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Is the Indian Shield hotter than other Gondwana shields?
Earth and Planetary Science Letters, 115 (1993) 275-285
275
Elsevier Science Publishers B.V., Amsterdam [DT]
Is the Indian Shield hotter than other Gondwana shields? Mohan L. Gupta Geothermal Group, National Geophysical Research Institute, Hyderabad, 500 007, India
Received November 7, 1991; revision accepted December 15, 1992
ABSTRACT Geothermal data on various Precambrian terrains from the African, Australian, Indian and South American (Brazil only) Gondwana landmasses have been compiled, synthesised and statistically analysed. The results do not support the prevailing notion that the Indian Shield is hotter than other shields. The study clearly shows that the mean surface heat flow values from the various Precambrian cratons and mobile belts of the Indian landmass for which the data have become available are either equal to, or even lower in some cases, than that in similar terrains from other Gondwana continents. Further, on the basis of available data, it is found that the Moho and the reduced heat flow values and Moho temperatures in the South Indian, South African, Western Australian and Brazilian shields fall within a narrow range, thus indicating, within the error limits of the estimation, the similarity of these shields in terms of these characteristics. In conclusion it is shown that the Indian landmass is not hotter than the other Gondwana landmasses, including even the presently immobile African continent, and that the "super-mobility" of the Indian landmass does not appear to be associated with its thermal characteristics. The cause of the latter lies elsewhere.
I. Introduction Understanding the temperature distribution within t h e e a r t h so as to o b t a i n an insight into its t h e r m a l a n d d y n a m i c e v o l u t i o n has b e e n a m a i n goal o f g e o t h e r m a l r e s e a r c h over t h e last t h r e e d e c a d e s . In a d d i t i o n , t h e i n t e r n a l h e a t o f t h e earth, which is a m a j o r e n e r g y c o m p o n e n t in g e o d y n a m i c m o t i o n , c o n t r o l s t h e physical state and the tectonic processes of the lithosphere. Consequently, the heterogeneties of the earth's s t r u c t u r e a r e r e f l e c t e d in t h e d i s t r i b u t i o n of t h e h e a t flow field. Efforts, t h e r e f o r e , to d e t e r m i n e t h e surface h e a t flow in an o p t i m u m n u m b e r o f l o c a t i o n s in v a r i o u s g e o t e c t o n i c units of t h e e a r t h have b e e n vigorously u n d e r t a k e n t h r o u g h o u t t h e world, ever since t h e first r e l i a b l e m e a s u r e m e n t s w e r e m a d e in S o u t h A f r i c a by B u l l a r d [1] a n d in E n g l a n d by B e n f i e l d [2]. T h e global surface h e a t flow d a t a s e t has greatly e x p a n d e d d u r i n g the last two d e c a d e s , a n d this is also t r u e for s o m e o f t h e Gondwana landmasses. A p a r t f r o m acting as a d i a g n o s t i c m e a n s o f studying t h e t e c t o n i c n a t u r e o f a given p a r t o f t h e earth, w h e n c o m b i n e d a n d s t u d i e d a g a i n s t t h e
b a c k d r o p o f o t h e r geological, geophysical a n d g e o c h e m i c a l i n f o r m a t i o n surface h e a t flow d a t a a r e very i l l u m i n a t i n g in a s c e r t a i n i n g t h e evolut i o n a r y p r o g r e s s i o n o f various e a r t h processes. H o w e v e r , t h e r e a r e v a r i o u s d a n g e r s t h a t m u s t be avoided, a n d t h e lack o f an a d e q u a t e d a t a s e t c o m b i n e d with o t h e r factors can s o m e t i m e s l e a d to fantasies t h a t w h e n s t u d i e d in d e p t h t u r n o u t to b e u n f o u n d e d . Such has b e e n t h e case with the t h e r m a l c h a r a c t e r i s t i c s of t h e I n d i a n l a n d m a s s . E a r l i e r workers, n o t a b l y R a o et al. [3] i n d i c a t e d a " h o t t e r " u p p e r m a n t l e b e n e a t h the I n d i a n Shield, while N e g i et al. [4] a t t e m p t e d to show t h a t t h e I n d i a n l i t h o s p h e r e is quite hot, a n d t h a t its thickness is a b o u t o n e t h i r d of t h a t of o t h e r shields, including t h e i m m o b i l e , cold, thick A f r i c a n lithos p h e r e , an i n f e r e n c e which a c c o r d i n g to t h e m [4] has i m p o r t a n t c o n s e q u e n c e s for the mobility of the Indian subcontinent. W i t h t h e i n c r e a s e in surface h e a t flow d a t a m e n t i o n e d a b o v e in m i n d an a t t e m p t is m a d e in this p a p e r to compile, synthesise a n d c o m p a r e t h e surface h e a t flow a n d o t h e r r e l e v a n t d a t a o f t h e various P r e c a m b r i a n t e r r a i n s o f G o n d w a n a l a n d m a s s e s as a first s t e p to a s c e r t a i n i n g w h e t h e r
0012-821X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
276
M.L. G U P T A
TABLE 1 Surface heat flow in various geological provinces of Gondwana landmasses S. No., landmass and geological province/unit
Age
N
mWm
2
Mean surface heat flow
S.D.
Range
A. India
1. Dharwar Craton (Cr) 1.1 Platform cover over Dharwar Craton 1.1.1 Cuddapah Basin 1.1.2 Deccan Traps 1.2 Values for all of the above 2. Granulite Terrane, South India 3. Bundelkhand-Aravalli Cr 4. Bastar Craton 5. Singhbhum Thrust Z o n e 6. Delhi Mobile Belt 7. Trans-Aravalli region 8. All Precambrian except 1.1.2 and 7 9. Phanerozoic cover 9.1 Gondwana grabens
9.2 Oil basins 9.3 Trap cover 9.4 Values for all of the above from 9.1 to 9.3 10. All data for the above (1 to 9)
Archaean
15
38
5.4
28-46
M i d - L . Prot. U. C r e t - E . Paleo.
53 44 41 55
20.1 4.2 11.0
27-75 41-47 27-75
Proterozoic
4 2 21 1
A r . - E . Prot. Proterozoic Proterozoic M i d - L . Prot. L. Prot.? Precambrian
9 3 3 10 1 45
41 56 61 62 95 48
2.1 7.2 2.0 12.5
38-44 51-64 59-63 44-74
13.4
27-75
Prot. a n d / o r PermoCarb.-L. Cret. sediments Tertiary U. Cret.-E. Paleo
18
64
15.5
49-107
12 2 32
71 44 65
14.1 4.24 15.6
50-93 41-47 41-107
78
56
17.0
27-107
15 1
37.3 46.0
6.0
29-54
4 6 5 3
77.8 82.3 73.8 80.7
26.7 24.0 19.4 4.9
48-113 49-109 45-96 75-84
32 38
83.0 72.0
19.6 26.0
48-125 45-121
91 53
68.0 65.0
33.0 24.0
29-125 29-113
7
33.0
1.5
31-36
11 71
45.5 52.1
5.5 5.97
36-54 36-72
B. Australia
11. Western Shield 11.1 Yilgarn C r a t o n / B l o c k 11.2 Hamersley Basin 12. Central Shield 12.1 Northern Craton 12.2 Gawler Craton 12.3 Phanerozoic cover 12.4 Mt. Isa Mobile Belt 12.5 All available values from Central Shield 13. Eastern Australia 14. Australian landmass 14.1 All values 14.2 All Precambrian areas C. Africa 15. Kaapvaal Craton, South Africa 15.1 Witwatersrand Basin (WB) 15.1.1 Hartebeesfontein and Ventersdrop granitic domes 15.1.2 Classical data * WB 15.1.3 Estimates from documented temperature data in WB
Archaean E. Prot. Proterozoic Proterozoic Precam. basement Mid Prot. Proterozoic Phanerozoic (Palaeozoic?)
Precambrian
Archaean
L. A r . - E . Prot. L. A r . - E . Prot.
277
IS THE INDIANSHIELDHOTTERTHAN OTHER GONDWANASHIELDS? TABLE 1 (continued) S. No., landmass and geological province/unit
Age
15.1.4 Within the WB after removing the effect of strata 15.1.5 All heat flow data in WB 16. Zimbabwe Craton, S. Africa 16.1 Tati Greenstone Belt 16.2 Near the border of a mobile belt in granitic gneisses 17. Mobile belts, S. Africa 17.1 Namaqva-Natal Belt 17.2 Ganzi-Chobe Belt 17.3 Damara-Katanga Belt 18. West African ShieM / Craton 18.1 Man Shield 18.2 Man Shield 18.3 Platform cover 18.4 All values for the above 19. All values from Africa 19.1 All Precambrian areas, excluding areas of Pan-African Orogeny
L. Ar.-E. Prot.
Ar.-E. Prot.
D. South Africa 20.1 S~o Francisco Craton (SFC) 20.2 Mobile belt encircling SFC 20.3 All values for the above 20.4 Parana Basin 20.5 Pocosde Caldas
mW m -2
N
Mean surface heat flow
S.D.
Range
81
43.5
7.8
36-72
89
50.0
7.8
31-72
1 1
37.0 65.0
16 5 17
62.0 63.0 66.0
11.4 6.0 10.0
39-81 56-71 54-92
5 15 19 39 313 142
39.0 30.0 71.0 51.0 58.0 46.0
10.0 27.0 27.0 24.0 17.0
21-194 31-81
Ar.-E. Prot.
3
42.0
8.0
37-51
Proterozoic
8
55.0
11.0
38-69
11 3 5
51.0 68.0 74.0
11.3 13.9 10.0
37-69 52-77 53-94
Archaean Archaean
Proterozoic Proterozoic Pan-African Archaean Mid Prot. U. Prot.-Palaeoz.
Phanerozoic Intrusive, Permian-E. Tert.
6.0
Ar. = Archaean; Prot. = Proterozoic; Permo-Carb. = Permo-Carboniferous; Palaeoz. = Palaeozoic; Cret. = Cretaceous; Tert. = Tertiary; Paleo. = Paleocene. Qs values have been compiled from a large number of original papers, data summary papers and data compilations: India [11,12,13,34,35]; Australia [14]; South America [15,16]; Africa [courtesy of H.N. Pollack, and 5-10]. * Determined from temperature logs and thermal conductivity data.
the presently prevailing belief that the Indian S h i e l d is h o t t e r a n d t h i n n e r t h a n t h e o t h e r s h i e l d s is j u s t i f i e d .
2. Pattern of surface heat flow field in G o n d w a n a Precambrian shields H e a t f l o w d a t a sets h a v e g r e a t l y e x p a n d e d in t h e last d e c a d e e s p e c i a l l y f o r t h e A f r i c a n a n d I n d i a n l a n d m a s s e s [5-13]. Sass a n d L a c h e n b r u c h [14] h a v e g i v e n a d e t a i l e d a c c o u n t o f t h e t h e r m a l
r e g i m e o f t h e A u s t r a l i a n c o n t i n e n t a l crust, a n d V i t o r e l l o e t al. [15] a n d H a m z a [16] h a v e p r o v i d e d the data for the Brazilian Coastal Shield. The h e a t f l o w d a t a in all t h e s e w o r k s , t o g e t h e r w i t h a compilation of heat flow data for the African c o n t i n e n t as p r o v i d e d by H . N . P o l l a c k , h a v e b e e n u s e d in t h e p r e s e n t p a p e r . I n d i v i d u a l r e f e r e n c e s will n o t b e e x t e n s i v e l y c i t e d in t h e text, a n d t h e r e a d e r s h o u l d r e f e r to t h e s e p u b l i c a t i o n s for further details on the data. A common feature of the growth of the conti-
278
M.L. GUPTA
nental crust is a craton-mobile belt association. Most Precambrian shields of the earth exhibit numerous elliptical to subcircular Archaean or Proterozoic cratons that are exposed over great distances (600-1500 km) together with younger pericratonic mobile belts [17]. This aspect is borne in mind while classifying the heat flow data of the Precambrian terrains of the Gondwana landmasses. A mean heat flow value calculated giving weight to the areal extent of different rock units
present in a region of interest is a most appropriate unit for comparison. However, bearing in mind the obvious practical difficulties, such an attempt has not been made. The general practice, followed by almost all, is to use, for various purposes, simple arithmetic mean values of the heat flow data sets. The same are, therefore, computed for the Indian, Australian, South American (Brazilian Coastal Shield only) and African shields for their separate segments (i.e., the Archaean-Early Proterozoic cratons, Pro-
TABLE 2 Surface heat flow in various Precambrian terranes and platform regions of Gondwana landmasses S. No. and geological p r o v i n c e / u n i t
Mean surface heat flow ( m W m -2) N
Qs
Data points in Fig. 2
S.D.
1. Archaean-Early Proterozoic cratons
1.1 Dharwar Craton (India) 1.2 Bundelkhand-Aravalli Craton (India) 1.3 Yilgarn C r a t o n / B l o c k (West Australia) 1.4 Kaapvaal Craton (South Africa) 1.5 West African Craton (Man Shield, Archaean) 1.6 Silo Francisco Craton (South America)
15 9 15 89 5 3
38 41 37 50 39 42
5.4'1 2.1 6.0 7.8 6.0 8.0
3 6 4
56 82 78
7.2 24.0 26.7
10 3 16 5 17
62 61 62 63 66
12.5~ 2.0 11.4 6.0 10.0
8 3
55 81
11.0 4.9)
32 5 19 3
65 74 71 68
15.6 19.4 27.3 13.9
Points 15, 16 and 17 are for data pairs of 4.1 with 4.2, 4.3 and 4.4 respectively
45 142 11 53
48 46 51 65
13.4 17.0 11.3 24.0
Points 18, 19 and 20 are for data pairs of 5.1 with 5.2, 5.3 and 5.4 respectively
78 91 313
56 68 58
17.0 33.0 24.0
Points 1, 2, 3 and 4 are for data pairs of 1.1 with 1.3, 1.4, 1.5 and 1.6 respectively. Points 5, 6, 7 and 8 are for data pairs of 1.2 with 1.3, 1.4, 1.5 and 1.6 respectively
2. Proterozoic cratons
2.1 Bastar Craton (India) 2.2 Gawler Craton (Central Australia) 2.3 Northern Craton (Central Australia) 3. Proteroznic mobile belts (MB) 3.1 Delhi MB (India) 3.2 Singhbhum Thrust Zone MB (India) 3.3 N a m a q u a - N a t a l MB (South Africa) 3.4 G a n z i - C h o b e MB (South Africa) 3.5 D a m a r a - K a t a n g a MB (Pan-African Orogeny) 3.6 Late Precambrian Brazilian metamorphic b e l t / M B encircling craton 3.7 Mt. Isa Mobile Belt (Central Australia)
Points 9 and 10 are for data pairs of 2.1 with 2.2 and 2.3 respectively
Points 11, 12, 13 and 14 are for data pairs of 3.1 with 3.3, 3.4, 3.5 and 3.7 respectively
4. Phanerozoic cover
4.1 4.2 4.3 4.4
India Central Australia Cover over West African Craton Parana Basin, Brazil
5. All Precambrian areas
5.1 5.2 5.3 5.4
India Africa Brazil (South America) Australia
6. All data
6.1 India 6.2 Australia 6.3 Africa
Points 21 and 22 are for data pairs of 6.1 with 6.2 and 6.3 respectively, and point 23 is that for a data pair of 6.2 and 6.3
IS THE INDIAN SHIELD HOTTER THAN OTHER GONDWANA SHIELDS?
terozoic cratons, and Proterozoic mobile belts, Table 1 and 2). Mean values for the Phanerozoic parts and of the total data set of the above-mentioned landmasses are also given in Tables 1 and 2. Mean heat flow values together with their standard deviations are shown in Fig. 1. The heat flow data for lakes Malawi, Tanganayika and Kivu of the African landmass have not been considered as the values are of variable quality and show large scatter due to various structural and other features.
279
than the value read from the tables of t-distribution we reject the hypothesis that /,~ = tx 2. The higher the calculated value of t than its theoretical value, the more reasonable it would be to reject the hypothesis ~1 =/*z in favour of the alternative that IZl >/*2 at the specified significance level. A curve showing t values read from the table of t-distribution for the appropriate degrees of freedom at the 5% significance level and the values of t calculated by using eq. (2) for various pairs of data (Table 2) is plotted in Fig. 2.
3. Statistical treatment of the data 4. Moho heat flow
A hypothesis concerning the difference between the means of two populations can be statistically tested. We assume that each heat flow data set as given in Tables 1 and 2 represents random samples drawn from normal populations. Let Q1 and Q2 be the arithmetic means of the two available heat flow data sets treated as two random samples drawn from two normal populations with mean heat flow values of /z I and /~2 respectively. We now test the null hypothesis, H 0 that /x 1 = ~ 2. According to Anderson and Bancroft [18] the test criterion on the assumption that the hypothesis tested (/z 1 =/*2) is true is: I Q1 - Q2[
t= S(l/nl
+ l/n2
(1)
where $2 =
(nl
-- 1)$12 -f- (/'/2 -- 1 ) $ 2
(n 1 + n 2 - 2) (where n 1 and 172 are the number of observations in each dataset having sample variances S~ and S 2 respectively), or t=
I Q1 - Q2 I Cnln2(nl + n 2 -
2)
(2)
¢(n 1 + n2) • ¢(n 1 - 1 ) S ( + (n 2 - 1)S 2 In practice, by using eq. (2) the above t statistic is computed for the available number of heat flow data pairs and compared with the theoretical value read from tables of the t-distribution for n I + n 2 - - 2 degrees of freedom at the 5% significance level. If the computed value of t is greater
Despite the fact that estimation of heat flow into the base of the cust is not a straightforward process attempts have been made to obtain reasonable estimate of its magnitude. According to Roy et al. [19,20] and Lachenbruch [21], reduced heat flow (Qr) is the contribution from the deep crustal a n d / o r mantle sources. However, it has been shown [22], based on numerical simulations of crustal thermal models, that Qr appears to be the mean heat flow at the characteristic depth D and that the general conclusion that Qr is an acceptable estimate for continental mantle heat flow does not appear to be valid. Ashwal et al. [23] have also indicated that lower crust heat production would be represented in the reduced heat flow component, and Moho heat flow could be significantly (8-10 mW m - z ) lower than Qr. Values of Qr and reasonable estimates of Moho heat flow are available only for some of the shield units of the Gondwana landmasses (Table 3). 5. Thermal structure of the crust and Moho temperatures
The one-dimensional steady-state crustal geotherms and Moho temperatures in a conductive regime for and beneath various cratonic blocks were calculated. Standard procedures [21,14,12,6] for calculating crustal temperatures were used and the available radioactive heat production models for the respective cratons were considered (Table 3). The crust is assumed to have a uniform conductivity of 2.5 W m - I K - 1 . As the models of radioactive heat source distribution
280
M.L. GUPTA
~
A
6~ -
6
b
fl:
120 I.-
o~ ~E
296
100
,u =
"2.
E~
u;80
~o
~o "6
_
6O
~
~
o
i
~
O
O
i
4O
I
o~dd
d
20
Arch I EP'°'I r°'cro,on. Cro,o°.
pr0,.ob,..,,.
II ,on.r=0c0., II
A 0°'°
I
Fig. 1. Surface heat flow in various regions. Vertical bars represent the mean _+1 S.D. For the Kaapvaal Craton the data plotted are those of 15.1.4 and 15.1.5 in Table 1. For details, refer to this table and Table 2. B U N D - A R A V = Bundelkhand-Aravalli Craton. The South American Proterozoic mobile belt data are for Brazil.
for the various cratons of the present study show no abnormal distribution [6,12,14,15] their thermal structure would be adequately represented by the Moho temperatures. The latter, therefore,
600 023
200
- -
°21 o6
tOO
E 0
m
Fig. 2. Comparison of mean heat flow values of various data pairs of Gondwana landmasses. The curve shows the theoretical values of t for various degrees of freedom at the 5% significance level. © = computed values of t using eq. (2) for various mean heat flow data pairs and for appropriate degrees of freedom (see last column of Table 2) for the data pairs:
¢
4o °17
°15 o 13
2o
o3
o ~"
o 02 20
A r c h a e a n - E a r l y Proterozoic eratons: l ( D C - Y C); 2 ( D C K C ) ; 3 (D C - M S); 4 (D C-SFC); 5 ( B - A C - Y C); 6 (B-
o7
12 o8
IO-
D
o5
% °14
-
~9
4 -
2I
0.1
I
[ I I I Ill 0'4 I'0
~ t
I 2"0
II 6"0
A C - K C); 7 ( B - A C - M S); 8 ( B - A C-SFC). Proterozoie eratons: 9 (B C - G a C); 10 (B C - N o C). Proterozaic mobile belts: 11 (D M B - N - N MB); 12 (D M E - G - C MB); 13 (D M B - D - K MB); 14 (D MB-Mt. Isa MB). Phanernzoic cover: 15 (Ind.-C. Aust.); 16 (Ind.-W. Af. C); 17 (Ind.-Pa. Brz.). All P r e e a m b r i a n areas: 18 (Ind.-Af.); 19 (Ind.-Brz.); 20 (Ind.-Aust.). All data: 21 (Ind.-Aust.); 22 (Ind.-Af.); 23 (Aust.-Af.). The abbreviations of the various cratons (C) and mobile belts (MB) may be obtained by inspecting the full n a m e s in Fig. 1. W. Af, C = W e s t Africa Craton; Pa. Brz. = Parana Basin, Brazil. Other abbreviations are self-explanatory.
281
IS T H E INDIAN SHIELD H O T I ' E R T H A N O T H E R G O N D W A N A SHIELDS?
TABLE 3 Thermal parameters and Moho temperatures at 40 km deep for and beneath Archaean/Archaean-Early Proterozoic cratons of Gondwana landmasses Craton
1. Dharwar Craton (India) 2. Kaapvaal Craton (South Africa) 3. Yilgarn Craton (Western Australia) 4. Silo Francisco Craton (Brazil)
Heat flow (mW m-2) Qs + S.D. Qr Qs-Qr
Qm
D (km)
11.5
Moho temperature (TmX°C)based on:
OX
QrXX
332-404
368-445
Temperature variation w.r.t. DC
38 + 6
23
15
15-17
50 _+8
-
-
12-17
39 _+8
26
13
-
4.5
-
416-471
+13- +6%
42+8 ~
28
14
-
13.1
-
448-552
+22-+24%
-
360-457
-
+ 8-+13%
x Moho temperatures were calculated using the layer model [12] for two values of surface heat flow (Qs) and using a Moho heat flow of Om= 17 mW m -2. For Dharwar Craton Qs = 29 and 40 mW m -2 and crustal radioactivity models as given by Gupta et al. [12] have been considered. For Kaapvaal Craton Qs = 33 and 51 mW m 2 and crustal radioactivity models as considered by Jones [6] have been used. xxMoho temperatures were calculated after considering exponential radioactive heat source distribution and using the given values of the heat flow parameters (Qr and D). The lower Moho temperature for each craton is obtained for the lowest possible value of As, i.e. A s = 0; and the higher Moho temperature is for the reported highest value of A s in the relevant Qs-As pairs. For Dharwar Craton A s = 1.5/xW m -3 [12]; for Yilgarn Craton A s = 6.82/xW m 3 [14]; for Sio Francisco Craton As = 1.6/xW m 3 [15]. (A s = radioactive heat production in surface/near-surface rocks.) a Calculated using data from Archaean-Early Proterozoic terrains only.
have b e e n c o n s i d e r e d for discussion a n d are given in T a b l e 3.
6. Discussion I n o r d e r to ascertain w h e t h e r the I n d i a n Shield is hotter t h a n o t h e r G o n d w a n a Shields we shall now e x a m i n e the following questions: (i) w h e t h e r the surface heat flow in the I n d i a n Shield is higher t h a n in the o t h e r G o n d w a n a shields a n d (ii) how the M o h o heat flow, the r e d u c e d heat flow a n d the M o h o t e m p e r a t u r f s b e n e a t h the G o n d w a n a shields differ from each other. T h e heat flow data s u m m a r i s e d in T a b l e 1 a n d 2 a n d p l o t t e d in Fig. 1 show that the heat flow m e a n s (Qs) for similar geological terrains of various G o n d w a n a l a n d m a s s e s mostly fall within o n e s t a n d a r d deviation of each o t h e r a n d t h e r e f o r e c a n n o t be c o n s i d e r e d as significantly different from each other. T h e A r c h a e a n - E a r l y Proterozoic c r a t o n s of the four G o n d w a n a l a n d m a s s e s are c h a r a c t e r i s e d by low heat flow a n d generally by a small v a r i a t i o n of values a r o u n d their means. T h e surface heat flow values from Proterozoic mobile belts a n d P h a n e r o z o i c p l a t f o r m covers
show n o a p p r e c i a b l e differences. F u r t h e r , the results of statistical t r e a t m e n t of the surface heat flow data have clearly shown that the calculated values of the t statistic are lower t h a n the values of t read from the tables a n d therefore plot o n the l e f t - h a n d side of the theoretical curve of the t statistic (Fig. 2), t h e r e b y d e m o n s t r a t i n g that there is n o significant difference b e t w e e n the m e a n heat flow values of such data pairs. This has b e e n tested at the 5% significance level. I n the p r e s e n t case, this implies that the Qs values of similar geological terrains (i.e., of the A r c h a e a n a n d Proterozoic cratons, of the Proterozoic mobile belts a n d of the P h a n e r o z o i c areas) of the I n d i a n , A u s t r a l i a n , A f r i c a n a n d Brazil l a n d m a s s e s are mostly equal at the 5% significance level. However, there are c e r t a i n data pairs, r e p r e s e n t e d by points 2, 6, 14, 20, 21 a n d 23, which plot away from the theoretical t curve o n its r i g h t - h a n d side (Fig. 2), t h e r e b y i n d i c a t i n g that the m e a n s of the data sets r e p r e s e n t e d by these respective pairs (2, 6, 14, 21, 20 a n d 23) are not equal. If we c o m b i n e the above i n f e r e n c e d r a w n o n the basis of statistical analysis of data with values given in T a b l e 2, we find the following:
282
(i) The Qs value of the Kaapvaal Craton (South Africa) is significantly higher than the Qs values of the Dharwar and Bundelkhand-Aravalli cratons of the Indian landmass (Fig. 2, points 2 and
6). (ii) The Qs value for the Proterozoic Mt. Isa Mobile Belt (Central Australian Shield) is higher than for the Proterozoic Delhi Mobile Belt (India) (Fig. 2, point 14). (iii) Qs values for all Precambrian sites, including those for all the available sites in the Australian landmass, are statistically higher than the values for the terrains of the Indian and African landmasses (Table 1 and 2; points 20, 21 and 23; Fig. 2). The heat flow data of the West African Shield show a peculiar distribution. Whereas elsewhere the Archaean-Early Proterozoic terrains are associated with lower heat flow values than in the Middle-Late Proterozoic terranes, the data reveal that this is not the case for the West African Shield, where its former terrain types show values that are higher than in its latter terrain types (the pattern of heat flow is reversed). The Archaean terrains and Phanerozoic platform areas of the West African Shield, however, are characterised by the same order of heat flow values as their counterpart terrains in India and Australia and Brazil (Table 2). It is worth noting that if one considers the surface heat flow values both for all the available sites and separately for all the Precambrian terrains in the Indian and the African landmasses the Q~ values for these terrains (Table 2) are statistically equal (Fig. 2, points 22 and 18). The foregoing discussion leads to the clear result that the mean surface heat flow values for the various geological terrains of the Indian landmass, for which data are available, are either equal to or lower than in terrains of similar geological character of the African and Australian landmasses and of Brazil. The exception to this observation, which is not restricted to India but may also be applied to Australia, Brazil and other cratonic parts of the African landmass is that the Middle Proterozoic part of the Man Shield (West African Craton) is characterised by Q~ values that are lower than in such terrains of the other Gondwana landmasses. However, this observation does require careful examination of
M.L. GUPTA
all the available data for the West African Craton [10,24], as a high heat flow value of 59 mW m 2 has been reported in its Upper Proterozoic terrains [24]. The Moho heat flow (Qm) and the reduced heat flow (Qr) and the radioactive component of heat flow in the top layers of the crust (Qs - Qr) for all four A r c h a e a n / A r c h a e a n - E a r l y Proterozoic cratons of the Gondwana landmasses (see Table 3) are more or less equal. It is worth noting that these four cratons (Table 3), which include large parts of the Indian, Australian, African and South American landmasses, clearly demonstrate similarity in their deep thermal characteristics. The temperature at the Moho (Tm) beneath the Dharwar Craton (India) is generally lower (by about 6-24%) than beneath the other A r c h a e a n / A r c h a e a n - E a r l y Proterozoic cratons of the Gondwana landmasses (Table 3). However, considering the range of values of the estimated Tm and the inherent problems faced and assumptions made in making such estimates, it is quite obvious that based on the present state of knowledge the values of Tm for all the four Precambrian Shields (South Indian, South African, Western Australian and Brazilian) are equal, as their values fall within the error limits of estimation. Therefore, based on Tm too it is not possible to infer that the Indian Shield is hotter than the other three Gondwana shields. Jones [5] and Gupta et al. [12] have reported that the thickness of the thermal lithosphere beneath the South African and South Indian shields is over 200 km. Additional support for thick lithosphere beneath the Dharwar Craton has come from the studies of Srinagesh et al. [25]. Through their analysis of teleseismic P-wave arrivals recorded at South Indian seismological stations they have inferred the presence of an anomalously thick high-velocity region in the depth range of 100-300 km below the Dharwar Craton (with the exception of beneath the westernmost part of the Deccan Traps region). According to Ganguly et al. [26] the peridotite nodules from the Proterozoic kimberlite pipes of South India are derived from various depths between 125 and 200 km. The presence of Late Proterozoic kimberlite pipes in the Bundelkhand-Aravalli Craton also points towards the presence of a thick lithosphere beneath it at that time. In addition, xenolith
IS T H E I N D I A N S H I E L D H O T F E R
THAN OTHER
GONDWANA
SHIELDS?
studies have confirmed the presence of thick root zones beneath the Kaapvaal Craton (South Africa) [27]. Based on the results of the synthesis of the surface heat flow data it may be easily understood, and can also be easily shown that the thicknesses of the thermal lithosphere beneath the Indian, African and Australian landmasses and of Brazil may not be significantly different from each other. From the above discussion it is concluded that the "super-mobility" of the Indian landmass cannot be ascribed to its thermal characteristics, as its lithosphere is neither hotter nor thinner than the lithosphere of the other Gondwana landmasses, including that of immobile Africa. Had the two plausible tracks in the form of La Reunion and Keruguelen hotspots, which the Indian landmass encountered during its northward movement, acted as "rails" and accelerated its movement? According to Raval [28], these hotspots have influenced the geotectonic evolution of the Indian landmass since at least the Jurassic. From the thermal standpoint the Indian landmass in fact shows only a normal thermal structure. In this respect it is worth noting that the part of the Indian Shield subducted beneath the Himalayas was also cold, with a thick lithosphere [29]. Further, it is also worth noting that while almost all parts of Peninsular India are unaffected by the Pan-African orogeny (500 + 150 Ma), major parts of the African landmass have been affected by it. It has also been suggested that the Pan-African areas of Africa are underlain by a fertile lithosphere and its cratonic parts, which escaped the Pan-African Orogeny, are characterised by depleted lithosphere [23]. Further, its reactivated areas show both Neogene uplift and volcanism, while its cratonic areas display only uplift. However, these features developed after the African plate came to rest with respect to sublithospheric mantle circulation patterns at about 30 m.y.B.P. The heat flow data synthesised and discussed are from the Precambrian terrains and Phanerozoic platform covers that developed in the unified Gondwanaland mostly prior to 150 Ma, when the initial separation of the Gondwana plate into a two-plate system, with spreading between Western Gondwanaland (Africa and South America)
283
and Eastern Gondwanaland (Antarctica-Austral i a - I n d i a - M a l a y a - N e w Zealand), began. It can be shown that the super-mobility of the Indian landmass, which began sometime after its detachment from Western Gondwanaland, decreased significantly after its terminal collision with the Eurasian plate [30,31,32]. As mentioned before, the terrains of the Indian landmass, which developed prior to the rifting of the Gondwana landmass, are not hotter than their counterpart terrains in Africa, Australia and Brazil. Despite this, the Indian landmass has acquired, subsequent to its separation, a high relative movement compared with the other Gondwana plates. The thermal characteristics of the plates in themselves, therefore, could not have been responsible for the differing rates of movement of the Gondwana landmasses. As regards the plate movements, it appears that the thermal structure of the plates have an insignificant part to p l a y - - i f a n y - - a n d the continental plates have been, and are, more or less silent spectators of the earth dynamism controlled by its vigorously active internal and interdependent processes (although of course, the plates in themselves have not been totally unaffected as they have often undergone deformation, uplift and magmatism). Some external force(s) would have played a major role in the super-mobility of the Indian plate. This point obviously needs no further elucidation as the nature of the forces responsible for plate motions are quite clear, and the two types of edge forces acting at plate boundaries (ridge-push acting at spreading centres and slab-pull in the subduction zone where the denser downgoing slab pulls the plate behind) have been widely advocated [32]. Despite this, however, the reason why the Indian Plate attained relatively a very high velocity remains unresolved. Could it be that appropriately placed (spatially and temporally) plausible plume traces in the path of the northward flight of the Indian lithosphere and assisting contributions by the forces due to slab-pull and ridge-push worked together? It is worth noting in this respect, however, that Patriat and Achache [32], in their study of the chronology of the India-Eurasia collision, have inferred that ridge-push associated with the thermal structure of the plate is not a dominant driving force.
284
M.L. GUPTA
7. Conclusion This study has unequivocally shown that the Indian landmass has not been, and is not, hotter than other Gondwana landmasses, especially compared to the Australian and the more or less stable African continent, and that we must look elsewhere for the reason behind the super-mobility of India after its detachment from Gondwanaland.
12
13
14
15
Acknowledgements I am grateful to Sri G. Ramacharyulu for help during this investigation, Dr. U. Raval for stimulating discussion, Dr. D.D. Sharma for his help and comments on the statistical treatment of the data, and Prof. John J.W. Rogers for his valuable suggestions on some parts of an early draft of this paper. The Director of the N G R I authorised the publication of this paper. References 1 E.C. Bullard, Heat flow in South Africa. Proc. R. Soc. London Set. A 173, 474-502, 1939. 2 A.E. Benfield, Terrestrial heat flow in Great Britain. Proc. R. Soc. London Set. A 173, 428-450, 1939. 3 R.U.M. Rao, G.V. Rao and H. Narain, Radioactive heat generation and heat flow in the Indian Shield, Earth Planet. Sci. Lett. 30, 57-64, 1976. 4 J.G. Negi, O.P. Pandey and P.K. Agrawal, Super-mobility of hot Indian lithosphere, Tectonophysics 131, 147-156, 1986. 5 M.Q.W. Jones, Heat flow and heat production in the Namaqua Mobile Belt, South Africa, J. Geophy. Res. 92, 6273-6289, 1987. 6 M.Q.W. Jones, Heat flow in the Witwatersrand basin and environs and its significance for the South African Shield geotherm and lithosphere thickness, J. Geophys. Res. 93 (B4), 3243-3260, 1988. 7 S. Ballard, H.N. Pollack and N.J. Skinner, Terrestrial heat flow in Botswana and Nambia, J. Geophys. Res. 92 (B7), 6291-6300, 1987. 8 J.H. Sass and J.C. Behrendt, Heat flow from the Liberian Precambrian Shield, J. Geophys. Res. 85, 3159-3162, 1980. 9 D.S. Chapman and H.N. Pollack, Heat flow and heat production in Zambia: Evidence for lithospheric thinning in Central Africa, Tectonophysics 41, 79-100, 1977. 10 F.A. Lucazeau, A. Lesquer and G. Vasseur, Trends of heat flow density from West Africa, in: Terrestrial Heat Flow and Lithosphere Structure, V. Cermak and L. Rybach, eds., pp. 417-425, Springer, Berlin, 1991. 11 M.L. Gupta, S.R. Sharma, A. Sundar and S.B. Singh, Geothermal studies in the Hyderabad granitic region and
16
17
18 19
20
21
22
23
24
25
26
27
28
29
the crustal thermal structure of the Southern Indian Shield, Tectonophysics 140, 257-264, 1987. M.L. Gupta, A. Sundar and S.R. Sharma, Heat flow and heat generation in the Archaean Dharwar cratons and implications for the Southern Indian Shield geotherm and lithospheric thickness, Tectonophysics 194, 107-122, 1991. M.L. Gupta, Heat Flow in the Indian Peninsula--its geological and geophysical implications, Tectonophysics 83, 71-90, 1982. J.H. Sass and A.H. Lachenbruch, Thermal regime of the Australian continental crust, in: The Earth--its Origin, Structure and Evolution, M.W. McElhinny, ed., pp. 301351, Academic Press, New York, 1979. I. Vitorello, V.M. Hamza and H.N. Pollack, Terrestrial heat flow in the Brazilian Highlands, J. Geophys. Res. 85 (B7), 3778-3788, 1980. V.M. Hamza, Terrestrial heat flow in the alkaline intrusive complex of Pocos de Caldas, Brazil, Tectonophysics 83, 45-62, 1982. A.M. Goodwin, Archaean basin--craton complexes and the growth of Precambrian shields, Can. J. Earth Sci. 14, 2737-2759, 1977. R.L. Anderson and T.A. Bancroft, Statistical Theory in Research, McGraw-Hill, New York, 1952. R.F. Roy, D.D. Blackwell and F.M. Birch, Heat generation of plutonic rocks and continental heat flow provinces, Earth Planet. Sci. Lett. 5, 1-12, 1968. R.F. Roy, D.D. Blackwell and E.R. Decker, Continental heat flow, in: The Nature of Solid Earth, E.C. Robertson, ed., pp, 506-543, McGraw-Hill, New York, 1972. A.H. Lachenbruch, Crustal temperature and heat production: Implications of the linear heat flow relation, J. Geophys. Res. 75 (17), 3291-3300, 1970. K.P. Furlong and D.S. Chapman, Crustal heterogeneities and thermal structure of the continental crust, Geophys. Res. Lett. 14, 314-315, 1987. L.D. Ashwal, P. Morgan, S.A. Kelley and J.A. Percival, Heat production in an Archaean crust profile and implications for heat flow and mobilization of heat producing elements, Earth Planet. Sci. Lett. 85,439-450, 1987. A. Lesquer, J.C. Villeneuve and G. Bromer, Heat flow data from the western margin of the West African Craton (Mauritania), Phys. Earth Planet. Inter. 66, 320-329, 1991. D. Srinagesh, S.S. Rai, V.K. Gaur and C.V.R. Rao, Evidence for thick continental roots beneath South Indian Shield, Geophys. Res. Lett. 16 (9), 1055-10581, 1989. J. Ganguly, P.K. Bhatacharya and P.K. Murty, Petrology and comparative regimes of garnet peridotite nodules from the Proterozoic kimberlite pipes of South India, EOS, 64 (5), 903, 1983. F.R. Boyd, J.J. Gurney and S.H. Richardson, Evidence for a 150-200 km thick Archaean lithosphere from diamond inclusion thermobarometry, Nature 315, 387-389, 1985. U. Raval, On hotspots, Meso-Cenozoic tectonics and possible thermal networking beneath the Indian continent, in: Proc. Semin. Advances in Geophysical Research in India, pp. 314-330, Indian Geophys. Union, Hyderabad, 1990. P. Molnar, A review of geophysical constraints on the deep structure of the Tibetan Plateau, the Himalaya and the Karakoram and their tectonic implications, Philos. Trans. R. Soc. London A 326, 33-88, 1988.
IS THE INDIAN SHIELD HOTTER THAN OTHER GONDWANASHIELDS? 30 P. Molnar and P. Tapponier, Cenozoic tectonics of Asia: effects of a continental collision, Science 189, 419-156, 1988. 31 C.J. Klootwijk, A summary of palaeomagnetic data from extra-peninsular Indo-Pakistan and South Central Asia: Implications for collision tectonics, in: Structural Geology of the Himalaya, P.S. Saklani, ed., pp. 307-360, Today and Tomorrow Publ., New Delhi, 1979. 32 P. Patriat and J. Achache, India-Eurasia collision chronology has implications for crustal shortening and driving mechanism of plates, Nature 311,615-621, 1984.
285 33 D. Forsyth and S. Uyeda, On the relative importance of the driving forces of plate motions, Geophys. J. R. Astron. Soc. 43, 163-200, 1975. 34 M.L. Gupta, Surface heat flow values in Gondwana grabens of Peninsular India: their significance and implications, in: Proc. Natl. Symp. Gondwana of India, in press, Nagpur Univ., Nagpur, India, 1993. 35 M.L. Gupta, S.B. Singh, S.R. Sharma, A. Sundar, G. Ramacharyulu and A. Yadgiri, Thermal structure of the Indian lithosphere. Phase I project terminal report NGRI3/Lithos/GTH, Natl. Geophys. Res. Inst., 1987 (Unpubl.).
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Elsevier Science Publishers B.V., Amsterdam [DT]
Is the Indian Shield hotter than other Gondwana shields? Mohan L. Gupta Geothermal Group, National Geophysical Research Institute, Hyderabad, 500 007, India
Received November 7, 1991; revision accepted December 15, 1992
ABSTRACT Geothermal data on various Precambrian terrains from the African, Australian, Indian and South American (Brazil only) Gondwana landmasses have been compiled, synthesised and statistically analysed. The results do not support the prevailing notion that the Indian Shield is hotter than other shields. The study clearly shows that the mean surface heat flow values from the various Precambrian cratons and mobile belts of the Indian landmass for which the data have become available are either equal to, or even lower in some cases, than that in similar terrains from other Gondwana continents. Further, on the basis of available data, it is found that the Moho and the reduced heat flow values and Moho temperatures in the South Indian, South African, Western Australian and Brazilian shields fall within a narrow range, thus indicating, within the error limits of the estimation, the similarity of these shields in terms of these characteristics. In conclusion it is shown that the Indian landmass is not hotter than the other Gondwana landmasses, including even the presently immobile African continent, and that the "super-mobility" of the Indian landmass does not appear to be associated with its thermal characteristics. The cause of the latter lies elsewhere.
I. Introduction Understanding the temperature distribution within t h e e a r t h so as to o b t a i n an insight into its t h e r m a l a n d d y n a m i c e v o l u t i o n has b e e n a m a i n goal o f g e o t h e r m a l r e s e a r c h over t h e last t h r e e d e c a d e s . In a d d i t i o n , t h e i n t e r n a l h e a t o f t h e earth, which is a m a j o r e n e r g y c o m p o n e n t in g e o d y n a m i c m o t i o n , c o n t r o l s t h e physical state and the tectonic processes of the lithosphere. Consequently, the heterogeneties of the earth's s t r u c t u r e a r e r e f l e c t e d in t h e d i s t r i b u t i o n of t h e h e a t flow field. Efforts, t h e r e f o r e , to d e t e r m i n e t h e surface h e a t flow in an o p t i m u m n u m b e r o f l o c a t i o n s in v a r i o u s g e o t e c t o n i c units of t h e e a r t h have b e e n vigorously u n d e r t a k e n t h r o u g h o u t t h e world, ever since t h e first r e l i a b l e m e a s u r e m e n t s w e r e m a d e in S o u t h A f r i c a by B u l l a r d [1] a n d in E n g l a n d by B e n f i e l d [2]. T h e global surface h e a t flow d a t a s e t has greatly e x p a n d e d d u r i n g the last two d e c a d e s , a n d this is also t r u e for s o m e o f t h e Gondwana landmasses. A p a r t f r o m acting as a d i a g n o s t i c m e a n s o f studying t h e t e c t o n i c n a t u r e o f a given p a r t o f t h e earth, w h e n c o m b i n e d a n d s t u d i e d a g a i n s t t h e
b a c k d r o p o f o t h e r geological, geophysical a n d g e o c h e m i c a l i n f o r m a t i o n surface h e a t flow d a t a a r e very i l l u m i n a t i n g in a s c e r t a i n i n g t h e evolut i o n a r y p r o g r e s s i o n o f various e a r t h processes. H o w e v e r , t h e r e a r e v a r i o u s d a n g e r s t h a t m u s t be avoided, a n d t h e lack o f an a d e q u a t e d a t a s e t c o m b i n e d with o t h e r factors can s o m e t i m e s l e a d to fantasies t h a t w h e n s t u d i e d in d e p t h t u r n o u t to b e u n f o u n d e d . Such has b e e n t h e case with the t h e r m a l c h a r a c t e r i s t i c s of t h e I n d i a n l a n d m a s s . E a r l i e r workers, n o t a b l y R a o et al. [3] i n d i c a t e d a " h o t t e r " u p p e r m a n t l e b e n e a t h the I n d i a n Shield, while N e g i et al. [4] a t t e m p t e d to show t h a t t h e I n d i a n l i t h o s p h e r e is quite hot, a n d t h a t its thickness is a b o u t o n e t h i r d of t h a t of o t h e r shields, including t h e i m m o b i l e , cold, thick A f r i c a n lithos p h e r e , an i n f e r e n c e which a c c o r d i n g to t h e m [4] has i m p o r t a n t c o n s e q u e n c e s for the mobility of the Indian subcontinent. W i t h t h e i n c r e a s e in surface h e a t flow d a t a m e n t i o n e d a b o v e in m i n d an a t t e m p t is m a d e in this p a p e r to compile, synthesise a n d c o m p a r e t h e surface h e a t flow a n d o t h e r r e l e v a n t d a t a o f t h e various P r e c a m b r i a n t e r r a i n s o f G o n d w a n a l a n d m a s s e s as a first s t e p to a s c e r t a i n i n g w h e t h e r
0012-821X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
276
M.L. G U P T A
TABLE 1 Surface heat flow in various geological provinces of Gondwana landmasses S. No., landmass and geological province/unit
Age
N
mWm
2
Mean surface heat flow
S.D.
Range
A. India
1. Dharwar Craton (Cr) 1.1 Platform cover over Dharwar Craton 1.1.1 Cuddapah Basin 1.1.2 Deccan Traps 1.2 Values for all of the above 2. Granulite Terrane, South India 3. Bundelkhand-Aravalli Cr 4. Bastar Craton 5. Singhbhum Thrust Z o n e 6. Delhi Mobile Belt 7. Trans-Aravalli region 8. All Precambrian except 1.1.2 and 7 9. Phanerozoic cover 9.1 Gondwana grabens
9.2 Oil basins 9.3 Trap cover 9.4 Values for all of the above from 9.1 to 9.3 10. All data for the above (1 to 9)
Archaean
15
38
5.4
28-46
M i d - L . Prot. U. C r e t - E . Paleo.
53 44 41 55
20.1 4.2 11.0
27-75 41-47 27-75
Proterozoic
4 2 21 1
A r . - E . Prot. Proterozoic Proterozoic M i d - L . Prot. L. Prot.? Precambrian
9 3 3 10 1 45
41 56 61 62 95 48
2.1 7.2 2.0 12.5
38-44 51-64 59-63 44-74
13.4
27-75
Prot. a n d / o r PermoCarb.-L. Cret. sediments Tertiary U. Cret.-E. Paleo
18
64
15.5
49-107
12 2 32
71 44 65
14.1 4.24 15.6
50-93 41-47 41-107
78
56
17.0
27-107
15 1
37.3 46.0
6.0
29-54
4 6 5 3
77.8 82.3 73.8 80.7
26.7 24.0 19.4 4.9
48-113 49-109 45-96 75-84
32 38
83.0 72.0
19.6 26.0
48-125 45-121
91 53
68.0 65.0
33.0 24.0
29-125 29-113
7
33.0
1.5
31-36
11 71
45.5 52.1
5.5 5.97
36-54 36-72
B. Australia
11. Western Shield 11.1 Yilgarn C r a t o n / B l o c k 11.2 Hamersley Basin 12. Central Shield 12.1 Northern Craton 12.2 Gawler Craton 12.3 Phanerozoic cover 12.4 Mt. Isa Mobile Belt 12.5 All available values from Central Shield 13. Eastern Australia 14. Australian landmass 14.1 All values 14.2 All Precambrian areas C. Africa 15. Kaapvaal Craton, South Africa 15.1 Witwatersrand Basin (WB) 15.1.1 Hartebeesfontein and Ventersdrop granitic domes 15.1.2 Classical data * WB 15.1.3 Estimates from documented temperature data in WB
Archaean E. Prot. Proterozoic Proterozoic Precam. basement Mid Prot. Proterozoic Phanerozoic (Palaeozoic?)
Precambrian
Archaean
L. A r . - E . Prot. L. A r . - E . Prot.
277
IS THE INDIANSHIELDHOTTERTHAN OTHER GONDWANASHIELDS? TABLE 1 (continued) S. No., landmass and geological province/unit
Age
15.1.4 Within the WB after removing the effect of strata 15.1.5 All heat flow data in WB 16. Zimbabwe Craton, S. Africa 16.1 Tati Greenstone Belt 16.2 Near the border of a mobile belt in granitic gneisses 17. Mobile belts, S. Africa 17.1 Namaqva-Natal Belt 17.2 Ganzi-Chobe Belt 17.3 Damara-Katanga Belt 18. West African ShieM / Craton 18.1 Man Shield 18.2 Man Shield 18.3 Platform cover 18.4 All values for the above 19. All values from Africa 19.1 All Precambrian areas, excluding areas of Pan-African Orogeny
L. Ar.-E. Prot.
Ar.-E. Prot.
D. South Africa 20.1 S~o Francisco Craton (SFC) 20.2 Mobile belt encircling SFC 20.3 All values for the above 20.4 Parana Basin 20.5 Pocosde Caldas
mW m -2
N
Mean surface heat flow
S.D.
Range
81
43.5
7.8
36-72
89
50.0
7.8
31-72
1 1
37.0 65.0
16 5 17
62.0 63.0 66.0
11.4 6.0 10.0
39-81 56-71 54-92
5 15 19 39 313 142
39.0 30.0 71.0 51.0 58.0 46.0
10.0 27.0 27.0 24.0 17.0
21-194 31-81
Ar.-E. Prot.
3
42.0
8.0
37-51
Proterozoic
8
55.0
11.0
38-69
11 3 5
51.0 68.0 74.0
11.3 13.9 10.0
37-69 52-77 53-94
Archaean Archaean
Proterozoic Proterozoic Pan-African Archaean Mid Prot. U. Prot.-Palaeoz.
Phanerozoic Intrusive, Permian-E. Tert.
6.0
Ar. = Archaean; Prot. = Proterozoic; Permo-Carb. = Permo-Carboniferous; Palaeoz. = Palaeozoic; Cret. = Cretaceous; Tert. = Tertiary; Paleo. = Paleocene. Qs values have been compiled from a large number of original papers, data summary papers and data compilations: India [11,12,13,34,35]; Australia [14]; South America [15,16]; Africa [courtesy of H.N. Pollack, and 5-10]. * Determined from temperature logs and thermal conductivity data.
the presently prevailing belief that the Indian S h i e l d is h o t t e r a n d t h i n n e r t h a n t h e o t h e r s h i e l d s is j u s t i f i e d .
2. Pattern of surface heat flow field in G o n d w a n a Precambrian shields H e a t f l o w d a t a sets h a v e g r e a t l y e x p a n d e d in t h e last d e c a d e e s p e c i a l l y f o r t h e A f r i c a n a n d I n d i a n l a n d m a s s e s [5-13]. Sass a n d L a c h e n b r u c h [14] h a v e g i v e n a d e t a i l e d a c c o u n t o f t h e t h e r m a l
r e g i m e o f t h e A u s t r a l i a n c o n t i n e n t a l crust, a n d V i t o r e l l o e t al. [15] a n d H a m z a [16] h a v e p r o v i d e d the data for the Brazilian Coastal Shield. The h e a t f l o w d a t a in all t h e s e w o r k s , t o g e t h e r w i t h a compilation of heat flow data for the African c o n t i n e n t as p r o v i d e d by H . N . P o l l a c k , h a v e b e e n u s e d in t h e p r e s e n t p a p e r . I n d i v i d u a l r e f e r e n c e s will n o t b e e x t e n s i v e l y c i t e d in t h e text, a n d t h e r e a d e r s h o u l d r e f e r to t h e s e p u b l i c a t i o n s for further details on the data. A common feature of the growth of the conti-
278
M.L. GUPTA
nental crust is a craton-mobile belt association. Most Precambrian shields of the earth exhibit numerous elliptical to subcircular Archaean or Proterozoic cratons that are exposed over great distances (600-1500 km) together with younger pericratonic mobile belts [17]. This aspect is borne in mind while classifying the heat flow data of the Precambrian terrains of the Gondwana landmasses. A mean heat flow value calculated giving weight to the areal extent of different rock units
present in a region of interest is a most appropriate unit for comparison. However, bearing in mind the obvious practical difficulties, such an attempt has not been made. The general practice, followed by almost all, is to use, for various purposes, simple arithmetic mean values of the heat flow data sets. The same are, therefore, computed for the Indian, Australian, South American (Brazilian Coastal Shield only) and African shields for their separate segments (i.e., the Archaean-Early Proterozoic cratons, Pro-
TABLE 2 Surface heat flow in various Precambrian terranes and platform regions of Gondwana landmasses S. No. and geological p r o v i n c e / u n i t
Mean surface heat flow ( m W m -2) N
Qs
Data points in Fig. 2
S.D.
1. Archaean-Early Proterozoic cratons
1.1 Dharwar Craton (India) 1.2 Bundelkhand-Aravalli Craton (India) 1.3 Yilgarn C r a t o n / B l o c k (West Australia) 1.4 Kaapvaal Craton (South Africa) 1.5 West African Craton (Man Shield, Archaean) 1.6 Silo Francisco Craton (South America)
15 9 15 89 5 3
38 41 37 50 39 42
5.4'1 2.1 6.0 7.8 6.0 8.0
3 6 4
56 82 78
7.2 24.0 26.7
10 3 16 5 17
62 61 62 63 66
12.5~ 2.0 11.4 6.0 10.0
8 3
55 81
11.0 4.9)
32 5 19 3
65 74 71 68
15.6 19.4 27.3 13.9
Points 15, 16 and 17 are for data pairs of 4.1 with 4.2, 4.3 and 4.4 respectively
45 142 11 53
48 46 51 65
13.4 17.0 11.3 24.0
Points 18, 19 and 20 are for data pairs of 5.1 with 5.2, 5.3 and 5.4 respectively
78 91 313
56 68 58
17.0 33.0 24.0
Points 1, 2, 3 and 4 are for data pairs of 1.1 with 1.3, 1.4, 1.5 and 1.6 respectively. Points 5, 6, 7 and 8 are for data pairs of 1.2 with 1.3, 1.4, 1.5 and 1.6 respectively
2. Proterozoic cratons
2.1 Bastar Craton (India) 2.2 Gawler Craton (Central Australia) 2.3 Northern Craton (Central Australia) 3. Proteroznic mobile belts (MB) 3.1 Delhi MB (India) 3.2 Singhbhum Thrust Zone MB (India) 3.3 N a m a q u a - N a t a l MB (South Africa) 3.4 G a n z i - C h o b e MB (South Africa) 3.5 D a m a r a - K a t a n g a MB (Pan-African Orogeny) 3.6 Late Precambrian Brazilian metamorphic b e l t / M B encircling craton 3.7 Mt. Isa Mobile Belt (Central Australia)
Points 9 and 10 are for data pairs of 2.1 with 2.2 and 2.3 respectively
Points 11, 12, 13 and 14 are for data pairs of 3.1 with 3.3, 3.4, 3.5 and 3.7 respectively
4. Phanerozoic cover
4.1 4.2 4.3 4.4
India Central Australia Cover over West African Craton Parana Basin, Brazil
5. All Precambrian areas
5.1 5.2 5.3 5.4
India Africa Brazil (South America) Australia
6. All data
6.1 India 6.2 Australia 6.3 Africa
Points 21 and 22 are for data pairs of 6.1 with 6.2 and 6.3 respectively, and point 23 is that for a data pair of 6.2 and 6.3
IS THE INDIAN SHIELD HOTTER THAN OTHER GONDWANA SHIELDS?
terozoic cratons, and Proterozoic mobile belts, Table 1 and 2). Mean values for the Phanerozoic parts and of the total data set of the above-mentioned landmasses are also given in Tables 1 and 2. Mean heat flow values together with their standard deviations are shown in Fig. 1. The heat flow data for lakes Malawi, Tanganayika and Kivu of the African landmass have not been considered as the values are of variable quality and show large scatter due to various structural and other features.
279
than the value read from the tables of t-distribution we reject the hypothesis that /,~ = tx 2. The higher the calculated value of t than its theoretical value, the more reasonable it would be to reject the hypothesis ~1 =/*z in favour of the alternative that IZl >/*2 at the specified significance level. A curve showing t values read from the table of t-distribution for the appropriate degrees of freedom at the 5% significance level and the values of t calculated by using eq. (2) for various pairs of data (Table 2) is plotted in Fig. 2.
3. Statistical treatment of the data 4. Moho heat flow
A hypothesis concerning the difference between the means of two populations can be statistically tested. We assume that each heat flow data set as given in Tables 1 and 2 represents random samples drawn from normal populations. Let Q1 and Q2 be the arithmetic means of the two available heat flow data sets treated as two random samples drawn from two normal populations with mean heat flow values of /z I and /~2 respectively. We now test the null hypothesis, H 0 that /x 1 = ~ 2. According to Anderson and Bancroft [18] the test criterion on the assumption that the hypothesis tested (/z 1 =/*2) is true is: I Q1 - Q2[
t= S(l/nl
+ l/n2
(1)
where $2 =
(nl
-- 1)$12 -f- (/'/2 -- 1 ) $ 2
(n 1 + n 2 - 2) (where n 1 and 172 are the number of observations in each dataset having sample variances S~ and S 2 respectively), or t=
I Q1 - Q2 I Cnln2(nl + n 2 -
2)
(2)
¢(n 1 + n2) • ¢(n 1 - 1 ) S ( + (n 2 - 1)S 2 In practice, by using eq. (2) the above t statistic is computed for the available number of heat flow data pairs and compared with the theoretical value read from tables of the t-distribution for n I + n 2 - - 2 degrees of freedom at the 5% significance level. If the computed value of t is greater
Despite the fact that estimation of heat flow into the base of the cust is not a straightforward process attempts have been made to obtain reasonable estimate of its magnitude. According to Roy et al. [19,20] and Lachenbruch [21], reduced heat flow (Qr) is the contribution from the deep crustal a n d / o r mantle sources. However, it has been shown [22], based on numerical simulations of crustal thermal models, that Qr appears to be the mean heat flow at the characteristic depth D and that the general conclusion that Qr is an acceptable estimate for continental mantle heat flow does not appear to be valid. Ashwal et al. [23] have also indicated that lower crust heat production would be represented in the reduced heat flow component, and Moho heat flow could be significantly (8-10 mW m - z ) lower than Qr. Values of Qr and reasonable estimates of Moho heat flow are available only for some of the shield units of the Gondwana landmasses (Table 3). 5. Thermal structure of the crust and Moho temperatures
The one-dimensional steady-state crustal geotherms and Moho temperatures in a conductive regime for and beneath various cratonic blocks were calculated. Standard procedures [21,14,12,6] for calculating crustal temperatures were used and the available radioactive heat production models for the respective cratons were considered (Table 3). The crust is assumed to have a uniform conductivity of 2.5 W m - I K - 1 . As the models of radioactive heat source distribution
280
M.L. GUPTA
~
A
6~ -
6
b
fl:
120 I.-
o~ ~E
296
100
,u =
"2.
E~
u;80
~o
~o "6
_
6O
~
~
o
i
~
O
O
i
4O
I
o~dd
d
20
Arch I EP'°'I r°'cro,on. Cro,o°.
pr0,.ob,..,,.
II ,on.r=0c0., II
A 0°'°
I
Fig. 1. Surface heat flow in various regions. Vertical bars represent the mean _+1 S.D. For the Kaapvaal Craton the data plotted are those of 15.1.4 and 15.1.5 in Table 1. For details, refer to this table and Table 2. B U N D - A R A V = Bundelkhand-Aravalli Craton. The South American Proterozoic mobile belt data are for Brazil.
for the various cratons of the present study show no abnormal distribution [6,12,14,15] their thermal structure would be adequately represented by the Moho temperatures. The latter, therefore,
600 023
200
- -
°21 o6
tOO
E 0
m
Fig. 2. Comparison of mean heat flow values of various data pairs of Gondwana landmasses. The curve shows the theoretical values of t for various degrees of freedom at the 5% significance level. © = computed values of t using eq. (2) for various mean heat flow data pairs and for appropriate degrees of freedom (see last column of Table 2) for the data pairs:
¢
4o °17
°15 o 13
2o
o3
o ~"
o 02 20
A r c h a e a n - E a r l y Proterozoic eratons: l ( D C - Y C); 2 ( D C K C ) ; 3 (D C - M S); 4 (D C-SFC); 5 ( B - A C - Y C); 6 (B-
o7
12 o8
IO-
D
o5
% °14
-
~9
4 -
2I
0.1
I
[ I I I Ill 0'4 I'0
~ t
I 2"0
II 6"0
A C - K C); 7 ( B - A C - M S); 8 ( B - A C-SFC). Proterozoie eratons: 9 (B C - G a C); 10 (B C - N o C). Proterozaic mobile belts: 11 (D M B - N - N MB); 12 (D M E - G - C MB); 13 (D M B - D - K MB); 14 (D MB-Mt. Isa MB). Phanernzoic cover: 15 (Ind.-C. Aust.); 16 (Ind.-W. Af. C); 17 (Ind.-Pa. Brz.). All P r e e a m b r i a n areas: 18 (Ind.-Af.); 19 (Ind.-Brz.); 20 (Ind.-Aust.). All data: 21 (Ind.-Aust.); 22 (Ind.-Af.); 23 (Aust.-Af.). The abbreviations of the various cratons (C) and mobile belts (MB) may be obtained by inspecting the full n a m e s in Fig. 1. W. Af, C = W e s t Africa Craton; Pa. Brz. = Parana Basin, Brazil. Other abbreviations are self-explanatory.
281
IS T H E INDIAN SHIELD H O T I ' E R T H A N O T H E R G O N D W A N A SHIELDS?
TABLE 3 Thermal parameters and Moho temperatures at 40 km deep for and beneath Archaean/Archaean-Early Proterozoic cratons of Gondwana landmasses Craton
1. Dharwar Craton (India) 2. Kaapvaal Craton (South Africa) 3. Yilgarn Craton (Western Australia) 4. Silo Francisco Craton (Brazil)
Heat flow (mW m-2) Qs + S.D. Qr Qs-Qr
Qm
D (km)
11.5
Moho temperature (TmX°C)based on:
OX
QrXX
332-404
368-445
Temperature variation w.r.t. DC
38 + 6
23
15
15-17
50 _+8
-
-
12-17
39 _+8
26
13
-
4.5
-
416-471
+13- +6%
42+8 ~
28
14
-
13.1
-
448-552
+22-+24%
-
360-457
-
+ 8-+13%
x Moho temperatures were calculated using the layer model [12] for two values of surface heat flow (Qs) and using a Moho heat flow of Om= 17 mW m -2. For Dharwar Craton Qs = 29 and 40 mW m -2 and crustal radioactivity models as given by Gupta et al. [12] have been considered. For Kaapvaal Craton Qs = 33 and 51 mW m 2 and crustal radioactivity models as considered by Jones [6] have been used. xxMoho temperatures were calculated after considering exponential radioactive heat source distribution and using the given values of the heat flow parameters (Qr and D). The lower Moho temperature for each craton is obtained for the lowest possible value of As, i.e. A s = 0; and the higher Moho temperature is for the reported highest value of A s in the relevant Qs-As pairs. For Dharwar Craton A s = 1.5/xW m -3 [12]; for Yilgarn Craton A s = 6.82/xW m 3 [14]; for Sio Francisco Craton As = 1.6/xW m 3 [15]. (A s = radioactive heat production in surface/near-surface rocks.) a Calculated using data from Archaean-Early Proterozoic terrains only.
have b e e n c o n s i d e r e d for discussion a n d are given in T a b l e 3.
6. Discussion I n o r d e r to ascertain w h e t h e r the I n d i a n Shield is hotter t h a n o t h e r G o n d w a n a Shields we shall now e x a m i n e the following questions: (i) w h e t h e r the surface heat flow in the I n d i a n Shield is higher t h a n in the o t h e r G o n d w a n a shields a n d (ii) how the M o h o heat flow, the r e d u c e d heat flow a n d the M o h o t e m p e r a t u r f s b e n e a t h the G o n d w a n a shields differ from each other. T h e heat flow data s u m m a r i s e d in T a b l e 1 a n d 2 a n d p l o t t e d in Fig. 1 show that the heat flow m e a n s (Qs) for similar geological terrains of various G o n d w a n a l a n d m a s s e s mostly fall within o n e s t a n d a r d deviation of each o t h e r a n d t h e r e f o r e c a n n o t be c o n s i d e r e d as significantly different from each other. T h e A r c h a e a n - E a r l y Proterozoic c r a t o n s of the four G o n d w a n a l a n d m a s s e s are c h a r a c t e r i s e d by low heat flow a n d generally by a small v a r i a t i o n of values a r o u n d their means. T h e surface heat flow values from Proterozoic mobile belts a n d P h a n e r o z o i c p l a t f o r m covers
show n o a p p r e c i a b l e differences. F u r t h e r , the results of statistical t r e a t m e n t of the surface heat flow data have clearly shown that the calculated values of the t statistic are lower t h a n the values of t read from the tables a n d therefore plot o n the l e f t - h a n d side of the theoretical curve of the t statistic (Fig. 2), t h e r e b y d e m o n s t r a t i n g that there is n o significant difference b e t w e e n the m e a n heat flow values of such data pairs. This has b e e n tested at the 5% significance level. I n the p r e s e n t case, this implies that the Qs values of similar geological terrains (i.e., of the A r c h a e a n a n d Proterozoic cratons, of the Proterozoic mobile belts a n d of the P h a n e r o z o i c areas) of the I n d i a n , A u s t r a l i a n , A f r i c a n a n d Brazil l a n d m a s s e s are mostly equal at the 5% significance level. However, there are c e r t a i n data pairs, r e p r e s e n t e d by points 2, 6, 14, 20, 21 a n d 23, which plot away from the theoretical t curve o n its r i g h t - h a n d side (Fig. 2), t h e r e b y i n d i c a t i n g that the m e a n s of the data sets r e p r e s e n t e d by these respective pairs (2, 6, 14, 21, 20 a n d 23) are not equal. If we c o m b i n e the above i n f e r e n c e d r a w n o n the basis of statistical analysis of data with values given in T a b l e 2, we find the following:
282
(i) The Qs value of the Kaapvaal Craton (South Africa) is significantly higher than the Qs values of the Dharwar and Bundelkhand-Aravalli cratons of the Indian landmass (Fig. 2, points 2 and
6). (ii) The Qs value for the Proterozoic Mt. Isa Mobile Belt (Central Australian Shield) is higher than for the Proterozoic Delhi Mobile Belt (India) (Fig. 2, point 14). (iii) Qs values for all Precambrian sites, including those for all the available sites in the Australian landmass, are statistically higher than the values for the terrains of the Indian and African landmasses (Table 1 and 2; points 20, 21 and 23; Fig. 2). The heat flow data of the West African Shield show a peculiar distribution. Whereas elsewhere the Archaean-Early Proterozoic terrains are associated with lower heat flow values than in the Middle-Late Proterozoic terranes, the data reveal that this is not the case for the West African Shield, where its former terrain types show values that are higher than in its latter terrain types (the pattern of heat flow is reversed). The Archaean terrains and Phanerozoic platform areas of the West African Shield, however, are characterised by the same order of heat flow values as their counterpart terrains in India and Australia and Brazil (Table 2). It is worth noting that if one considers the surface heat flow values both for all the available sites and separately for all the Precambrian terrains in the Indian and the African landmasses the Q~ values for these terrains (Table 2) are statistically equal (Fig. 2, points 22 and 18). The foregoing discussion leads to the clear result that the mean surface heat flow values for the various geological terrains of the Indian landmass, for which data are available, are either equal to or lower than in terrains of similar geological character of the African and Australian landmasses and of Brazil. The exception to this observation, which is not restricted to India but may also be applied to Australia, Brazil and other cratonic parts of the African landmass is that the Middle Proterozoic part of the Man Shield (West African Craton) is characterised by Q~ values that are lower than in such terrains of the other Gondwana landmasses. However, this observation does require careful examination of
M.L. GUPTA
all the available data for the West African Craton [10,24], as a high heat flow value of 59 mW m 2 has been reported in its Upper Proterozoic terrains [24]. The Moho heat flow (Qm) and the reduced heat flow (Qr) and the radioactive component of heat flow in the top layers of the crust (Qs - Qr) for all four A r c h a e a n / A r c h a e a n - E a r l y Proterozoic cratons of the Gondwana landmasses (see Table 3) are more or less equal. It is worth noting that these four cratons (Table 3), which include large parts of the Indian, Australian, African and South American landmasses, clearly demonstrate similarity in their deep thermal characteristics. The temperature at the Moho (Tm) beneath the Dharwar Craton (India) is generally lower (by about 6-24%) than beneath the other A r c h a e a n / A r c h a e a n - E a r l y Proterozoic cratons of the Gondwana landmasses (Table 3). However, considering the range of values of the estimated Tm and the inherent problems faced and assumptions made in making such estimates, it is quite obvious that based on the present state of knowledge the values of Tm for all the four Precambrian Shields (South Indian, South African, Western Australian and Brazilian) are equal, as their values fall within the error limits of estimation. Therefore, based on Tm too it is not possible to infer that the Indian Shield is hotter than the other three Gondwana shields. Jones [5] and Gupta et al. [12] have reported that the thickness of the thermal lithosphere beneath the South African and South Indian shields is over 200 km. Additional support for thick lithosphere beneath the Dharwar Craton has come from the studies of Srinagesh et al. [25]. Through their analysis of teleseismic P-wave arrivals recorded at South Indian seismological stations they have inferred the presence of an anomalously thick high-velocity region in the depth range of 100-300 km below the Dharwar Craton (with the exception of beneath the westernmost part of the Deccan Traps region). According to Ganguly et al. [26] the peridotite nodules from the Proterozoic kimberlite pipes of South India are derived from various depths between 125 and 200 km. The presence of Late Proterozoic kimberlite pipes in the Bundelkhand-Aravalli Craton also points towards the presence of a thick lithosphere beneath it at that time. In addition, xenolith
IS T H E I N D I A N S H I E L D H O T F E R
THAN OTHER
GONDWANA
SHIELDS?
studies have confirmed the presence of thick root zones beneath the Kaapvaal Craton (South Africa) [27]. Based on the results of the synthesis of the surface heat flow data it may be easily understood, and can also be easily shown that the thicknesses of the thermal lithosphere beneath the Indian, African and Australian landmasses and of Brazil may not be significantly different from each other. From the above discussion it is concluded that the "super-mobility" of the Indian landmass cannot be ascribed to its thermal characteristics, as its lithosphere is neither hotter nor thinner than the lithosphere of the other Gondwana landmasses, including that of immobile Africa. Had the two plausible tracks in the form of La Reunion and Keruguelen hotspots, which the Indian landmass encountered during its northward movement, acted as "rails" and accelerated its movement? According to Raval [28], these hotspots have influenced the geotectonic evolution of the Indian landmass since at least the Jurassic. From the thermal standpoint the Indian landmass in fact shows only a normal thermal structure. In this respect it is worth noting that the part of the Indian Shield subducted beneath the Himalayas was also cold, with a thick lithosphere [29]. Further, it is also worth noting that while almost all parts of Peninsular India are unaffected by the Pan-African orogeny (500 + 150 Ma), major parts of the African landmass have been affected by it. It has also been suggested that the Pan-African areas of Africa are underlain by a fertile lithosphere and its cratonic parts, which escaped the Pan-African Orogeny, are characterised by depleted lithosphere [23]. Further, its reactivated areas show both Neogene uplift and volcanism, while its cratonic areas display only uplift. However, these features developed after the African plate came to rest with respect to sublithospheric mantle circulation patterns at about 30 m.y.B.P. The heat flow data synthesised and discussed are from the Precambrian terrains and Phanerozoic platform covers that developed in the unified Gondwanaland mostly prior to 150 Ma, when the initial separation of the Gondwana plate into a two-plate system, with spreading between Western Gondwanaland (Africa and South America)
283
and Eastern Gondwanaland (Antarctica-Austral i a - I n d i a - M a l a y a - N e w Zealand), began. It can be shown that the super-mobility of the Indian landmass, which began sometime after its detachment from Western Gondwanaland, decreased significantly after its terminal collision with the Eurasian plate [30,31,32]. As mentioned before, the terrains of the Indian landmass, which developed prior to the rifting of the Gondwana landmass, are not hotter than their counterpart terrains in Africa, Australia and Brazil. Despite this, the Indian landmass has acquired, subsequent to its separation, a high relative movement compared with the other Gondwana plates. The thermal characteristics of the plates in themselves, therefore, could not have been responsible for the differing rates of movement of the Gondwana landmasses. As regards the plate movements, it appears that the thermal structure of the plates have an insignificant part to p l a y - - i f a n y - - a n d the continental plates have been, and are, more or less silent spectators of the earth dynamism controlled by its vigorously active internal and interdependent processes (although of course, the plates in themselves have not been totally unaffected as they have often undergone deformation, uplift and magmatism). Some external force(s) would have played a major role in the super-mobility of the Indian plate. This point obviously needs no further elucidation as the nature of the forces responsible for plate motions are quite clear, and the two types of edge forces acting at plate boundaries (ridge-push acting at spreading centres and slab-pull in the subduction zone where the denser downgoing slab pulls the plate behind) have been widely advocated [32]. Despite this, however, the reason why the Indian Plate attained relatively a very high velocity remains unresolved. Could it be that appropriately placed (spatially and temporally) plausible plume traces in the path of the northward flight of the Indian lithosphere and assisting contributions by the forces due to slab-pull and ridge-push worked together? It is worth noting in this respect, however, that Patriat and Achache [32], in their study of the chronology of the India-Eurasia collision, have inferred that ridge-push associated with the thermal structure of the plate is not a dominant driving force.
284
M.L. GUPTA
7. Conclusion This study has unequivocally shown that the Indian landmass has not been, and is not, hotter than other Gondwana landmasses, especially compared to the Australian and the more or less stable African continent, and that we must look elsewhere for the reason behind the super-mobility of India after its detachment from Gondwanaland.
12
13
14
15
Acknowledgements I am grateful to Sri G. Ramacharyulu for help during this investigation, Dr. U. Raval for stimulating discussion, Dr. D.D. Sharma for his help and comments on the statistical treatment of the data, and Prof. John J.W. Rogers for his valuable suggestions on some parts of an early draft of this paper. The Director of the N G R I authorised the publication of this paper. References 1 E.C. Bullard, Heat flow in South Africa. Proc. R. Soc. London Set. A 173, 474-502, 1939. 2 A.E. Benfield, Terrestrial heat flow in Great Britain. Proc. R. Soc. London Set. A 173, 428-450, 1939. 3 R.U.M. Rao, G.V. Rao and H. Narain, Radioactive heat generation and heat flow in the Indian Shield, Earth Planet. Sci. Lett. 30, 57-64, 1976. 4 J.G. Negi, O.P. Pandey and P.K. Agrawal, Super-mobility of hot Indian lithosphere, Tectonophysics 131, 147-156, 1986. 5 M.Q.W. Jones, Heat flow and heat production in the Namaqua Mobile Belt, South Africa, J. Geophy. Res. 92, 6273-6289, 1987. 6 M.Q.W. Jones, Heat flow in the Witwatersrand basin and environs and its significance for the South African Shield geotherm and lithosphere thickness, J. Geophys. Res. 93 (B4), 3243-3260, 1988. 7 S. Ballard, H.N. Pollack and N.J. Skinner, Terrestrial heat flow in Botswana and Nambia, J. Geophys. Res. 92 (B7), 6291-6300, 1987. 8 J.H. Sass and J.C. Behrendt, Heat flow from the Liberian Precambrian Shield, J. Geophys. Res. 85, 3159-3162, 1980. 9 D.S. Chapman and H.N. Pollack, Heat flow and heat production in Zambia: Evidence for lithospheric thinning in Central Africa, Tectonophysics 41, 79-100, 1977. 10 F.A. Lucazeau, A. Lesquer and G. Vasseur, Trends of heat flow density from West Africa, in: Terrestrial Heat Flow and Lithosphere Structure, V. Cermak and L. Rybach, eds., pp. 417-425, Springer, Berlin, 1991. 11 M.L. Gupta, S.R. Sharma, A. Sundar and S.B. Singh, Geothermal studies in the Hyderabad granitic region and
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