Movement Of Indian Plate Research Articles

Upper crustal stress and seismotectonics of the Garhwal Himalaya using small-to-moderate earthquakes: Implications to the local structures and free fluids

The work presents new focal-mechanism data of small-to-moderate (3.0⩾ML⩽5.0) upper crustal earthquakes for the Garhwal Himalaya from a local seismic network installed in July 2007. Majority of the epicenters of these earthquakes are located close to the Main Central Thrust (MCT) zone. We retrieved Moment Tensor (MT) solutions of 26 earthquakes by waveform inversion. The MT results and 11 small-to-moderate earthquakes from the published records are used for stress inversions. The MT solutions reveal dominatingly thrust mechanisms with few strike slip earthquakes near Chamoli. The seismic cross sections illustrate that, these earthquakes are located around the Mid-Crustal-Ramp (MCR) in the detachment. The optimally oriented faults from stress inversions suggest that, the seismogenic fault in this region is similar to a fault plane having dip angle between 12 and 25 degrees, which is compatible with the dip angle of the MCR (∼16°) in this region. P-axes and the maximum horizontal compressive stress are NE-SW oriented; the direction of the relative motion of Indian plate with respect to the Eurasian plate. The Friction Coefficient estimated from stress inversions show that the Chamoli region having low friction in comparison to the overall values. The free fluids trapped beneath the detachment are penetrating into the local faults, hence, decreasing the frictional strength and altering the prevailing stress conditions of the surroundings. The present study reveals that the MCR structure is seismogenically active and producing the small-moderate earthquakes in the region, while the MCT is probably dormant at present.

Eastern Indian Ocean microcontinent formation driven by plate motion changes

The roles of plate tectonic or mantle dynamic forces in rupturing continental lithosphere remain controversial. Particularly enigmatic is the rifting of microcontinents from mature continental rifted margins, with plume-driven thermal weakening commonly inferred to facilitate calving. However, a role for plate tectonic reorganisations has also been suggested. Here, we show that a combination of plate tectonic reorganisation and plume-driven thermal weakening were required to calve the Batavia and Gulden Draak microcontinents in the Cretaceous Indian Ocean. We reconstruct the evolution of these two microcontinents using constraints from new paleontological samples, 40Ar/39Ar ages, and geophysical data. Calving from India occurred at 101–104 Ma, coinciding with the onset of a dramatic change in Indian plate motion. Critically, Kerguelen plume volcanism does not appear to have directly triggered calving. Rather, it is likely that plume-related thermal weakening of the Indian passive margin preconditioned it for microcontinent formation but calving was triggered by changes in plate tectonic boundary forces.

40 Ar/ 39 Ar ages of alkaline and tholeiitic rocks from the northern Deccan Traps: implications for magmatic processes and the K–Pg boundary

The Deccan large igneous province in India was emplaced temporally close to the Cretaceous–Palaeogene (K–Pg) boundary and is formed by tholeiitic flood basalts and less abundant alkaline rocks. Definition of the origin of Deccan magmatism and of its environmental impact relies on precise and accurate geochronological analyses. We present new 40 Ar/ 39 Ar ages from the northern sector of the province. In this area, tholeiitic and alkaline rocks were contemporaneously emplaced at 66.60 ± 0.35 to 65.25 ± 0.29 Ma in the Phenai Mata area, whereas rocks from Rajpipla and Mount Pavagadh yielded ages ranging from 66.40 ± 2.80 to 64.90 ± 0.80 Ma. The indistinguishable ages for alkaline and tholeiitic magmatism suggest that distinct mantle sources were synchronously active. The new ages are compared with previous ages, which were carefully screened and filtered and then recalculated to be comparable. The entire dataset of geochronological data does not support a time-related migration of the magmatism related to the northward Indian plate movement relative to the Reunion mantle plume. The main phase of magmatism, including the newly dated rocks from the northern Deccan, occurred at the K–Pg boundary. This suggests a causal link between the emplacement of the province and the K–Pg mass extinction. Supplementary material: Whole-rock and mineral compositions and the complete 40 Ar/ 39 Ar dataset are available at https://doi.org/10.6084/m9.figshare.c.2674441 .

Shear-wave splitting in Eastern Indian Shield: Detection of a Pan-African suture separating Archean and Meso-Proterozoic terrains

Based on fast axes orientation (ψ) and delay time (δt) measurements at fifteen broadband stations, the Archean Eastern Indian Craton (EIC) is classified into three main tectonic zones viz. Singhbhum-Odisha craton (SOC) [ψ=(82±8)° and δt=(1.5±0.2)s], Chotanagpur granitic gneissic terrain (CGGT) [ψ=(63±9)° and δt=(1.4±0.3)s] and Eastern Ghats mobile belt (EGMB) [ψ=(70±10)° and δt=(1.3±0.2)s]. The results in the Archean province of EIC depict a moderate to large delay times (1.0–1.9s) with azimuths that reveals an upper mantle anisotropy with a mean orientation of (74.5°±5.6°) sub-parallel to the E-W trending structural grain of the Archean craton, evidencing frozen lithospheric anisotropy. The direction of anisotropy (∼N39°E) in northern Meso-Proterozoic CGGT is found to be consistent with the direction of current absolute plate motion of Indian plate. Regional variation of observed orientation of fast axes near the flanking Paleo- and late- Archean orogenic belts is attributed to the tectonic control rather than current plate motion. Nevertheless, estimated large mean delay times (1.41±0.27s) do also suggest a contribution from the anisotropy associated with asthenospheric mantle flow. Most interestingly, our study detects a transition zone (coinciding with the location of Damodar Graben) separating south (late Archean) and north (Meso-Proterozoic) CGGT across which the fast axes orientation changes from E-W (ψ=71°) to NE (ψ=39°), which could be evidencing the signatures of the eastward extension of Pan African (550–500Ma) suture as also observed in Antarctica.

“印度板块”研究回顾及思维再完善—换个方式审视地球暨庆祝中国地质博物馆建馆100周年(1916-2016)

依据当今非洲大裂谷的发育过程可以推断出，2亿多年前由于受到裂谷系的作用，致使冈瓦纳古陆全面解体，出现了南美洲板块、非洲板块、澳大利亚板块、南极洲板块、印度-印支板块；之后裂谷系又逐渐发展成大西洋海岭、印度洋海岭、太平洋海岭。古生物学又证实，“印度板块”与“印支板块”都生存有三叠纪恐龙动物，这表明“印支板块”曾经是冈瓦纳大陆的一部分，与“印度板块”曾经也是一个相互连接的整体，即“印度-印支板块”。从冈瓦纳大陆分离出来，“印度-印支板块”于晚三叠世后期至早侏罗世初期，在赤道附近同“华南大陆板块”发生撞击，自此并入欧亚板块，并将冈瓦纳大陆的海生爬行动物鱼龙、棘皮类海百合及陆生恐龙等生物先后带入亚洲南部的贵州、云南、四川等地区。随着印度-印支板块与欧亚板块向北移动，印度-印支板块又受到来自“东经九十度海岭”的切割，新的裂谷系将其由南至北分成西部的印度板块和东部的印支板块。被分割的印度板块在“东经九十度海岭”和马尔代夫海岭合力推动下，向北移动，至喜马拉雅地区，再次与欧亚板块发生撞击，形成当今的地貌景观。两次撞击都与裂谷系相关联，因此对裂谷系的认识，有助重塑印度板块的运动轨迹，有助重新解释中生代以来众多的地质现象。 At about 200 ma ago, Gondwanaland was split by a rift system, according to the development process of present East African Rift system, into South America Plate, Africa Plate, Australia Plate, Antarctic Plate and India-Indochina Plate. The rift system itself developed to the present Mid-Atlantic Ridge, Indian Ridges, and Pacific Ridges gradually. Triassic dinosaurs, found at both Indian and Indo-China plates, showed that Indo-China Plate was also a part of Gondwanaland and that Indian Plate and Indo-China Plate were located together as Indian-Indo-China Plate. When separated from Gondwanaland, and collided with South-China Plate, it was merged in Eurasian Plate at the Equator in Late Triassic-Early Jurassic period. The collision had brought Gondwana fauna like sea reptiles Ichthyosaurus, crinoids, and terrestrial dinosaurs to South China (Guizhou, Yunnan, Sichuan, etc.). In its further movement northward, Indian-Indo-China Plate was split by the Ninety East Ridge into two parts from south to north: Indo-China Plate at its east and Indian Plate at its west. Indian Plate, driven jointly by the Ninety East Ridge and Maldive Ridge, moved northward and collided with Eurasian Plate again, at the region of present Himalaya, forming present Himalaya topography. The two collisions were caused by the forming of rift systems. So understanding the formation of the rift systems is the key to reconstruct the route of Indian Plate movement which in turns helps to interpret many geological phenomena happened since Mesozoic.

Seismotectonics and crustal stress field in the Kumaon–Garhwal Himalaya

We present fault plane solutions of 94 well located small-to-moderate sized (1.5≤ML≤5.4) earthquakes, which occurred in the Kumaon–Garhwal Himalaya during 2005–2008, using P-wave polarity and body wave amplitudes. These earthquakes show a mixture of thrust, normal and strike-slip type mechanism, with a majority of thrust type. Most of the thrust earthquakes occur at a depth of 8–22km in the Main Central Thrust (MCT) zone and the Lower Himalaya. The spatial distribution of these earthquakes suggest that the strain resulting from the ongoing collision of the Indian plate with the Eurasian plate is being consumed by thrust fault movement mainly on the north dipping Munsiari Thrust and south dipping Tons Thrust. The strike-slip earthquakes are mainly observed in the Lower Himalaya as well as around the Munsiari region in the MCT zone. The normal earthquakes are also observed in different parts of the Kumaon–Garhwal Himalaya and the Gangetic plain. Their occurrence is attributed to the local structure(s) as well as the flexure of the Indian plate. Stress tensor inversion of the calculated fault plane solutions indicates that the maximum compressive stress in the Gangetic plain is N–S directed and near vertical; whereas in the Kumaon–Garhwal Himalaya, it is near horizontal and NNE–SSW directed, and correlating with the prevailing stress condition due to northward movement of Indian plate.

Depth‐variant azimuthal anisotropy in Tibet revealed by surface wave tomography

AbstractAzimuthal anisotropy derived from multimode Rayleigh wave tomography in China exhibits depth‐dependent variations in Tibet, which can be explained as induced by the Cenozoic India‐Eurasian collision. In west Tibet, the E‐W fast polarization direction at depths <100 km is consistent with the accumulated shear strain in the Tibetan lithosphere, whereas the N‐S fast direction at greater depths is aligned with Indian Plate motion. In northeast Tibet, depth‐consistent NW‐SE directions imply coupled deformation throughout the whole lithosphere, possibly also involving the underlying asthenosphere. Significant anisotropy at depths of 225 km in southeast Tibet reflects sublithospheric deformation induced by northward and eastward lithospheric subduction beneath the Himalaya and Burma, respectively. The multilayer anisotropic surface wave model can explain some features of SKS splitting measurements in Tibet, with differences probably attributable to the limited back azimuthal coverage of most SKS studies in Tibet and the limited horizontal resolution of the surface wave results.

Magma production rate along the Ninetyeast Ridge and its relationship to Indian plate motion and Kerguelen hot spot activity

AbstractThe Ninetyeast Ridge, a linear trace of the Kerguelen hot spot in the Indian Ocean, was emplaced on a rapidly drifting Indian plate. Magma production rates along the ridge track are computed using gravity‐derived excess crustal thickness data. The production rates change between 2 and 15 m3/s over timescales of 3–16 Myr. Major variations in magma production rates are primarily associated with significant changes in the Indian plate velocity with low‐production phases linked to high plate velocity periods. The lowest magma production rate (2 m3/s) at 62 Ma is associated with the rapid northward drift of Indian plate under the influence of the Reunion mantle plume. The contemporaneous slowing of the African plate coincides with increase in magma production rate along the Walvis Ridge in the Atlantic Ocean. The present study suggests that variations in the Indian plate motion and frequent ridge jumps have a major role in controlling the magma production, particularly on long‐period cycles (~16 Myr). Short‐period variations (~5 Myr) in magma productions may be associated with intrinsic changes in the plume, possibly due to the presence of solitary waves in the plume conduit.

Ridge push, mantle plumes and the speed of the Indian plate

SUMMARY The buoyancy of lithospheric slabs in subduction zones is widely thought to dominate the torques driving plate tectonics. In late Cretaceous and early Paleogene times, the Indian plate moved more rapidly over the mantle than freely subducting slabs sink within it. This signal event has been attributed to arrival of the Deccan-Rmantle plume beneath the plate, but itisunknowninwhichproportionstheplumeactedtoalterthebalanceofexistingplatedriving torques and to introduce torques of its own. Our plate kinematic analysis of the Mascarene Basin yields a detailed Indian plate motion history for the period 89-60Ma. Plate speed initially increases steadily until a pronounced acceleration in the period 68-64Ma, after which it abruptly returns to values much like those beforehand. This pattern is unlike that suggested to result from the direct introduction of driving forces by the arrival of a thermal plume at the base of the plate. A simple analysis of the gravitational force related to the Indian plate's thickening away from its boundary with the African plate suggests instead that the sudden acceleration and deceleration may be related to uplift of part of that boundary during a period when it was located over the plume head. In this instance, torques related to plate accretion and subduction may have contributed in similar proportions to drive plate motion.

Post-seismic deformation associated with the 2001 Bhuj earthquake

We report the results of GPS measurements of post-seismic deformation due to the 2001 Bhuj earthquake in the Kachchh region, western India. The estimated horizontal velocity vectors in ITRF05 are in the range of 48–49 mm/year in N46–50°E. The observed velocity at the Gandhinagar permanent site, a far off site from the earthquake source region and probably unaffected by the post-seismic deformation, is 49 ± 1 mm/year in N47°E, which is consistent with the predicted motion of Indian plate at Gandhinagar. At other sites in the source region, transient post-seismic deformation is found to be low; it attenuated rapidly within 3–4 years of the earthquake and is much low now. Our results support the idea that mantle rheology is weak in the region.

Seismic Vulnerability in the Himalayan Region

The frequently occurring strong earthquakes in the Himalayan region signify the seismic vulnerability in the region. The continued northward movement of Indian plate is generating large amount of stress at the plate boundary which is being released in form of large and great earthquakes (M≥7). Absence of such great events in the Himalayan front for last six decades and in some segments for last two centuries envisages the region as a high potential zone for future seismic hazard. In this paper we studied the larger events in the central Himalayan region.Key words: Central Himalaya; Large earthquakes; Seismic hazardsThe Himalayan Physics Vol.1, No.1, May, 2010Page: 14-17Uploaded Date: 28 July, 2011

Acceleration and deceleration of India-Asia convergence since the Cretaceous: Roles of mantle plumes and continental collision

[1] A strong 50–35 Ma decrease in India-Asia convergence is generally ascribed to continent-continent collision. However, a convergence rate increase of similar magnitude occurred between ∼65–50 Ma. An earlier increase occurred at ∼90 Ma. Both episodes of accelerated convergence followed upon arrival of a mantle plume below and emplacement of a large igneous province (LIP) on the Indian plate. We here first confirm these convergence rate trends, reassessing the Indo-Atlantic plate circuits. Then, using two different numerical models, we assess whether plume head arrival and its lateral asthenospheric flow may explain the plate velocity increases and whether decreased plume flux and increasing continent-plume distance may explain deceleration, even without continental collision. The results show that plume head arrival can indeed lead to absolute Indian plate motion accelerations on the order of several cm/yr, followed by decelerations on timescales similar to the reconstructed fluctuations. The 90 Ma increase could potentially be explained as response to the Morondova mantle plume alone. The 65–50 Ma convergence rate increase, however, is larger than can be explained by plume head spreading alone. We concur with previous hypotheses that plume-induced weakening of the Indian continental lithosphere-asthenosphere coupling and an increased slab pull and ridge push efficiency are the most likely explanations for the large convergence rate increase. The post-50 Ma decrease is best explained by orogeny-related increased trench resistivity, decreased slab pull due to continental subduction, and possibly restrengthening of lithosphere-asthenosphere coupling upon plume demise.

Details of the Doublet Moho Structure beneath Lhasa, Tibet, Obtained by Comparison of P and S Receiver Functions

The collision of the Indian and Asian plates since about 55 Ma has created the largest plateau with the thickest crust on Earth. There is, however, no general agreement on the modes of the crustal thickening. The crustal thickness of the Tibetan plateau still remains poorly determined. It is generally accepted that the Tibetan crust has roughly double normal thickness and that it thins somewhat toward the north, but individual observations of Moho depth vary spatially and for different techniques at the same place by greater than 20 km. In this work we compare P and S receiver functions at station LSA, located in the southern Lhasa terrane, to determine the crustal thickness beneath the station. A doublet Moho structure with two significant interfaces at about 60 and 80 km depth in the lower crust is seen clearly in the P receiver functions. The deeper phase (Moho) is, however, absent in the S receiver function data. Here we model the observed P and S receiver functions by a strong Moho topography that dips to the east at an angle of 30°. This result may indicate that the Moho structure beneath Tibet can be very complicated and has strong lateral variations and suggests that a detailed map of Moho depth is only possible with 2D dense seismic experiments. The Moho dips locally perpendicular to the direction of the Indian plate motion, suggesting that the lower crustal deformation is decoupled from the underlying Indian mantle lithosphere. The extremely strong Moho variation beneath and east of station LSA may also imply a crust-mantle interaction, specifically delamination or foundering of lower crust down to the upper mantle.

Monsoon speeds up Indian plate motion

Short-term plate motion variations on the order of a few Myr are a powerful probe into the nature of plate boundary forces, as mantle-related buoyancies evolve on longer time-scales. New reconstructions of the ocean-floor spreading record reveal an increasing number of such variations, but the dynamic mechanisms producing them are still unclear. Here we show quantitatively that climate changes may impact the short-term evolution of plate motion by linking explicitly the observed counter-clockwise rotation of the Indian plate since ~ 10 Ma to increased erosion and reduced elevation along the eastern Himalayas, due to temporal variations in monsoon intensity. By assimilating observations into empirical relations for the competing contributions of erosion and mountain building, we estimate the first-order decrease in elevation along the eastern Himalayas since initial strengthening of the monsoon. Furthermore, we show with global geodynamic models of the coupled mantle/lithosphere system that the inferred reduction in elevation is consistent with the Indian plate motion record over the same period of time, and that lowered gravitational potential energy in the eastern Himalayas following stronger erosion is a key factor to foster plate convergence in this region. Our study implicates lateral variations in plate coupling and their temporal changes as an efficient source to induce an uncommon form of plate motion where the Euler pole falls within its associated plate.

Transverse Tectonics in the Sikkim Himalaya: Evidence from Seismicity and Focal-Mechanism Data

Abstract In the present study, about 356 local earthquakes in the region of the Sikkim Himalaya have been accurately located and analyzed using 2181 P travel times and 2161 S travel times from a network of 11 broadband seismic stations operated by the National Geophysical Research Institute during January 2006 to November 2007. Further refinement of the hypocentral parameters using the hypoDD relocation program resulted in 198 well-constrained locations. Interestingly, this study reveals several characteristic features that distinguish Sikkim from the rest of the Himalaya. The seismicity distribution is found to be confined mostly between the main boundary thrust (MBT) and the main central thrust (MCT) but not quite associated with either. While the entire Himalayan front is generally characterized by shallow-angle thrust faulting, focal mechanisms in this region are predominantly of strike-slip type in conformity with a right-lateral strike-slip mechanism along the northwest-trending Tista and Gangtok lineaments. The P -axis trends of earthquake focal mechanisms are clearly oriented north-northwest, marking a clear transition from the ambient north-northeast trending direction of Indian plate motion with respect to the Eurasian plate all along the Himalayan front. Moderate-sized earthquakes occur down to 70 km depth in this region, compared to an average focal depth of 15–20 km in the rest of the Himalaya. Also, a high average crustal P velocity of 6.66 km/sec and a fairly low b value of 0.83±0.04 are obtained indicating the probability of occurrence of a higher magnitude earthquake in the future. A north–south section in the Sikkim region shows a relatively flat topography, unlike in the rest of the Himalayan mountain chain and suggestive of lower rates of convergence in the recent geologic past. It is proposed that crustal shortening in the Sikkim Himalaya has been substantially accommodated by transverse tectonics rather than underthrusting in recent times.

Anisotropy of the Indian continental lithospheric mantle

SUMMARY Due to the paucity of seismological data available in the public domain, the structure of the Indian lithosphere is still little known. We investigate the lithospheric structure and potential mechanical coupling between the crust and upper mantle along the Himalayan arc and underneath peninsular India using seismic anisotropy. Shear wave splitting measurements are performed on core-refracted phases. For each event recorded at a given seismological station we measured the orientation of the polarization plane of the fast S wave (phi), assumed to be a proxy for the orientation of the a axis of olivine, and the delay (dt) between the arrival time of the fast and slow S waves. We present a very comprehensive data set recorded at 86 seismological stations, deployed from the Himalayas to the southern tip of the Indian peninsula, in a joint effort by the National Geophysical Research Institute, Hyderabad, India, the University of Cambridge and the Indian Institute of Astrophysics. The unprecedented data set we present sheds light on the mechanisms involved in the India–Eurasia continental collision in a region along the Himalayan arc, south of the Indus-Tsangpo suture zone. At the scale of the Indian plate, the majority of the stations show a NNE–SSW orientation of phi over hundreds of kilometres, from Sri Lanka to the northern part of the Dharwar craton. This direction closely parallels the trend of the Indian plate motion, with respect to a fixed Eurasian plate, as defined through the NUVEL1A plate model. Along the Himalayan arc, from Ladakh in the northwest, to Bhutan and the Shillong plateau in the east, the orientation of phi rotates to become ∼EW, perpendicular to the plate motion as defined through NUVEL1A. Unlike previous studies, we do find strong evidence for seismic anisotropy south of the Indus Tsangpo suture zone. A large number of null results have been computed, with consistent orientation of the two fast polarization directions (phi) across the subcontinent. We demonstrate the potential value of the too often neglected null measurements in the interpretation of seismic anisotropy. From these results, we infer the dominance, beneath the Indian lithosphere, of the asthenospheric flow in aligning minerals in the sheared lithosphere–asthenosphere boundary layer, masking any compression induced anisotropy expected to be normal to this direction. Closer to the collision front in northern India, the anisotropy may in part, be due to the foliation planes of the Himalayan fold and thrust belt aligning the a axis of olivine perpendicular to the compression axis, but more likely to the turning of the relative asthenospheric flow along the strike caused by the downthrusting Indian lithosphere acting as a barrier. The continent-wide consistency of results strengthens the understanding that the Indian lithosphere has distinct anisotropic signatures, contrary to the hitherto assumed isotropy and allows one to interpret the results in a coherent framework of Indo-Eurasian convergence.

Compressive tectonics around Tibetan plateau edges

Various earthquake fault types, mechanism solutions, stress field, and other geophysical data were analyzed for study on the crust movement in the Tibetan plateau and its tectonic implications. The results show that numbers of thrust fault and strike-slip fault type earthquakes with strong compressive stress near NNE-SSW direction occurred in the edges around the plateau except the eastern boundary. Some normal faulting type earthquakes concentrate in the Central Tibetan plateau. The strikes of fault planes of thrust and strike-slip faulting earthquakes are almost in the E-W direction based on the analyses of the Wulff stereonet diagrams of fault plane solutions. This implies that the dislocation slip vectors of the thrust and strike-slip faulting type events have quite great components in the N-S direction. The compression motion mainly probably plays the tectonic active regime around the plateau edges. The compressive stress in N-S or NE-SW directions predominates earthquake occurrence in the thrust and strike-slip faulting event region around the plateau. The compressive motion around the Tibetan plateau edge is attributable to the northward motion of the Indian subcontinent plate. The northward motion of the Tibetan plateau shortened in the N-S direction encounters probably strong obstructions at the western and northern margins.

Constraints on Indian plate motion since 20 Ma from dense Russian magnetic data: Implications for Indian plate dynamics

We use more than 230,000 km of Russian marine magnetic and bathymetric data from the Carlsberg and northern Central Indian ridges, comprising one of the most geographically extensive, dense shipboard surveys anywhere in the ocean basins, to describe in detail seafloor spreading since 20 Ma along the trailing edge of the Indian plate. India‐Somalia plate rotations for ∼1 Myr intervals over the past 20 Myr are derived from inversions of more than 6600 crossings of 20 magnetic reversals and ∼1400 crossings of fracture zones that offset these two ridges. Statistical analysis of the numerous data indicates that outward displacement of reversal boundaries due to finite seafloor emplacement widths and correlated noise for anomaly crossings from individual spreading segments constitute two distinct sources of systematic bias in the locations of magnetic anomaly crossings, contrary to the often‐made assumption that random, Gaussian‐distributed noise dominates the error budget. Seafloor spreading rates slowed gradually by 30% from 20 Ma to 10 ± 1 Ma about a relatively stationary pole of rotation. From 11 Ma to 9 Ma the rotation axis migrated several angular degrees toward the plate boundary, modestly increasing the spreading gradient along the plate boundary. India‐Somalia kinematic data for times since ∼9 Ma are consistent with remarkably steady motion, with no evidence for a change in either the rotation pole or rate of angular opening within the few percent precision of our data. The timing and nature of changes in India‐Somalia motion since 20 Ma closely resemble those for the Capricorn‐Somalia plate pair, indicating that India and Capricorn plate motions are strongly coupled. We speculate that the slowdown in seafloor spreading at the trailing edges of the Indo‐Capricorn composite plate from 20 Ma to 10 ± 1 Ma resulted from the increasing amount of work that was needed to build topography in the Himalayan collisional zone. The transition to stable India‐Somalia and Capricorn‐Somalia seafloor spreading at ∼10–9 Ma corresponds well with the onset at 8 Ma of folding and faulting across an equatorial plate boundary separating the Indian and Capricorn plates, suggesting that the latter may have played a fundamental role in restoring equilibrium between the torques that were driving and resisting the northward motions of the Indian and Capricorn plates.

Two- and three-dimensional gravity modeling along western continental margin and intraplate Narmada-Tapti rifts: Its relevance to Deccan flood basalt volcanism

The western continental margin and the intraplate Narmada-Tapti rifts are primarily covered by Deccan flood basalts. Three-dimensional gravity modeling of +70mgal Bouguer gravity highs extending in the north-south direction along the western continental margin rift indicates the presence of a subsurface high density, mafic-ultramafic type, elongated, roughly ellipsoidal body. It is approximately 12.0 ±1.2 km thick with its upper surface at an approximate depth of 6.0 ±0.6 km, and its average density is {dy2935} kg/m3. Calculated dimension of the high density body in the upper crust is 300 ±30 km in length and 25 ±2.5 to 40 ±4 km in width. Three-dimensional gravity modeling of +10mgal to -30mgal Bouguer gravity highs along the intraplate Narmada-Tapti rift indicates the presence of eight small isolated high density mafic bodies with an average density of {dy2961} kg/m3. These mafic bodies are convex upward and their top surface is estimated at an average depth of 6.5 ±0.6 (between 6 and 8km). These isolated mafic bodies have an average length of 23.8 ±2.4km and width of 15.9 ±1.5km. Estimated average thickness of these mafic bodies is 12.4±1.2km. The difference in shape, length and width of these high density mafic bodies along the western continental margin and the intraplate Narmada-Tapti rifts suggests that the migration and concentration of high density magma in the upper lithosphere was much more dominant along the western continental margin rift. Based on the three-dimensional gravity modeling, it is conjectured that the emplacement of large, ellipsoidal high density mafic bodies along the western continental margin and small, isolated mafic bodies along the Narmada-Tapti rift are related to lineament-reactivation and subsequent rifting due to interaction of hot mantle plume with the lithospheric weaknesses (lineaments) along the path of Indian plate motion over the Reunion hotspot. Mafic bodies formed in the upper lithosphere as magma chambers along the western continental margin and the intraplate Narmada-Tapti rifts at estimated depths between 6 and 8 km from the surface (consistent with geological, petrological and geochemical models) appear to be the major reservoirs for Deccan flood basalt volcanism at approximately 65 Ma.

Current deformation of the Himalaya–Tibet–Burma seismic belt: inferences from seismic activity and strain rate analysis

A new measure of seismic activity in a region is defined, which is based on the concept of integrated fault surface area of earthquakes in the region. Using this concept, we analyze the Harvard CMT data of the Himalaya–Tibet–Burma seismic belt to estimate the relative proportions of seismic activity corresponding to reverse, strike-slip and normal faulting in the region. Further, strain rates are computed through summation of moment tensor elements of earthquakes. The study indicates that the deformation patterns of the Himalaya and the adjoining Tibetan plateau regions are distinct. For instance, the seismic activity of the reverse fault category changes from 93% in the Himalaya to a mere 2% in Tibet, which is dominated instead, by strike-slip faulting (59%). Strain rate computation indicates predominant crustal thickening in the Himalaya with a clear transition to crustal thinning in the Tibetan plateau region, just across the Indus-Tsangpo suture zone where EW extension is the predominant mechanism. A model of a thinning seismic upper crust in the EW direction decoupled from a thickening aseismic lower crust, both in equilibrium, in the Tibetan plateau, is proposed. In the Burmese arc region, crustal thickening is indicated, but coupled with NS compression. The observed seismic activity is predominantly of the strike-slip type (56%), uncharacteristic of subduction zones, which generally display up to 75% of seismic activity of the reverse fault category. This has implications for Indian plate motion along the Burmese arc, rather than in the direction of the subducted slab.