Evidence of underplating from seismic and ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, B12311, doi:10.1029/2003JB002764, 2004

Evidence of underplating from seismic and gravity studies in the Mahanadi delta of eastern India and its tectonic significance Laxmidhar Behera, Kalachand Sain, and P. R. Reddy National Geophysical Research Institute, Hyderabad, India Received 28 August 2003; revised 16 August 2004; accepted 26 August 2004; published 29 December 2004.

[1] We have imaged the crustal configuration of the Mahanadi delta of eastern India by

modeling both wide-angle seismic and Bouguer gravity data. This delta has undergone several stages of rifting, subsidence, sedimentation, and uplift. It is one of the important Gondwana sedimentary basins of India where widespread volcanic activity occurred during the Early Cretaceous along the rift zones. This volcanic activity corresponds to the breakup of greater India from east Gondwana (e.g., present Antarctica and Australia). We have derived a five-layer crustal model with velocities of 6.0, 6.5, 6.0, 7.0, and 7.5 km/s having estimated densities of 2.7, 2.8, 2.65, 2.9, and 3.05 g/cm3, respectively. The velocity-density relation along with heat flux and other geological/geochronological information indicates typical rift-related evolution of the delta with a midcrustal lowvelocity (6.0 km/s) and low-density (2.65 g/cm3) zone and a 10 km thick high-velocity (7.5 km/s) and high-density (3.05 g/cm3) layer at the base of the crust, presumably due to underplating. The Moho upwarping or crustal thinning in the rift zone and emplacement of thick, high-velocity material at the base of the crust strongly suggests basaltic underplating probably due to the Kerguelen hot spot activity. These activities are synchronous with 117 Ma Rajmahal volcanism in northeast India and the Lambert graben in East Antarctica and are closely associated with the Gondwana breakup in the IndiaAntarctica sector. The rifting stages are closely correlated with the sedimentation phases of the lower and upper Gondwana deposits in the Mahanadi delta, which evolved parallel to the INDEX TERMS: 7299 Seismology: General or miscellaneous; Lambert graben in East Antarctica. 8157 Tectonophysics: Plate motions—past (3040); 9320 Information Related to Geographic Region: Asia; KEYWORDS: wide-angle seismic, Moho, Gondwana, Kerguelen hot spot, Lambert graben, Rajmahal volcanism Citation: Behera, L., K. Sain, and P. R. Reddy (2004), Evidence of underplating from seismic and gravity studies in the Mahanadi delta of eastern India and its tectonic significance, J. Geophys. Res., 109, B12311, doi:10.1029/2003JB002764.

1. Introduction [2] Wide-angle reflections from a boundary, even with a small impedance contrast, appear as fairly strong arrivals in the postcritical ranges because of total internal reflections and are extensively used in crustal seismics. Wide-angle seismic reflection data acquired in the Mahanadi delta along three deep seismic sounding (DSS) profiles have been utilized here to image the intracrustal horizons including the Moho with the help of two-dimensional (2-D) ray-based inversion technique [Zelt and Smith, 1992]. The conformity of the strong phases corresponding to different reflectors is shown by 2-D synthetic seismogram generation. The deep crustal structures imaged from the wide-angle reflection data are further corroborated by gravity modeling. The velocity-density model along with the available heat flux and other geological/geochronological information provides vital input to understand the tectonic processes involved in the evolution of the Mahanadi delta.

Copyright 2004 by the American Geophysical Union. 0148-0227/04/2003JB002764$09.00

[3] The Mahanadi delta is located in Orissa around the confluence of the Mahanadi River with the Bay of Bengal (Figure 1). It is a classical arcuate-type delta that lies between longitudes 85250E and 8700E and latitudes 19300N and 20400N. The delta is widely covered with recent alluvium that obscures the deeper geological features. Toward the western side of the delta, there are vast tracts of rocks such as lower Gondwanas (Lower Triassic to Upper Carboniferous), laterites (Pliocene-Pleistocene), granites/ gneisses (Archean), khondalites (Precambrian metamorphic rocks), and charnokites/anorthosites (Precambrian igneous rocks) exposed on the surface (Figure 1). These exposures belong to the Eastern Ghat group and are disposed mostly in the form of detached hillocks striking in the ENE-WSW direction bordering the Mahanadi basin. The iron ore formations exist toward the north of the Eastern Ghat group. Upper Gondwana rocks (Early Cretaceous) have a strike of E-W to ENE-WSW with very low dips and are exposed between the towns of Cuttack and Bhubaneswar and form the beginning of the delta. [4] Complex geology and tectonic setting (Figure 2) of the delta are largely controlled by the Eastern Ghat orogeny during the Jurassic period. The delta has undergone several

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BEHERA ET AL.: EVIDENCE OF UNDERPLATING IN MAHANADI DELTA

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Figure 1. Geological map of the Mahanadi delta showing three deep seismic sounding (DSS) profiles, i.e., profile I (Konark-Mukundpur), profile II (Baliamba-Jagannathpur), and profile III (ParadipKabatabandha), along with the respective shot point (SP) locations represented by open circles with dots at the center. Various SPs are indicated by their respective numbers. stages of rifting and subsidence, followed by sediment deposition accompanied by uplift due to tensional forces during the Late Jurassic [Sastri et al., 1974]. Fuloria [1993] has suggested the presence of the Gondwana graben and horst features in the delta. He has also reported widespread volcanic activity along the rift zones of the Mahanadi basin (Figure 2) associated with the breakup of greater India from Antarctica and Australia during the Early Cretaceous. During the Tertiary, deltaic, transitional marine sediments, carbonates, and clastic sediments were deposited because of the northward drift of greater India and subsidence of continental margins along with the fall of sea levels due to epeirogenic movements. Until the Jurassic the Mahanadi basin was an intracontinental pull-apart basin. After the breakup of the Gondwana it became pericratonic [Rao, 1993]. [5] Burke and Dewey [1973] proposed the concept of mantle plume-generated triple junction of ridges along which continents broke up and separated out. Two of the three ridges opened up the ocean and finally joined to form a plate boundary, while the third arm failed to open, leading to the formation of a rift along which the major rivers flowed down to the newly opened basin and formed deltas. On the basis of these facts we propose that the Mahanadi

delta developed in this way. On the basis of available seismic, gravity, aeromagnetic, heat flow, and geological information, earth scientists prefer a rift-related evolution of the Mahanadi basin. The basin was formed either as a single rift valley with synsedimentation and postsedimentation faulting episodes [e.g., Fox, 1934; Thakur et al., 1993] or as a multiple rift system [Choudhury, 1979], or it combined with the Lambert graben in the Indo-Antarctic rift [e.g., Fedorov et al., 1982; Hofmann, 1996]. The later model, that of a common intra-Gondwana rift system, is supported by numerous paleontologic, stratigraphic, and structural observations [e.g., Balme and Playford, 1967; Barrett et al., 1972; Fedorov et al., 1982; Tewari and Veevers, 1993] and serves as the basis for many recent Gondwana reconstructions. [6] The Mahanadi deltaic terrain has four distinct geological stages in its evolution [Bharali et al., 1992]: (1) the formation of Eastern Ghat crystallines belonging to the Archean age forming the basement of the delta, (2) the formation of the coal-bearing Gondwanas in grabens within the basement crystallines, (3) the formation of east coast basins due to the rifting of greater India from Australia and Antarctica during the end of Mesozoic, and (4) the deposition of marine and deltaic sediments beginning from Upper

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Figure 2. Tectonic map of the Mahanadi basin (modified after Fuloria [1993]) showing various faults, depressions, and ridges, along with the zone of rifting on which three DSS profiles are superimposed. Cretaceous until recent time. The history of the east coast marine basin also shows four important stages: (1) prerift stage, (2) rift stage, e.g., rifting of the continents during Upper Cretaceous associated with the volcanic eruption and deposition of clastic sediments both on land and offshore, (3) postrift carbonate platform stage, e.g., development of Paleogenes representing thick carbonate buildup under a stable shelf condition and establishment of continental shelf-slope divide, and (4) middle-late Tertiary marine clastic/deltaic sedimentation stage, e.g., deposition of thick Neogene sediments under deltaic to marine conditions following a major hiatus during Pliocene – early Pleistocene. About this time the Mahanadi River appeared in the scene, and sedimentation during upper Pleistocene– Holocene gave rise to the modern Mahanadi delta superimposing itself on all other earlier geological events and phenomena. Before knowing the details of the Quaternary Mahanadi delta it is essential to know the origin and development of the Mahanadi coastal basin in which the Mahanadi River deposited its sediments to form the delta. The Mahanadi coastal basin forms an integral part of the east coast sedimentary basins of India and has a similar origin to others, e.g., Godavari, Krishna, and Cauvery basins. These basins have been extensively studied by many workers specially connected to the exploration of hydrocarbons

[e.g., Bharali et al., 1987, 1991, 1998; Govindan, 1966; Jagannathan et al., 1983; Kaila et al., 1987; Mishra, 1984; Mohinuddin et al., 1993; Ramamohan Rao, 1988; Rao, 1993; Sastri et al., 1973, 1974; K. Das, Report on the gravity and magnetic survey in Cuttack-BhubaneswarKendrapara area of the Mahanadi basin (Orissa), ONGC Technical Report OR-1/G. M., unpublished, Oil and Natural Gas Corporation, 1966; K. V. S. Murti et al., Hydrocarbon prospects of Mahanadi basin, India, unpublished report, Oil and Natural Gas Corporation, 1973; K. S. Shahid, Report on gravity cum magnetic survey in Athgarh-Cuttack-Bhadrak area, Orissa, ONGC Technical Report 3/G. M., unpublished, Oil and Natural Gas Corporation, 1968 (hereinafter referred to as Shahid, unpublished report, 1968)]. Ramamohan Rao [1988] has given an excellent review of the geological events leading to the development of east coast sedimentary basins. A tectonic map based on earlier works on the Mahanadi delta [Kaila et al., 1987; Baishya and Singh, 1986; Fuloria, 1993; Shahid, unpublished report, 1968] showing basement structure and different fault patterns is displayed in Figure 2. This shows a sequence of predrift depressions such as Cuttack depression and ridges such as the Bhubaneswar ridge. Superimposed on them are the postdrift coastal depressions such as Konark depression, Paradip depression, and Kendrapara depression. It is be-

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lieved that the Cuttack-Kendrapara depression (e.g., Mahanadi Gondwana graben) containing Gondwana sediments was contiguous with the Lambert rift of Antarctica [Hofmann, 1996]. [7] A careful study indicates that the heat flux values in the Indian Gondwana basins surrounding the Mahanadi basin range from 49 to 107 mW/m2 with a mean value of 77 mW/m2 [Rao and Rao, 1980, 1983]. This is very high compared to that (average of 30 mW/m2) of the stable shield and other cratonic regions of India [Rao et al., 1982]. Magmatic underplating has been invoked in recent years as a mechanism of crustal growth in which new material is added to the base of existing crust during the periods of high heat flow and crustal extension [Furlong and Fountain, 1986; White and McKenzie, 1989]. Though previously thought to be a dominant mechanism of crustal growth and differentiation in the Precambrian, the process is now cited as active throughout the Phanerozoic. Verifying the process and estimating its significance to crustal growth has been difficult because of the lack of exposed materials representative of the lower crust and upper mantle [Fountain et al., 1990]. The presence of underplated material through seismic imaging can provide some clues to the genesis and emplacement characteristics but as yet has usually been dependent on bulk velocity determinations from wide-aperture seismic data rather than from the highresolution near-vertical seismic data [Deemer and Hurich, 1994]. Underplating along volcanic/rift margins can be inferred by identifying areas with seismic velocity intermediate between typical crust and mantle as measured from wide-angle seismic data [White and McKenzie, 1989]. Hence, by suitable combination of wide-angle seismic and gravity results along with heat flux values in the study region we attempt to establish probable presence of deepseated magmatic underplating and to shed light on related phenomenon.

2. Methodology 2.1. Seismic Method [8] The methodology used in the seismic modeling is the ray-based inverse approach of Zelt and Smith [1992]. Briefly, the inversion scheme is based on the method of model parameterization followed by ray tracing suited to the forward step of the inverse approach. The model is parameterized by an irregular grid of velocity and boundary nodes, between which linear interpolation is made. A smooth layer boundary simulation is applied to avoid the scattering and focusing of ray paths and to obtain a more stable inversion result. Within each layer an arbitrary number and spacing of upper and lower boundary velocity nodes specify the P wave velocity field. The travel times and their partial derivatives with respect to the model parameters at specified nodes are calculated using an efficient numerical solution [Zelt and Ellis, 1988] of raytracing equations [Cˇerveny´ et al., 1977]. The forward response of the starting model is compared with the observed data, and the model parameters are updated using the correction vectors obtained from the damped least squares inversion [Zelt and Smith, 1992] of travel time residuals between the data being inverted and the corresponding forward response. The process is repeated until a satisfac-

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tory fit corresponding to a normalized c2 value of almost 1 is achieved. 2.2. Gravity Method [9] For gravity modeling the model parameterization of Zelt and Smith [1992] has been adopted to compute the gravity responses utilizing the 2-D gravity modeling algorithm of Talwani et al. [1959]. The Bouguer gravity data along the DSS profiles have been obtained at irregular intervals from the analog contour map (Figure 3) using the weighted sum method based on Lagrange interpolation [Cheung and Yeo, 1979; Zienkiewicz, 1979] in such a way to have maximum gravity features in the data. The mean error or uncertainty associated with the observed gravity data is of the order of 4 mGal. [10] Density blocks were defined by interface nodes of the velocity model, and the measured seismic velocities within each polygon were converted to densities per the standard curve (Figure 4) obtained by the empirical relationship of Nafe and Drake [1957] and Ludwig et al. [1970]. The structures at the edges of the model were continued to large distances to reduce the edge effects. The layers of the P wave velocity model define respective layers of the gravity model. The densities are changed in such a way that the calculated anomaly satisfactorily matches the general trend of the observed gravity anomaly. Care has been taken that the interface geometry of the derived seismic model was not changed, except for incorporating lateral variation of density. The minimum rootmean-square (RMS) gravity residual and c2 value of almost 1 have been used to fit the gravity data.

3. Crustal Modeling 3.1. Seismic Data [11] The Mahanadi delta is affected by various processes such as rifting, sedimentation, and erosion, making the area complex in nature. Not much is known about the evolution of the area because of limited geophysical data and other information. Crustal velocity and density models along the three DSS profiles, namely, (1) Konark-Mukundpur (profile I), (2) Baliamba-Jagannathpur (profile II), and (3) Paradip-Kabatabandha (profile III) (Figure 1), in conjunction with heat flux values in the surrounding region are used to understand various aspects of the geotectonic settings. In order to map the basement configuration along with the overlying sedimentary formations and to gain insight into the deep crustal features, Kaila et al. [1987] presented a 1-D velocity-depth function based on effective velocity analysis [Kaila and Krishna, 1979]. The derived velocity model was not (1) verified with the theoretical responses and (2) assessed in terms of errors and resolutions. As the delta is complex, 1-D velocity models are not sufficient to explain the different crustal elements. Hence the 2-D ray-based inverse approach of Zelt and Smith [1992] has been applied to the wide-angle seismic data with a view to (1) build up the 2-D velocity structures, (2) provide a measure of resolutions and uncertainties of estimated model parameters, and (3) assure data fit according to a least squares norm. [12] Originally, the data were recorded in analog form. However, we have digitized the analog data and assembled these into composite record sections with amplitude nor-

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Figure 3. Composite Bouguer anomaly contour map of the Mahanadi delta (modified after National Geophysical Research Institute [1978] and Shahid (unpublished report, 1968)) on which three DSS profiles are superimposed. malization. Such record sections have been generated for different shots along the profiles. Noisy traces are removed while digitizing the data. To obtain the crustal seismic model, only wide-angle reflection data along the three DSS profiles (Figure 1) have been utilized. The shot interval is 10 km, whereas the average receiver interval is 400 – 500 m after digitization. The shallow structure down to the

basement was modeled using the first-arrival refraction data [Behera et al., 2002; Behera, 2003]. The most important aspects of modeling are the phase identification and classification. Arrivals that are identified on the basis of amplitude coherency and the reciprocity test have been used. Five wide-angle reflected phases (P1, P2, P3, P4, P5, etc.) from below the basement are identified on the basis of the

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Figure 4. Mean velocity-density relationship (standard curve (thin line)) in the middle of the minimum and maximum density curves (solid lines) used by Ludwig et al. [1970], commonly used for gravity calculations from seismically derived velocities (shown by horizontal and vertical hairlines). The estimated densities after the final gravity model are shown by solid triangles for the Mahanadi delta. coherency of phases from the digitized record sections. Travel times of wide-angle reflected phases are picked from all shots. The data uncertainties and fitting play an important role in modeling. Hence it is necessary to assign uncertainties to the travel time picks while modeling to avoid overfitting or underfitting. Uncertainties are often assigned by inspection, taking into account the signal-tonoise ratio and frequency content of the data using a constant value for each phase [Zelt, 1999]. On the basis of 50 and 100% of the dominant period (e.g., 50 ms) of the data, uncertainties of ±25 and ±50 ms are assigned to the shallower phases (P1 and P2) and deeper phases (P3, P4, and P5), respectively, as were used by Sain et al. [2000]. Note that the uncertainty estimate is subjective in nature. [13] We have derived 1-D velocity-depth functions below various wide-angle shots using the damped least squares inversion technique of Sain and Kaila [1994] in a layer-bylayer stripping fashion. For this we have retained the shallow averaged 1-D models down to the basement taken from Behera et al. [2002] and Behera [2003]. The basic purpose of this 1-D inversion is to ascertain the real existence of different velocity layers and more specifically

the velocity layer just above the Moho. The model responses and the derived 1-D velocity-depth functions are shown in Figure 5. The velocity of the layer lying above the Moho varies between 7.3 and 7.6 km/s. The responses clearly show that 2-D modeling is required to fit the data satisfactorily. By smoothly joining the 1-D velocity models (Figure 5) we have derived pseudo 2-D velocity models (Figure 6) that serve as good starting models for 2-D inversion [Zelt and Smith, 1992; Zelt, 1999]. [14] The inversion parameters chosen are 1.0 for the overall damping factor, 0.1 km/s for a priori error of velocity nodes (e.g., velocity uncertainty), and 0.1– 0.5 km for a priori error of boundary nodes (e.g., depth uncertainty). The subbasement crustal structures are determined from the travel time inversion of wide-angle reflected phases in a layer-stripping fashion. The model is parameterized by a minimum number of velocity and boundary nodes (parameters) to avoid unnecessary structures. The final ray trace model along with the observed data (represented by vertical bars) for various shots is shown in Figure 7. The number of rays traced through the models, the overall RMS travel time residuals, and the

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Figure 5. Travel time fit (solid lines) of the observed wide-angle data (vertical bars) along with the derived one-dimensional (1-D) velocity-depth functions for various wide-angle shots (indicated at top) along three DSS profiles obtained by using the damped least squares technique [Sain and Kaila, 1994]. The travel times are plotted in a reduced timescale with a reduction velocity of 7.0 km/s. normalized chi-square (c2) values associated with various phases corresponding to all shots are displayed in Tables 1, 2, and 3, respectively, for three profiles. Ideally, an overall normalized c2 value of 1 should be achieved [Bevington, 1969]. However, in cases where the observed data include relatively short-wavelength variations with offset, a fine

spacing of velocity or depth nodes may be required to obtain c2 = 1. The velocity-depth images of the crustal models are shown in Figure 8. Note that the shallow velocity variation (1.7 – 5.0 km/s) down to the basement [Behera et al., 2002; Behera, 2003] is not shown here to expand the crustal velocity variations between 5.0 and

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Figure 6. Pseudo 2-D velocity model derived by smoothly joining (dashed lines) the 1-D velocity-depth functions (solid lines) obtained for different shots along (a) profile I, (b) profile II, and (c) profile III. Locations of shots are marked by inverted triangles. The velocity scale (km/s) is indicated below each velocity-depth function. 8.0 km/s. The final velocity models are derived on the basis of (1) minimum RMS travel time residual between the data and the model response, (2) ability to trace rays to almost all observations, (3) parameterization represented by minimum number of velocity and boundary nodes, and (4) high parameter resolution. The model parameters used in the inversion are shown in Figure 9. Inversion with a single velocity node for various crustal layers shows that the data can be fit satisfactorily without invoking velocity variation within a layer that may be unnecessary.

[15] The ‘‘hit counts’’ are a measure of the number of rays sampling each cell of the model. By increasing the cell size, the hit counts also increase as the number of rays passing through the cells increases. However, the corresponding resolutions get degraded. The cell size is to be judiciously selected such that each cell has a moderate number of rays through it. Thus the hit counts are subjective in nature. We have used rectangular cells of size 8 by 3 km for computation of hit counts, and the values are shown in Figure 10. The hit count plots indicate that the shallow crustal models have

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Table 1. Crustal Modeling Results Along Profile I Phases

Number of Data Picked

Data Uncertainty, ms

RMS Residual, s

Normalized c2 Value

Number of Rays Traced Through the Model

P1 P2 P3 P4 P5

713 797 617 479 240

25 25 50 50 50

0.027 0.036 0.044 0.048 0.030

1.170 2.029 0.968 0.912 0.332

713 797 609 476 238

more hit counts represented by dark grey shading compared to the deeper part of the models. The hit counts or ray density plots provide some insight for indirect model assessments for various velocity and boundary nodes (Figure 9) although less reliably [Zelt, 1999]. [16] The most important aspect of model assessment is to provide a measure of resolutions and uncertainties of estimated model parameters from the diagonal elements of resolution and covariance matrices, respectively [Zelt and Smith, 1992]. Generally, resolution values range between 0 and 1 and depend on the relative number of rays sampling each model parameter. The model is parameterized (Figure 9) with a minimum number of model parameters (e.g., velocity and boundary nodes) having resolution values >0.7 (Tables 4, 5, and 6) except at a few boundary nodes toward the edge and in the deeper part of the models because of the insufficient ray coverage. This indicates that the model is well resolved and reliable. [17] The standard deviations (Tables 4, 5, and 6) are the statistical errors associated with model parameters, and they represent the lower bounds of parameter uncertainties. The covariance matrix is analyzed under a linear assumption. To account for the nonlinearity of travel time inversion and to provide significant insight into the model constraint, a uniparameter uncertainty test [Zelt and Smith, 1992; Zelt, 1999] is performed. Applying this type of error analysis to all velocity and boundary nodes could take longer than the time required in obtaining the final model. Therefore we have performed both positive and negative parameter perturbation tests on one representative boundary and velocity node of each layer. To obtain the absolute uncertainty, we have perturbed its value from that in the final model and held it fixed while inverting the observed data involving all other parameters that were determined at the same time as the perturbed parameter during the inversion for the final model. Then, we have increased the perturbation until the final model so obtained is unable to fit the observed data. The maximum perturbation of the parameter that allows a comparable fit to the observed data becomes an estimate of its absolute uncertainty as described by Zelt and Smith [1992] and Zelt [1999]. Figure 11 displays the absolute uncertainty estimate for the lower crustal velocity (7.52 km/s) node and the Moho boundary (36.75 km) node at 70 km profile distance along profile I. This shows that the absolute velocity uncertainty lies in the range of 0.14 to

+0.2 km/s corresponding to the 50 ms RMS residual (Figure 11a) equal to assigned uncertainties of travel time picks. Similarly, the absolute uncertainty for the Moho depth is ±0.8 km, corresponding to the 50 ms RMS residual (Figure 11b). Tests on absolute velocity uncertainties for other crustal layers show the values lying between ±0.15 and ±0.25 km/s along the three DSS profiles. The absolute uncertainties of boundary depths for various crustal layers lie in the range between ±0.6 and ±1.0 km along the three profiles. Note that the same damping factor of 1.0 is used for all of the above tests. [18] Besides travel time fit the validity of a model is checked through the computation of synthetic seismogram and the correlation of the amplitudes of observed phases with those of theoretical ones. This has been displayed for different wide-angle shots in Figures 12– 16. The phases, which are convincingly strong and correlated for long distances, are taken for modeling. The reflections from the top of different crustal boundaries are shown as P6.0, P7.0, P7.5, and P8.1, corresponding to wide-angle reflected phases P2, P3, P4, and P5, respectively, and are marked on the observed record sections. 3.2. Gravity Data [19] The derived 2-D crustal seismic models have been utilized for modeling the Bouguer gravity data along three DSS profiles to further constrain the geologic and tectonic settings. The Bouguer gravity data (Figure 3) taken for gravity modeling show alternate ‘‘highs’’ and ‘‘lows.’’ The gravity highs lie over the shallow basement ridges, and the lows lie over the basement depressions in which thick sediments are deposited. The Bouguer gravity values are quite high in this region (20 to 50 mGal), compared to the Indian shield (20 to 100 mGal) or central India (30 to 70 mGal). These anomalous high gravity values may be attributed to the reduction of crustal thickness and the presence of relatively thick high-density material at the base of the crust, a possible cause for underplating. In this study, an effort has been made to get the complete crustal structure of the delta by combining the seismic results with the gravity data. There is no clear relationship between the observed gravity data and the shallow structure, depth of basement, or Moho. Proper care has been taken to fit the observed gravity data by varying densities without changing the seismically derived interface structures. The final grav-

Figure 7. Rays traced through the crustal models for different shots such as (a) SPs 10, 50, 90, and 120 along profile I, (b) SPs 1, 51, 91, and 121 along profile II, and (c) SPs 12, 32, 52, and 92 along profile III. The top plot for each SP represents the comparison of observed (vertical bars) and theoretical (solid lines) travel times of wide-angle reflections (P1, P2, P3, P4, and P5) from different layers. Prefr represents the refracted phase from the basement. The lengths of vertical bars correspond to picking uncertainties, and the data are plotted in reduced scale with a reduction velocity of 7.0 km/s. The bottom plot displays the reflected rays traced through the final crustal models. Rays are traced at the fourth interval for clarity. 10 of 25


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Table 2. Crustal Modeling Results Along Profile II Phases



RMS Residual, s

Normalized c2 Value


P1 P2 P3 P4 P5

614 813 878 632 295

25 25 50 50 50

0.026 0.030 0.052 0.044 0.035

1.084 1.412 1.077 0.767 0.488

614 812 878 632 295

ity model constrained from the seismic model along with the data fit is shown in Figure 17. The estimated density values are shown by triangles in Figure 4, which fall within the minimum and maximum density bounds. The normalized c2 value has been minimized in the modeling so as to obtain an optimum fit between the observed data and the computed response. This provides a test for whether the proposed velocity model can explain other geophysical data such as gravity. The gravity modeling also fills the gap in seismic data, and the combined model vindicates the presence of various crustal features, which lead to an understanding of the processes involved in the formation and evolution of the Mahanadi delta.

4. Results and Discussion 4.1. Crustal Velocity Structure 4.1.1. Profile I [20] The aim of modeling the wide-angle seismic data in this study is to delineate the velocity variations from below the basement down to the Moho, which has implications for the tectonic and geodynamic evolution of the Mahanadi delta. The basement layer along this profile shows a lateral variation of velocity (5.9 – 6.1 km/s) from south to north (Figure 8a) due to transition of basement materials from granite to khondalite (a high-grade metamorphic rock of Eastern Ghat orogeny). In the southern part this layer is thin (3 km) below the Konark depression and thick (6 km) below the Bhubaneswar ridge. Below the Gondwana graben (e.g., Mahanadi Gondwana graben) the thickness is 3 km and increases toward the north to a value of 4.5 km near Mukundpur below the Chandikhol ridge. The basement layer is underlain by a very thick (10 km) high-velocity layer (6.5 km/s), and together these layers constitute the upper crustal column. This velocity jump from 6.0 to 6.5 km/s at an average depth of 5.5 km can be attributed to a change in composition from granitic/khondalitic to charnokitic/gabbroic/anorthositic materials, as corroborated by magnetic signatures [Anand et al., 2002]. DSS studies conducted over the Lambert glacier [Fedorov et al., 1982] in East Antarctica also show a similar velocity structure (6.4 –6.6 km/s) at a depth of around 5– 8 km and have been related to the magnetic signatures in the upper crust [Anand

et al., 2002]. These results are consistent with the Lambert graben in the Mac Robertson Land of East Antarctica, a part of the Mahanadi graben prior to the breakup of Gondwanaland. This high-velocity (6.5 km/s) layer is underlain by a low-velocity (6.0 km/s) layer (LVL) at an average depth of 16 km, followed by a high-velocity layer (7.1 km/s) at 20 km depth. The amplitude of reflection from the top of the 7.1 km/s velocity layer is large in the wide-angle range shown in the record sections of shot points 10 and 120 and has been compared with synthetic seismograms (Figures 12 and 13). An increase in velocity at 16 km depth will give rise to small velocity contrast at 20 km depth, which cannot explain the large amplitudes of reflections from this boundary, whereas the presence of midcrustal LVL would cause a sufficient velocity contrast to explain the amplitude of reflections from both of the discontinuities at 16 and 20 km depths. The LVL could be due to the presence of fluid released during the process of magmatism. [21] Continental rift zones are commonly associated with magmatic underplating [Deemer and Hurich, 1994; Peirce et al., 1996]. Since the three DSS profiles studied here are located in such a rift zone, it is appropriate to check for the presence of underplating. The model analysis including the generation of synthetic seismograms reveals the presence of a high-velocity (7.5 km/s) layer below the 7.1 km/s velocity layer at an average depth of 27 km extending down to an average depth of 37 km. This high-velocity layer may be due to the underplated materials at the base of the crust as suggested by Mishra et al. [1999] from gravity study. The upper mantle velocity below the underplated layer is assumed to be 8.1 km/s, as is used in other sedimentary basins of India [Kaila and Sain, 1997]. Because of insufficient length of the profile the Pn wave could not be recorded. Wide-angle reflections marked with considerably strong amplitude corresponding to P8.1 represent the Moho phase (PmP) at distances beyond 80 km (Figures 12b and 13b). Kaila et al. [1987] presented crustal structure of the Mahanadi delta, but because of poor phase correlation on analog records the Moho could not be mapped properly. 4.1.2. Profile II [22] The crustal depth section (Figure 8b) along this profile shows lateral variation of basement velocity (5.9 –

Table 3. Crustal Modeling Results Along Profile III Phases



RMS Residual, s

Normalized c2 Value


P1 P2 P3 P4 P5

289 263 249 201 53

25 25 50 50 50

0.051 0.045 0.049 0.050 0.048

3.374 2.568 0.959 0.988 0.925

279 262 249 201 53

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Figure 8. Velocity-depth image of the final crustal model derived along (a) profile I, (b) profile II, and (c) profile III. Regions not sampled by rays are shown without color. The numbers within each model represent localized average velocities (km/s). Inverted solid triangles at the top of the model show the locations of various shots along the profiles. The color scale represents the range of crustal velocity (5.0 – 8.0 km/s). 12 of 25

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Figure 9. Velocity (circles) and boundary (squares) nodes representing the model parameterization along (a) profile I, (b) profile II, and (c) profile III. The average velocity values of each layer are shown. 6.1 km/s) toward the west and represents the transition from granitic to high-grade metamorphic khondalitic rocks. The granitic basement below the Gondwana graben in the middle is thin (2.5 km) and extends toward the east. The basement is underlain by a relatively thick (9 km) high-velocity (6.5 km/s) layer. The jump in velocity from 6.0 to 6.5 km/s at an average depth of 5 km in this highgrade setting most probably represents the change in composition from granitic/khondalitic to gabbroic/charnokitic/ anorthositic materials. This upper crustal high-velocity layer is underlain by a LVL (6.0 km/s) at an average depth of 13 km. This layer may indicate the presence of fluids released during the process of rifting and magmatism. Farther down, the velocity increases to a value of 7.0 km/s at an average depth of 18 km, followed by a high-velocity layer (7.5 km/s) at the base of the crust. This high-velocity layer is slightly undulating and extends downward to an average Moho depth

of 37.5 km. The same upper mantle velocity of 8.1 km/s has been used as described in section 4.1.1. 4.1.3. Profile III [23] In the crustal depth section (Figure 8c) along this profile the basement is quite thick (4– 5 km) in the NNW part, and the velocity varies laterally from 5.9 to 6.1 km/s. As before the change in velocity represents a transition from granitic to metamorphic khondalitic composition. The basement below the Gondwana graben is thin (2.5– 3 km) and is underlain by a very thick (8 km) high-velocity layer (6.5 km/s), followed by an 8 km thick LVL (6.0 km/s), which, in turn, is underlain by a 7.0 km/s velocity layer at an average depth of 19 km. The presence of a high-velocity (7.5 km/s) presumably underplated layer at an average depth of 27 km as noticed along the other two profiles has also been identified along this profile. The upper mantle velocity, as usual, is maintained as 8.1 km/s. The Moho is

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Figure 10. (a) Ray hit count plots along (a) profile I, (b) profile II, and (c) profile III. Grey scale indicates the values of hit counts. 14 of 25

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Table 4. Resolution Estimates Along Profile I Offset, km

Depth Nodes, km

5.0 30.0 60.0 85.0 115.0 5.0 20.0 50.0 60.0 80.0 115.0 5.0 30.0 50.0 90.0 115.0 5.0 30.0 50.0 80.0 115.0 5.0 30.0 70.0 115.0

5.85 5.49 6.85 4.54 4.25 15.45 14.70 14.28 16.04 15.04 17.33 20.77 20.66 20.93 19.93 20.29 27.83 26.28 26.66 26.42 26.73 34.72 35.91 36.72 39.69

Offset, km 5.0 60.0 110.0 60.0 60.0 60.0 60.0

Depth Resolutions

Standard Deviations

0.98 0.99 0.99 0.95 0.74 0.95 0.99 0.99 0.99 0.98 0.95 0.98 0.99 0.99 0.99 0.85 0.91 0.99 0.99 0.99

0.023 0.012 0.014 0.041 0.100 0.040 0.010 0.011 0.019 0.020 0.040 0.055 0.022 0.024 0.031 0.093 0.141 0.026 0.027 0.035

0.64 0.97 0.98 0.38

0.298 0.081 0.057 0.392

Velocity Nodes, km/s

Velocity Resolutions

Standard Deviations

5.9 6.0 6.1 6.5 6.0 7.1 7.5

0.85 0.99 0.90 0.99 0.96 0.98 0.98

0.0195 0.0025 0.0125 0.0025 0.0191 0.0139 0.0129

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(2.7 g/cm3). Below the basement, there is a thick highdensity material (2.8 g/cm3) representing the igneous intrusives, followed by a comparatively thinner low-density (2.65 g/cm3) material that may be due to the presence of fluids in the midcrustal level (Figure 17a). [25] Below the low-density (2.65 g/cm3) layer lies a highdensity layer (2.9 g/cm3) followed by a thick higher-density (3.05 g/cm3) magmatic lower crust corresponding to underplating. This layer extends to an average depth of 37 km as constrained from seismic modeling. The upper mantle density has been kept fixed to 3.3 g/cm3 as deduced from the velocity and density relationship (Figure 4). The large drop in gravity anomaly of the order of 40 mGal over the Gondwana graben (Figure 17a) is attributed to the combined effect of thick low-density Gondwana (2.4 g/cm3) sediments in the graben and almost exposed high-density basement ridges (2.7 – 2.75 g/cm3) on both sides of the graben along with the effect of deep crustal features, including the highdensity (3.05 g/cm3) magmatic underplating. To assess the effect of underplating and its contribution to the Bouguer gravity anomaly, we have recomputed the gravity response (represented by dashed lines in Figure 17a) by replacing the high-density (3.05 g/cm3) layer with the overlying material (2.9 g/cm3). This shows a considerable misfit (19.8 mGal) between the observed and the theoretical gravity responses

Table 5. Resolution Estimates Along Profile II

upwarped significantly along this profile and deepens from 33 to 36 km within a span of 30 km from the NNW to SSE direction. 4.2. Crustal Density Structure 4.2.1. Profile I [24] The Bouguer gravity anomaly along this N-S trending profile (Figure 3) has alternate lows and highs having gravity values varying between 40 and +18 mGal. Two centers of gravity lows lie over two depressions, namely, the Konark depression toward the coast in the south and the Gondwana graben in the continent toward the north of the profile (Figure 17a). The Konark depression shows Tertiary sediments having densities of 2.2 and 2.35 g/cm3 overlying the basement. On the other hand, the Gondwana graben has high-density basalts of 2.5 g/cm3 and Gondwana sediments of 2.4 g/cm3 above the basement. The top layer above the Konark depression and the Gondwana graben has a density of 1.8 g/cm3 corresponding to alluvium and recent sediments. Between these depressions, lies the Bhubaneswar ridge associated with a gravity high. Another high lies in the northern end of the profile near Mukundpur called the Chandikhol ridge. The surface geology shows that metamorphic khondalite rocks of the Eastern Ghats are exposed toward the north of the delta. The Chandikhol ridge may thus be associated with the exposed khondalitic basement having relatively high density (2.75 g/cm3) compared to the Bhubaneswar ridge granitic basement having less density

Offset, km

Depth Nodes, km

5.0 25.0 60.0 80.0 120.0 5.0 20.0 30.0 40.0 60.0 80.0 90.0 120.0 5.0 20.0 45.0 70.0 90.0 120.0 5.0 40.0 70.0 120.0 5.0 40.0 80.0 120.0

4.12 5.61 6.02 6.28 4.41 12.93 13.66 14.01 13.93 13.40 14.23 12.99 13.78 17.57 18.11 17.67 16.21 18.03 18.83 25.34 26.91 27.86 24.52 38.99 36.51 36.77 36.00

Depth Resolutions

Standard Deviations

0.97 0.99 0.98 0.98 0.93 0.88 0.98 0.99 0.99 0.99 0.99 0.98 0.91 0.75 0.97 0.98 0.99 0.97 0.79 0.97 0.99 0.99 0.88 0.77 0.98 0.96

0.031 0.015 0.024 0.022 0.049 0.068 0.020 0.017 0.014 0.010 0.013 0.023 0.057 0.099 0.030 0.022 0.018 0.034 0.090 0.082 0.026 0.027 0.172 0.239 0.068 0.088

Offset, km

Velocity Nodes, km/s


10.0 62.5 110.0 62.5 62.5 62.5 62.5

6.1 6.0 5.9 6.5 6.0 7.0 7.5

0.92 0.99 0.95 0.99 0.98 0.99 0.98

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Standard Deviations 0.0192 0.0024 0.0135 0.0016 0.0128 0.0044 0.0138


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Table 6. Resolution Estimates Along Profile III Offset, km 5.0 20.0 40.0 60.0 90.0 100.0 5.0 10.0 35.0 55.0 75.0 100.0 5.0 20.0 60.0 100.0 5.0 10.0 70.0 100.0 5.0 20.0 40.0 60.0 100.0 Offset, km 5.0 30.0 80.5 51.0 51.0 51.0 51.0

Depth Node, km 4.36 4.28 6.30 5.56 3.16 3.50 13.10 10.75 13.67 12.80 8.99 9.54 18.00 17.64 18.35 19.00 27.00 27.41 25.89 27.00 35.00 26.53 35.29 33.26 37.00 Velocity Nodes, km/s 5.9 6.0 6.1 6.5 6.0 7.0 7.5

Depth Resolutions

Standard Deviations

0.97 0.99 0.98 0.93

0.034 0.018 0.022 0.051

0.97 0.98 0.98 0.88

0.029 0.021 0.025 0.069

0.99 0.99

0.044 0.034

0.99 0.98

0.039 0.064

0.84 0.83 0.60

0.201 0.203 0.314


Standard Deviations

0.98 0.99 0.99 0.99 0.94 0.97 0.68

0.0115 0.0040 0.0035 0.0089 0.0237 0.0158 0.0589

corresponding to a normalized c2 value of 6.4, although the general trend of the computed gravity response correlates well with the observed gravity data. This implies the need for a thick high-density material at the base of the crust to fit the data appropriately. The maximum misfit between the observed and the computed responses by incorporating the high-density underplated material (3.05 g/cm3), as constrained from the seismic model, comes down to a value of 4.3 mGal corresponding to a normalized c2 value of 1.16. 4.2.2. Profile II [26] The gravity model constrained from the seismic model derived along this profile is shown in Figure 17b. The maximum misfit between the observed and the computed gravity response is 4.7 mGal corresponding to a normalized c2 value of 1.24. This profile has a significant gravity low that extends laterally over large distances toward the east of the profile. This gravity low lies over the Gondwana graben in which thick Gondwana sediments (2.4 g/cm3) are deposited. A gravity high is observed toward the west of the profile. The gravity values vary from 50 to 10 mGal along this profile. The basement corresponding to the Eastern Ghat metamorphic khondalite rocks, which are almost exposed at the western end of the profile, has a gravity high. The density of the basement (2.75 g/cm3) in the western part is higher than that (2.7 g/cm3) toward the middle and eastern part of the profile. It is clear that the beginning of the Gondwana graben is demarcated by

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the boundary of the Eastern Ghat khondalitic basement. Beyond this the effect of the Eastern Ghat ceases, and basement becomes granitic in nature. Since the density model is constrained from the seismic model, the presence of a low-density layer (2.65 g/cm3) in the midcrust and a thick high-density underplated layer (3.05 g/cm3) at the base of the crust is well defined along with other crustal layers. To assess the effect and presence of the high-density underplated layer, remodeling as described for profile I was done. The large misfit (13.8 mGal) corresponding to a normalized c2 value of 4.93 demands the presence of a high-density material at the base of the crust. The density of upper mantle was kept fixed to 3.3 g/cm3 in the gravity modeling. The high-density (3.05 g/cm3) underplated layer shows considerable lateral variations with Moho upwarp, as evidenced in a rift basin. The extent of upwarping is significantly less because the zone of rifting is away from this profile toward the NE end of the delta (Figure 2). The large drop of gravity anomaly of the order of 40 mGal over the Gondwana graben (Figure 17b) is due to the combined effect of thick low-density (2.4 g/cm3) Gondwana sediments within it and high-density (2.75 g/cm3) khondalite rocks of

Figure 11. (a) Root-mean-square (RMS) travel time residual as a function of velocity perturbation of the underplated layer for profile I. The absolute velocity uncertainty lies between 7.38 and 7.72 km/s (0.14 to +0.2 km/s), corresponding to a 50 ms travel time residual. (b) RMS travel time residual as a function of depth perturbation for the Moho boundary. The absolute depth uncertainty lies between 36 and 37.5 km (±0.8 km), corresponding to a 50 ms travel time residual.

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Figure 12. (a) Two-dimensional synthetic seismogram for SP 10 along profile I showing wide-angle reflected phases P7.1, P7.5, and P8.1. P7.5 indicates reflection from the top of the 7.5 km/s velocity layer. (b) Observed seismogram of SP 10 showing the respective wide-angle phases superimposed with the model responses (solid lines). Seismograms are plotted with a reduction velocity of 7.0 km/s. (c) Rays traced through the model from SP 10. the Eastern Ghats exposed outside the graben and the deep crustal features. 4.2.3. Profile III [27] The gravity model along this profile, constrained from the seismic model, is shown in Figure 17c. The maximum misfit between the observed and the model response is of the order of 3.8 mGal corresponding to a normalized c2 value of 0.97. The gravity values along this profile vary from 40 to +15 mGal, and there are two gravity lows within sedimentary formations, one toward the coast near Paradip and the other in the continent near Kendrapara. The gravity high between Kendrapara and Kabatabandha can be explained as being due to the emplacement of a 5 km thick layer of high-density (2.75 g/cm3) khondalites of the Eastern Ghats along with the effect of Moho upwarp. The significant Moho upwarping along this profile and minor undulations observed along the other two profiles indicated close proximity of the rift zone (Figure 2). The effect of the high-density underplated layer has been tested by modeling the gravity data without this high-density layer (3.05 g/cm3) as was described in sections 4.2.1 and 4.2.2 for profiles I and II. The computed gravity response indicates a significant misfit of 24.7 mGal and a large normalized c2 value of 10.24 without the highdensity materials (3.05 g/cm3), although the general trend of the gravity response correlates well with the observed data. This clearly indicates that the seismically derived highvelocity layer at the base of the crust, represented by the

high-density underplated layer, is required for fitting other geophysical data such as gravity. The density of the upper mantle was kept fixed at 3.3 g/cm3, as usual. Similar study by Peirce et al. [1996] has deciphered the presence of highvelocity and high-density magmatic underplating in the Coˆte d’Ivoire-Ghana continental rift margin. The large drop in gravity values of 40 mGal near Kendrapara can be explained in a similar way that has been described in sections 4.2.1 and 4.2.2 for the other two profiles over the Gondwana graben. The high gravity effect of the Chandikhol ridge along with deep structural features is conspicuous toward the NNW of the profile.

5. Tectonic and Geodynamic Implications [28] A shallow crust (35 – 37 km) with high-velocity (7.5 km/s) and high-density (3.05 g/cm3) materials at the base of the crust and low-velocity (6.0 km/s) and lowdensity (2.65 g/cm3) materials at the midcrustal level indicate a typical continental rift environment and underplating. The Mahanadi rift basin along the east coast of India and the continental shelf may be related to the breakup of greater India from Antarctica during the Early Cretaceous [Scotese et al., 1988]. They were juxtaposed with the Amery ice shelf, Prydz Bay, and adjoining regions of Antarctica. The coastal part of the Mahanadi basin and the continental shelf is characterized by Early Cretaceous and older Gondwana sediments overlain by volcanic rocks

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Figure 13. (a) Two-dimensional synthetic seismogram for SP 120 along profile I, (b) observed seismogram, and (c) ray diagram. For details, see Figure 12. [Behera et al., 2002], which are considered contemporary to the Rajmahal, Bengal, and Sylhet traps [Fuloria, 1993; Mishra, 1984; Baksi, 1995]. This indicates that the Mahanadi rift resembles an active continental rift with magmatic activities along the coastal part and the continental shelf to the Lambert rift and Prydz Bay of Antarctica (Figure 18). The lower crustal rocks exposed along the Eastern Ghats show petrological and geochemical similarities to those exposed over Antarctica in the corresponding section of Prince Charles Mountain and Enderby Land [Rao et al., 1995]. Hence we can infer that the Mahanadi rift basin of India and the Lambert rift of Antarctica fall in the same line in most of the reconstructions [Scotese et al., 1988; Sahabi, 1993] and form conjugate structures on the two continents [Fedorov et al., 1982; Mishra, 1984]. These structures are characterized by underplated lower crust and significant magmatic activities of the Early Cretaceous, especially along the coast and the adjoining continental shelves of

Mahanadi basin of eastern India. There are several other magmatic activities in east India corresponding to the breakup of greater India and Antarctica. Both alkaline and tholeiitic magmatism of approximately 130– 120 Ma representing the Rajmahal, Bengal, and Sylhet traps are also reported [Baksi, 1995]. These magmatic activities have been related to the Kerguelen hot spot at the time of the breakup of India and Antarctica [Mahoney et al., 1983; Storey, 1995; Kent, 1991]. The thermal source responsible for this may be related to the Kerguelen hot spot, which was located for a long time under East Antarctica [Morgan, 1981; Storey, 1995] and erupted during the Early Cretaceous resulting in the break of greater India from Antarctica. This was followed by large-scale magmatism over east India and Antarctica forming the Early Cretaceous volcanic provinces of the two continents (Figure 18). The convincing support for the rifting model suggested here comes from the Lambert rift, which has been interpreted to be in conjunc-

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Figure 14. (a) Two-dimensional synthetic seismogram for SP 1 along profile II, (b) observed seismogram, and (c) ray diagram. For details, see Figure 12.

tion with the Mahanadi graben and can be considered as a transcontinental rift structure [Fedorov et al., 1982; Hofmann, 1996]. Besides this the wide-angle seismic and gravity modeling presented here for the Mahanadi delta, supported by an anomalous high heat flux of 77 mW/m2 [Rao and Rao, 1980, 1983], indicates the presence of highvelocity (7.5 km/s) and high-density (3.05 g/cm3) magmatic material at the base of the crust. This, in turn, suggests that the rift margins might have been affected by continental breakup. However, this evidence is not sufficient on its own, since unstretched continental crust can sometimes exhibit similar high seismic velocities [Meissner, 1986]. It is only in such regions where the adjacent unstretched continental crust does not have a high-velocity layer that the high velocities developed on the margins can be attributed with

confidence to igneous accretion. The underplating produced by the later Kerguelen hot spot activity could be one triggering mechanism for the gentle domal uplift of eastern Gondwana in the Late Carboniferous [Kent, 1991] and probably reduced the crustal thickness due to extension caused by upwelling of plume head as depicted in the schematic cartoon (Figure 19). Below preexisting zones of weakness, mantle upwarping and propagation of a rift commenced in various regions of Gondwana such as Lambert rift of Antarctica and Karoo, Parana, and Perth basins of southern Africa, South America, and Australia, respectively [White and McKenzie, 1989]. Fission track analysis of 9 zircon and 49 apatite samples also suggests a two-stage rifting process for the Mahanadi graben in the Late Permian to Early Cretaceous [Lisker and Fachmann,

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Figure 15. (a) Two-dimensional synthetic seismogram for SP 121 along profile II, (b) observed seismogram, and (c) ray diagram. For details, see Figure 12.

2001]. Both rifting stages correlate closely with the sedimentation phases of the lower and upper Gondwana sediments, respectively, and they evolved parallel to the rifting of the Lambert graben in East Antarctica. In addition to this, younger tectonic activities in the Mahanadi region manifest as mafic dykes and the hydrothermal overprint of faults and thrust zones. These activities are synchronous with Rajmahal volcanism in northeastern India and around the Lambert graben in East Antarctica and are obviously related to the Gondwana breakup in the India-Antarctica sector. The cartoon (Figure 19) represents all the tectonic features cited above, indicating the plume head and the underplated layer along with the midcrustal magmatic body. The various highdensity magmatic sills present in the Gondawana graben of the Mahanadi delta are also clearly indicated along with different paths (e.g., weak zones or conduits) through which

the high-density magmatic body has come up to the shallow level.

6. Conclusions [29] Here we have derived a unified crustal structure using wide-angle seismic and gravity data along three DSS profiles in the Mahanadi delta. The main feature of this study is the delineation of a high-velocity (7.5 km/s) and high-density (3.05 g/cm3) layer above the Moho, which is attributed to the emplacement of mafic underplating material at the base of the crust via mantle diapirism in this pull-apart rift basin. The underplating is supported by anomalous high heat flux (77 mW/m2) in the region as compared to that (average 30 mW/m2) of the stable shield and other cratonic regions of India [Rao et al., 1982]. The

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Figure 16. (a) Two-dimensional synthetic seismogram for SP 12 along profile III, (b) observed seismogram, and (c) ray diagram. For details, see Figure 12.

reduced crustal thickness (35 – 37 km) compared to the average Moho depth (42 km) of adjacent areas of the shield, Godavari graben, and east of Narmada lineament in central India [Kaila and Sain, 1997] also bolster the crustal underplating in the Mahanadi delta. Petrophysical and geochemical similarities between the lower crustal rocks exposed along the Eastern Ghats hills and those over the Prince Charles Mountain and Enderby Land in Antarctica [Scotese et al., 1988; Rao et al., 1995] indicate the juxtaposition of the Mahanadi basin of India and the Lambert graben in Antarctica before rifting and breakup. The thermal source required for the mafic underplated material may be related to the Kerguelen hot spot, which was located for a long time under the India-Antarctica sector [Morgan, 1981; Storey, 1995] and erupted during the Early Cretaceous. This resulted in the breakup of greater India from Antarctica followed by a large-scale magmatism closely correlated with the outburst of Rajmahal volcanism in east India and the magmatic activities along the continental shelf of the Lambert rift and Prydz Bay of East Antarctica (Figure 18).

[30] Because of the extensional force caused by upwelling of plume head (Figure 19), part of the high-velocity and high-density mafic material might have been transported to the shallow depth through weak zones/conduits and formed as intrusive. This appears as a high-velocity (6.5 km/s) and high-density (2.8 g/cm3) magmatic body in the upper crust of the Mahanadi delta. Fluids released during the process of magmatism may get trapped at some depth, and this may have formed the low-velocity (6.0 km/s) and low-density (2.65 g/cm3) layer at the midcrustal level. A thin (200 m) lid of high-velocity (5.5 km/s) volcanic rock at very shallow depth (150 m) underlain by a thick (1.75 km) pile of Gondwana sediments [Behera et al., 2002; Behera, 2003] may represent the sill-type structure (Figure 19). This indicates intense volcanism that has affected the entire crustal column in the form of undulating intracrustal boundaries including the Moho. [31] The shallow structure down to the basement is highly complex with numerous faults forming alternate horsts/ ridges and grabens/depressions. The lateral variation of velocity (5.9 – 6.1 km/s) and density (2.7 – 2.75 g/cm3) of

Figure 17. (top) Observed (red stars) and computed (blue lines) Bouguer gravity response (bottom) from the final density model along (a) profile I, (b) profile II, and (c) profile III. The dashed black line shows the gravity response calculated for the model in which the higher-density (3.05 g/cm3) underplated materials are replaced by overlying low-density (2.9 g/cm3) materials, showing a noticeable mismatch between the observed and the calculated response. Color scale indicates the variation of density (g/cm3) values. 21 of 25

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Figure 17 22 of 25

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Figure 18. Relative positions of India and Antarctica showing the Gondwana rift valleys (modified after Fedorov et al. [1982]). Dot indicates the probable position of the Kerguelen hot spot [Storey, 1995]. Dot-dashed line indicates master faults of the rift basins of the two continents. Lambert and Mahanadi basins are the conjugate and show similar crustal structures. Early Cretaceous volcanic provinces inferred for the two continents are shown.

Figure 19. A cartoon derived from the seismic and gravity model proposed for the evolution of the Mahanadi delta indicating a thick underplated layer at the base of the crust and the conjectural source marked by a plume head. Conduits through which the magmatic materials have come up to form sills in shallow Gondwana graben and a magmatic body (mafic intrusives) in the upper crust are shown. The Moho configuration is represented by the thick line. 23 of 25

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the basement rocks is due to a change in composition. It has been inferred that the rocks of the Bhubaneswar ridge are crystalline (granities/gneissies) in nature, while those of the Chandikhol ridge mainly comprise khondalitic (high-grade metamorphic) rocks of Eastern Ghat orogeny. [32] Acknowledgments. We are grateful to the Director of NGRI for his permission to publish this paper. We express our deep sense of gratitude to H. K. Gupta, Secretary, DOD, Government of India, for his encouragement throughout the progress of the work. C. A. Zelt of Rice University is gratefully acknowledged for providing the 2-D Rayinvr code. P. R. Reddy thanks C. S. I. R. (India) for the Emeritus Scientist position. Thanks are due to Roberto de Franco, an anonymous reviewer, and Giorgia Spada, the Associate Editor, for their valuable comments and suggestions to improve the quality of the manuscript.

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L. Behera, P. R. Reddy, and K. Sain, National Geophysical Research Institute, CSSP, Uppal Road, Hyderabad, Andhra Pradesh 500 007, India. ([email protected])

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