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3-D Structure and geodynamic evolution of accreted igneous layer in the Narmada-Tapti region (India)
J. Geodwmmics Vol. 25, No. 2, pp. 129-141. 1998 0 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0264-3707/98 $19.00+0.00 PII: SO264-3707@7)00017-3
3-D STRUCTURE AND GEODYNAMIC EVOLUTION OF ACCRETED IGNEOUS LAYER IN THE NARMADA-TAPTI REGION (INDIA) A. P. SINGH National Geophysical Research Institute, Uppal Road, Hyderabad, 5OOCO7India. (Received 30 May 1996; revised 24 December 1996; accepted 28 January 1997)
Abstract-The
Narmada-Tapti rift system is an unusual feature crossing the west coast of India. A recent 2-D analysis of Bouguer gravity anomaly combined with four deep seismic sounding profiles in the region has revealed a 15-20 km thick high-density (3.02 g cm -3) accreted igneous layer at the base of the crust. For a more realistic approximation a 3-D density model of the proposed underplated layer is obtained. The thickness of the lower crustal accreted igneous layer varies from 8 km beneath the eastern part to about 16 km beneath the central part and about 24 km beneath Navsari in the westernmost part of the region. The greater thickness of the magmatic body beneath the Navsari gravity high presents itself as one of the potential feeder channels for the Deccan flood basalts. The study further deciphers a normal Moho located at a depth of about 38 km in the southwestern part of the region. The geothermal signatures show an upwarped asthenosphere indicating the existence of partial melting conditions at very shallow depths. It is suggested that the massive eruption of the Deccan flood basalt on the west coast of India has modified the deep crustal structure of the Narmada-Tapti region in agreement with rheological boundary conditions. 0 1997 Elsevier Science Ltd
INTRODUCTION lineament (NSL), straddling the Indian shield, is the most conspicuous linear feature on the geological map of west-central India (Fig. 1). It is a mid-continental rift system (Ravi Shanker, 1991) which divides the shield into two halves (Radhakrishna and Naqvi, 1986; Biswas, 1987, 1993; Powar, 1993). The Narmada-Tapti rift system, which constitutes the western part of the NSL, is covered by a thick pile of Deccan lava flows and is characterized by several hidden tectonic structures, magmatic crustal accretion and complex geophysical signatures. The area has been the subject of extensive geological and geophysical investigations for many years (Auden, 1949; West, 1962; Qureshy, 1964; Choubey, 1971; Mishra, 1977; Crawford, 1978; Biswas, 1982, 1987; Ravi Shanker, 1987, 1991; Kaila and Krishna, 1992; Verma and Banerjee, 1992; Powar, 1993; Singh and Meissner, 1995; Bhattacharji er al., 1996). A controversy, however, still exists as to whether it is an ancient rift zone, a zone of persistent weakness and tectonism or the consequence of a strong plume head mushrooming beneath the region. Tectonically the area is located close to the west coast fault (Fig. 2), which is a major
The Natmada-Son
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geofracture zone related to the breaking away of the Indian plate from the Gondwanaland. The NSL is a fundamental tectonic line of Precambrian origin (Auden, 1949; West, 1962). According to Jain er al. (1984), Kale (1985), Radhakrishna and Naqvi (1986), and Biswas (1993) the origin of this major line of geological discontinuity is a direct consequence of collision tectonics. The suture zone of collision was later transformed into a rift which now is the NSL. Geological and geophysical data adduced and collected by Crawford (1978), Ravi Shanker (1987, 1991) and Verma and Banerjee (1992) lead to the conclusion that the faults have been rejuvenated periodically and the region has undergone vertical movements from time to time. The other important tectonic element to the north is the NNW-SSE oriented Cambay graben. The Narmada-Tapti rift system and the Cambay graben cross each other in the Gulf of Cambay region, and together with the west coast fault, define an area which has been identified as a triple junction (Burke and Dewey, 1973). The radial drainage pattern together with the rifted
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Fig. 1. Present configuration of the western continental margin of India and adjoining Arabian sea with the location of the Narmada-Son lineament (NSL), the Deccan traps and the trail of volcanic ridges and islands left by the Reunion plume as it migrated away from its initial position under India. Figures along the trail show the average age of the rocks in Ma. Abbreviations NTR. CR, WCF and TJ stands for the NarmadaTapti rift, the Cambay rift, the West Coast fault and the Triple junction, respectively (modified after White and McKenzie, 1989b).
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crest is another characteristic feature of the west coast encompassing the triple junction (Cox, 1989). Given that the present configuration shows a mantle updoming in the Gulf of Cambay region, the concept of a plume-associated triple junction (Biswas, 1987) is an acceptable hypothesis for the present-day configuration of the west coast megastructure (Powar, 1993) encompassing the Cambay graben, the Narmada-Tapti region and the triple junction. Surface expression of the proposed plume is the Deccan flood basalts on the western continental margin of India (Fig. 2). Around 65 Ma ago, when Indian subcontinent passed over the Reunion hot spot, the crust opened on the west coast of India and produced one of the largest flood basalt provinces on the Earth’s surface. More than 2,000,OOOkm3 of lava were spread in less than half a million years (White and McKenzie, 1989a) blanketing over 500,000 km2 of the Indian landmass. When correlative basalts identified offshore (Arabian Sea) are included and an estimate is made for eroded lavas, the province may have covered more than 1,500,000 km2 (Richards et al., 1989). Evidently, the event which produced such a large amount of basaltic magma on the continental surface must have had a profound influence on the crustal evolution including the present-day configuration of the continental lithosphere. It has been suggested that igneous intrusion at the base of the crust may underlie all shallow flood basalts (White and McKenzie, 1989a). According to Meissner (1986), zones of low viscosity, such as the continental lower crust and the asthenosphere, are weak and therefore act as the zones of 70’1
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Fig. 2. Tectonic map of the western margin of India showing the three rift basins (Kutch, Cambay and Narmada-Tapti), the major Precambrian tectonic trends and the four DSS profiles across the NarmadaTapti region (modified after Biswas, 1982).
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decoupling, attracting lateral movements of intruding plume material. The Narmada-Tapti region of Deccan volcanic province is no exception. Coffin and Eldholm (1994) presumed a lower crustal body in the form of magmatic underplating beneath the region. Recently Singh and Meissner (1995) based on a 2-D density model along the four deep seismic sounding (DSS) profiles in the region, have suggested a 15-20 km thick accreted igneous layer at the base of the crust. The observation of a high VP-layer in the lower crust along the DSS profiles was used as a strong argument for such a high-density layer and its interpretation as a massive mafic intrusion beneath the region. To better account for the large scale geometry of the proposed crustal accretion, a 3-D density model of the underplated layer is obtained. The model is then used to elucidate an integrated geodynamical evolutionary history of the region.
ANOMALOUS
GRAVITY SlGNATURE
The Bouguer anomaly (BA) map of the Narmada-Tapti region bounded between latitudes 20 to 22”45’ N and longitudes 72”30’ to 79” E along with the four DSS profiles and course of the rivers Narmada and Tapti is shown in Fig. 3. A broad relative gravity high is aligned in the E-W direction between the rivers Narmada and Tapti. The Navsari gravity high, with a maximum value of +40 mgal, forms the western extension of this more-or-less linear gravity high, whereas in the east it is terminated by a NNW-SSE trending gravity low over Pachmarhi. The relative high gravity anomaly, of nearly 100 km in width and extending over a length of about 600 km, is thought to be caused by the magmatic underplating at the base of the crust (Singh and Meissner. 1995).
3-D density model of the accreted igneous layer Delineation of crustal structures responsible for the observed BA is the aim of any gravity modelling. The present investigation is subjected to the 3-D approximation of the proposed underplated layer, expected to be responsible for the referenced anomaly. The shallow structural features are of no interest here, nor would it be appropriate to decipher on such a large scale in
Fig. 3. Bouguer anomaly map of the Narmada-Tapti region along with the DSS profiles and the course of the rivers Narmada and Tapti (modified after Verma and Banerjee, 1992).
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the absence of detailed geological/geophysical information. Their effects are therefore needed to be adequately removed from the observed BA. It has been proposed by Jacobsen (1987) to use upward continuation as a standard suboptimum filter which can solve a wide span of separation problems when applied to real, non-random anomalies. According to him the optimum filter for the extraction of the field associated to sources below a certain depth (Z,,) is the upward continuation to the height (2Z,,) above the measurement plane. A thickness of 15 km for the horizontal layer representing the upper crust is chosen as a compromise suggested by the 2-D density modelling of Singh and Meissner (1995). The observed BA map is then subjected to the upward continuation to a height of 30 km for the extraction of the field associated to sources below 15 km depth (Fig. 4). Obviously, the separation of the observed BA by upward continuation to the elevation of 30 km is an over simplification of the true geology of the upper crust. At the same time, the attribution of high frequency features at shallow depth, i.e. the upper crust, is strictly valid, it is not necessarily valid to attribute low frequency features to deep-seated structures such as the proposed under-plated layer. It is possible that the low frequency feature of the BA map is not associated with the underplated layer, but in fact caused by some broad, shallower feature within the upper crust. However, in the present case, with the 2-D density modelling along the four DSS profiles (Singh and Meissner, 1995), the uncertainty about the underplated layer responsible for the referenced anomaly is considerably reduced. For a detailed investigation of this filtered component of the BA, the concept of interactive forward gravity modelling developed by Gijtze and Lahmeyer (1988) is applied. In this method the effect on gravity of a homogeneous polyhedra of suitable geometry and density, constrained from the a priori information, is calculated by transforming a volume integral into a sum of line integrals. The DSS and the correlative 2-D density modellings provided the necessary a priori information to well constrain the initial 3-D density model of the underplated layer. Appropriate density values such as 2.87 and 3.22 g cmm3 are attributed to the lower crust and the upper mantle respectively and were held constant during the modelling. Through cooling and contraction, the crustal accretion is expected to produce a homogeneous body at the base of the crust (Meissner and Kopnick, 1988). A density of 2.99 to 3.07 g cm -3 is suggested for such
Fig. 4. 30 km upward continued Bouguer anomaly map of the Narmada-Tapti region.
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accreted igneous layers (White and McKenzie, 1989b). An average density of 3.02 g cm -3 is therefore assigned to the proposed under-plated layer. Initially the geometry of the layer is constrained from the 2-D gravity study of Singh and Meissner (1995). Except in the southwestern part of the region, the computed gravity values with minor adjustments in its subsurface geometry matched fairly well with the filtered BA. The calculated gravity values for the given structure of the underplated layer together with an upwarped Moho in the southwestern part of the region did not conform well with this part of the filtered BA. Trusting the Moho geometry as delineated by the DSS study Singh and Meissner (1995) matched this part of the observed BA by restricting the underplated layer up to the Tapti river, In contrast Gupta and Mishra (1992) and Verma and Banerjee ( 1993) envisaged a greater depth for the underplated layer lying above an almost normal Moho. The underplated layer is therefore extended in southwestern part of the region and its upper and lower boundaries were suitably varied to match the anomaly. Care was, however, taken that the depth to the interfaces were compatible with the earlier gravity studies. The resulting 3-D configuration of the accreted igneous layer is shown in Fig. 5. It clearly depicts the underplated layer with its thickness decreasing gradually towards the east. The thickness of the inferred magmatic layer varies from 8 km beneath the eastern part
Fig. 5. Three dimensional density model of the accreted igneous layer at the base of the crust beneath the Narmada-Tapti region. Contours (in km) show the Moho configuration.
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to about 16 km beneath the central part of the region. A thickness of about 24 km below the Navsari window is another local but prominent feature of the underplated layer. Another characteristic feature of the presented 3-D density model is the normal Moho observed beneath the region. A Moho of about 38 km deep beneath the southwestern part of the region is the most significant departure from the DSS observations. A similar depth is also suggested by Gupta and Mishra (1992) and Verma and Banerjee (1993) beneath that region. In contrast the DSS results show a shallow Moho of less than 25 km deep in this region (Fig. 6a). The calculated gravity field in the Navsari-Billimora region for the delineated DSS structure is of the order of +250 mgal (Fig. 6a). Since the DSS result as presented is unable to explain the nature of the observed gravity field, the presence of a very shallow Moho in this region is most unlikely. Though any structural model derived by gravity modelling alone is ambiguous, the negation of a seismic model on the basis of its gravity effect is possible (Holliger and Kissling, 1992; Kissling, 1993). With single-sided wide angle reflection data it may not be possible for the DSS study to resolve the Moho at the end part of the profile. Possibly what has been assumed to be the Moho in the DSS profile-I, actually represents the high-velocity/high-density discontinuity where the crust transforms into the transitional zone. To recheck the presented structure of the underplated layer in southwestern part of the region a unified 2-D infinite length density model with combining the upper crustal structure of Singh and Meissner (1995) along the DSS profile-I is obtained (Fig. 6b). As is apparent, the observed Bouguer gravity field along the profile is well accounted for by the present unified 2-D infinite length density model. It not only favours a normal Moho, about 38 km deep beneath the Navsari-Billimora region, but also suggests that the greater thickness of the inferred magmatic intrusion is the obvious source of the strong positive gravity anomaly over Navsari.
GEOTHERMAL
REGIME
The Cenozoic volcanic continental rifts with igneous intrusion into the rifted crust are characterized by high heat flow (Morgan, 1982). The thermal anomalies in such regions are caused by magmatic mass transfer into the rifted crust and modified by heat transport through fluid convection (Lysak, 1992). Interestingly, the Narmada-Tapti region with Late Cretaceous magmatic crustal accretion is also characterized by a relatively high heat flow regime (Fig. 7). An extensive geothermal regime encompassing the Cambay graben, the Bombay offshore, the Konkan geothermal province and the Narmada-Tapti region appears to be a regional phenomenon in the western margin. Another closely related indicator of intense subsurface thermal activity is the distribution of the hot springs. Their manifestation along the west coast is seen as a region of elevated temperature at depths. It was assumed by Murthy (198 1) that the magma chamber beneath that region has not cooled down completely. For Bose (1972) the observed heat flow values in part of the west coast imply an active mantle similar to mid-oceanic ridges at a relatively shallow depth. The present-day high heat regime, localization of eruptive centres and basic igneous complexes along the faulted western margin and associated rifts (Biswas, 1982) indicate a possible thinning of the lithosphere. In fact Negi et al. (1986) found an upwarped asthenosphere in the Narmada-Tapti region up to a mean depth of about 57 km. The thermal iithosphere beneath the triple junction is found to be extremely thin ( - 40 km; Negi er al., 1992). It is quite probable that the thermal indentation at crustal level caused by upward flow of Deccan magma would subsequently cool losing all of its original heat. However, due to
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Fig. 6. a. Crustal depth section along the Mehmadabad-Billimora (Line- I ) as obtained from the DSS data (modified after Kaila et al., 1981). The Iower refractor segments are shown with velocity in km s - ‘. b. Modified two-dimensional density model of crustal cross-sectiod along the DSS Profile-I. Densities are in g cm -‘. R stands for the river.
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the extremely thinned thermal lithosphere with possible passive margin conditions at very
shallow depth, the temperature is expected to be still higher over and around the region. DISCUSSION The present gravity analysis and the existing geothermal regime together with the other relevant geophysical information provide a comprehensive evolutionary picture of the under-plated layer beneath the Narmada-Tapti region. One of the most characteristic features of the area is the broad relative gravity high despite the presence of high Satpura mountains (Fig. 2). It is an anomalous feature since a high region is supposed to have a BA low due to expected compensation. On the basis of the qualitative study of the BA along the NSL, Qureshy (1964) concluded that the relative gravity high over the Satpura mountain range indicates a horst type structure. Later the crustal upliftment was associated to the movement due to incorporation of material from the upper mantle into the crust (Qureshy, 1971). In order to satisfy the observed BA, Vet-maand Banerjee (1992) introduced a high-density basic intrusive material at mid-crustal levels without giving any plausible explanation. On the other hand, Singh and Meissner (1993, based on the 2-D density modelling along the four DSS profiles, suggested the igneous crustal 7oq
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Fig. 7. Heat flow distribution (in mW m -*) over western India. Solid circles denote the values observed conventionally (Singh and Meissner, 1995) and open circles denote the geochemically estimated heat flow values (Ravi Shanker, 1988). Hot springs are indicated by stars (Krishnaswamy and Ravi Shanker, 1980).
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accretion at the base of the crust beneath the region. The presented 3-D gravity modelling provides a more realistic structure than previous 2-D gravity modelling by gaining third dimension of the underplated layer. The study further suggests a normal crustal thickness along the west coast of India contrary to earlier belief that it represents a highly upwarped Moho beneath that region. On the basis of the DSS study and the correlative 2-D density model, it was suggested that the shallow Moho beneath the southwestern part of the region is the cause for the Bouguer gravity high over Navsari (Singh and Meissner, 1995). In contrast the presented 3-D and 2-D density models clearly indicate that the main contributor to the Navsari gravity high is the greater thickness of the high-density magmatic body reaching up to a depth of about 15 km. The presence of the accreted igneous layer at the base of the crust is also compatible with the high geothermal regime of the Narmada-Tapti region (Fig. 7). According to Ravi Shanker (1988) the high heat flow observations represent an anomalous hot mantle and/or a zone of intense lower crust and upper mantle interaction. A 2-D magnetometer array study has also shown that the Satpura ranges is a locus of internal current concentration for parts of its length (Arora and Reddy, 1991). In view of the known relation between high heat flow and electrical conductivity (Adam, 1978) it was associated with a partial melt zone in the crust or upper mantle beneath the area of the triple junction and under the Satpura ranges (Arora and Reddy, 1991). This intra-crustal thermally disturbed layer with high current concentration conforms well with the present accreted igneous layer. The extremely thinned thermal lithosphere with possible presence of partial melting conditions at very shallow depth are further corroborated by teleseismic tomographic observation in the Deccan volcanic province. The 3-D tomographic study revealed a large scale low velocity zone which persists till at least 200 km beneath the westernmost part of the Deccan volcanic province (Ramesh et al., 1993). A similar low velocity fossil mantle plume beneath South America is recently interpreted as the thermal remnant of the original plume conduit that supplied the Parana plume head (VanDecar et al., 1995). Origin and evolution of the proposed underplated layer in such a thermally disturbed condition may be seen in the Late Cretaceous geodynamics of the Indian subcontinent. The series of southward decreasing age of the submarine volcanic lineaments (the Laccadive, Chagos and Mascarene ridges; Fig. 1) links the Deccan flood basalts to the Reunion hot spot (Richards er al., 1989). Around 65 Ma ago the Deccan plume head mushrooming beneath the west coast modified the former crustal vis-a-vis sublithospheric structure of the Narmada-Tapti region together with the west coast in the low viscosity environment (Singh and Meissner, 1995). The delineated 3-D structure of the underplated layer beneath the Narmada-Tapti region and the fossil mantle plume beneath the west coast are thus the imprint of the Deccan magmatism along the west coast of India. The conceptual model presented in Fig. 8 gives a general 3-D view of the igneous crustal accretion at the base of the crust caused by a rising deep mantle plume. One consequence of the model is the crustal contamination of intruding plume material by getting mixed with melting crustal material of lower melting temperature. The upward flow of this contaminated shallow secondary magma was later facilitated by feeder channels present all along the NSL (Bhattacharji et al., 1996). The model thus provides an alternative explanation for the highly fractionated nature of the Deccan flood basalts (Mahoney, 1988). CONCLUSIONS
The results obtained from the present study are in general accordance with earlier gravity studies. It, however, provides a more realistic approximation than previous 2-D gravity
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Fig. 8. Three dimensional conceptual model of the accreted igneous layer at the base of the crust beneath the survey area. modelling by gaining the third dimension of the underplated layer at the base of the crust. The greater thickness of the accreted igneous layer beneath the Navsari gravity high, the main contributor to the referenced anomaly, presents itself as one of the potential sites for the Deccan magma evolution. The study further modifies the upwarped Moho to a normal depth in the Navsari-Billimora region. The conceptual model for the underplated layer (Fig. 8) provides the right kind of geometrical configuration for the crustal accretion beneath the Narmada-Tapti region. Acknowkdgemenrs-I am grateful to Prof. R. Meissner for his support and encouragement to carry out this work.
Thanks are due to Prof. P Janle for helpful discussions and two anonymous reviewers for useful suggestions. The permission accorded by Prof. H. J. Gbtze to use his computer software for this study is gratefully acknowledged. The work was supported by a grant from Deutscher Akademisher Austauschdienst (DAAD), Germany.
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3-D STRUCTURE AND GEODYNAMIC EVOLUTION OF ACCRETED IGNEOUS LAYER IN THE NARMADA-TAPTI REGION (INDIA) A. P. SINGH National Geophysical Research Institute, Uppal Road, Hyderabad, 5OOCO7India. (Received 30 May 1996; revised 24 December 1996; accepted 28 January 1997)
Abstract-The
Narmada-Tapti rift system is an unusual feature crossing the west coast of India. A recent 2-D analysis of Bouguer gravity anomaly combined with four deep seismic sounding profiles in the region has revealed a 15-20 km thick high-density (3.02 g cm -3) accreted igneous layer at the base of the crust. For a more realistic approximation a 3-D density model of the proposed underplated layer is obtained. The thickness of the lower crustal accreted igneous layer varies from 8 km beneath the eastern part to about 16 km beneath the central part and about 24 km beneath Navsari in the westernmost part of the region. The greater thickness of the magmatic body beneath the Navsari gravity high presents itself as one of the potential feeder channels for the Deccan flood basalts. The study further deciphers a normal Moho located at a depth of about 38 km in the southwestern part of the region. The geothermal signatures show an upwarped asthenosphere indicating the existence of partial melting conditions at very shallow depths. It is suggested that the massive eruption of the Deccan flood basalt on the west coast of India has modified the deep crustal structure of the Narmada-Tapti region in agreement with rheological boundary conditions. 0 1997 Elsevier Science Ltd
INTRODUCTION lineament (NSL), straddling the Indian shield, is the most conspicuous linear feature on the geological map of west-central India (Fig. 1). It is a mid-continental rift system (Ravi Shanker, 1991) which divides the shield into two halves (Radhakrishna and Naqvi, 1986; Biswas, 1987, 1993; Powar, 1993). The Narmada-Tapti rift system, which constitutes the western part of the NSL, is covered by a thick pile of Deccan lava flows and is characterized by several hidden tectonic structures, magmatic crustal accretion and complex geophysical signatures. The area has been the subject of extensive geological and geophysical investigations for many years (Auden, 1949; West, 1962; Qureshy, 1964; Choubey, 1971; Mishra, 1977; Crawford, 1978; Biswas, 1982, 1987; Ravi Shanker, 1987, 1991; Kaila and Krishna, 1992; Verma and Banerjee, 1992; Powar, 1993; Singh and Meissner, 1995; Bhattacharji er al., 1996). A controversy, however, still exists as to whether it is an ancient rift zone, a zone of persistent weakness and tectonism or the consequence of a strong plume head mushrooming beneath the region. Tectonically the area is located close to the west coast fault (Fig. 2), which is a major
The Natmada-Son
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geofracture zone related to the breaking away of the Indian plate from the Gondwanaland. The NSL is a fundamental tectonic line of Precambrian origin (Auden, 1949; West, 1962). According to Jain er al. (1984), Kale (1985), Radhakrishna and Naqvi (1986), and Biswas (1993) the origin of this major line of geological discontinuity is a direct consequence of collision tectonics. The suture zone of collision was later transformed into a rift which now is the NSL. Geological and geophysical data adduced and collected by Crawford (1978), Ravi Shanker (1987, 1991) and Verma and Banerjee (1992) lead to the conclusion that the faults have been rejuvenated periodically and the region has undergone vertical movements from time to time. The other important tectonic element to the north is the NNW-SSE oriented Cambay graben. The Narmada-Tapti rift system and the Cambay graben cross each other in the Gulf of Cambay region, and together with the west coast fault, define an area which has been identified as a triple junction (Burke and Dewey, 1973). The radial drainage pattern together with the rifted
I
/f
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6
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REiiNlON
Fig. 1. Present configuration of the western continental margin of India and adjoining Arabian sea with the location of the Narmada-Son lineament (NSL), the Deccan traps and the trail of volcanic ridges and islands left by the Reunion plume as it migrated away from its initial position under India. Figures along the trail show the average age of the rocks in Ma. Abbreviations NTR. CR, WCF and TJ stands for the NarmadaTapti rift, the Cambay rift, the West Coast fault and the Triple junction, respectively (modified after White and McKenzie, 1989b).
3-D
structure of accreted igneous layer in NTR
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crest is another characteristic feature of the west coast encompassing the triple junction (Cox, 1989). Given that the present configuration shows a mantle updoming in the Gulf of Cambay region, the concept of a plume-associated triple junction (Biswas, 1987) is an acceptable hypothesis for the present-day configuration of the west coast megastructure (Powar, 1993) encompassing the Cambay graben, the Narmada-Tapti region and the triple junction. Surface expression of the proposed plume is the Deccan flood basalts on the western continental margin of India (Fig. 2). Around 65 Ma ago, when Indian subcontinent passed over the Reunion hot spot, the crust opened on the west coast of India and produced one of the largest flood basalt provinces on the Earth’s surface. More than 2,000,OOOkm3 of lava were spread in less than half a million years (White and McKenzie, 1989a) blanketing over 500,000 km2 of the Indian landmass. When correlative basalts identified offshore (Arabian Sea) are included and an estimate is made for eroded lavas, the province may have covered more than 1,500,000 km2 (Richards et al., 1989). Evidently, the event which produced such a large amount of basaltic magma on the continental surface must have had a profound influence on the crustal evolution including the present-day configuration of the continental lithosphere. It has been suggested that igneous intrusion at the base of the crust may underlie all shallow flood basalts (White and McKenzie, 1989a). According to Meissner (1986), zones of low viscosity, such as the continental lower crust and the asthenosphere, are weak and therefore act as the zones of 70’1
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I
78’1
Fig. 2. Tectonic map of the western margin of India showing the three rift basins (Kutch, Cambay and Narmada-Tapti), the major Precambrian tectonic trends and the four DSS profiles across the NarmadaTapti region (modified after Biswas, 1982).
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decoupling, attracting lateral movements of intruding plume material. The Narmada-Tapti region of Deccan volcanic province is no exception. Coffin and Eldholm (1994) presumed a lower crustal body in the form of magmatic underplating beneath the region. Recently Singh and Meissner (1995) based on a 2-D density model along the four deep seismic sounding (DSS) profiles in the region, have suggested a 15-20 km thick accreted igneous layer at the base of the crust. The observation of a high VP-layer in the lower crust along the DSS profiles was used as a strong argument for such a high-density layer and its interpretation as a massive mafic intrusion beneath the region. To better account for the large scale geometry of the proposed crustal accretion, a 3-D density model of the underplated layer is obtained. The model is then used to elucidate an integrated geodynamical evolutionary history of the region.
ANOMALOUS
GRAVITY SlGNATURE
The Bouguer anomaly (BA) map of the Narmada-Tapti region bounded between latitudes 20 to 22”45’ N and longitudes 72”30’ to 79” E along with the four DSS profiles and course of the rivers Narmada and Tapti is shown in Fig. 3. A broad relative gravity high is aligned in the E-W direction between the rivers Narmada and Tapti. The Navsari gravity high, with a maximum value of +40 mgal, forms the western extension of this more-or-less linear gravity high, whereas in the east it is terminated by a NNW-SSE trending gravity low over Pachmarhi. The relative high gravity anomaly, of nearly 100 km in width and extending over a length of about 600 km, is thought to be caused by the magmatic underplating at the base of the crust (Singh and Meissner. 1995).
3-D density model of the accreted igneous layer Delineation of crustal structures responsible for the observed BA is the aim of any gravity modelling. The present investigation is subjected to the 3-D approximation of the proposed underplated layer, expected to be responsible for the referenced anomaly. The shallow structural features are of no interest here, nor would it be appropriate to decipher on such a large scale in
Fig. 3. Bouguer anomaly map of the Narmada-Tapti region along with the DSS profiles and the course of the rivers Narmada and Tapti (modified after Verma and Banerjee, 1992).
3-D structure of accreted igneous layer in NTR
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the absence of detailed geological/geophysical information. Their effects are therefore needed to be adequately removed from the observed BA. It has been proposed by Jacobsen (1987) to use upward continuation as a standard suboptimum filter which can solve a wide span of separation problems when applied to real, non-random anomalies. According to him the optimum filter for the extraction of the field associated to sources below a certain depth (Z,,) is the upward continuation to the height (2Z,,) above the measurement plane. A thickness of 15 km for the horizontal layer representing the upper crust is chosen as a compromise suggested by the 2-D density modelling of Singh and Meissner (1995). The observed BA map is then subjected to the upward continuation to a height of 30 km for the extraction of the field associated to sources below 15 km depth (Fig. 4). Obviously, the separation of the observed BA by upward continuation to the elevation of 30 km is an over simplification of the true geology of the upper crust. At the same time, the attribution of high frequency features at shallow depth, i.e. the upper crust, is strictly valid, it is not necessarily valid to attribute low frequency features to deep-seated structures such as the proposed under-plated layer. It is possible that the low frequency feature of the BA map is not associated with the underplated layer, but in fact caused by some broad, shallower feature within the upper crust. However, in the present case, with the 2-D density modelling along the four DSS profiles (Singh and Meissner, 1995), the uncertainty about the underplated layer responsible for the referenced anomaly is considerably reduced. For a detailed investigation of this filtered component of the BA, the concept of interactive forward gravity modelling developed by Gijtze and Lahmeyer (1988) is applied. In this method the effect on gravity of a homogeneous polyhedra of suitable geometry and density, constrained from the a priori information, is calculated by transforming a volume integral into a sum of line integrals. The DSS and the correlative 2-D density modellings provided the necessary a priori information to well constrain the initial 3-D density model of the underplated layer. Appropriate density values such as 2.87 and 3.22 g cmm3 are attributed to the lower crust and the upper mantle respectively and were held constant during the modelling. Through cooling and contraction, the crustal accretion is expected to produce a homogeneous body at the base of the crust (Meissner and Kopnick, 1988). A density of 2.99 to 3.07 g cm -3 is suggested for such
Fig. 4. 30 km upward continued Bouguer anomaly map of the Narmada-Tapti region.
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accreted igneous layers (White and McKenzie, 1989b). An average density of 3.02 g cm -3 is therefore assigned to the proposed under-plated layer. Initially the geometry of the layer is constrained from the 2-D gravity study of Singh and Meissner (1995). Except in the southwestern part of the region, the computed gravity values with minor adjustments in its subsurface geometry matched fairly well with the filtered BA. The calculated gravity values for the given structure of the underplated layer together with an upwarped Moho in the southwestern part of the region did not conform well with this part of the filtered BA. Trusting the Moho geometry as delineated by the DSS study Singh and Meissner (1995) matched this part of the observed BA by restricting the underplated layer up to the Tapti river, In contrast Gupta and Mishra (1992) and Verma and Banerjee ( 1993) envisaged a greater depth for the underplated layer lying above an almost normal Moho. The underplated layer is therefore extended in southwestern part of the region and its upper and lower boundaries were suitably varied to match the anomaly. Care was, however, taken that the depth to the interfaces were compatible with the earlier gravity studies. The resulting 3-D configuration of the accreted igneous layer is shown in Fig. 5. It clearly depicts the underplated layer with its thickness decreasing gradually towards the east. The thickness of the inferred magmatic layer varies from 8 km beneath the eastern part
Fig. 5. Three dimensional density model of the accreted igneous layer at the base of the crust beneath the Narmada-Tapti region. Contours (in km) show the Moho configuration.
3-D structure of accreted
igneous layer in NTR
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to about 16 km beneath the central part of the region. A thickness of about 24 km below the Navsari window is another local but prominent feature of the underplated layer. Another characteristic feature of the presented 3-D density model is the normal Moho observed beneath the region. A Moho of about 38 km deep beneath the southwestern part of the region is the most significant departure from the DSS observations. A similar depth is also suggested by Gupta and Mishra (1992) and Verma and Banerjee (1993) beneath that region. In contrast the DSS results show a shallow Moho of less than 25 km deep in this region (Fig. 6a). The calculated gravity field in the Navsari-Billimora region for the delineated DSS structure is of the order of +250 mgal (Fig. 6a). Since the DSS result as presented is unable to explain the nature of the observed gravity field, the presence of a very shallow Moho in this region is most unlikely. Though any structural model derived by gravity modelling alone is ambiguous, the negation of a seismic model on the basis of its gravity effect is possible (Holliger and Kissling, 1992; Kissling, 1993). With single-sided wide angle reflection data it may not be possible for the DSS study to resolve the Moho at the end part of the profile. Possibly what has been assumed to be the Moho in the DSS profile-I, actually represents the high-velocity/high-density discontinuity where the crust transforms into the transitional zone. To recheck the presented structure of the underplated layer in southwestern part of the region a unified 2-D infinite length density model with combining the upper crustal structure of Singh and Meissner (1995) along the DSS profile-I is obtained (Fig. 6b). As is apparent, the observed Bouguer gravity field along the profile is well accounted for by the present unified 2-D infinite length density model. It not only favours a normal Moho, about 38 km deep beneath the Navsari-Billimora region, but also suggests that the greater thickness of the inferred magmatic intrusion is the obvious source of the strong positive gravity anomaly over Navsari.
GEOTHERMAL
REGIME
The Cenozoic volcanic continental rifts with igneous intrusion into the rifted crust are characterized by high heat flow (Morgan, 1982). The thermal anomalies in such regions are caused by magmatic mass transfer into the rifted crust and modified by heat transport through fluid convection (Lysak, 1992). Interestingly, the Narmada-Tapti region with Late Cretaceous magmatic crustal accretion is also characterized by a relatively high heat flow regime (Fig. 7). An extensive geothermal regime encompassing the Cambay graben, the Bombay offshore, the Konkan geothermal province and the Narmada-Tapti region appears to be a regional phenomenon in the western margin. Another closely related indicator of intense subsurface thermal activity is the distribution of the hot springs. Their manifestation along the west coast is seen as a region of elevated temperature at depths. It was assumed by Murthy (198 1) that the magma chamber beneath that region has not cooled down completely. For Bose (1972) the observed heat flow values in part of the west coast imply an active mantle similar to mid-oceanic ridges at a relatively shallow depth. The present-day high heat regime, localization of eruptive centres and basic igneous complexes along the faulted western margin and associated rifts (Biswas, 1982) indicate a possible thinning of the lithosphere. In fact Negi et al. (1986) found an upwarped asthenosphere in the Narmada-Tapti region up to a mean depth of about 57 km. The thermal iithosphere beneath the triple junction is found to be extremely thin ( - 40 km; Negi er al., 1992). It is quite probable that the thermal indentation at crustal level caused by upward flow of Deccan magma would subsequently cool losing all of its original heat. However, due to
A. P. Singh
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SOL
(b)
‘?-
N
0 IO 20 30 40
50
Fig. 6. a. Crustal depth section along the Mehmadabad-Billimora (Line- I ) as obtained from the DSS data (modified after Kaila et al., 1981). The Iower refractor segments are shown with velocity in km s - ‘. b. Modified two-dimensional density model of crustal cross-sectiod along the DSS Profile-I. Densities are in g cm -‘. R stands for the river.
3-D structure of accreted igneous layer in NTR
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the extremely thinned thermal lithosphere with possible passive margin conditions at very
shallow depth, the temperature is expected to be still higher over and around the region. DISCUSSION The present gravity analysis and the existing geothermal regime together with the other relevant geophysical information provide a comprehensive evolutionary picture of the under-plated layer beneath the Narmada-Tapti region. One of the most characteristic features of the area is the broad relative gravity high despite the presence of high Satpura mountains (Fig. 2). It is an anomalous feature since a high region is supposed to have a BA low due to expected compensation. On the basis of the qualitative study of the BA along the NSL, Qureshy (1964) concluded that the relative gravity high over the Satpura mountain range indicates a horst type structure. Later the crustal upliftment was associated to the movement due to incorporation of material from the upper mantle into the crust (Qureshy, 1971). In order to satisfy the observed BA, Vet-maand Banerjee (1992) introduced a high-density basic intrusive material at mid-crustal levels without giving any plausible explanation. On the other hand, Singh and Meissner (1993, based on the 2-D density modelling along the four DSS profiles, suggested the igneous crustal 7oq
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4! +
‘70
J
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4p
Fig. 7. Heat flow distribution (in mW m -*) over western India. Solid circles denote the values observed conventionally (Singh and Meissner, 1995) and open circles denote the geochemically estimated heat flow values (Ravi Shanker, 1988). Hot springs are indicated by stars (Krishnaswamy and Ravi Shanker, 1980).
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accretion at the base of the crust beneath the region. The presented 3-D gravity modelling provides a more realistic structure than previous 2-D gravity modelling by gaining third dimension of the underplated layer. The study further suggests a normal crustal thickness along the west coast of India contrary to earlier belief that it represents a highly upwarped Moho beneath that region. On the basis of the DSS study and the correlative 2-D density model, it was suggested that the shallow Moho beneath the southwestern part of the region is the cause for the Bouguer gravity high over Navsari (Singh and Meissner, 1995). In contrast the presented 3-D and 2-D density models clearly indicate that the main contributor to the Navsari gravity high is the greater thickness of the high-density magmatic body reaching up to a depth of about 15 km. The presence of the accreted igneous layer at the base of the crust is also compatible with the high geothermal regime of the Narmada-Tapti region (Fig. 7). According to Ravi Shanker (1988) the high heat flow observations represent an anomalous hot mantle and/or a zone of intense lower crust and upper mantle interaction. A 2-D magnetometer array study has also shown that the Satpura ranges is a locus of internal current concentration for parts of its length (Arora and Reddy, 1991). In view of the known relation between high heat flow and electrical conductivity (Adam, 1978) it was associated with a partial melt zone in the crust or upper mantle beneath the area of the triple junction and under the Satpura ranges (Arora and Reddy, 1991). This intra-crustal thermally disturbed layer with high current concentration conforms well with the present accreted igneous layer. The extremely thinned thermal lithosphere with possible presence of partial melting conditions at very shallow depth are further corroborated by teleseismic tomographic observation in the Deccan volcanic province. The 3-D tomographic study revealed a large scale low velocity zone which persists till at least 200 km beneath the westernmost part of the Deccan volcanic province (Ramesh et al., 1993). A similar low velocity fossil mantle plume beneath South America is recently interpreted as the thermal remnant of the original plume conduit that supplied the Parana plume head (VanDecar et al., 1995). Origin and evolution of the proposed underplated layer in such a thermally disturbed condition may be seen in the Late Cretaceous geodynamics of the Indian subcontinent. The series of southward decreasing age of the submarine volcanic lineaments (the Laccadive, Chagos and Mascarene ridges; Fig. 1) links the Deccan flood basalts to the Reunion hot spot (Richards er al., 1989). Around 65 Ma ago the Deccan plume head mushrooming beneath the west coast modified the former crustal vis-a-vis sublithospheric structure of the Narmada-Tapti region together with the west coast in the low viscosity environment (Singh and Meissner, 1995). The delineated 3-D structure of the underplated layer beneath the Narmada-Tapti region and the fossil mantle plume beneath the west coast are thus the imprint of the Deccan magmatism along the west coast of India. The conceptual model presented in Fig. 8 gives a general 3-D view of the igneous crustal accretion at the base of the crust caused by a rising deep mantle plume. One consequence of the model is the crustal contamination of intruding plume material by getting mixed with melting crustal material of lower melting temperature. The upward flow of this contaminated shallow secondary magma was later facilitated by feeder channels present all along the NSL (Bhattacharji et al., 1996). The model thus provides an alternative explanation for the highly fractionated nature of the Deccan flood basalts (Mahoney, 1988). CONCLUSIONS
The results obtained from the present study are in general accordance with earlier gravity studies. It, however, provides a more realistic approximation than previous 2-D gravity
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Fig. 8. Three dimensional conceptual model of the accreted igneous layer at the base of the crust beneath the survey area. modelling by gaining the third dimension of the underplated layer at the base of the crust. The greater thickness of the accreted igneous layer beneath the Navsari gravity high, the main contributor to the referenced anomaly, presents itself as one of the potential sites for the Deccan magma evolution. The study further modifies the upwarped Moho to a normal depth in the Navsari-Billimora region. The conceptual model for the underplated layer (Fig. 8) provides the right kind of geometrical configuration for the crustal accretion beneath the Narmada-Tapti region. Acknowkdgemenrs-I am grateful to Prof. R. Meissner for his support and encouragement to carry out this work.
Thanks are due to Prof. P Janle for helpful discussions and two anonymous reviewers for useful suggestions. The permission accorded by Prof. H. J. Gbtze to use his computer software for this study is gratefully acknowledged. The work was supported by a grant from Deutscher Akademisher Austauschdienst (DAAD), Germany.
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