due to the subducting plate is cooler and therefore more dense than the mantle into which it descends. Thus, the ridge-push force and slab-pull forces are the main driving forces till the shallower part of the mantle along consuming boundaries. With subsequent motion into the deeper part of the mantle, the tip of the lithosphere experience increasing viscous drag forces which resists its motion. The total slab-pull force near the margin is invariably recorded to be about of order N/m in magnitude and ridge-push force is estimated to be N/m. Both these forces are caused by the difference in density between hot and cold mantle. In our present study, a force of an order of N/m, equivalent to ridge-push force, is applied to the plate in NW direction keeping the opposite face fixed. We also have examined the stress field under other boundary conditions fixing the opposite face in different ways (Fig. 2). DISCUSSION We have critically examined the stress field in different segments along the profile under three different boundary conditions (Fig. 2). Under the initial boundary condition when the entire opposite edge

5 5 towards the northwestern end of the profile remains fixed (Fig. 2a), and the frontal edge (i.e., southeastern end) is under the application of force of amount N/m, only the small shallow segments of all the heterogeneities come under compression (Fig. 3a), whereas horizontal extension is more predominant in the deeper level. An extensional stress field is developed at intermediate depth-level in between the heterogeneities and particularly more dominant in the Shillong Plateau area. However, the region beneath the Moho accounts for complex tectonics. Under the second condition when the Indian lithospheric part remains fixed (Fig. 2b), stress fields are observed to be compressive in the regions all through the discontinuities (Fig. 3b), and also conformable with their respective dip. A distinct change in stress fields between crust and mantle beneath the Shillong Plateau is noted and might be associating a decoupling layer around that area. Under the third boundary condition when the Eurasian crust remains fixed (Fig. 2c), shearing motion/deformation is more predominant along the boundaries between layers (Fig. 3c). While the regions between discontinuities document compression, is oriented along the strike of the profile. Further, a rotation of the orientation of maximum compressive stress field is apparently noted along the zone between the upper crusts of Eurasian and Indian plates and the Indian lower crust. Although the tectonics and deformation pattern of the study area are complicated because of the differential and oblique movement of the plates and the existence of diverge geologic units in the region (Evans, 1964; Chen and Molnar, 1990; Khan, 2005a; Khan and Chakraborty, 2007; Khan et al., 2011), the study of Angelier and Baruah (2009) reveals that widespread N-S compression is quite active in the whole area from the Eastern Himalaya to the Bengal Basin, through the Shillong-Mikir Massif and Upper Assam Valley, and consistent in the direction of India-Eurasia convergence. The N-S compression is also conformable with the eastward extrusion of the Tibetan and Himalayan mass. The present study shows that the major N-S compression also is well-extended into the descending Indian lithosphere as well as overriding Eurasian crust. This N-S compression receives support from focal mechanism solutions and analysis of borehole breakouts (Kayal, 2001; Gowd et al., 1992; Heidbach et al., 2007), and also from the geodetic studies (Bilham and Gaur, 2000; Jouanne et al., 2004; Jade et al., 2007). It is thus may be proposed that the third model (Fig. 2c), where the shallower part dominated by horizontal compression and an evolving separating zone between two zones of distinctly different stress fields marked by boundary between lower and upper crusts, is more compatible with the existing tectonics of the study area. CONCLUSIONS Oblique convergence of the Indian plate against the Eurasian plate along the Himalaya and Indo- Mayanmar margins resulted in the complex tectonics during the Tertiary. However, the focal mechanism solutions and the other studies discussed above clearly advocate a N-S compression all through the northeast part of India up to the interior of the Himalaya. While the deeper region along the eastern subduction margin is normally dominated by E-W thrusting. The significant contribution of the present study might be the evolution of the decoupling layer evolved when the Indian lithosphere is allowed to move freely below the Eurasian plate, and turns the shallower part of the converging boundary under both compression and shearing. Under such condition, evolution of a decoupling layer between the upper and lower crusts is not incongruous and also observed in other parts of the Himalaya. ACKNOWLEDGEMENTS First author is thankful to the Director, Indian School of Mines, Dhanbad and Head, Department of Applied Geophysics for providing the infrastructure facilities. This work has been supported fully by the grant of the Ministry of Earth Sciences, Govt. of India, New Delhi. REFERENCES

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7 23. Joshi, J.R. and Hayashi, D. (2008). Numerical modeling of Neotectonic movements and state of stresses in the Central Seismic Gap Region, Garhwal Himalaya, J. Mt. Sci. 5(4), Jouanne, F.et al. (2004). Current shortening across the Himalayas of Nepal, Int. J. Geophys., 157(1), Kayal, J.R. (1987). Microseismicity and source mechanism study: Shillong Plateau, northeast India, Bull. Seismol. Soc. Am. 77(1), Kayal, J.R.(2001). Microearthquake activity in some parts of the Himalayaand the tectonic model, Tectonophysics339, Keary, P. and Frederick, J.F. (1998). Global tectonics. Blackwell science publishing company, p Khan, A.A. and Hoque, M.A. (2006). Crustal dynamics, seismicity and seismotectonics of the Bengal Basin, Proceeding of 1st Bangladesh earthquake symposium. Earthquake Society, Bangladesh, pp Khan, P.K., Chakraborty, P.P. (2009). Bearing of plate geometry and rheology on shallow-focus mega-thrust seismicity with special reference to 26 December 2004 Sumatra event, J. Asian Earth Sci., 34, Khan, P.K. (2005a). Variation in dip-angle of the Indian plate subducting beneath the Burma plate and its tectonic implications, Int. Geosci. J. 9(3), Khan, P.K. (2005b). Mapping of b-value beneath the Shillong Plateau, Gond. Res. 8(2), Khan, P.K., and Chakraborty, P.P. (2007). The seismic b value and its correlation with Bouguer gravity anomaly over the Shillong plateau area: a new insight for tectonic implication, J. Asian Earth Sci. 29, Khan, P.K., Ghosh, M., Chakraborty, P.P. and Mukherjee, D. (2011). Seismic b-value and the assessment of ambient stress in Northeast India, Pure and Applied Geophysics, 168, , doi: /s x. 34. Kumar, N., Sarkar, S. and Mandal, N.(2010). Numerical Modeling of Flow Patterns around Subducting Slabs in a Viscoelastic Medium and its Implications in the Lithospheric Stress Analysis, J. Geol. Soc. India. 75(1), Le Dain, A.Y., Tapponnier, P., and Molnar, P. (1984). Active faulting and tectonics of Burma and surrounding regions, J. Geophys. Res. 89(B1), Liu, Z. and Bird, P. (2002). Finite element modeling of neotectonics in New Zealand, Journal of Geophysical Research 107, B12, Lowrie, W., Fundamentals of Geophysics (Cambridge Press, 1997). 38. Mishra, U.K. and Sen, S. (2001). Dinosaur bones from Megalaya, Current Science 80(8), Molnar, P. and Tapponnier, P. (1975). Cenozoic tectonics of Asia: effects of a continental collision, Sci., 189 (4201), Molnar, P. (1987). The distribution of intensities associated with the great 1897 Assam earthquake and constraint on the extent of rupture, J. Geol. Soc. India 30, Monsalve, G., McGovern, P. and Sheehan, A. (2009). Mantle fault zones beneath the Himalayan collision: Flexure of the continental lithosphere, Tectonophysics 477(1-2), Mukhopadhyay, M. (1984). Seismotectonics of transverse lineaments in the eastern Himalaya and its foredeep, Tectonophysics 109(3-4), Radha Krishna, M., and Sanu, T.D. (2000). Seismotectonics and rates of active crustal deformation in the Burmese arc and adjacent regions, J. Geodyn. 30(4), Oldham, R. D. (1899). Report on the Great Earthquake of 12 June 1897, Mem. Geol. Soc. of India, 29, pp Geol. Surv. India, Calcutta. 45. Rajesekhar, R.P. and Mishra, D.C. (2008). Crustal structure of Bengal Basin and Shillong Plateau: Extension of Eastern Ghat and Satpura mobile belts to Himalyan fronts and seismotectonics, Gondwana Research 14,

8 8 46. Santosh, M. (1999). Intregated geological studies in the deep continental crust of southern India, Gondwana Research 2(2), Sikder, A.M. and Alam, M.M. (2003). 2-D modelling of the anticlinal structures and development of the eastern fold belt of the Bengal basin, Bangladesh, Sedimentary Geology 155(3), Srivastava, R.K. and Sinha, A.P. (2004). Geochemistry of Early Cretaceous alkaline ultramafic mafic complex from Jasra, Karbi Anglong, Shillong Plateau, Northeastern India, Gondwana Research 7(2), Tandon, A.N. (1954). Study of the great Assam earthquake of August, 1950 and its aftershocks, Indian J. Met. Geophys. 5, Verma, R.K., Mukhopadhyay, M., and Ahluwalia, M.S. (1976). Seismicity, Gravity and Tectonics of Northeast India and Northern Burma, Bull. Seismol. Soc. Am. 66(5), Watts, A.B., and Burov, E.B. (2003). Lithospheric strength and its relationship to the elastic and seismogenic layer thicknesses, Earth Planet. Sci. Lett. 213(1-2), Table 1. Material properties used for numerical modeling. Layer No. Region Density (Kg/m 3 ) Young modulus (GPa) Cohesion (MPa) Frictional co-efficient 1 Upper mantle Indian crust MBT MCT Himalaya Oldham fault Dauki fault Shillong plateau Tripura fold belt Burma crust Assam valley

9 Figure 1. Geological Map of Northeast India showing the major crustal elements (after Khan and Chakraborty, 2007). MBT: Main Boundary Thrust; MCT: Main Central Thrust; DF: Dauki Fault; OF: Oldham Fault; TF: Tista Fault; PF: Padma Fault; TFB: Tripura Fold Belt; NT: Naga Thrust; DT: Disang Thrust; HF: Halflong fault; LT: Lohit Thrust; TS: Tidding Suture; MT: Mishmi Thrust. Solid stars indicate the locations of Historical large earthquakes (Magnitude between 7-8). Solid converging arrows represent the direction of compressive stress field (after Radha Krishna and Sanu, 2000). A-A' is the NW- SE trending profile along which a vertical section is chosen for 2D finite element modeling and simulation. 9

10 Figure 2. Simplified 480 km long vertical geological cross-section passing through MCT, MBT, Assam valley, Oldham fault, Shillong plateau, Dauki fault, and some parts of Tripura fold-thrust belt. Small circles on the left edge of the 84 km thick profile represent its fixing or zero displacement whereas the arrows on the right edge of the profiles indicate the direction of application of force. Layer 1: Upper Mantle; Layer 2: Indian Crust; Layer 3: MBT; Layer 4: MCT; Layer 5: Upper Eurasian Crust, Layer 6: Oldham Fault; Layer 7: Dauki Fault; Layer 8: Shillong Plateau; Layer 9: Tripura Fold Belt; Layer 10: Burmese Crust; Layer 11: Assam Valley. (a) Both the crustal and mantle parts remain fixed, (b) only Indian lithosphere remains fixed, and (c) only Eurasian crustal part remains fixed. 10

11 11 Figure 3. Plots illustrating the stress distribution along the 480 km long profile over the northeast part of India under the boundary conditions of (a) both the crustal and mantle parts remain fixed, (b) only Indian lithosphere remains fixed, and (c) only Eurasian crustal part remains fixed. Note the compressive stress regime near MBT and MCT.