Movement Of Indian Plate Research Articles

Earth’s longest preserved linear volcanic ridge generated by a moving Kerguelen hotspot

Recent seismic tomography unveiled complex mantle plume structures diverging from the originally proposed single, narrow, and vertically-oriented plume conduits, which necessitates new perspectives on the mechanism of hotspot motion. While several recent endeavours have focused on Pacific hotspots’ motion, knowledge of others remains limited. Here we constrain the motions of the Kerguelen hotspot within the Indian Ocean by obtaining robust 40Ar/39Ar ages for the Ninetyeast Ridge, Earth’s longest linear volcanic ridge. These data indicate varying volcanic progression rates along the ridge, contrasting to a constant rate as previously documented. Combined with constraints on the Indian Plate motion and seafloor spreading, we reveal four periods of motions of the hotspot caused by its interactions with the Indian–Antarctic spreading ridge. This suggests that mantle plume lateral flows are susceptible to changes in shallow mantle convection due to the existence of horizontal ponding zones and vertical conduits, especially in the shallow asthenosphere.

Unveiling the Mechanisms of the 1819 M 7.7 Kachchh Earthquake, India: Integrating Physics‐Based Simulation and Strong Ground Motion Estimates

AbstractThis study provided a comprehensive understanding of the source process of the 1819 M 7.7 Kachchh Indian earthquake using physics‐based dynamic rupture modeling and strong ground motion simulations. We successfully simulated the spontaneous dynamic rupture along a curved non‐planar fault using the 3‐D curved‐grid finite‐difference method (CGFDM). The estimated earthquake magnitude is around 7.6, consistent with previous estimations. Our simulations accurately replicated macroscopic rupture patterns and surface deformation, showing agreement with observed data along the Allah Bund fault (ABF) with a maximum displacement ∼5.5 m at the Earth's surface. The maximum modeled coseismic slip on the fault was approximately 7.5 m. Notably, the ABF exhibited characteristics of a weak barrier (leaky barrier) at the bending part, allowing the rupture to propagate further. Despite limitations in surface deformation calculations, the modeled values aligned with the trend of surface fault slip, with a slight deviation in the epicenter toward the east compared to earlier studies. We observed a homogeneous principal stress oriented N25°E, consistent with the present day Indian plate motion. The estimated horizontal peak ground velocities (PGVh) and the maximum value of Intensity X+ aligns well with observations. Furthermore, conducting thorough case studies on significant earthquakes and potential seismic scenarios in stable continental regions is crucial. Such studies play a vital role in validating and improving dynamic rupture models. When combined with statistical methods, this research holds great promise for advancing seismic hazard assessments, earthquake engineering, and strategies for disaster management.

Slab buckling as a driver for rapid oscillations in Indian plate motion and subduction rate

Plate tectonics is primarily driven by the constant gravitational pull of slabs where oceanic lithosphere sinks into the mantle at subduction zones. Under stable plate boundary configurations, changes in plate motion are then thought to occur gradually. Surprisingly, recent high-resolution Indian plate reconstructions revealed rapid (2–3 Million-year) plate velocity oscillations of ±50 %. We show, through numerical experiments, that the buckling of slabs in the mantle transition zone causes such oscillations. This buckling results from the deceleration of slabs as they sink into the lower mantle. The amplitude and period of buckling-associated oscillations depend on average subduction velocity and the available space in the mantle transition zone. The oscillations also affect the upper plate which may explain enigmatic observations of episodic deformation and fluid flow in subduction-related orogens. We infer that the slab pull that drives plate tectonics is generated in just the top few hundred kilometers of the mantle.

Periodicity in the Deccan Volcanism Modulated by Plume Perturbations at the Mid‐Mantle Transition Zone

AbstractIn peninsular India, the Deccan Traps record massive, continental‐scale volcanism in a sequence of magmatic events that corresponds with the timing of mass extinction at the Cretaceous‐Paleogene boundary. Although the Deccan volcanism is linked with the Réunion hotspot, the origin of its periodic magmatic pulses is still debated. We developed a numerical model replicating the geodynamic scenario of the African superplume underneath a moving Indian plate to explore the mechanism of magmatic pulse generation during the Deccan volcanism. Our model results revealed a connection between the Réunion hotspot and the African large low shear‐wave velocity province (LLSVP), suggesting that the pulses were produced from a thermochemical plume originated in the lower mantle. The ascending plume had stagnation at 660 km due to phase changes in the transition zone, and its head eventually underwent detachment from the tail under the influence of Indian plate movement to produce sequentially four major pulses (periodicity: 5–8 Ma), each giving rise to multiple secondary magmatic pulses at a time interval of ∼0.15–0.4 Ma. This study sheds a new light on the mechanism of periodic hotspot activities from a global perspective.

Understanding the causes and consequences of the northward extension of the tropical monsoon in Asia in the Eocene

Previous geological studies have revealed that a zonal climate pattern with an arid zonal band prevailed in the mid-latitude of East Asia in the Paleogene. On the south of the arid band, a tropical monsoon climate controlled South China, Indochina, and India. In the Eocene, the boundary between the arid and the tropical monsoon climate largely moved northward. However, the reason behind this change remains unclear. Here, with the fast version of the Norwegian Earth System Model (NorESM1-F), we conduct experiments under different Eocene paleogeography conditions to analyze the evolution of the tropical Asian monsoon from the early Eocene to the late Eocene. Our results demonstrate that the northward movement of Indian plate causes a reorganization in low-level westerlies in summer. After the full merging of the India-Asia continents, modern-like low-level tropical circulations come into being and lead to the wetting in southern China.

Deep seismogenic tectonics of Yangbi MS6.4 on 21 May 2021 in the SE margin of the Tibetan plateau from earthquake sequence relocation, stress field and seismic anisotropy

The MS6.4 earthquake occurred in Yangbi County, Yunnan Province, China, on 21 May, 2021. The earthquake is located on the west side of the Red River fault, the western boundary of the Sichuan-Yunnan block in the SE margin of the Tibetan Plateau. The Yangbi MS6.4 earthquake has aroused great concern due to its unique geographical location and frequent strong earthquakes in Yunnan. In this study, a total of 4695 local earthquakes of the Yangbi MS6.4 earthquake sequence are relocated using the double-difference location method, including the mainshock source location of 99.879°E, 25.687°N, and depth of 9.67 km. The mainshock has a moment magnitude of 6.1, and one of the fault-plane solutions is 137°, 83° and − 163° in strike, dip, rake respectively. Combined with the relocation and focal mechanism solutions results, it is suggested that the seismogenic fault of the Yangbi MS6.4 earthquake is a NW-SE trending fault on the southwest side of the Weixi-Qiaohou fault (WQF), with high dip of SW, which is a previously-unknown blind fault. The regional 2D stress field shows that the principal compressive stress around the Yangbi earthquake is NNW-SSE and nearly N-S direction, which appears to be deflected counterclockwise by a small angle on its east side and changed to N-S and NNE-SSW direction on its west side, while both the north and south sides of the Yangbi earthquake show directional coherence. Strong differential deformation distorts in the upper crust to the northeast of the seismic source as reflected by tectonic stress field and the upper crust seismic anisotropy, indicating strong influence of Lijiang-Xiaojinhe fault (LXF). The stress field in the source area is jointly controlled by southeastward movement of southeastern margin of Tibetan Plateau, the northeastward movement of Indian plate and the differential movement between the different tectonic blocks.

Seismotectonic Analysis of the 7 October 2021 Mw 5.9 Harnai Earthquake, Pakistan

ABSTRACTAn earthquake of moderate magnitude 5.9 occurred northeast of Harnai (Baluchistan), Pakistan, on 07 October 2021. This event caused several deaths and injuries due to the collapse of many mud houses in the epicentral area according to the Provincial Disaster Management Authority (PDMA). The event occurred along the Quetta syntaxis, which is one of the most seismically active zones of the country. The source mechanism of the mainshock based on the moment tensor inversion technique was determined using seismic data from the local network. Stress patterns and active tectonics within the Harnai area were investigated on the basis of recent event and previously available focal mechanism solutions. The 2021 earthquake source mechanism was oblique slip with 12 km focal depth and appears to be associated with the Harnai–Karahi fault zone. Interestingly, the nodal plane oriented in the east–west direction having a high dip corresponds to reverse faulting, whereas the low dip nodal plane trending northwest–southeast-depicts a strike-slip mechanism. The upper crustal stress field and current tectonic deformation in this region are governed by the oblique convergence between the Indian and Eurasian plates along the Chaman fault zone marking the transform plate boundary. The local tectonic stress pattern anomalously supports the low dip nodal plane of the determined fault plane solution representing transpressional environment in which originally produced reverse fault is being reactivated with a dominantly strike-slip sense of movement. In this area, two stress regimes, strike-slip and thrust mechanism, were identified based on homogenous stress patterns. The recent Harnai earthquake occurred in a region where thrust-faulting environment due to horizontal principal compression (SHmax) directed in north-northwest–south-southeast act as first order stress owing to the collision of the Indian and Eurasian plates. The SHmax direction of other subset areas has a similar orientation but different stress ratio (R′), which depicts strike-slip faulting. The presence of a rigid (Katawaz) block along the plate boundary, which impedes the Indian plate motion, causes the second-order stresses that result in strike-slip sense of movement in the pre-existing reverse faults. The same stress seems to be responsible for bending of the existing faults to produce en echelon structures within the Sulaiman range and generates connecting faults in the form of a bookshelf model.

Deterministic and Probabilistic Seismic Hazard Analysis of Tindharia, Darjeeling Sikkim Himalaya, India

Abstract The simulation of appropriate ground motion is the main objective in earthquake-resistant designing to mitigate the impact of landslides and structural damages. The seismic hazard studies serve as the basis for quantifying the hazard associated within the particular area over a given time period. In this study, the detailed site specific hazard analysis of Tindharia site as well as Darjeeling Sikkim Himalaya (DSH) region has been carried out to assure the safety of human and infrastructure against seismic risk. The study area is mountainous region on north western side of India, falls in seismic zone IV and V, and is one of the most seismically active regions with vulnerable earthquakes along with fatal landslides. The future large earthquakes are likely to occur in the region due to constant movement of Indian plate towards Eurasian plate. Therefore, the heightened seismic risk increases the demand for seismic studies near the Himalayan belt. Thus, the assessment of associated seismic hazard in the study area has been carried out by deterministic and probabilistic seismic hazard approaches. From the results, the seismo-tectonic map of a 300km radial distance of the study area is developed. The deterministic hazard distribution in terms of peak ground acceleration (PGA) and the PGA and peak spectral acceleration (PSA) hazard maps for 2% and 10% probability of exceedance in 50 years at different time periods have been generated. The updated hazard maps developed in this study play pivotal roles in mitigation measures with better pre-disaster prevention by earthquake resistant design and in preparedness and post disaster rescue in the study area.

Seismic anisotropy beneath the Kumaun–Garhwal Himalaya using core-refracted shear wave phases

SUMMARY We investigate the nature of seismic anisotropy in the Kumaun–Garhwal Himalaya by analysing the core-refracted shear wave splitting phases (SK(K)S, PKS) recorded at 42 broad-band seismic stations. The mean fast anisotropy directions are mostly in ∼E–W and a few in the ∼NE–SW directions. The mean delay time decreases progressively from ∼1.0 s beneath the Sub Himalaya to ∼0.5 s at the Higher Himalaya. The observed seismic anisotropy suggests that the ongoing deformation beneath the Kumaun–Garhwal Himalaya has a large contribution by the relative rotation of the asthenospheric flow by the compression along the Indian Plate motion direction in this collision zone. Also, the southward transportation of the mid-crustal flow is probably responsible for the reduction in the delay times in the Main Central Thrust zone and further north than in the Sub-to-Lower Himalaya. The splitting parameters indicate a complex pattern of deformation beneath the Kumaun–Garhwal Himalaya with contribution from crust, lithospheric mantle and asthenospheric mantle.

Shear wave birefringence and current configuration of active tectonics of Shillong plateau: an appraisal of Indian plate motion and regional structures

Shear wave birefringence and current configuration of active tectonics of Shillong plateau: an appraisal of Indian plate motion and regional structures

Crustal deformation analysis across Garhwal Himalaya: Part of western Himalaya using GPS observations

Past earthquakes in the Himalayas are mainly due to tectonic activities along the thrust faults parallel to the Himalayan belt. In order to study the ongoing tectonic process, crustal deformation at a selected transect in a part of western Himalayas as well as of the Indian plate, then: are studied by the analyses of GPS data processed in a GAMIT/GLOBK platform. The data were collected from four local continuous operating reference stations (CORS), six International Global Navigation Satellite System Service (IGS) stations and five surveyed based GPS campaign stations transverse to major faults system. Results show an annual crustal motion of 19.18 mm Near Southern Tibetan Detachment (STD), 18.72 mm near Main Central Thrust (MCT), 11.93 mm near Main Boundary Thrust (MBT) and 11.53 mm near Himalayan Frontal Thrust (HFT) in India fixed reference. The present day Indian plate motion was estimated to be 55.33 mm/yr. In the study region at western Himalaya, crustal convergence of 13.83 mm/yr was observed in Dehradun-Badrinath sector in Uttarkhand. GPS sites near to north of HFT shows a westward movements and sites at peninsular India shows eastward movements suggesting possible counter clockwise rotation of the Indian plate. The annual motion across HFT varies between 0.1 and 2.0 mm, suggesting that HFT is locked in Garhwal region of western Himalaya. Insignificant movement across HFT indicates greater strain accumulation and the possibility of earthquakes in near future. The study suggests requirement of continuous monitoring of deformation across major thrusts in the Himalaya with dense network of GPS stations.

Upper Mantle Anisotropy beneath the Western Segment, NW Indian Himalaya, Using Shear Wave Splitting

Abstract This study investigates the upper mantle deformation pattern beneath the Indo-Eurasia collision zone utilizing the core-refracted (S(K)KS) phases from 167 earthquakes recorded by 20 broadband seismic stations deployed in the Western Himalaya. The 76 new shear wave splitting measurements reveal that the fast polarization azimuths (FPAs) are mainly oriented in the ENE-WSW direction, with the delay times varying between 0.2 and 1.7 s. The FPAs at most of the stations tend to be orthogonal to the major geological boundaries in the Western Himalaya. The average trend of the FPAs at each station indicates that the seismic anisotropy is primarily caused due to strain-induced deformation in the top ~200 km of the upper mantle as a result of the ongoing Indo-Eurasian collision. A contribution from the mantle flow in the direction of the Indian plate motion is possible. The mantle strain revealed in the present study may be due to a combination of basal shear resulting from plate motion and ductile flow along the collision front due to compression.

Does pulsed Tibetan deformation correlate with Indian plate motion changes?

Models that aim to explain the causes of the significant Indian plate motion acceleration around 70 Ma, and the subsequent deceleration around 52 Ma predict different scenarios regarding crustal shortening of the Tibetan Plateau, which can be tested by precisely determining the timing of regional shortening events in Tibet. Here we attempt to determine this timing by presenting a high-resolution magnetostratigraphy of a ∼3.5 km thick sedimentary sequence in the syn-contractional Gonjo Basin, east-central Tibet. We successfully isolated the primary remanence as confirmed by positive fold and reversal tests. Correlation to the geomagnetic polarity time scale reveals a 69–41.5 Ma age for the Gonjo Basin sedimentary succession. Average sedimentation rates indicate two episodes of enhanced sediment accumulation rate at 69–64 Ma and 52–48 Ma, which coincide with periods of vertical axis rotation recorded in the basin fill. This coincidence suggests a tectonic cause, which given regional structures we interpret as shortening pulses. Our results are similar to those from basins elsewhere in southern, central and northern Tibet, suggesting plateau-wide, synchronous shortening pulses at ∼69–64 Ma and ∼52–48 Ma. These pulses are synchronous with major acceleration and deceleration of India-Asia convergence rate, suggesting that both the acceleration and deceleration of India-Asia convergence may be associated with enhanced crustal deformation in Tibet, which we use to evaluate previous dynamic models explaining the Indian plate motion changes and India-Asia collision processes.

Cenozoic paleostress evolution in south central Vietnam: Implication for changing dynamics of faulting along the eastern Indochina continental margin

The tectonics of the Indochina continental margin before and after the Neogene-Quaternary basalt eruption is poorly understood. The analysis of 1500 striated faults observed in different rocks, mostly from volcano-plutonic rocks with ages ranging from Cretaceous to Quaternary, at more than 140 locations in south central Vietnam by direct inversion method allowed us to determine four paleostress regimes that affected eastern Indochina from Oligocene up to present day. The first tectonic paleostress regime with Oligocene age was responsible for the sinistral and dextral motions along the NW-SE and NE-SW striking faults respectively. This stress state is typical for a strike-slip tectonic regime with maximum principal stress (σ1) and minimum principal stress (σ3) axes oriented nearly in E-W and N-S direction and both plunging at average angle of 80 and 100, respectively. The paleostress ellipsoid shape ratio Φ (Φ = (σ2 − σ3)/(σ1 − σ3)) ranges from 0.23 to 0.77 with an average of 0.50 ± 0.15. The subsequent strike-slip tectonic stress state prevailing from the Late Oligocene to very Early Miocene, was characterized by σ1 and σ3 axes trending in NNW-SSE and ENE-WSW and plunging at 120 ± 90, and 120 ± 130, respectively. The paleostress ellipsoid shape ratio Φ ranges from 0.12 to 0.88 with a 0.52 ± 0.23 average. Under this paleostress regime, the N1100 to N1400 and N1600 to N200 striking faults moved with dextral and sinistral movements, respectively. The third paleostress state with assigned Early Miocene to Late Miocene age was responsible for dextral and sinistral motions along N1400 to N1600 and N200 to N400 striking faults, coeval with emplacement of N-S striking basaltic dykes. This pattern shows consistency and homogeneity of the σ1 and σ3 axes in N-S trend, plunging at 120 ± 90 and σ3 in E-W trend, plunging at 130 ± 110, respectively. The paleostress ellipsoid shape ratio Φ varies from 0.41 to 0.89 with an average of 0.53 ± 0.16. The fourth tectonic stress regime occurring since the Late Miocene to the present day is characterized by a consistency of sub-horizontal NNE striking σ1, plunging 110 ± 100 and ESE striking σ3, plunging at 130 ± 11, and an average of ellipsoid shape ratio Φ = 0.45 ± 0.24. Under the latest tectonic stress state, the N-S and N450 to N600 striking faults continue to move as dextral and sinistral faults, respectively. The first three paleostress regimes have evolved clockwise, and were induced by the force exerted from the eastern Himalayan syntaxis during the northward Indian plate motion, while the fourth tectonic stress regime is most likely linked to the force originated from the Sunda subduction.

Mantle deformation patterns and signatures of rift, beneath Eastern Ghats Mobile Belt

The Eastern Ghats Mobile Belt (EGMB) and the adjacent Archaean Singhbhum and Bastar craton contact collisional sutures zones in southeastern India are investigated for seismic anisotropic signatures. The EGMB-craton contact boundary is a witness to the episode of rifting related to the breakup of super-continent Columbia, followed by reassembly and breakup of Rodinia, and eventual episode of collision of eastern India with Antarctica during the Pan-African age. With the intent to capture signatures of the collision, 22 broadband seismic stations were installed along two distinct profiles covering cratons and Eastern Ghats contact boundaries. Shear wave splitting analysis using core refracted phases, SKS, SKKS and PKS, waveforms resulted in 202 high-quality measurements, with variation in delay times (δt) from 0.4 s to 2.0 s, and predominant NE trend for the fast polarization directions (FPDs). The anisotropic parameters along the two profiles depict the influence of two dominant mechanisms. First, Absolute Plate Motion (APM) related strain where mantle drag at the base of lithosphere combined with other plate driving forces creates a NE dominance of fast-axis orientations in the direction of the Indian plate motion. Second, fossil anisotropy of the lithospheric origin, as fast axis is not completely in the direction of the Indian plate motion, specially across Rengali Province and Mahanadi Rift basin. Fast-axis orientations parallel to the surface geological features indicate large-scale pervasive deformation related to the collision and rifting episodes of India and East Antarctica. The composite stacked FPDs and δt in the Rengali domain is manifestation of the NE trend of the Indian Plate motion, combined with the WNW–ESE to E–W oriented fabrics associated to the EGMB-cratons contact shear zones. The Mahanadi Rift has a predominance of the rift parallel signatures due to the ancient preserved fabrics, which correlates the Mahanadi Rift with the Lambert Rift of East Antarctica, and is of key importance in plate-tectonic reconstructions.

Shear-wave splitting in the Rajasthan craton, India

The shear-wave splitting parameters (i.e. fast axes orientation (ψ) and delay time (δt)) are measured at four broadband stations in Rajasthan, using SKS/SKKS core phases from three-component broadband waveforms of 30 events recorded during 2014–16. The thicknesses of lithosphere at these four stations are also inferred from the estimated delays. The average splitting parameters (ψ, δt) at the northern-most (JAI) and southern-most (BAN) stations are modeled to be 36.44° ± 08.36° and 1.48 ± 0.32 s, and 35.72° ± 10.47° and 1.86 ± 0.34 s, respectively. While these parameters at UDI and BUN stations are found to be 41.08° ± 08.54° and 1.61 ± 0.26 s, and 43.86° ± 08.96° and 1.68 ± 0.27 s, respectively. Thus, our modeling reveals a mean fast axes orientation of (39° ± 9°) with a mean delay of 1.7 ± 0.3 s, for Rajasthan, which could be attributed to the frozen lithospheric anisotropy in the direction of current absolute plate motion of the Indian plate. A slight regional variation of observed orientation of fast axes ranging from 35.7°(at BAN) to 43.9° (at BUN)could be attributed to the tectonic control rather than current plate motion. In addition, the mean orientations of fast axes are measured to be 36.4o at UDI and 41.1o at JAI, respectively. Thus, this observed coherent variation of fast axes orientation across the Rajasthan suggests a significant contribution from the asthenospheric flow. Further, estimated mean delay time (1.7 ± 0.3 s) does also suggest a contribution from the anisotropy associated with asthenospheric flows. The maximum delay (∼1.86 s) is estimated at southernmost BAN station, suggesting a thick anisotropic layer of ∼214 km, while minimum delay (∼1.48 s) is modeled at northernmost JAI station, suggesting a 170-km thick anisotropic layer. While asthenospheric layer thicknesses at UDI and BUN are found to be 185 and 193 km, respectively. This observation suggests a relatively more anisotropic and thick lithosphere in the south in comparison to that in the north. The measured deviation of fast axis orientation from the absolute Indian plate motion varies from 1 to 4.3°, which could be related to the changes in asthenosphric flow pattern due to the presence of topography at the base of lithosphere.

Oblique slip faulting associated with evolving central Indo-Burmese region from Early Pleistocene deformational sequences

The Indo-Burmese Ranges (IBR) marks as the boundary between the rigid Burmese Plate and the north-easterly moving, obliquely converging Indian plate. This results in upliftment of Mizoram fold belt alongside of IBR, in the eastern most part of the Indian subcontinent. NS trending fault generated fold belt appears around the region due to the NE stress component from Indian plate movement. There are many linear tectonic features formed due to this process. On the eastern margin of the fold belt in the greater elevated part of IBR, E–W and NW–SE lineament patterns intersect the NS lineaments, and deformed the ridges obliquely to create complex geotectonic settings. In this study, a pristine attempt has been made to understand the geodynamic evolutionary mechanisms for the area to reveal the cause for such surficial convolutions. The arcuate subduction zone along the IBR causes the slab to flex and bend at depth. This relates to differential dipping of the slab and the greater dip surface occurred beneath the study area. The easterly dipping slab can accommodate great amount of sediments and therefore the region having maximum dip upholds the highest Tertiary sediment thickness, above the basement. Champhai district of Mizoram, India had been considered for this study as it has in the zone of convergence with highest slab dip underneath. The surface exposures have sufficient neotectonic evidences which inferred the signs from Early Pleistocene neotectonic to present active tectonic deformational history. Sequential offset of the antecedent rivers carries the signatures of the varying stress component within them. The geodynamic processes produce some tectonic features through the mode of earthquake generation. Epicentral plots and focal mechanism solutions for the area indicate seismic activity associated with the thrust and oblique strike-slip movement along with their correlation for the lineament distribution. The evolutionary model indicates syn-tectonic upliftment of the study area along with evolving IBR during Late Oligocene thrusting events. A Paleo-lake adjacent to the Champhai town is believed to be the source of some northerly flowing rivers that was located on the ridge. Late Pleistocene seismic events produce some contractional strike-slip faults that later transformed to oblique slip component that shaped the area. This study is also important for understanding the earthquake events and related deformation pattern in IBR. This also might be used to locate potential geohazard sites for safer construction as well as to understand the petroleum migration pathways.

Earthquake Focal Mechanism Studies in Koyna-Warna Region in the Last Five Decades — Current Understanding on Tectonics and Seismogenesis

ABSTRACT On 10th December 1967, the world’s largest reservoir triggered seismic (RTS) event of magnitude 6.3 shook the Koyna region, the prime site of RTS globally. Ever since, several studies have attempted to infer the seismotectonics and to comprehend the actual causative mechanism of triggered seismicity in this region. Initial studies, including those of the 1967 Koyna main shock and its aftershocks, were based on the conventional P wave polarity or the first motion approach. These studies provided the first ever understanding of a predominantly strike-slip environment in the Koyna region, concurrent with the direction of ambient stress field due to the Indian plate motion. Subsequent studies pointed to a normal faulting environment in the Warna region further south, subsequent to impoundment later in 1985. A few studies did report solutions based on composite focal mechanisms, which however, only represent the average picture of the region. More recent studies based on modelling of seismic broadband waveform data provided more accurate focal mechanisms with unprecedented location accuracies including focal depths. A catalog of 50 focal mechanism solutions is now available for the earthquakes of magnitude ∼4 and larger that occurred during the last 50 years, which has paved way for a clear understanding of the stress field and the causative model of seismogenesis in this active intra-plate seismic RTS zone in western India. Based on stress inversion using this catalog, a new tectonic model depicting a periodically varying stress field and hence faulting mechanism has been inferred.

Provenance and paleo–weathering of Tertiary accretionary prism–forearc sedimentary deposits of the Andaman Archipelago, India

In order to understand the provenance and tectono–sedimentary processes occurring in the Andaman Subduction Zone (ASZ), the Late Cretaceous to Oligocene sedimentary records from the Andaman Islands have been studied. These sedimentary records are considered to have preserved the history of the India–Asia collision, evolution of the Himalayas, climatic development and palaeo–drainage reorganizations on the Indian and Asian plates. About 47 sandstones and mudstones (shales and siltstones) samples were analyzed for whole rock major, trace, and rare earth element compositions. The geochemical results suggest mixing of sediments derived from the mafic igneous sources comprising local ophiolites and volcanic arc of the ASZ and an older Archean to Proterozoic age felsic cratonic source with compositions similar to average granodiorite or upper continental crustal sources. The compositions were dominated by sources of the mafic arc during deposition of the Mithakhari Group, whereas they were controlled by continental sources during deposition of the Andaman Flysch Group. The Hope Town Conglomerate unit of the Mithakhari Group was mainly derived from weathering and erosion of the subaerially exposed local ophiolite thrust sheets, whereas its Namunagarh unit contains significant detritus from volcanic arcs. The Andaman Flysch turbidites were deposited with a greater supply of sediments from first–cycle active continental margin sources probably located in the Tibetan and eastern Myanmar region and recycled quartzose sedimentary sources within the nascent Himalayas. The sediments supplied to both the Mithakhari and the Andaman Flysch Groups were characterized by varying values of CIA, PIA and W. These variable values were either due to non–steady state weathering conditions in the sources or the changing climatic conditions owing to the motion of Indian plate with reference to the equator. The uniformly high CIA and W values in the Andaman Flysch rocks can be related to high precipitation and strong chemical weathering associated with the initiation of the Indian monsoon.

Constraining quantitatively the timing and process of continent-continent collision using magmatic record: Method and examples

Based on the main driving force of plate motion (the slab pull force generated by the descent of the oceanic plate in subduction zones) and the three primary mechanisms for magma generation (adding fluid, increasing temperature, and decreasing pressure), the continent-continent collisional process has been divided into three stages, including initial collision, ongoing collision, and tectonic transition. These stages are characterized by normal calc-alkaline andesitic magma (dehydration of the oceanic crust to release fluids), the migration of calc-alkaline magma toward the trench (dehydration of the oceanic crust or an increase in temperature) or small-scale crust-derived peraluminous magma (heat from intra-crustal shearing), and extensive magmatism with compositional diversity induced by slab break-off (increasing temperature and decreasing pressure), respectively. On the basis of the obtained age of slab break-off, the timing of the initial continent-continent collision can be quantitatively back-dated using the convergence rate, depth of slab break-off, and subduction angle. The spatio-temporal migration of the magmatic activity of the Gangdese Batholith, the onset of magmatic flare-up, and the increase of magma temperature at 52–51 Ma documented by the volcanic rocks of the Linzizong Pana Formation were most likely the result of the break-off of the Yarlung-Zangbo Neo-Tethyan oceanic lithosphere at approximately 53 Ma. This proposed age of slab break-off suggests that the initial India-Asia collision likely occurred at approximately 55–54 Ma, which is consistent with the collision ages constrained by other abundant geological data (60–55 Ma). This magmatic method has been applied to the Bitlis orogenic belt in southern Turkey in the Arabia-Eurasia continental collision zone, yielding an age range of approximately 29–22 Ma for the initial Arabia-Asia continental collision that is close to the collision ages recently obtained by apatite fission-track dating (approximately 20 Ma) and regional tectonic shortening (approximately 27 Ma). The intense folding of the Upper Cretaceous and the angular unconformity between the overlying Linzizong volcanic rocks in the southern Lhasa Terrane (90−69 Ma) are not related to the initial continental collision between India and Asia, but can be interpreted as the consequences of the strong coupling between the hot and young subducting oceanic crust immediately south of the spreading ridge and the overriding lithosphere or the subduction of the Neo-Tethys oceanic plateaux or seamounts. The tectonic event documented by the angular unconformity between the Linzizong Dianzhong Formation and the Nianbo Formation lasted approximately 3 Ma and likely marks the initial India-Asia collision. The significant deceleration of the Indian continent at approximately 51 Ma can be attributed to the disappearance of the slab pull force in the subduction zone due to the break-off of the Yarlung-Zangbo Neo-Tethyan oceanic lithosphere. The descent of the eclogitized lower crust of the northern Indian continent provides the main driving force for the current northward motion of Indian plate. The weak deformation of the lithospheric plate in the overriding plate of the India-Asia collisional zone between 60 and 40 Ma can be attributed to the high-angle subduction related to the rollback of the Yarlung-Zangbo Neo-Tethyan oceanic lithosphere after the initial India-Asia continental collision, the presence of the thick crust and high elevation on the southern margin of the Lhasa Terrane, and the decoupling between the mid-upper and lower crust and between the lower crust and lithospheric mantle of the Indian continent.