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Average crustal density of the Indian Lithosphere—An inference from gravity anomalies and deep seismic soundings
J. GeodynomicsVol. 23, No. I, pp. I-4. 1997 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved P11:S026&3707(%)00025-7 02643707/97 $17.00+0.00
Pergamon
AVERAGE CRUSTAL DENSITY OF THE INDIAN LITHOSPHERE-AN INFERENCE FROM GRAVITY ANOMALIES AND DEEP SEISMIC SOUNDINGS H. V. RAM BABU National Geophysical
Research Institute, Hyderabad-500 007, India
(Received 23 February 1996; accepted in revised form 28 May 1996)
Abstract-In
India, the depths of the Mohorovicic discontinuity at few deep seismic sounding (DSS) profiles are available. A plot of Bouguer gravity anomaly at respective locations has been fitted to a provided a value of 440 kg/m3 for the density contrast between the mantle. For a standard subcrustal density of 3300 kg/m3, the average out to be 2860 kg/m3. Copyright 0 1996 Elsevier Science Ltd
several locations along a these depths against the straight line whose slope crust and the uppermost density of the crust turns
The density structure of the Earth has been derived from the relationships (Nafe and Drake, 1963) established between the seismic wave velocity and the measured densities of various surface and subsurface (from drill cores) rock samples. The average densities of the crust and the subcrustal layer estimated from these relationships are about 2850 and 3300 kg/m3, respectively. In this note, we present the average density of the crust of the Indian lithosphere estimated from Bouguer gravity anomalies and seismically determined crustal thickness. A worldwide study (Woolard, 1959; Sazhina and Grushinsky, 1971) on the relationship between Bouguer gravity anomaly (Ag) and seismically determined crustal thickness (H) revealed that they are linearly related, indicating that isostasy prevails on a regional scale. The linear relationship between Ag and H may be established in the following way. Let H, and H, be the Moho depths at two locations, P,and P,,with corresponding gravity anomalies Ag, and Ag, (Fig. 1). Let a, and a;, be the average densities of the crust and the upper mantle, respectively. Since H,- H,,H, and H,4x,where x is the horizontal extent of the structure involved, the maximum gravity anomaly caused by the plate of thickness H2- H, is given by
(1)
Ag,-Ag,=2nC(u,-~~,)(H,-H,), where G is the Universal
gravitation
constant. It follows from equation (1) that (2)
H,=H,+M(&-Ag,), where M= 1/27rG(u,,, - Us). If H,, is the Moho depth corresponding
to Ag=O, equation (2) may be generalized
H=H,+MAg.
as
(3)
H. V. Ram Babu
H2 Upper
mantle
I
cr,
I
Upper
mantle
Fig. I, The gravity anomaly over a density interface. The gravity anomalies at P, and Pz are related to H, and H? by equation (I ).
Equation (3) represents a straight line with slope M and intercept Ho and forms the basis for the present study. It is interesting to note that the constant M is directly related to the density contrast (u,,, - gC). The constants M and H,,have been estimated by Sazhina and Grushinsky (1971) from 287 stations covering the entire globe and are furnished in Table 1. This table shows that the constants vary from one part of the globe to the other. In India, since 1977, the National Geophysical Research Institute conducted over 20 DSS profiles in various parts of the country and the crustal sections showing the location of the Moho were presented in various publications (Kaila et al., 1979, 1981a, b, 1987). These Moho depths and corresponding Bouguer gravity anomalies for about 60 stations distributed over the Cuddapah basin, the Mahanadi basin, Narmada-Son lineament belt, Deccan traps, Bengal basin, etc., are plotted one against the other and presented in Fig. 2. As expected, the graph of Ag vs H is a straight line with slope M= - 0.054 km/mGal and H,=34 km. The density contrast (g,,, - a,) determined from the constant M is equal to 440 kg/m”. If the average density of the Table 1. Computed coefficients
H,, and M for various parts of the Earth (after Sazhina and Grushinsky,
1971) Number of stations used
Area ..~_ The Earth Land (total) All seas America Eurasia and Africa India (present method)
~_~ ~~_.
~.~ 287 181 106 36 35 60
H0 (km)
M (km/mGal)
35.0 37.5 30.8 31.1 41.4 34.0
0.073 0.059 0.062 0.102 0.033 0.054
Average crustal density of the indian lithosphere
I
0 tD I
0 N I
0
0 (v
0
H. V. Ram Babu
4
uppermost mantle is about 3300 kg/m-‘, the average density of the crust of the Indian lithosphere, estimated from the present analysis, is about 2860 kg/m’. The linear relationship between Ag and H given by equation (3) holds good in the case of a homogeneous crust of density cr,. In reality, the upper crust is highly inhomogeneous resulting in a change of density both laterally and vertically. As the densities of igneous and metamorphic rocks are more than those of sedimentary rocks, the anomalies observed over these terrains are relatively more positive, and vice versa. Therefore. in real situations, the Ag data, when plotted against H, deviate from the straight-line path and appear as scatter (Fig. 2). The data from highdensity regions act to decrease the slope M while the data from the sedimentary regions act to increase the slope. For a well-distributed dataset such as that for India, the overall effect of these inhomogeneties could be minimized by linear least-squares regression. AcknoM,led~emenrs-The author is grateful to the Director, National Geophysical Research Institute, for his kind permission to publish this work. The diagrams were neatly traced by Sri G. Ramachandar Rao.
REFERENCES Kaila K. L., Chowdhury K. R., Reddy P. R., Krishna V. G.. Harinarain, Subbotin S. I., Sollogub V. B., Chekunov A. V., Kharetchko G. E., Lazarenko M. A. and Ilchenko T. V. (1979) Crustal structure along Kavali-Udipi profile in the Indian peninsular shield from deep seismic sounding. J. Geol. Sot. India 20, 307-333. Kaila K. L., Murthy P. R. K., Rao V. K. and Kharetchko G. E. (198la) Crustal structure from deep seismic soundings along the Koyana-II (Kelsi-Loni) profile in the Deccan trap area. Tectonophysics 73, 365-384.
Kaila K. L., Reddy P. R., Dixit M. M. and Lazarenko M. A. (198lb) Deep crustal structure at Koyna, Maharashtra, indicated by deep seismic soundings. J. Geol. Sot. India 22, 1- 16. Kaila K. L., Tewari H. C. and Mall D. M. (1987) Crustal structure and delineation of Gondwana basin in the Mahanadi delta area, India, from deep seismic soundings. J. Geol. Sot. India 29, 293-308.
Nafe J. E. and Drake C. L. (1963) In: The Sea, Vol. 3, pp. 794-815. Interscience, New York. Sazhina N. and Grushinsky N. (197 1) Gravity Prospecting. Mir Publ. Moscow. Woolard G. P. (1959) Crustal structure from gravity and seismic measurements. J. Geophys. Rex 64, 1524- 1544.
Pergamon
AVERAGE CRUSTAL DENSITY OF THE INDIAN LITHOSPHERE-AN INFERENCE FROM GRAVITY ANOMALIES AND DEEP SEISMIC SOUNDINGS H. V. RAM BABU National Geophysical
Research Institute, Hyderabad-500 007, India
(Received 23 February 1996; accepted in revised form 28 May 1996)
Abstract-In
India, the depths of the Mohorovicic discontinuity at few deep seismic sounding (DSS) profiles are available. A plot of Bouguer gravity anomaly at respective locations has been fitted to a provided a value of 440 kg/m3 for the density contrast between the mantle. For a standard subcrustal density of 3300 kg/m3, the average out to be 2860 kg/m3. Copyright 0 1996 Elsevier Science Ltd
several locations along a these depths against the straight line whose slope crust and the uppermost density of the crust turns
The density structure of the Earth has been derived from the relationships (Nafe and Drake, 1963) established between the seismic wave velocity and the measured densities of various surface and subsurface (from drill cores) rock samples. The average densities of the crust and the subcrustal layer estimated from these relationships are about 2850 and 3300 kg/m3, respectively. In this note, we present the average density of the crust of the Indian lithosphere estimated from Bouguer gravity anomalies and seismically determined crustal thickness. A worldwide study (Woolard, 1959; Sazhina and Grushinsky, 1971) on the relationship between Bouguer gravity anomaly (Ag) and seismically determined crustal thickness (H) revealed that they are linearly related, indicating that isostasy prevails on a regional scale. The linear relationship between Ag and H may be established in the following way. Let H, and H, be the Moho depths at two locations, P,and P,,with corresponding gravity anomalies Ag, and Ag, (Fig. 1). Let a, and a;, be the average densities of the crust and the upper mantle, respectively. Since H,- H,,H, and H,4x,where x is the horizontal extent of the structure involved, the maximum gravity anomaly caused by the plate of thickness H2- H, is given by
(1)
Ag,-Ag,=2nC(u,-~~,)(H,-H,), where G is the Universal
gravitation
constant. It follows from equation (1) that (2)
H,=H,+M(&-Ag,), where M= 1/27rG(u,,, - Us). If H,, is the Moho depth corresponding
to Ag=O, equation (2) may be generalized
H=H,+MAg.
as
(3)
H. V. Ram Babu
H2 Upper
mantle
I
cr,
I
Upper
mantle
Fig. I, The gravity anomaly over a density interface. The gravity anomalies at P, and Pz are related to H, and H? by equation (I ).
Equation (3) represents a straight line with slope M and intercept Ho and forms the basis for the present study. It is interesting to note that the constant M is directly related to the density contrast (u,,, - gC). The constants M and H,,have been estimated by Sazhina and Grushinsky (1971) from 287 stations covering the entire globe and are furnished in Table 1. This table shows that the constants vary from one part of the globe to the other. In India, since 1977, the National Geophysical Research Institute conducted over 20 DSS profiles in various parts of the country and the crustal sections showing the location of the Moho were presented in various publications (Kaila et al., 1979, 1981a, b, 1987). These Moho depths and corresponding Bouguer gravity anomalies for about 60 stations distributed over the Cuddapah basin, the Mahanadi basin, Narmada-Son lineament belt, Deccan traps, Bengal basin, etc., are plotted one against the other and presented in Fig. 2. As expected, the graph of Ag vs H is a straight line with slope M= - 0.054 km/mGal and H,=34 km. The density contrast (g,,, - a,) determined from the constant M is equal to 440 kg/m”. If the average density of the Table 1. Computed coefficients
H,, and M for various parts of the Earth (after Sazhina and Grushinsky,
1971) Number of stations used
Area ..~_ The Earth Land (total) All seas America Eurasia and Africa India (present method)
~_~ ~~_.
~.~ 287 181 106 36 35 60
H0 (km)
M (km/mGal)
35.0 37.5 30.8 31.1 41.4 34.0
0.073 0.059 0.062 0.102 0.033 0.054
Average crustal density of the indian lithosphere
I
0 tD I
0 N I
0
0 (v
0
H. V. Ram Babu
4
uppermost mantle is about 3300 kg/m-‘, the average density of the crust of the Indian lithosphere, estimated from the present analysis, is about 2860 kg/m’. The linear relationship between Ag and H given by equation (3) holds good in the case of a homogeneous crust of density cr,. In reality, the upper crust is highly inhomogeneous resulting in a change of density both laterally and vertically. As the densities of igneous and metamorphic rocks are more than those of sedimentary rocks, the anomalies observed over these terrains are relatively more positive, and vice versa. Therefore. in real situations, the Ag data, when plotted against H, deviate from the straight-line path and appear as scatter (Fig. 2). The data from highdensity regions act to decrease the slope M while the data from the sedimentary regions act to increase the slope. For a well-distributed dataset such as that for India, the overall effect of these inhomogeneties could be minimized by linear least-squares regression. AcknoM,led~emenrs-The author is grateful to the Director, National Geophysical Research Institute, for his kind permission to publish this work. The diagrams were neatly traced by Sri G. Ramachandar Rao.
REFERENCES Kaila K. L., Chowdhury K. R., Reddy P. R., Krishna V. G.. Harinarain, Subbotin S. I., Sollogub V. B., Chekunov A. V., Kharetchko G. E., Lazarenko M. A. and Ilchenko T. V. (1979) Crustal structure along Kavali-Udipi profile in the Indian peninsular shield from deep seismic sounding. J. Geol. Sot. India 20, 307-333. Kaila K. L., Murthy P. R. K., Rao V. K. and Kharetchko G. E. (198la) Crustal structure from deep seismic soundings along the Koyana-II (Kelsi-Loni) profile in the Deccan trap area. Tectonophysics 73, 365-384.
Kaila K. L., Reddy P. R., Dixit M. M. and Lazarenko M. A. (198lb) Deep crustal structure at Koyna, Maharashtra, indicated by deep seismic soundings. J. Geol. Sot. India 22, 1- 16. Kaila K. L., Tewari H. C. and Mall D. M. (1987) Crustal structure and delineation of Gondwana basin in the Mahanadi delta area, India, from deep seismic soundings. J. Geol. Sot. India 29, 293-308.
Nafe J. E. and Drake C. L. (1963) In: The Sea, Vol. 3, pp. 794-815. Interscience, New York. Sazhina N. and Grushinsky N. (197 1) Gravity Prospecting. Mir Publ. Moscow. Woolard G. P. (1959) Crustal structure from gravity and seismic measurements. J. Geophys. Rex 64, 1524- 1544.