I'm currently a JC Bose National Fellow, Professor, and Founding-Chair of the Earth and Climate Science department at the Indian Institute of Science Education and Research, Pune. Address: Hyderabad, Andhra Pradesh, India
We present a shear velocity image across the early Archaean (>3.4-3.0 Ga) granite-greenstone c... more We present a shear velocity image across the early Archaean (>3.4-3.0 Ga) granite-greenstone crust of the western Dharwar craton of the south Indian shield. We determine this image through a receiver function analysis of teleseismic waveforms recorded at six broadband seismographs deployed for a year along a 200 km long profile across the granite-greenstone belt. We analyze more than 60
We present an improved shear velocity model for the crust and upper mantle of the Indian shield, ... more We present an improved shear velocity model for the crust and upper mantle of the Indian shield, an assembly of Precambrian cratons bounded by mobile belts and paleo-rifts, obtained by inverting receiver function data and a set of recently constructed Rayleigh wave group velocity maps of India and the surrounding regions. The latter provide a higher resolution dataset than previous
Teleseismic P‐wave arrivals recorded at South Indian seismological stations have been used to con... more Teleseismic P‐wave arrivals recorded at South Indian seismological stations have been used to construct a 3‐D velocity image of the upper mantle beneath this region. Analysis of a suite of models lead us to conclude that an anomalously high velocity region (1–6% contrast) exists in the upper mantle beneath the whole of South Indian shield including the Deccan Traps in the depth range of 60–300 km. In contrast, the westernmost part of the Deccan Traps and its subjacent mantle in the northwest is characterised by a relatively low velocity. This observed low velocity zone may be a still warm remnant of the spreading centre offset eastwards from the Carlsberg ridge, which might have been the dominant source of the flood basalts that flowed over the Deccan Plateau 65 Ma ago. The high velocity upper mantle of the entire South Indian shield is viewed as a continental root formed by a process of geochemical reordering which happens to be more stable.
We investigate crustal properties beneath the deep crustal (∼35 km) Jabalpur earthquake of 21 May... more We investigate crustal properties beneath the deep crustal (∼35 km) Jabalpur earthquake of 21 May 1997, Narmada‐Son Lineament (NSL), central India, in search of a possible cause of stress accumulation in the region. Teleseismic receiver functions computed at nine digital seismographs along a 250 km long profile suggests Moho downwarp to ∼52 km across the width of the lineament, in contrast with an average 40 km depth elsewhere. In addition, the crust beneath the NSL has higher Vp/Vs of 1.84 compared to ∼1.73 in the surrounding, suggestive of a high‐density mafic mass at depth that compensates the crustal root, also supported by small topographic variation (200 m) across the lineament. Presence of such an anomalous mass in the deep crust may lead to gravitationally induced stresses in the lower crust that contribute to the failure of rock along the pre‐existing Narmada‐Son fault leading to the earthquake in deep crust.
ABSTRACT The diverse geological fabric of south India provides a unique opportunity to study/veri... more ABSTRACT The diverse geological fabric of south India provides a unique opportunity to study/verify different models of continental crustal evolution. We examine these models using constraints such as variation in crustal thickness, average composition and the S velocity variation in depth beneath these geological terrains. These parameters are modeled from teleseismic receiver functions computed for 32 broadband seismograph sites. The important results are as follows: The crustal thickness in the Eastern Dharwar Craton (EDC), Deccan Volcanic Province (DVP) and Cuddapah Basin (CB) varies from 33-39 km similar to the global average. The most unexpected result is the anomalous present day crustal thickness of 48-54 km beneath the early/mid Archaean (3.4-3.0 Ga) greenstone belt of the Western Dharwar Craton (WDC). Beneath the granulite terrain, the crustal thickness varies between 36-52 km. The Poisson’s ratio ranges between 0.25-0.28 beneath the study region, indicating the intermediate crustal composition. In EDC and DVP, the shear velocity image shows a simple and transparent crust with an average thickness of 35 2 km and the average S velocity to be 3.79 0.09 km/s. Across the obliquely exposed Archaean continental crust of the WDC, Moho depth increases progressively with the metamorphic grade from ~40 km in greenschist to ~65 km in Nilgiri hills (garnet-granulite). These observations of a thickened crust suggest significant crustal shortening in south India during the late Archaean. The most prominent structure in the granulite terrain is a high velocity zone (S velocity 4.0 km/s) in the upper crust overlying a low velocity (< 3.7 km/s) in the lower crust. Transition from amphibolite to granulite terrain is marked by a significantly low S velocity ( 3.5 km/s) in the mid crust. Such a prominent low velocity in an over- thickened crust could be imprint of fluid, trapped during metamorphism at ~2.6 Ga. The shear velocity image across EDC-WDC shows crustal thickening from ~35 km beneath late Archaean crust (EDC) to over 54 km beneath the oldest nucleus in WDC. Also, we observe significantly lower S velocity (3.3-3.5 km/s) with high Poisson's ratio (0.30) in the lower crust beneath the early Archaean greenstone belt, indicative of a hydrated and serpentanized mantle in a subduction environment. These results provide the first seismological imprint of early Archaean plate tectonics that facilitated the formation of cratonic keel beneath the Dharwar craton.
We present the analysis of shear-wave splitting of the mantle phases and modelling of the partiti... more We present the analysis of shear-wave splitting of the mantle phases and modelling of the partitioning of the energy on the radial and tangential components of the receiver function for the anisotropy in the Indian crust and upper mantle. We estimated the crustal contribution in the shear-wave splitting by the analysis of Moho converted Ps phases and removed this effect
The Pan-African orogeny was a major tectono-thermal event which occurred at the Proterozoic-Palae... more The Pan-African orogeny was a major tectono-thermal event which occurred at the Proterozoic-Palaeozoic boundary (~600 Ma) and is now marked by zones of high-grade metamorphism distributed from the Arabian-Nubian shield through Tanzania, Mozambique, Southern India and Sri Lanka to the eastern Antarctic. Teleseismic earthquake recordings from stations in these areas have been analyzed to compare the seismic characteristics of the
Most intraplate continental earthquakes occur in the upper ˜15 km of the crust but occasionally e... more Most intraplate continental earthquakes occur in the upper ˜15 km of the crust but occasionally earthquakes occur at deeper levels beneath the continents in some regions. This observation coupled with laboratory measurements of rock properties as a function of temperature and pressure, led to a model for the continental crust consisting of a brittle upper layer and a weak lower
We present group velocity dispersion results from a study of regional distance range fundamental ... more We present group velocity dispersion results from a study of regional distance range fundamental mode Rayleigh waves propagating across the Indian region. One-dimensional path average dispersion measurements have been made for 1001 source-receiver paths and these have been combined to produce tomographic images between 15s and 60s period. These results have significantly higher lateral resolution for the Indian region than
We present group velocity dispersion results from a study of regional fundamental mode Rayleigh a... more We present group velocity dispersion results from a study of regional fundamental mode Rayleigh and Love waves propagating across India and surrounding regions. Data used in this study comes from broadband stations operated in India by us in addition to data from seismograms in the region whose data is archived at the IRIS Data Management Centre. The large amount of
The January 26, 2001 mb 6.9 Bhuj mainshock was well recorded at both teleseismic and regional dis... more The January 26, 2001 mb 6.9 Bhuj mainshock was well recorded at both teleseismic and regional distances. Many of the larger aftershocks were also well recorded at regional distances by digital broadband seismographs operated by the National Geophysical Research Institute of India, the University of Cambridge and the Indian Meteorological Office. We have modeled the teleseismic P- and SH-waveforms to
The Dharwar craton has a 3-4 - 3.1 Ga old continental nucleus wrapped by a series of arcs and bac... more The Dharwar craton has a 3-4 - 3.1 Ga old continental nucleus wrapped by a series of arcs and back arc basins developed around 2-7-2.5 Ga to the east of the nucleus. The craton is divided into two parts west and east by a N-S elongated 2.5 Ga closepet granitic intrusion. The western Dharwar craton evolved during early - mid
Q measurements from the coda of Lg waves provide excellent constraint on the attenuation structur... more Q measurements from the coda of Lg waves provide excellent constraint on the attenuation structure along source-receiver paths. Lg coda Q values are strongly dependent on crustal structure and generally correlate very well with the tectonic histories of the regions sampled -- high Lg coda Q is typical of tectonically inactive regions (e.g., cratons) whilst low Lg coda Q is
We present a shear velocity image across the early Archaean (>3.4-3.0 Ga) granite-greenstone c... more We present a shear velocity image across the early Archaean (>3.4-3.0 Ga) granite-greenstone crust of the western Dharwar craton of the south Indian shield. We determine this image through a receiver function analysis of teleseismic waveforms recorded at six broadband seismographs deployed for a year along a 200 km long profile across the granite-greenstone belt. We analyze more than 60
We present an improved shear velocity model for the crust and upper mantle of the Indian shield, ... more We present an improved shear velocity model for the crust and upper mantle of the Indian shield, an assembly of Precambrian cratons bounded by mobile belts and paleo-rifts, obtained by inverting receiver function data and a set of recently constructed Rayleigh wave group velocity maps of India and the surrounding regions. The latter provide a higher resolution dataset than previous
Teleseismic P‐wave arrivals recorded at South Indian seismological stations have been used to con... more Teleseismic P‐wave arrivals recorded at South Indian seismological stations have been used to construct a 3‐D velocity image of the upper mantle beneath this region. Analysis of a suite of models lead us to conclude that an anomalously high velocity region (1–6% contrast) exists in the upper mantle beneath the whole of South Indian shield including the Deccan Traps in the depth range of 60–300 km. In contrast, the westernmost part of the Deccan Traps and its subjacent mantle in the northwest is characterised by a relatively low velocity. This observed low velocity zone may be a still warm remnant of the spreading centre offset eastwards from the Carlsberg ridge, which might have been the dominant source of the flood basalts that flowed over the Deccan Plateau 65 Ma ago. The high velocity upper mantle of the entire South Indian shield is viewed as a continental root formed by a process of geochemical reordering which happens to be more stable.
We investigate crustal properties beneath the deep crustal (∼35 km) Jabalpur earthquake of 21 May... more We investigate crustal properties beneath the deep crustal (∼35 km) Jabalpur earthquake of 21 May 1997, Narmada‐Son Lineament (NSL), central India, in search of a possible cause of stress accumulation in the region. Teleseismic receiver functions computed at nine digital seismographs along a 250 km long profile suggests Moho downwarp to ∼52 km across the width of the lineament, in contrast with an average 40 km depth elsewhere. In addition, the crust beneath the NSL has higher Vp/Vs of 1.84 compared to ∼1.73 in the surrounding, suggestive of a high‐density mafic mass at depth that compensates the crustal root, also supported by small topographic variation (200 m) across the lineament. Presence of such an anomalous mass in the deep crust may lead to gravitationally induced stresses in the lower crust that contribute to the failure of rock along the pre‐existing Narmada‐Son fault leading to the earthquake in deep crust.
ABSTRACT The diverse geological fabric of south India provides a unique opportunity to study/veri... more ABSTRACT The diverse geological fabric of south India provides a unique opportunity to study/verify different models of continental crustal evolution. We examine these models using constraints such as variation in crustal thickness, average composition and the S velocity variation in depth beneath these geological terrains. These parameters are modeled from teleseismic receiver functions computed for 32 broadband seismograph sites. The important results are as follows: The crustal thickness in the Eastern Dharwar Craton (EDC), Deccan Volcanic Province (DVP) and Cuddapah Basin (CB) varies from 33-39 km similar to the global average. The most unexpected result is the anomalous present day crustal thickness of 48-54 km beneath the early/mid Archaean (3.4-3.0 Ga) greenstone belt of the Western Dharwar Craton (WDC). Beneath the granulite terrain, the crustal thickness varies between 36-52 km. The Poisson’s ratio ranges between 0.25-0.28 beneath the study region, indicating the intermediate crustal composition. In EDC and DVP, the shear velocity image shows a simple and transparent crust with an average thickness of 35 2 km and the average S velocity to be 3.79 0.09 km/s. Across the obliquely exposed Archaean continental crust of the WDC, Moho depth increases progressively with the metamorphic grade from ~40 km in greenschist to ~65 km in Nilgiri hills (garnet-granulite). These observations of a thickened crust suggest significant crustal shortening in south India during the late Archaean. The most prominent structure in the granulite terrain is a high velocity zone (S velocity 4.0 km/s) in the upper crust overlying a low velocity (< 3.7 km/s) in the lower crust. Transition from amphibolite to granulite terrain is marked by a significantly low S velocity ( 3.5 km/s) in the mid crust. Such a prominent low velocity in an over- thickened crust could be imprint of fluid, trapped during metamorphism at ~2.6 Ga. The shear velocity image across EDC-WDC shows crustal thickening from ~35 km beneath late Archaean crust (EDC) to over 54 km beneath the oldest nucleus in WDC. Also, we observe significantly lower S velocity (3.3-3.5 km/s) with high Poisson's ratio (0.30) in the lower crust beneath the early Archaean greenstone belt, indicative of a hydrated and serpentanized mantle in a subduction environment. These results provide the first seismological imprint of early Archaean plate tectonics that facilitated the formation of cratonic keel beneath the Dharwar craton.
We present the analysis of shear-wave splitting of the mantle phases and modelling of the partiti... more We present the analysis of shear-wave splitting of the mantle phases and modelling of the partitioning of the energy on the radial and tangential components of the receiver function for the anisotropy in the Indian crust and upper mantle. We estimated the crustal contribution in the shear-wave splitting by the analysis of Moho converted Ps phases and removed this effect
The Pan-African orogeny was a major tectono-thermal event which occurred at the Proterozoic-Palae... more The Pan-African orogeny was a major tectono-thermal event which occurred at the Proterozoic-Palaeozoic boundary (~600 Ma) and is now marked by zones of high-grade metamorphism distributed from the Arabian-Nubian shield through Tanzania, Mozambique, Southern India and Sri Lanka to the eastern Antarctic. Teleseismic earthquake recordings from stations in these areas have been analyzed to compare the seismic characteristics of the
Most intraplate continental earthquakes occur in the upper ˜15 km of the crust but occasionally e... more Most intraplate continental earthquakes occur in the upper ˜15 km of the crust but occasionally earthquakes occur at deeper levels beneath the continents in some regions. This observation coupled with laboratory measurements of rock properties as a function of temperature and pressure, led to a model for the continental crust consisting of a brittle upper layer and a weak lower
We present group velocity dispersion results from a study of regional distance range fundamental ... more We present group velocity dispersion results from a study of regional distance range fundamental mode Rayleigh waves propagating across the Indian region. One-dimensional path average dispersion measurements have been made for 1001 source-receiver paths and these have been combined to produce tomographic images between 15s and 60s period. These results have significantly higher lateral resolution for the Indian region than
We present group velocity dispersion results from a study of regional fundamental mode Rayleigh a... more We present group velocity dispersion results from a study of regional fundamental mode Rayleigh and Love waves propagating across India and surrounding regions. Data used in this study comes from broadband stations operated in India by us in addition to data from seismograms in the region whose data is archived at the IRIS Data Management Centre. The large amount of
The January 26, 2001 mb 6.9 Bhuj mainshock was well recorded at both teleseismic and regional dis... more The January 26, 2001 mb 6.9 Bhuj mainshock was well recorded at both teleseismic and regional distances. Many of the larger aftershocks were also well recorded at regional distances by digital broadband seismographs operated by the National Geophysical Research Institute of India, the University of Cambridge and the Indian Meteorological Office. We have modeled the teleseismic P- and SH-waveforms to
The Dharwar craton has a 3-4 - 3.1 Ga old continental nucleus wrapped by a series of arcs and bac... more The Dharwar craton has a 3-4 - 3.1 Ga old continental nucleus wrapped by a series of arcs and back arc basins developed around 2-7-2.5 Ga to the east of the nucleus. The craton is divided into two parts west and east by a N-S elongated 2.5 Ga closepet granitic intrusion. The western Dharwar craton evolved during early - mid
Q measurements from the coda of Lg waves provide excellent constraint on the attenuation structur... more Q measurements from the coda of Lg waves provide excellent constraint on the attenuation structure along source-receiver paths. Lg coda Q values are strongly dependent on crustal structure and generally correlate very well with the tectonic histories of the regions sampled -- high Lg coda Q is typical of tectonically inactive regions (e.g., cratons) whilst low Lg coda Q is
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