Eastern Ghats Mobile Belt Research Articles

Crustal architecture and rift tectonics across the Visakhapatnam Bay basin, central-east Indian margin: Insights from multichannel seismic and potential field data

The Visakhapatnam Bay (VB) basin is a passive margin rift basin located at the non-basinal segment of the Eastern Continental Margin of India (ECMI) and formed during the rift–drift events associated with the breakup of eastern Gondwanaland. In the present study, integrated analysis of potential field and multi-channel seismic reflection (MCS) data were carried out to understand the rift tectonics, crustal configuration and onshore–offshore structural continuity across this basin. The study revealed the following: (i) crustal models derived through joint gravity-magnetic modelling show limited stretching with 36–40 thick crust below the Eastern Ghat Mobile Belt (EGMB) thinning down to 16–20 km at the Ocean Continent Transition (OCT), (ii) extension of Charnockitic basement associated with the Eastern Ghat Mobile Belt (EGMB) into the offshore region. Comparison of the crustal configuration across the VB basin with that across the adjacent thick sedimentary area of the Krishna–Godavari shows that upper crustal configuration is significantly different in the VB area. The observed limited rift related structuration (horst-graben morphology), the mapped high angle break-away fault with large offset in the seismic data and narrow width (70–90 km) of extended crustal domain in VB basin suggests that this segment acts as transfer zone between Krishna–Godavari and Mahanadi rift zones. Further, long curve-linear trend of magnetic anomalies associated with Pudimadaka Lineament (PKL) demarcate the VB basin from the adjacent Krishna–Godavari rift zone.

Sedimentologic and stratigraphic constraints on the orientation of the Late Triassic northern Indian passive continental margin

The orientation and distribution of the northern Indian passive continental margin basin during different epochs constrained the geometric characteristics of the hyperextended northern Indian edge (“Greater India”). Traditional paleogeographic reconstructions envisage the Late Triassic northern Indian passive continental margin as E/W-trending, whereas paleomagnetic data support an ~55° counterclockwise rotation during northward drifting. This case study of Upper Triassic units from the northern Indian margin, based on facies analysis, sandstone petrography, heavy mineral assemblages, U-Pb detrital zircon geochronology, and stratigraphic correlation from the Zanskar Range to southern Tibet, provides new constraints and suggests that the northern Indian passive continental margin might have been N/S-trending in the Late Triassic. Given the dominance of quartz sandstones with zircon, tourmaline and rutile—indicating a provenance from continental blocks and the recycling of older quartz-rich sandstones—and detrital zircon age spectra characterized by a Neoproterozoic detrital zircon U-Pb age peak (921 ± 13 Ma), the Upper Triassic sandstones widespread along the Tethyan Himalayas may have been partly derived from the Eastern Ghats Mobile Belt in eastern India and the Rayner Orogen in eastern Antarctica. The Upper Triassic Quartzite Series comprises feldspatho-quartzose to pure quartzose sandstones with intercalated bioclastic oosparites deposited in backshore, foreshore and shoreface coastal environments. Without considering the subsequent counterclockwise rotation of India, the consistent pattern of the Upper Triassic bidirectional E-W paleocurrent observed throughout the Himalaya suggests that the paleoshoreline was striking near N-S, approximately perpendicular to the regional orientation of the northern Indian margin. Progressive eastward thickening of the sandstone layers and eastward younging of the Quartzite Series suggest diachronous sedimentation. Such stratal patterns and the strike of the Late Triassic paleoshoreline were attributed to the Neo-Tethys transgression from the Zanskar Range to southern Tibet.

Estimation of ground response and local site effects for Vishakhapatnam, India

Ground motion intensity due to an earthquake changes as it disseminates through the soil media from bedrock to the surface. As the ground motion intensity and damage levels mainly depend upon the local site conditions, it is mandatory to carry out the detailed site-specific hazard studies to assure safety of the structure against seismic risk. In this research paper, an effort has been made to estimate seismic hazard associated with the city of Vishakhapatnam. The city lies in east coast region of southern India and falls under seismic zone II (IS 1893-2016 in Criteria for earthquake-resistant design of structures: part 1—general provisions and buildings, Bureau of Indian Standards, New Delhi, 2016). Seismic activity in the Eastern Ghats mobile belt region has increased due to subduction of Burma plate toward the Bay of Bengal, which resulted in activation of inactive faults and new fault development in the region. Therefore, increasing seismic risk and importance of the study area has motivated the researchers to carry out probabilistic seismic hazard assessment and estimation of local site effects using ground response analysis and microtremor testing. From the results, hazard maps were generated in terms of peak ground acceleration (surface, bedrock level), H/V frequency and H/V amplitude. The seismic hazard parameters, uniform hazard response spectrum and hazards curves from probabilistic seismic hazard assessment are further useful in design and construction of prominent structures. The peak ground acceleration at surface and bedrock, predominant frequency and H/V amplitude indicate the variation in local site conditions and will be of great help in seismic design of structures as well as retrofitting of the existing structures to withstand against seismic hazards. Hazard maps from the study will be helpful in further seismic microzonation studies and also identifying zones of potential seismic risk.

Seismic imaging of the deep crustal structure beneath Eastern Ghats Mobile Belt (India): Crustal growth in the context of assembly of Rodinia and Gondwana supercontinents

We present the first high resolution seismic images illuminating the hitherto-elusive crustal architecture beneath the Eastern Ghats Mobile Belt (EGMB) using teleseismic receiver functions. Data were collected using 27 broadband seismic stations operated in a continuous mode covering two distinct seismic profiles (∼550 km long) during 2015–2018. Several interesting observations and inferences are made through analysis of the receiver functions such as (a) a very thick crust (>40 km) with oppositely dipping Moho beneath the EGMB and Archean Bastar Craton, (b) EGMB formed from amalgamation of different crustal domains thrust over one another possibly during the Pan-African orogeny, (c) the Archean Bastar Craton crust extends (∼75 km) eastward beneath the EGMB, from its surficial geological boundary, (d) there is a sharp contrast in the crustal structure (with ∼20 km Moho offset) at the contact between the Rengali Province and Singhbhum Craton which does not support southward growth of the Singhbhum Craton through accretion, (e) anorthosite complexes may possibly be created by rising diapirs channeled through the weak zones in the crust, from the magma chambers developed by melting of frontal portion of the underthrusting lower crust. We report a significant change in the crustal architecture just east of the most elevated topography observed along the profile covering the Bastar Craton and the EGMB. It requires further careful petrological investigations to ascertain the relationship of high elevation and its linkages with the deep crust, forming a separate domain. Our results do not support or discard a Grenvillian age (∼1 Ga) docking of the EGMB with Proto-India, though it is preferred to explain the present day crustal features with intense Pan-African (0.5–0.6 Ga) reorganization.

Estimation of coda Q for the eastern Indian craton

We herein present new frequency-dependent coda-\(Q\,\,(Q_\mathrm{c})\) relations (\(Q_\mathrm{c} =Q_{\text {0}}f ^{n})\) (frequency ranges between 2 and 18 Hz) for three regions of the eastern Indian craton (EIC), viz., the Singhbhum Odisha craton (SOC) and the Eastern Ghat mobile belt (EGMB), comprising the Mahanadi basin and the Chotanagpur granitic gneissic terrain (CGGT). The frequency-dependent coda-\(Q_\mathrm{c}\) relations are obtained through the single backscattering model for coda waves (\(Q_\mathrm{c})\) of local earthquakes which are recorded on 15 three-component broadband seismograph stations in the regions. In this work, we pay special attention to test the lapse time (\(t_\mathrm{L})\) dependency of coda-Q (\(Q_\mathrm{c})\) estimates for the three regions. Lapse time signifies the sample area of the coda wave of the study region. Generally, the sample area increases with lapse time. To test the lapse time (\(t_\mathrm{L})\) dependency, nine different lapse time windows (\(t_\mathrm{L})\) from 10 to 90 s with 10 s interval are considered. On the ground of estimated poor correlation coefficients, only six lapse time windows (\(t_\mathrm{L})\) from 40 to 90 s with 10 s interval are considered. Our results suggest more heterogeneity in EGMB than that of the SOC and CGGT region. Estimates of \(Q_{\text {0}}\) and n for the three regions of EIC (SOC, EGMB and CGGT) are found to be consistent with the results of \(Q_{\text {0}}\) and n for mildly active less heterogeneous seismic zones in different parts of the world. By assuming entirely intrinsic attenuation characteristics, actual hazard parameters, i.e., extinction distance and anelastic attenuation coefficients are also computed for the three regions. The extinction distance (\(L_\mathrm{e})\) provides an idea of the distribution of scatterers in the lithosphere and anelastic attenuation coefficients signify the anelasticity of the medium, i.e., fluid movement and grain distribution. The estimate of extinction distance and attenuation coefficients suggests that for all three study regions, the upper mantle is relatively less heterogeneous and attenuation below 110–126 km depth is also comparatively lower. Coda Q indicates the degree of fracture and heterogeneity in the lithosphere related to seismicity. A higher estimate of \(Q_{\text {0}}\) values in the Archaean SOC region and the Proterozoic CGGT region is found when compared with that of the sedimentary-rich EGMB. It can be inferred that seismically less active cratons in general comprise high \(Q_{\text {0}}\) values, whereas the sedimentary-rich EGMB is more attenuative, characterised by a low coda \(Q_{\text {0}}\) value. Moreover, it is found that the estimated \(Q_{\text {0}}\) values for CGGT region are a little bit higher than that for the SOC region. This can be explained as a comparatively less disturbed and less heterogonous land mass that is present in the CGGT region as compared to the SOC region, which comprises different minerals, ore bodies, fault scarps and shear zones. The developed \(Q_\mathrm{c}\) relation for the EIC region could be useful for the study of hazards and ground motion prediction.

Crustal architecture and Moho topography beneath the eastern Indian and Bangladesh margins – new insights on rift evolution and the continent–ocean boundary

The thick sedimentary cover (≤18 km) along the Eastern Continental Margin of India (ECMI) and over the Bay of Bengal has hindered understanding of the mechanisms of rifting that operated on the eastern Indian and Bangladesh margins. Analysis of multichannel seismic reflection data, together with 3D gravity inversion and 2D gravity forward modelling, illuminates basement configuration, crustal thickness and Moho topography, thereby revealing the modes of rift evolution and the location of the continent–ocean boundary (COB) along these margins. The basement, particularly in the shelf and slope regions of eastern peninsular India, is traversed by nearly coast-perpendicular graben, and their seaward continuity is delineated up to 125 km from the coastline. Three different types of continental margin are present along the ECMI: (1) sheared rift on the southern segment of the ECMI, up to 14° N latitude (offshore the Southern Granulite Terrain); (2) hyper-extended rift in the central segment between 14° N and 17° N (offshore the Dharwar Craton); and (3) hypo-extended rift on the northern segment between 17° N and 20° N (offshore the Eastern Ghats Mobile Belt). The graben terminations, crustal thickness and Moho topography of both the ECMI and offshore Bengal Basin clearly suggest that the COB runs nearly parallel to the coastline of peninsular India, but takes an orthogonal turn offshore in the Bengal Basin and connects the palaeo-continental shelf and the Rajmahal–Sylhet Line onshore in the Bengal Basin. Crustal thickness (8–10 km) and depth to the Moho discontinuity (≤28 km) beneath the Bangladesh margin are unusual because the crustal thickness increased through interaction of the Kerguelen plume with the existing oceanic lithosphere. The Moho deepened largely due to the load of the Bengal Fan sediments. The absence of rifted crustal blocks on the Bangladesh margin and the continuity of the COB into the onshore Bengal Basin together imply that the present Bangladesh region was under marine conditions at least until the beginning of Bengal Fan sedimentation (i.e. no less than 23 Ma).

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.

Magnetotelluric investigation of lithospheric electrical structure beneath the Dharwar Craton in south India: Evidence for mantle suture and plume-continental interaction

Broad-band and long period magnetotelluric measurements made at 63 locations along ∼500 km long Chikmagalur-Kavali profile, that cut across the Dharwar craton (DC) and Eastern Ghat Mobile Belt (EGMB) in south India, is modelled to examine the lithosphere architecture of the cratonic domain and define tectonic boundaries. The 2-D resistivity model shows moderately conductive features that intersperse a highly resistive background of crystalline rocks and spatially connect to the exposed schist belts or granitic intrusions in the DC. These features are therefore interpreted as images of fossil pathways of the volcanic emplacements associated with the greenstone belt and granite suite formation exposed in the region. A near vertical conductive feature in the upper mantle under the Chitradurga Shear Zone represents the Archean suture between the western and eastern blocks of DC. Although thick (∼200 km) cratonic (highly resistive) lithosphere is preserved, significant part of the cratonic lithosphere below the western DC is modified due to plume-continental lithosphere interactions during the Cretaceous–Tertiary period. A west-verging moderately conductive feature imaged beneath EGMB lithosphere is interpreted as the remnant of the Proterozoic collision process between the Indian land mass and East Antarctica. Thin (∼120 km) lithosphere is seen below the EGMB, which form the exterior margin of the India shield subsequent to its separation from East Antarctica through rifting and opening of the Indian Ocean in the Cretaceous.

Tectonothermal imprints in a suite of mafic dykes from the Chotanagpur Granite Gneissic complex (CGGC), Jharkhand, India: Evidence for late Tonian reworking of an early Tonian continental crust

Tectonothermal evolution of a set of mafic dykes in the felsic orthogneiss basement of the Chotanagpur Granite Gneissic Complex (CGGC) is presented. The gneissic basement rocks preserve high pressure granulite facies metamorphism (M2, ~800 °C and ~9 kbar) and develop pervasive planar fabric (S2) during a continent-continent collision event at 1000–950 Ma. Mafic dykes cut the S2 fabric of its basement it intrudes. Together with the S2, the mafic dykes have been deformed by three sets of folds viz. DkF1, DkF2 and DkF3. A conspicuous NS trending foliation defined by metamorphic amphibole is developed parallel to the axial plane of DkF1–2. The mafic dykes preserve relict igneous textures (e.g. intergranular texture) defined by clinopyroxene, orthopyroxene and plagioclase. Numerically computed phase diagram (pseudosection) with a representative bulk composition (with <10 vol% metamorphic minerals namely amphibole+ plagioclase rim + quartz) constrain the depth of emplacement of the mafic dykes (15–18 km). The geological features are consistent with rifting of the early Tonian basement of the CGGC and the then mantle beneath it. This lithospheric extension resulted in outpouring of basaltic magma that punctured the early Tonian basement rocks in the form of mafic dykes. Subsequent metamorphism that affected the mafic dykes and its basement culminated at ~700 °C and ~7.5 kbar (M3 metamorphism).The estimated physical conditions during the M3 metamorphic event suggest burial of the previously rifted basement possibly in a continent-continent collisional tectonic setting. The Th-U-total Pb ages of monazite grains from the host felsic orthogneiss date the M2 (ca. 964 Ma) and M3 (ca. 917 Ma) metamorphism. Based on commonality of geological and geochronological history among the CGGC, the Eastern Ghats Mobile Belt and Rayner complex (and its equivalent parts) in east Antarctica corroborates the view that Indo-Antarctic landmass were united and shared the similar geological histories during the entire Tonian period. During the Tonian period the Indo-Antarctic landmass witnessed two major phases of orogenesis (1000–950 Ma and < 920 Ma) that are punctuated by a phase of lithospheric extension manifested by the emplacement of mafic dykes.

Reassembly of the Dharwar and Bastar cratons at ca. 1 Ga: Evidence from multiple tectonothermal events along the Karimnagar granulite belt and Khammam schist belt, southern India

The northern part of the Nellore–Khammam schist belt and the Karimnagar granulite belt, which are juxtaposed at high angle to each other have unique U–Pb zircon age records suggesting distinctive tectonothermal histories. Plate accretion and rifting in the eastern part of the Dharwar craton and between the Dharwar and Bastar craton indicate multiple and complex events from 2600 to 500 Ma. The Khammam schist belt, the Dharwar and the Bastar craton were joined together by the end of the Archaean. The Khammam schist belt had experienced additional tectonic events at \(\sim \)1900 and \(\sim \)1600 Ma. The Dharwar and Bastar cratons separated during development of the Pranhita–Godavari (P–G) valley basin at \(\sim \)1600 Ma, potentially linked to the breakup of the Columbia supercontinent and were reassembled during the Mesoproterozoic at about 1000 Ma. This amalgamation process in southern India could be associated with the formation of the Rodinia supercontinent. The Khammam schist belt and the Eastern Ghats mobile belt also show evidence for accretionary processes at around 500 Ma, which is interpreted as a record of Pan-African collisions during the Gondwana assembly. From then on, southern India, as is known today, formed an integral part of the Indian continent.

Magnetotelluric evidence on the southward extension of the Eastern Ghats mobile belt from Ongole, India

There has been a longstanding and scientifically contentious issue, concerning whether or not the Eastern Ghats mobile belt (EGMB) extends southward beyond Ongole in the state of Andhra Pradesh, in south India. Various geological and geophysical investigations carried out in different campaigns could not conclusively address this important issue. Some argued that the EGMB that originates near Mahanadi in the northeast of India and extends along the eastern continental boundary, is abruptly cut near Ongole in the south, swerves eastward towards the Bay of Bengal and joins the erstwhile east Antarctica boundary. Others opined differently and debated about its southward extension beyond Ongole. In the present study, we provide a conclusive evidence for the first time on this perplexing issue, by determining the deep earth electrical resistivity structure of the EGMB region through broad-band magnetotelluric (MT) investigations carried out along two different profiles laid out one near the Ongole region (profile AA′ of 35 km length) and the other near the Nellore region (profile BB′ of 52 km length). The latter is located at a distance of about 125 km south of the former. The lengths of both these profiles are sufficiently far from the influences of the electrical nature of the subsurface of the adjoining geological terrains namely, the east Dharwar craton, the Cuddappah basin and the southern granulite terrain. This enables to determine the electrical nature of the EGMB alone and examine whether or not it extends south of Ongole. Two-dimensional inversion of MT data of both the profiles revealed a very thin top conductive layer of about 1–10 Ω-m with an average thickness of about 500 m, representing the alluvium. This layer is underlain by very high resistive subsurface with resistivity of about 107 Ω-m extending up to about 30 km and probably further beyond (not shown in the present study). Since the major rock types in the study area, namely, Khondalites, Calc-granulites, Charnockites and Granitic gneisses of Archaean age, have large overlapping resistivities, MT data could not explicitly discern the nature of the multilayered crust beneath the EGMB. As a result, the subsurface beneath both the regions shows up as a very thick single layer of very high resistivity. The high grade rocks of the Ongole domain and supracrustals of Nellore Schist Belt (NSB), which lies south of Ongole domain, have undergone synchronous tectonic activity at ca.1.6 By. This is evident by the Kanigiri and Kandra ophiolites within NSB that signify the Proterozoic subduction of the EGMB. This geological evidence together with the similar nature of the high electrical resistivity of the subsurface observed at Ongole and Nellore regions confirm the southward extension of the EGMB beyond Ongole.

Mineralogy of the Vattikod lamproite dykes, Ramadugu lamproite field, Nalgonda District, Telangana: A possible expression of ancient subduction-related alkaline magmatism along Eastern Ghats Mobile Belt, India

ABSTRACTThe mineralogy of nine recently discovered dykes (VL1:VL8 and VL10) in the vicinity of Vattikod village, Nalgonda district in Telangana State is described. The mineral assemblage present and their compositions are comparable to those of bona fide lamproites in terms of the presence of phlogopite (Ti-rich, Al-poor phlogopite and tetraferriphlogopite); amphiboles (potassic-arfvedsonite, potassic-richterite, potassic-ferro-richterite, potassic-katophorite, Ti-rich potassic-katophorite, Ti-rich potassic-magnesio-katophorite); Al-poor clinopyroxenes; feldspars (K-feldspar, Ba-K-feldspar and Na-feldspar) and spinels (chromite-magnetite and qandilite-ulvöspinel-franklinite). These dykes have undergone diverse and significant degrees of deuteric alteration as shown by the formation of secondary phases such as: titanite, allanite, hydro-zircon, calcite, chlorite, quartz and cryptocrystalline SiO2. On the basis of their respective mineralogy: the VL4 and VL5 dykes are classified as pseudoleucite-phlogopite lamproite; VL2 and VL3 dykes as pseudoleucite-amphibole-lamproite; and VL6, VL7 and VL8 as pseudoleucite-phlogopite-amphibole-lamproite. VL10 is extensively altered but contains fresh euhedral apatite microphenocrysts together with pseudomorphs after leucite and is classified as a pseudoleucite-apatite-(phlogopite?) lamproite. The mineralogy of the Vattikod lamproite dykes is compared with that of the Ramadugu, Somavarigudem and Yacharam lamproite dykes which also occur in the Ramadugu lamproite field. The lamproites from the Eastern Dharwar Craton are considered as being possible expressions of ancient subduction-related alkaline magmatism along the Eastern Ghats mobile belt.

Mapping of magnetic basement in Central India from aeromagnetic data for scaling geology

ABSTRACTThe Central Indian region has a complex geology covering the Godavari Graben, the Bastar Craton (including the Chhattisgarh Basin), the Eastern Ghat Mobile Belt, the Mahanadi Graben and some part of the Deccan Trap, the northern Singhbhum Orogen and the eastern Dharwar Craton. The region is well covered by reconnaissance‐scale aeromagnetic data, analysed for the estimation of basement and shallow anomalous magnetic sources depth using scaling spectral method. The shallow magnetic anomalies are found to vary from 1 to 3 km, whereas magnetic basement depth values are found to vary from 2 to 7 km. The shallowest basement depth of 2 km corresponds to the Kanker granites, a part of the Bastar Craton, whereas the deepest basement depth of 7 km is for the Godavari Basin and the southeastern part of the Eastern Ghat Mobile Belt near the Parvatipuram Bobbili fault. The estimated basement depth values correlate well with the values found from earlier geophysical studies. The earlier geophysical studies are limited to few tectonic units, whereas our estimation provides detailed magnetic basement mapping in the region. The magnetic basement and shallow depth values in the region indicate complex tectonic, heterogeneity, and intrusive bodies at different depths, which can be attributed to different thermo‐tectonic processes since Precambrian.

Seismic noise analysis of broadband stations in the Eastern Ghat Mobile Belt of India using power spectral density

ABSTRACTThe seismic background noise is studied at newly constructed sites of the Eastern Ghat Mobile Belts (EGMB), in Orissa. Ten broadband stations have been deployed along two profiles. The area has not been previously attempted for any broadband experiment. To assess the performance of the stations, we investigate the characteristics of different types of noises dominated at those sites. The power spectral densities of ambient background noise for each site are calculated and compared with the global noise model. The results suggest that the noise levels at all stations that belong to profile-1 are within the global high and low bounds. Along profile-2, the station PATN stabilizes after some time post-deployment while noise levels at other two stations are within the proposed limits. Results show that profile-1 is predominated by cultural noise, and the microseismic noise levels are observed to be low in both the profiles. Assessment of the temporal and seasonal variation of the noise has also been done, along with the calculation of the power spectral densities in the East and North components of the seismometer for this station. Overall, our observations suggest that installation of the seismometers have been successful in all the sites.

Modeling of Source Parameters and Moment Tensors of Local Earthquakes Occurring in the Eastern Indian Shield

ABSTRACT Earthquake source parameters and crustal Q are being estimated simultaneously through the inversion of S-wave displacement spectra from three-component recordings of ten local cratonic intraplate earthquakes from 3-6 broadband stations in the eastern Indian shield, wherein, an iterative Levenberg-Marquardt inversion technique is used. The estimated seismic moment (Mo) and source radii (r) vary from 7.4 x 1012 to 7.1 x 1014 N-m and 144.2 to 211.3 m, respectively, while estimated stress drops (Δσ) and multiplicative factor (Emo) values range from 0.11 to 4.13 MPa and 1.33 to 2.16, respectively. The corner frequencies range from 6.23 to 8.62 Hz while moment magnitudes vary from 2.44 to 3.57. The radiated seismic energy and apparent stresses range from 8.3 x 106 to 2.0 x 1010 Joules and 0.06 to 0.94 MPa, respectively, wherein the estimated corner frequencies and seismic moment satisfy the relation Mo ∞ fc−(3+ε) for ε = 12.7. Thus, the source scaling of these events clearly deviates from the self-similarity i.e. f−3. Estimated Zuniga parameters reveal that all selected events satisfy the partial stress drop model, which is in good agreement with the global observations. Our estimated crustal S-wave quality factors vary from 1091 to 4926 with an average of 3006, suggesting a less heterogeneous crustal structure underlying the study region. We also perform moment tensor inversion of five selected local events using ISOLA software, which reveals that the dominant deformation mode for the eastern Indian shield is left-lateral strike slip motion with minor normal dip-slip component on an almost vertical plane. This observation suggests that neotectonic vertical movements might have played a key role in generating these earthquakes. Our modeling also depicts that the seismically mildly active Singhbhum shear zone and Eastern Ghats mobile belt are characterized by the left-lateral strike motion while two events in the Chotanagpur half graben belt suggest a normal dip-slip motion along a south dipping plane. A north-south orientation of P-axis is found to be dominant in the area, which is consistent with the prevailing north–south compression over the Indian plate.

About this title ‐ Crustal Evolution of India and Antarctica: The Supercontinent Connection

The Proterozoic aeon involved at least three major continental readjustments. India and Antarctica appear in most models of supercontinent reconstructions, but their relative position has been the subject of debate. High-resolution petrological and geochronological data, especially from the Proterozoic mobile belts, provide the principal means of resolving this issue. The ice-covered nature of Antarctica allows only limited access to the rocks, and then only in coastal tracts, so detailed studies in more accessible Proterozoic terrains in India assume added significance. This volume, a follow-up to the XII International Symposium on Antarctic Earth Science, Goa (a SCAR symposium), provides new data from selected locations in east Antarctica (Enderby Land and Dronning Maud Land) and from India, including the Eastern Ghats Mobile Belt (EGMB), Chota Nagpur Gneissic Complex, the Khasi Hills and the Aravalli–Delhi Mobile Belt. The presented geochronological data, constrained by petrological studies, are expected to provide new insights, especially into the EGMB–east Antarctica connection and the rate of continental readjustments in the post-Rodinia break-up.

Seismic evidence of crustal low velocity beneath Eastern Ghat Mobile Belt, India

The Eastern Ghat Mobile Belt (EGMB), a tectonically active area extends along the eastern margin of Peninsular India, is divided into three provinces, namely, Eastern Ghat Province, the Jeypore Province, and the Krishna Province. The Ongole domain of Krishna Province is a seismically active region that has experienced four moderate earthquakes of magnitude ⩾5.0, of which largest one is of magnitude 5.4 occurred on 27th March 1967. The crustal shear wave velocity structure in the Eastern Ghat Mobile Belt has been investigated using joint inversion of receiver functions and Rayleigh wave group velocity at 5 locations in the study region. The results show crustal thickness variation from 37 to 42km and average shear velocity variation from 3.67 to 3.78km/s in the study region. A low velocity layer of variable thickness and velocities 3.54–3.7km/s) is also observed in the region. The low velocity layer in most of the stations is observed at a depth of ∼20km. This low velocity layer may be due to the presence of fluid in the crust, which also be one of the causes of the intraplate earthquakes in the study region.

Teleseismic receiver functions modeling of the eastern Indian craton

We estimate receiver functions (RFs) through the time-domain deconvolution using three-component broadband data of 100 teleseismic events (30°⩽∧⩽90°) from 15 seismographs in the eastern Indian craton. Estimated radial RFs show a positive phase at 4.6–5.8s delay time corresponding to the crustal thicknesses of 37–46km. Through the differential evolution (DE) waveform inversion modeling of radial receiver functions, we delineate the crustal structure at 15 broadband stations. On an average, the Archean Singhbhum Odisha Craton (SOC) is characterized by a thick crust of 43±3km in comparison to a relatively thin crust of 41±1km underlying the Proterozoic Chotanagpur Granite Gneissic terrain (CGGT). While, a thin crust of 38±1km characterizes the younger Eastern Ghats Mobile Belt (EGMB). The main results of our modeling reveal a 46km thick Archean crust underlying the Singhbhum granite (SG) of 3.6Ga, which is characterized by a 3km crustal thickening probably resulted from the Archean subduction process. Our modeling also detects a 2–3km crustal thinning with the thinnest crust of 37km below the region near South Singhbhum Shear Zone, which could be attributed to the 1.6Ga plume activity associated with Dalma volcanic. Our modeling also led to the delineation of a crustal thinning of 2–3km underlying the region in EGMB, which was influenced by a much younger (∼117Ma) Rajmahal magmatism associated with the Gondwana break-up episode. However, our study could not detect any age-dependent variation of crustal thicknesses in the eastern Indian craton. The main result of our modeling suggests a two-phase crustal evolution process for the SOC viz. older E-W crustal thickening due to E-W plate compression and later crustal thinning episodes associated with the Dalma volcanism in the north and the Rajmahal volcanism in the South.

Spatial relationship between earthquakes, hot-springs and faults in Odisha, India

Odisha is famous for Mineral rich Eastern-Ghat mobile belt, hot springs and cultural Heritage. The hot springs are known for centuries and are used by public as a place for worship. Odisha falls under the II and III seismic zones in India. Most of the seismicity in Odisha is due to motion along some active normal faults along the Mahanadi Graben. Therefore, it is necessary to identify the active faults and understand spatial distribution of seismic activity in Odisha. It is also important to understand the Earthquakes and their relation with the Geology of Odisha and understand the neo-tectonic activity. There are 7 major hot springs found along the North Odisha Boundary Fault and Mahanadi Shear Zone. The hot water percolates deep into the Earth through porous and permeable fractured rocks along the fault. Depth of source for most of the hot springs in Odisha must be some few feets to few meters; however most of these observations are not based on scientific geophysical data. Therefore, spatial relationship between thermal springs, earthquakes, and geology of Odisha may provide better understanding of the hot-spring setting. By using the earthquake and fault data, the sense of motion along faults can be easily interpreted. All these information can explain the spatial distribution and inter-relation between hot-springs, faults and earthquakes in Odisha.

Vertical density contrast and mapping of basement, Conrad and Moho morphologies through 2D spectral analysis of gravity data in and around Odisha, India

Power spectrum analysis of Complete Bouguer Anomaly (CBA) map of Eastern Ghat Mobile Belt (EGMB) and its surroundings in India through Two Dimensional (2D) spectral analysis has provided estimates of the ensemble average depths for the density discontinuities which represent crustal inhomogeneities. The spectral analysis method has helped to estimate the depths of a perturbing body sources which are obtained from the negative slopes of the linear relationship between the logarithmic power spectrum and the wave-numbers of the gravity field. The detailed analysis reveals three horizontal discontinuities (i) Phanerozoic sediment thickness (ii) Basement depth and (iii) Conrad discontinuity. The average thickness of Phanerozoic sediments is estimated to be 3km whereas depth of basement and Conrad discontinuity are at 7km and 14.5km respectively. Additionally Mohorovicic discontinuity also estimated at a depth of 32.8km in the study region.