Magnetovariational study over a seismically active area in the Deccan Trap province of western India

118

Physics of the Earth and Planetary Interiors, 66 (1991) 118—131 Elsevier Science Publishers WV., Amsterdam

Magnetovariational study over a seismically active area in the Deccan Trap province of western India * B.R. Arora and C.D. Reddy Indian Institute of Geomagnetism, Colaba, Bombay 400 005 (India) (Received March 23, 1990, revision accepted July 26, 1990)

ABSTRACT Arora, B.R. and Reddy, C.D., 1991. Magnetovariational study over a seismically active area in the Deccan Trap province of western India. Phys. Earth Planet. Inter., 66: 118—131. This paper presents the results of a magnetovariational study undertaken over a basalt-covered region of western India which has been experiencing a swarm of seismic activity since early 1986. The most striking feature of the transient variations is the anomalous enhancement of the east—west (Y) component at Stations bounding the seismically active belt. From the nature of the horizontal transfer functions and the anomalous induction arrows, it is inferred that this behaviour is related to the presence of a resistive body embedded in a north—south-oriented conductive belt. Consistent with an evolutionary model of Deccan volcanism and other geophysical data, it is suggested that the central resistive block coincident with the concentrated belt of earthquake epicentres represents either a dormant volcanic plug or a plutonic body rooted iii upwarped conducting mantle. To account for the phase relationship between the anomalous Y variations and the normal north—south (X) variations, it is proposed that the upwarped asthenosphere, with its associated high geothermal anomaly, acts as a conducting path to channel induced current from the region of the Cambay triple junction located towards the northwest of the array. The existence of the strong induced currents in the region of the triple junction conforms with the induction arrow pattern observed at the array stations.

1. Infroduction The Deccan Trap volcanic province of western India, by virtue of its location in the stable peninsular shield of the Indian subcontinent, has long been considered immune to seismic activity. However, in recent years, certain pockets of the Deccan province have exhibited mild to moderate seismicity. The most recent activity in this series is centred around the village of Ankalachh (ANK) of the Valsad District in South Gujarat, Western India (Fig. la and b), which started experiencing a swarm of seismic activity in February, 1986. The main shock, of magnitude 4.6, occurred on April

27, 1986. The activity is still continuing but is weak and less frequent. The majority of the epicentres determined from the network of seven MEQ stations are concentrated between 73°15’E, 73°20’ E and 20030~N, 20°42’N (Fig. lb). The other two regions of the Deccan province which have experienced seismicity in recent times are the Koyna and the Khardi-Bhatsa districts, shown in Fig. Ia. The seismic activity in these regions began soon after the impounding of water in reservoirs of hydroelectric dams and is considered to be reservoir-induced seismicity (Gupta and Rastogi, 1976; Rastogi et al., 1986). The association of seismic activity in the Valsad district with the very small reservoir (area 2.2 km2) behind the 26 m high Kelia dam has been ruled out by Rao et al. (1986). In view of the presence of three hot springs in the vicinity of the currently active area (Fig. =

*

Based on a paper presented at the IXth Electromagnetic Induction Workshop, Sochi, U.S.S.R.

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© 1991

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ib), these authors have raised the question as to whether or not the present sequence of earthquake swarms is of geothermal origin. Although the principal volcanic activity leading to the Deccan Trap province occurred 60—65 Ma ago, the presence of a number of hot springs on Deccan Trap terrain—these tend to be aligned with some major lineaments—has been considered by many researchers to be an indicator of high temperature at greater depths. Mapping of the higher-temperature zones in the crust and upper mantle by imaging zones of enhanced electrical conductivity has been one of the longstanding interests of magnetovariational studies (e.g. Lilley and Bennett, 1972). The present study is thus designed to probe the geothermal environment and related subsurface structures Qf the Valsad seismic zone by investigating the electrical conductivity distribution in and around the seismically affected area.

2. Tectonic environment of the study area The area under investigation is a small part of the extensive Deccan Trap province, which is covered with lava flows of Palaeocene age. Figure Ia gives the location of the study area in relation to various tectonic elements of western India superimposed on the heat flow map compiled by Ravi Shankar (1988). Tectonically, the area is located close to the west coast of India, which is~ an expression of a Miocene age fault (Krishnan, 1953). This fault is a major geofracture zone related to the breaking away of the Indian plate from Gondwanaland. The land—sea boundary of this coastline provides a natural boundary of electrical conductivity contrast. Concentration of induced currents on the seaward side of this electrical discontinuity is expected to influence the na-

BR. ARORA AND CD. REDDY

ture of transient variations. The Narmada—Tapti zone, generally interpreted as a continental rift (Agrawal and Gaur, 1972; Kailasam, 1976), is a major crustal feature to the north of the study area. Deep seismic soundings along several profiles across this zone (Fig. la) have indicated the presence of deep-seated faults, extending down to the Moho, along the Narmada and Tapti lineaments which have divided the crustal section into blocks. The vertical movements of these crustal blocks have given rise to horst and graben structures in the crustal cross-section (Kaila, 1986, 1988). The other important tectonic element to the north is the roughly NNW—SSE-oriented Cambay graben. Biswas (1987) is of the opinion that the Cambay graben, displaced westward by the rightlateral Narmada tear fault, continues southward parallel to the west coast. The east bounding fault of this extended graben is reckoned to correspond to the west coast fault and its position, as visualised by Biswas, is close to the present study area. Further south the monoclinal flexure, named the Panvel flexure, which runs parallel to the west coast belt from Surat to Ratnagiri, hosts a number of hot springs. Both the Narmada—Tapti and the west coast tectonic belts are characterised by positive gravity anomalies, high gravity gradients, high heat flow and seismic activity (Kailasam et al., 1972; Qureshy, 1982; Gupta and Gaur, 1984; Ravi Shankar, 1988). An intense linear Bouguer anomaly along the west coast is considered to be related to deep-seated causes marking areas of crustal thinning and asthenospheric upwarping (Qureshy, 1981). This postulation has been largely substantiated by deep seismic sounding studies, which indicate that the region from Surat to Bombay is one of Moho upwarp (Kaila, 1986, 1988). Along its north—south central axis, the Moho is found as shallow as 18 km deep, and the top of the crustal basaltic layer in this region

Fig. I. (a) Map of the Deccan volcanic province of western India, showing the location of the present study area in relation to major tectonic elements and heat flow zones (based on Powar, 1981; Biswas, 1987; Ravi Shankar, 1988). Locations of deep seismic sounding (DSS) profiles and seismicity active zones are also shown. (b) Layout of the magnetometer sites in the study area in relation to the currently active seismic zone (hatched area). Locations of the hot springs and Landsat lineament are also shown.

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MAGNETOVARIATIONAL STUDY. INDIA

almost reaches 6-km deep. This led Kaila (1988) to suggest that during the Cretaceous period, the present-day west coast represented a transition type (continental to oceanic) of crust which acted as a major source of Deccan Trap flows. Such a visualisation corroborates a hypothesis of Bose (1972), who observed that high heat flow values in parts of the west coast, comparable to that in oceanic ridges, implies an active mantle at a relatively shallow depth. The manifestation of thermal activity in the form of hot springs is taken by Murthy (1981) to indicate that magma beneath the west coast chamber has not cooled completely. Based on gravity features, Krishna Brahmam and Negi (1973) have also postulated the existence of Kurduvadi and Koyna basement rifts. The connection of the Koyna and the Bhatsa seismic activity with these lineaments has been discussed by Rastogi et al. (1986).

3. The experiment and operational details In this study, three-component fluxgate magnetometers were operated at 12 sites, with an interstation spacing of 10—20 km, during November— December 1986. The layout of the magnetometer sites in relation to the concentrated belt of earthquake epicentres is shown in Fig. lb. The location of Kelia dam and three hot springs within the study area is also shown. The temperature of the hot spring near Unai (UNA) is highest, and is about 57°C. The temperature and discharge of the other two springs are much less than at Unai. The broken lines in Fig. lb represent the prominent lineaments inferred from Landsat imagery. The operational work was carried out in two phases. In phase I, nine magnetometers were deployed along two northern lines, and during phase II, the magnetometers from the three northernmost stations (CHI, UNA and WAG) were shifted to VAL, MAK and PIN. For technical reasons, most of the data for DHA were lost during phase II. The magnetometers measured temporal variations in three geomagnetic components along the north—south (X), east—west (Y) and vertical (Z) directions. The variations were sampled at 30-s

intervals by digital data-loggers and the data were stored on cassettes.

4. Magnetograms From the variety of magnetic activity recorded, several events containing disturbances of various periodicities and polarisations were selected and plots of stacked magnetograms were prepared for identification of normal and anomalous behaviour. Figures 2 and 3 illustrate the stacked plots for two such disturbance events recorded during phase I. Also included in Figs. 2 and 3 are the variations recorded at the two permanent observatories, Alibag (ABG) and Ujjain (UJJ). Their locations are also shown in Fig. Ia. The most striking feature of these variations is the strong variability of the Y variations across the array. The nature of the Y variations at a group of stations located within or in the immediate vicinity of the seismically active area, namely WAD, JAM, ANK and NIR—hereafter referred to as the central group—is distinctly different from that at the other stations. In Fig. 2, the Y variations at the outer group of stations are anomalously large compared with those at the central group of sta/986 DECEMBER /4 (O34OO73OL/~

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5. Anomalous magnetic variations The presence of two contrasting patterns in the Y component clearly shows strong anomalous ef-

tions. In Fig. 3, although the enhancement of the long-period variations in Y at the outer group of stations is conspicuous, certain short-period fluctuations, well developed at the central group of stations, are appreciably suppressed at the outer stations. This overall enhancement of the Y cornponent at long periods at the outer stations is pronounced at the northern (CHI, UNA and WAG), western (VAL and DHA) and the eastern (UMA) stations but tends to be only marginal at MAK and almost vanishes at PIN. As a result of. the loss of X and Z variations, the true characterisation of the anomalous features at PIN has been seriously hampered. An unusual aspect of the anomalous Y variations is the absence of correspondingly pronounced anomalies in Z, which on physical consideration is usually a more sensitive indicator of the internal conductivity distribution. The Z vanations are similar in form at all stations and show only selective enhancement at CHI, VAL, NIR and DHA. The marginal enhancement of Z at CHI and VAL is consistent with the geomagnetic coast effect or some other concentration of induced currents along the north—south-trending west coast fault. Nevertheless, the marked difference in the strength of the Z fluctuations at the

fects to be present within the array. However, due to the very localised dimension of the array, the identification of normal and anomalous behaviour is difficult. When the transient variations are compared with those at the two distant observatories, namely ABG and UJJ, one finds that the Y variations at the central group resemble better the observatory records (Figs. 2 and 3). Given that the external current systems associated with disturbance events of the types shown in Figs. 2 and 3 have a spatial scale length of at least several hundred kilometers at mid-latitudes, this similarity possibly suggests that the X and Y variations at the central group essentially represent a urnform normal field. The enhancement of Y at the outer group of stations could then be related to the anomalous induced currents associated with the subsurface electrical conductivity distribution. When the average pattern corresponding to the central group, taken as a measure of the normal field, is subtracted from the variations at individual stations, certain characteristic features of the anomalous part become much more apparent. A plot of the thus-defined ‘anomalous field’ comesponding to the event shown in Fig. 3 is given in Fig. 4. Anomalous Y variations at the outer stations show an almost one-to-one correspondence with the normal X variations (Fig. 3). This feature

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MAGNETOVARIATIONAL STUDY. INDIA

persists for all events examined, and constitutes the basic evidence to suggest that the anomalous internal currents causing the strong Y anomaly result from the leakage (channelling) of basically east—west-flowing internal currents through a roughly north—south-oriented conducting belt. Some additional features of the anomalous field are the weak reversal in the Z variations between NIR and UMA, and between NIR and ANK, as well as the presence of a weak anomalous X variation at UMA.

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A more complete description of the magnetic field variations across the array was also obtained by examining the data in the form of Fourier transform maps at selected frequencies. Figure 5 gives one such map corresponding to a period of 37 mm. Although the contour pattern is constrained by the limited number of simultaneously operating sites, the examination of several such maps brings out certain common characteristics: (1) the anomalous behaviour of the Y component tends to be more pronounced when the X and y variations are in phase, with the amplitude of the

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X variations being much stronger than that of the accompanying Y variations, and (2) the phase differences in the Y component at the central (normal) and outer (anomalous) groups exist only

For computation of the induction arrows, the vertical field transfer functions (Tzx and Tzy), which relate the station’s vertical field (taken to be entirely anomalous) to the components of the normal horizontal field (Xn, Yn), were derived by fitting the observed data to the usual equation:

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(1) where all quantities are complex and functions of frequency (period) and ez denotes an uncorrelated part of z. In the practical determination through the cross-spectral procedure, developed by Schmucker (1970), the normal field components were approximated by the mean values of the available X and Y from the central group of

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stations. Fourier transforms based on 7—14 disturbance events were used at various stations for the statistical evaluation of the transfer functions, except at VAL and MAK, where only five and four events respectively were available. Apart from providing a wide range of source field polarisations, the events used satisfied the conditions suggested by Gough and De Beer (1980) to ensure stable estimation of transfer functions free from source-field bias. The real and quadrature induction arrows at a given frequency were formed as follows: meal Iimag

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where i and j are unit Cartesian vectors pointing north and east respectively. From among the 10 sets of arrows, computed in the period range of 13—128 mm, three sets are shown in Fig. 6. Superimposed in the background are the contour lines corresponding the magnitude the consistent induction arrows. The realto induction arrowsofwith northwesterly orientation at all stations clearly indicate the presence ~fa high-conductivity zone to the northwest of the array. The spatial trends in the strength of the induction arrows are perturbed by the isolated localised anomaly, as a result of the features of single stations. Some reduction in the magnitude of the real arrows with increasing distance away from the western and northwestern corners can be inferred from the magnitudes of the contours. This feature is most conspicuous at a period of 30 mm. The directional and spatial pattern of the induction arrows shows fair stability even when the field values averaged over all or only the outer group of stations (rather than the mean from the central group of stations) are used in defining the normal field components (Xn and Yn) in eqn. (1). The features also remain unaltered when single-station transfer functions are used to construct the induction arrows. The complex spatial behaviour seen in Fig. 6 is compatible with the pattern which would result from the concentration of internal current flow along the western continental margin and some structural discontinuity following the strike of the Naronada—Tapti rift zone. Gothe et al. (1977) have shown that al-

though it might be possible to calculate theoretically the induction arrows that result from the combined effects of two conductors, the inverse task of decomposing observed arrows mto parts that would separate the combined effects of conductors is not possible, even if the strikes of the two conductors are known. The application of hypothetical event analysis (HEA) to the verticalfield transfer functions reveals that the induction effect is at its maximum when the incident horizontal field is polarised approximately at right angles to the strike of the Narmada rift, signifying that the induction in this structure with its dominant effect controls the directional pattern of the real induction arrows. In Fig. 7 the real part of the vertical field response, ZR, associated with an

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tune are effectively confined to its surface because of a relatively small skin-depth, or, alternatively, the conducting physically (Summer, 1981). Anstructure additionalisfeature seen in thin Fig. 6 as local highs or lows in the contour plots of the real arrows, as well as the substantially large quadrature arrows at DHA and UMA at a period of 18 mm, will be used later to define the edge of a block proposed to exist beneath the central part of the array.

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Fig. 7. Dots indicate the magnitude of the real part of the computed anomalous vertical field (ZR) at 60 mm, associated with an easterly polarized horizontal field of unit amplitude, for stations along the central east—west line of the array. The dotted and dashed curves represent the decay patterns of the geomagnetic coast effect across stable and tectonically active shield margins (after Everett and Hyndman, 1967).

easterly polarised horizontal field, obtained again through HEA, is plotted against the distance from an arbitrary coastline to facilitate comparison of the decay pattern with those observed across stable and tectonically active shield margins (Everett and Hyndman, 1967). Consistent with the geological evidence that the western coastal margin of India is tectonically active, the observed decay pattern shows better correspondence with the pattern observed across other tectonically active coast margins. This agreement of the decay pattern may be considered to indicate that the ocean/continent contrast may be adequate to account for the marginal enhancement of Z at VAL and CHI as well as the east—west gradient on the induction arrowmap. The quadrature arrows, both in direction and magnitude, are more scattered. At a period of 60 mm, where the pattern is best developed, the quadrature arrows at the respective stations generally have the azimuth of the accompanying real arrows. This parallelism is another indication that the induction effects relate to a common structure of high electrical conductivity (Banks and Ottey, 1974). The directional stability of the induction arrows over a wide range of frequencies with no pronounced period dependence of the magnitude of the real arrows, further implies that either the induced currents in the highly conducting struc-

8. The problem of anomalous horizontal fields In the region where anomalous horizontal fields are substantial fractions of the normal field, the use of horizontal field transfer functions, as well as anomalous induction arrows, as an aid to trace the flow path of an anomalous internal current system, has been advocated by Beamish (1977) and Jones (1983, 1986). The efficacy of these parameters in providing further insight into the anomalous behaviour of the Y component has been examined by constructing the little-employed perturbation arrows and the anomalous induction arrows as defined respectively by Schmucker (1970) and Jones (1983). 9. Perturbation arrows Complex perturbation arrows p and q were formed by combining the horizontal field transfer function (Txx, Txy, Tyx and Tyy) such that (3)

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for both real and quadrature parts. Defining the anomalous part of the horizontal field components, in the frequency domain, as the difference between the station’s and the normal field values (i.e. Xa X— Xn; Ya Y Yn), the horizontal field transfer functions, as defined below, were computed using the data base and the procedure employed in the computation of the vertical field transfer functions (eqn. (1)): =

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With respect to the definition of Schmucker (1970), the sign of the p arrows has been reversed, and, for their interpretation and pictorial presentation, both p and q arrows were rotated 90°anticlockwise so that they directly indicate the strength and direction of the anomalous internal current flow which is superimposed on the unperturbed normal current flow. The p arrows represent the perturba-~ tion current superimposed on the normal eastward and the q arrows on the normal northward directed flow. After Hibbs and Jones (1976), p and q arrows have been added vectorially to yield p + q arrows which then describe the overall flow pattern of the anomalous internal currents. Both the p and q arrows were found to be sufficiently large at stations where the Y variations appeared to be anomalous. The dominance of both p and q arrows suggests that the anomalous current flow possesses both north—south and east—west components. Maps of p + q arrows, shown in Fig. 8, clearly illustrate that the anomalous current flow is dominantly directed along the NNW—SSE direction and is relatively large along the eastern edge of the array and around the station DHA.

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In defining the anomalous Z at any given station as the departure from the average Z field over the central part of the array, in contrast to assuming the observed Z variations to be entirely anomabus, a large part of the effect associated with the currents concentrated near the Narmada—Tapti lineament is eliminated; hence the anomalous transfer functions provide more information about the local and weak conductive structures. Anomalous induction arrows, obtained by replacing Tzx and Tzy in eqn. (2) by T’zx and T’zy, were found to be negligibly small at periods > 30 mm. For shorter periods, they tend to be stronger. The set shown in Fig. 8b corresponds to a period of 18 mm. Introducing the concept of anomalous induction arrows, Jones (1983, 1986) noted that these arrows are more useful response functions for locating anomalous conductivity zones, but they do not indicate whether the anomalous zone is of higher or lower conductivity than the surrounding host medium. Certain additional features of the anomalous field must be taken into consideration in interpreting the anomalous induction arrows. Consistent with the behaviour of perturbation arrows, the anomalous induction arrows indicate the presence of an anomalous zone near the eastem edge of the array. The spatial gradients in the vertical field being proportional to the current density in the equivalent internal current system, the large east—west gradient in the anomalous vertical field response (Fig. 8b) coupled with the intense NNW—SSE-directed current flow, as deduced from the perturbation arrows, can therefore be associated with an exceptionally high conductivity contrast and/or with intense geometrical concentration of currents near DHA and in the vicinity of the eastern edge of array.

127

MAGNETOVARIATIONAL STUDY, INDIA

11. Interpretation of the anomalous features

to the western continental margin in conformity with the geomagnetic coast effect. The physical evidence that results from the interpretation of the p + q arrows i.e. that the anomalous internal currents entering the array from the north are predominantly directed along the NNW—SSE direction, is taken to indicate an electrical conductive structure aligned with segment III in Fig. 9. The observational evidence that the anomalous Y vanations in the time domain vary almost in phase with the normal X variations (Fig. 4) and the features noted on the Fourier transform maps, suggests that the generating mechanism for the anomalous flow along the structure marked III is

Figure 9 gives a qualitative picture of the anomalous currents compatible with the various features noted in the earlier sections. Although the observed magnetic variations contain the integrated effects of all the conductive zones present in the study region, the different sensitivities of the various interpretational techniques have at least permitted the possibility of locating them with fair precision. The segments I and II of the flow path are related to one concentration of internal currents in a massive conductor to the northwest of array, and to another flowing parallel

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Fig. 9. Inferred flow path of the concentrated internal currents superimposed on the tectonic map of India. The intensities of the current along different segments and within segment III increase with grade of shading. The unshaded portion embedded in segment III represents the resistive block coincident with the seismically active zone.

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more likely to be associated with the channelling of currents rather than with local induction in an elongated structure. Such an interpretation is consistent with the observations of Bailey et al. (1974) that the horizontal field polarisation, which is most effective in producing current channelling in an elongated conductor, may not bear any relation to the polarisation which is most effective in generating the same current by local induction. The flow path further south appears to be intricate and can be understood by considering that the central part of the array, coincident with the concentrated belt of earthquake epicentres, denotes an approximately north—south-oriented resistive block embedded in a zone of enhanced conductivity. In such a configuration, the channelled current from the north, as a result of obstruction by a highly resistive block, will be deflected into two narrow channels on either side of the block. Because of the conservation of current, the current density in the conducting zone near the edges of resistive block will be appreciably enhanced, leading to the enhancement of the V component at DHA and UMA. Furthermore, consistent with the observations, such intense geometrical concentration will not introduce any phase anomaly if normal X and V components are nearly in phase, but will produce a large spatial gradient in the vertical field as is clearly borne out on the map of the anomalous induction arrows. The large gradient near the eastem edge, coupled with the enhanced p + q arrow at UMA, suggests that the eastern edge of the central resistive block, marking the boundary of the large conductivity contrast, is located in the vicinity of UMA. On the western side, the spatial structure of the internal currents appears to be more detailed than could be resolved by the station spacing, but the enhanced anomalous induction arrow at JAM (Fig. 8b), the large p + q arrow at DHA, and the local deformation pattern, seen as local highs and lows centred respectively at DHA and JAM on the contour plot of the real induction arrows (Fig. 6), places the western boundary between JAM and DHA. At the central stations, such as ANK and WAD, parallel and nearly in-phase currents along the branched segments of current path III would produce opposite Z variation and may, thus, account for the ab-

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sence of Z anomaly or the existence of only a weak Z anomaly accompanying the strong Y anomaly. These two branched segments of channelled currents may complete their path either by joining and flowing to the ocean to the south of the study area or by spreading laterally into the host medium.

12. Geological and geophysical correlation of the conductive structures The above deductions on the nature of the internal current flow indicate that the overall induction pattern is controlled by multi-conductors which appear to be related to the larger tectonic features of the region. As noted earlier, the Narmada—Tapti zone is generally regarded as a continental rift. Magnetovariational and magnetotellunic studies carried out across various prominent rifts have suggested that the presence of a conductive zone within and/or beneath the rift valley is a common feature (for example, see Hutton, 1976; Jiracek et al., 1983). A two-dimensional magnetometer array study by Arora et al. (1989) over central India, east of the present study area, has shown that the Satpura ranges, interpreted as a southern lifted shoulder of the Narmada—Tapti rift system, is a locus of internal current concentration for part of its length. On the western extreme of the Narmada zone, the Satpura ranges, which here run between the Narmada and Tapti Valleys, is considered to be related to asthenospheric upwarping (Auden, 1949). The seismic resuits of Kaila et al. (1980) also indicate a shallow depth of the mantle, 20—25 km, near the junction of the Cambay and Narmada rifts. Further southwest, these two major conjugate rift systems cross each other in the Cambay Gulf region and, together with the west coast fault, define an area which has been identified by many as a triple junction (Burke and Dewey, 1973; Bose, 1980). Burke and Dewey (1973) have attributed the Maunitius—Reunion plume to be the cause of this junction. Given that the present configuration shows a mantle updoming in the Gulf of Cambay region with an associated high thermal anomaly, Biswas (1987) suggested that the concept of a

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plume-associated triple junction is an acceptable hypothesis. This contention is further supported by a magnetisation map, deduced from an inversion of the MAGSAT anomaly map (Singh et al., 1989), where the area of the triple junction is seen as a region of relatively low magnetisation, signifying a thin magnetic crust and/or an elevated temperature at depth. In view of the known relation between high heat flow and electrical conductivity (Adam, 1978), a zone of high electrical conductivity required in association with a line of induced currents, marked I on Fig. 9, can reasonably be associated with a partial melt zone in the crust or upper mantle beneath the area of the triple junction or under the Satpura ranges. The geological and geophysical evidence already discussed clearly indicates that the west coast belt in the vicinity of the present study area presents a region of Moho upwarp. It may also be strongly electrically conducting as a result of the prevailing high heat flow and thermal gradient. Further, its link with the triple junction provides the right kind of geometrical configuration to channel current out of the region of the triple junction along the flow path marked III in Fig. 9. Another conspicuous feature apparently related to the Deccan volcanism is the distinctive occurrence of plutonic bodies and volcanic plugs in western India (Biswas, 1988). The concentration is particularly heavy in Kutch, Saurashtra, Narmada, and along the west coast, and most of these bodies are associated with closely spaced dyke swarms. On seismic records, one such plutonic body, the Girnar hill, is identified as being a differentiate of basalt continuing to the upper mantle. Seismic signatures of some other plutomc bodies are shown in Ramanathan (1981). Consistent with such observations, a model is visualised in which the central part of the study area, coincident with a concentrated belt of epicentres, represents a dormant volcanic plug or plutonic body embedded in the conducting upwarped asthenosphere. The consolidation of magma in the plug, subsequent to volcanic activity, would cause a reduction of conductivity and may be seen as a highly resistive structure. The deflection of induced currents and their geometrical concentration in the ambient conducting medium in the

immediate vicinity may account for the anomalous enhancement of the V component.

13. Conclusion Examination and analyses of the geomagnetic variations from the Valsad region, using different methods of data presentation, have shown the existence of a complex internal current system which is controlled by a combination of conductive and resistive structures. In terms of the nature and magnitude of the induction anomalies, the situation is similar to that in the southern tip of the Indian peninsula, where the overall induction pattern is attributed to the combination of conductors connected with a triple junction between the Indo-Ceylon Graben, the Comonin ridge and the west coast rift (Thakur et al., 1986; Mareschal et al., 1987; Agarwal and Weaver, 1989). The region of the currently active seismic zone is identified as a resistive block embedded in the conductive environment. This is in contrast to the often reported good geographical correlation between zones of high electrical conductivity and high seismicity (Gough, 1974; Lilley, 1975; Arora and Mahashabde, 1987). The inverse correspondence seen here implies that the mechanism of thermal origin, involving conversion of thermal energy to strain energy, appears unlikely. However, the triggering of earthquake swarms in response to the thermal or structural adjustment of a resistive block located above a thermally upwarped mantle cannot be ruled out. In this paper, no quantitative geoelectnical model is proposed to explain the gross induction features. The reason is that the pattern is obviously related to a three-dimensional situation and the character of the induction taking place away from the study area in the region of the Cambay triple junction is not mapped adequately. Quantitative modelling would be further complicated because the induction effects are shown to be associated with current channelling rather than with local induction. To this end, the application of the thin sheet modelling approach (Vasseur and Weidelt, 1977; Agarwal and Weaver, 1989) and/or the equivalent current model technique developed

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by Banks (1986) may prove advantageous. However, more precise interpretation should follow by incorporating results from magnetotelluric investigations which have been initiated recently. Preliminary results obtained from five stations along the central east—west line confirm the existence of a large resistivity contrast between the seismically active zone and that outside (S.G. Gokarn, personal communication, 1990). The apparent resistivity sounding curves are similar at all stations except that the average levels differ inside and outside the seismic zone. They have been interpreted by one-dimensional modelling to result from lateral heterogeneity of the deep resistivity: 1300 ~2m at 10-km deep beneath JAM, ANK and NIR and — 130 ~2m at the same depth beneath VAL and UMA. These two blocks of contrasting resistivity are underlain by a 6 km thick conducting layer of 30 ~ m at a depth of 30 km. All these features are consistent with the magnetovariational data and provide a first estimate of the geoelectrical parameters of the mapped structures. Two-dimensional modelling of these and magnetotelluric data currently being collected should provide a better constraint on the geological interpretation of the mapped structures.

Acknowledgements We express our sincere thanks to Professor U. Schmucker and Professor B.P. Singh for many. stimulating discussions, to Dr. H.N. Srivastava for his encouragement in the work presented here, and to the Director of the Gujarat Engineering Research Institute, Baroda, for logistic support. Help received from D.T. Rao, Dr. B.B. Jambusaria and Dr. Hunchinal in organising fieldwork is deeply appreciated. References Adam, A., 1978. Geothermal effects in the formation of electrical conducting zones and temperature distribution in the earth. Phys. Earth Planet. Inter., 17: 21—28. Agarwal, AK. and Weaver, J.T., 1989. Regional electromagnetic induction around the Indian Peninsula and Sri Lanka: a three-dimensional numerical model study using the thin

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sheet approximation. Phys. Earth Planet. Inter., 54: 320— 331. Agrawal, P.N. and Gaur, V.K., 1972. Study of crustal deformation in India. Tectonophysics, 15: 287—296. Arora, BR. and Mahashabde, MV., 1987. A transverse conductive structure in the northwest Himalaya. Phys. Earth Planet. Inter., 45: 119—127. Arora, B.R., Mahashabde, MV. and Waghmare, S.Y., 1989. Electrical conductivity structures of central India and their relation to Narmada-Son lineament (abstract). Presented in session 1-9 of IAGA, Exeter, U.K., in press. Auden, J.B., 1949. Dykes in western India—a discussion of their relationship with the Deccan Traps. Trans. Nat. Inst. Sci. India, 3: 123—159. Bailey, R.C., Edwards, RN., Garland, G.D., Kurtz, R. and Pitcher, D., 1974. Electrical conductivity studies over a tectonically active area in eastern Canada. J. Geomagn. Geoelectr., 26: 125-146. Banks, R.J., 1986. The interpretation of the Northumberland Trough geomagnetic variation anomaly using two-dimensional current models. Geophys JR. Astron. Soc., 87: 595— 616. Banks, R.J. and Ottey, P. 1974. Geomagnetic deep sounding in and around the Kenya rift valley. Geophys. JR. Astron. Soc., 36: 321—335. Beamish, D., 1977. The mapping of induced currents around the Kenya rift: a comparison of techniques. Geophys. J.R. Astron. Soc., 50: 311-332. Biswas, 5K., 1987. Regional tectonic framework, structure evolution of the western marginal basins of India. Tectonophysics, 135: 307—327. Biswas, S.K., 1988. Structure of the western continental margin of India and related igneous activity. In: K.V. Subbarao (Editor), Deccan Flood Basalts. Geol. Soc. India Mem., 10: 371—390. Bose, M.K., 1972. Deccan basalts. Lithos, 5: 131—135. Bose, M.K., 1980. Magnetically defined tectonic framework of the Deccan volcanic province. Neues Mineral. Jb., 8: 373— 384. Burke, K. and Dewey, J.F., 1973. Plume generated triple junetions: key indicators in applying plate tectonics to old rocks. J. Geol., 81: 406—433. Everett, i.E. and Hyndman, RD., 1967. Geomagnetic variations and electrical conductivity structures in south-western Australia. Phys. Earth Planet. Inter., 1: 24—34. Gough, DI., 1974. Electrical conductivity under western North America in relation to heat flow, seismology and structure. J. Geomagn. Geoelectr., 26: 105—123. Gough, DI. and de Beer, J.H., 1980. Source field bias in geomagnetic transfer functions: a case history. J. Geomagn. Geolelectr., 32: 471—482. Gupta, ML. and Gaur, V.K., 1984. Surface heat flow and probable evolution of Deccan volcanism. Tectonophysics, 105: 309—318. Gupta, H.K. and Rastogi, BK., 1976. Dams and Earthquake. Elsevier, New York, 227 pp. Gothe, W., Porstendorter, G. and Roster, R., 1977. The prob-

MAGNETOVARIATIONAL STUDY. INDIA

lem of distinction of separate conductivity anomalies in the map of Wiese induction arrows. Acta Geodaet. Geophys. Montanist. Acad. Sci. Hung., 12: 295—299. Hibbs, RD. and Jones, F.W., 1976. The calculation of perturbation and induction arrows for a three-dimensional conductivity model. J. Geomagn. Geoelectr., 28: 283—293. Hutton, V.R.S., 1976. Induction studies in rifts and other active regions. Acta Geodaet. Geophys. Montanist. Acad. Sci. Hung., 11: 347—376. Jiracek, G.R., Gustafson, E.P. and Mitchell, P.S., 1983. Magnetotelluric results opposing magma origin of crustal environment in the Rio Grande Rift. Tectonophysics, 94: 299—326. Jones, A.G.,. 1983. The problem of ‘current channelling’: a critical review. Geophys. Surv., 6: 79—122. Jones, AG., 1986. Parkinson’s pointers’ potential perfidy! Geophys. JR. Astron. Soc., 87: 1215—1224. Kaila, K.L., 1986. Tectonic framework of Narmada—Son lineament—a continental rift system in central India, from deep seismic soundings. In: M. Barazangi and L. Brown (Editors), Reflection Seismology: A Global Prospective. Geodyn. Ser., Am. Geophys. Union PubI., 13: 133—150. Kaila, K.L., 1988. Mapping the thickness of Deccan Trap flows in India from DSS studies and inferences about a hidden Mesozoic basin in the Narmada—Tapti Region. In: K.V. Subbarao (Editor), Deccan Flood Basalts. Geol. Soc. India Mem., 10: 91—116. Kaila, K.L., Krishna, V.G. and Mall, D.M., 1980. Crustal structure along Mehmadabad—Billimora profile in the Cambay basin, India, from deep seismic soundings. Tectonophysics, 76: 99—130. Kailasam, L.N., 1976. Plateau uplift in Peninsular India. Tectonophysics, 61: 243—269. Kailasam, L.N., Murthy, B.G.K. and Chayanulu, A.V.S.R., 1972. Regional gravity studies of the Deccan Trap areas of peninsular India. Curr. Sci., 41: 403—407. Krishna Brahmam, N. and Negi, J.G., 1973. Rift valleys beneath Deccan Traps (India). Geophys. Res. Bull., 5: 207— 238. . Krishnan, MS., 1953. Structural and tectonic history of India. Mem. Geol. Surv. India, 81: 1—109. Lilley, F.E.M., 1975. Electrical conductivity anomalies and continental seismicity in Australia. Nature, 257: 381—382. Lilley, F.E.M. and Bennett, D.J., 1972. An array experiment with magnetic variometers near the coast of south-east Australia. Geophys. JR. Astron. Soc. 29: 49—64. Mareschal, M., Vesseur, G., Srivastava, B.J. and Singh, R.N., 1987. Induction models of southern India and the effect of off-shore geology. Phys. Earth Planet. Inter., 45: 137—148.

131

Murthy, M.V.N., 1981. Late Mesozoic—early Tertiary volcanism in the Trans-Deccan Trap areas of the Indian shield: a synthesis. In: K.V. Subbarao and RN. Sukheswala (Editors), Decan Volcanism. Geol. Soc. India Mem., 3: 93—100. Powar, KB., 1981. Lineament fabric and dyke pattern in the western part of the Deccan volcanic province. In: K.V. Subbarao and R.N. Sukheswala (Editors), Deccan Volcanism. Geol. Soc. India Mem., 3: 45—57. Qureshy, M.N., 1981. Gravity anomalies, isostacy, and crust— mantle relations in Deccan Trap and contiguous regions, India. In: K.V. Subbarao and R.N. Sukheswala, (Editors), Deccan Volcanism. Geol. Soc. India Mem., 3: 184—197. Qureshy, MN., 1982. Geophysical and Landsat lineament mapping—an approach illustrated from west—central and south India. Photogrammetria, 37: 161—184. Ramanathan, S., 1981. Some aspects of Deccan volcanism of western India shelf and Cambay basin. In: K.V. Subbarao and RN. Sukheswala (Editors), Deccan Volcanism. Geol. Soc. India Mem. 3: 198—217. Rao, D.T., Jambusaria, B.B., Rama Rao, J.V., Thatte, CD. and Srivastava, H.N., 1986. A seismological study of recent Valsad earth tremors. 8th Symp. Earthquake Engng., Roorkee, December 29—31, Vol. I, pp. 56—62. Rastogi, BK., Chadha, R.K. and Raju, I.P., 1986. Seismicity near Bhatsa reservoir, Maharashtra, India. Phys. Earth Planet. Inter., 44: 179—199. Ravi Shankar, 1988. Heat flow map of India and discussions on its geological and economic significance. Indian Minerals, 42: 89—110. Schmucker, U., 1970. Anomalies of geomagnetic variations in the southwestern United States. Bull. Scripps Inst. Oceanogr., No. 13: 1—165. Singh, B.P., Mita Rajaram and Basavaiah, N. 1989. Inversion of magnetic and gravity data in the Indian region. In: R.F. Mereu, S. Mueller and D.M. Fountain (Editors), Properties and Processes of Earth’s Lower Crust. Geophys. Monogr. 51 IUGG, 6: 271 —278. Summer, D.M., 1981. Interpreting the magnetic fields associated with two-dimensional induction anomalies. Geophys. JR. Astron. Soc., 65: 535—552. Thakur, N.K., Mahashabde, MV., Arora, BR., Singh, B.P., Srivastava, B.J. and Prasad, SN., 1986. Geomagnetic variation anomalies in peninsular India. Geophys. J.R. Astron. Soc., 86: 839—854. Vasseur, G. and Weidelt, P., 1977. Bimodel electromagnetic induction in non-uniform thin sheet with an application to the northern Pyrenean induction anomaly. Geophys. JR. Astron. Soc., 51: 669—690.