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First heat flow determination from the central Sahara: relationship with the Pan-African belt and Hoggar domal uplift
Journal of African Earth Sciences, Vol. 9, No. 1, pp. 41-48, 1989
0899-5362/89 $3.00 + 0.00 (~) 1989 Pergamon Press plc
Printed in Great Britain.
First heat flow determination from the central Sahara : relationship with the Pan-African belt and Hoggar domal uplift A. LESQUER* , A. BOURMATrE**,S. LY*** and J.M. DAU'rRIA*
* Centre G(mlogiqueet G~,ophysique,Place E. Bataillon, 34060 - MontpelIier Ceclex, France ** Craag, Bouzareah - Alger, Alg&ie *** Ecole Nationale d'Ing(mieurs, B.P. 242, Bamako, Mall Abstract - The first heat flow determinations in the Sahara (Algeria, Mall, Niger) at 13 widely spaced sites are described.Six sites are locatedin the Late Precambrianbasementof the Pan-Africanbelt (600 Ma) and seven sites in the Late Paleozoic and Mesozoic sedimentary cover. The average value 53 mWm2 is comparable to results obtained from other Precambrian belts. The dislrihution of heat flow data according to the main geologicalunits suggests a possible relation with crustal structure and tectonichistory of the major traits of the Pan-Africanbelt. No significantregional thermal disturbance is associatedwith the Hoggar Cenozoicintraplate volcanism. The likelihood that large scale sweUingof the Precambrian basement is of thermal origin appears questionable. We propose a hypothesis involving variation of the thickness of the heat productive upper crust linked to a continuous erosional process during Cenozoic and Mesozoic times.
INTRODUCTION
The long w a v e l e n g t h negative gravity a n o m a l y a s s o c i a t e d with t h e Hoggar volcanic province s u g g e s t s t h a t a t h e r m a l l y p e r t u r b e d lithosphere is a s s o c i a t e d w i t h b a s e m e n t uplift. To t e s t this hypothesis, h e a t flow m e a s u r e m e n t s were carried o u t in 1984, 1985, 1986, s o m e t i m e s with g r e a t difficulties, m a i n l y in b o r e h o l e s drilled for m i n e r a l exploration. Finding 2 0 y e a r s old b o r e h o l e s in t h e S a h a r a w a s a l w a y s a difficult Job a n d often u n s u c c e s s f u l . The t e m p e r a t u r e survey, a t t e m p t e d in m o r e t h a n one h u n d r e d boreholes, yielded 3 0 g e o t h e r m a l gradients, w h i c h c a n b e g r o u p e d in 13 widely s p a c e d sites. GEOLOGY
D u r i n g t h e Pan-African orogeny, a wide mobile belt w a s f o r m e d e a s t w a r d of t h e W e s t Pan-African craton. F r o m v a r i o u s s o u r c e s of evidence, a complete Wilson cycle h a s b e e n p r o p o s e d ; initiated a b o u t 8 0 0 Ma, it e n d e d a b o u t 6 0 0 Ma ago b y t h e collision b e t w e e n t h e p a s s i v e m a r g i n of t h e c r a t o n a n d t h e active m a r g i n of t h e Pan-African mobile belt (Bayer a n d L e s q u e r 1978; B l a c k et a t 1979; C a b y et a t 1981). In t h e Hoggar, f o u r s t r u c t u r a l d o m a i n s have b e e n defined, s e p a r a t e d b y m a j o r N-S l i n e a m e n t s . T h e y are from w e s t to e a s t : t h e s u t u r e zone, t h e P h a r u s i a n belt, the Central Hoggar, t h e E a s t e m H o g g a r (Fig. 1). The s u t u r e zone is c h a r a c t e r i z e d b y island arc a n d m a r g i n a l t r o u g h volcano-clastic a s s e m b l a g e s . 41
High p r e s s u r e - low t e m p e r a t u r e internal n a p p e s are t r a n s l a t e d into t h e c r a t o n margin. A string of u l t r a b a s i c to b a s i c r o c k s w a s e v i d e n c e d b y gravity surveys. The P h a r u s i a n belt c o m p r i s e s a w e s t e m a n d a n eastern branch both characterized by an a b u n d a n c e of U p p e r Proterozoic volcano-detritic m a t e r i a l s ('S6rie verte"). The In OuTJal a n d t h e Iforas u n i t s s e p a r a t e t h e s e two b r a n c h e s . The InOuzzal u n i t w h i c h m o s t l y i n c l u d e s g r a n u l i t e s of E b u m e a n age ( 2 0 0 0 Ma) h a s b e e n u n a f f e c t e d during t h e Pan-African o r o g e n y a n d c o n t r a s t s tectonically with t h e highly d e f o r m e d a d j a c e n t P h a r u s i a n belt. The P h a r u s i a n belt is b o r d e r e d e a s t w a r d b y t h e 4 ° 50 m e g a fault. The Central Hoggar is largely c o m p o s e d of g r a n u l i t e s a n d gneisses, E b u r n e a n in age (2075 + 2 0 Ma), reactivated a n d injected b y a b u n d a n t granitoids during t h e Pan-African o r o g e n y (Bertrand et a t , 1984, 1986). The Eastern Hoggar-T6n6r6 domain was stabilized at a n early s t a g e of t h e Pan-African episode a r o u n d 725 Ma ago. It i n c l u d e s a Late Pan-African ensialic linear belt (Tlririne belt) along its w e s t e r n m a r g i n (8 ° 50 m e g a fault). The collision induced intense intraplate d e f o r m a t i o n in N-S linear belt a c c o m p a n i e d b y c r u s t a l thickening, g e n e r a t i o n of granite a n d m a j o r lateral d i s p l a c e m e n t along m e g a s h e a r zones. After p e n e p l a n a t i o n d u r i n g C a m b r i a n times, t h e belt h a s b e e n b u r i e d u n d e r 3 to 7 k m of Paleozoic s e d i m e n t s . T h e s e s e d i m e n t s are n o w completely
42
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eroded in the Hoggar area and the scarce continental Cretaceous deposits are found directly on the Precambrian basement. This feature implies important pre-Cretaceous uplift and intensive erosion. The recent magmatic activity of the Hoggar b e g a n d u r i n g the U p p e r C r e t a c e o u s a n d Eocene (Remy, 1959; Rossi et al., 1979). The Miocene and Pliocene to Q u a t e m a r y volcanic activity is typical of intraplate alkali volcanism (Girod, 1971) and occurred in five m a i n districts (Tahalra, Atakor, Manzaz, Egg~r6, Adrar N'Ajjer). These d i s t r i c t s c o r r e s p o n d to z o n e s w h e r e the Precambrian b a s e m e n t has been uplifted (up to 2600 m in the Atakor) during the Miocene and Pliocene. These domes (150 k m wide) are superimposed on a very large b a s e m e n t swell (1000 k m wide) where altitude ranges from 350-400 m on the
outer part, to 1000-1200 m in the central part. DATA P R O C E S S I N G Temperature gradient
The t e m p e r a t u r e s have been m e a s u r e d (in water) in shallow (100-150 m) a n d sometimes tilted boreholes, at thermal equilibrium, at intervals of 5 or 10 meters, with a thermistor probe equipment (relative a c c u r a c y : 0.OI°C). The heat flow sites are plotted on the simplified tectonic m a p s of Fig. 1 and the t e m p e r a t u r e s profiles are presented in Fig. 2. Table 1 shows the data for each heat flow m e a s u r e m e n t . The lack of precise information on paleoclimatic and erosional history precludes the calculation of corrections for these effects, which are probably not negligible, especially for the uplifted and erod-
First heat flow determination from the central Sahara TEMPERATURE 2
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p r o f i l e s for e a c h b o r e h o l e s . N u m b e r s i n d i c a t e s i t e s a s s h o w n i n t a b l e 1 a n d Pig. I. Boreholes names are given at bottom.
ed a r e a of t h e Hoggar. For e x a m p l e w i t h i n F r e n c h M a s s i f C e n t r a l p a l e • c l i m a t i c c o r r e c t i o n s is e s t i m a t e d to 8 - 1 5 m W m -2 n e a r t h e s u r f a c e (Vasseur a n d L u c a z e a u , 1982). Most of t h e sites are located w i t h i n fiat a r e a s w h e r e t h e d i s t u r b a n c e d u e to t o p o g r a p h y is negligible. The b o r e h o l e s (1 to 20 y e a r s old) are p r o b a b l y n e a r to perfect t h e r m a l e q u i l i b r i u m a n d w i t h o u t a n y evident hydrological p e r t u r b a t i o n , except for t h e site of Malbaza (5) drilled for hydrological p u r p o s e a few d a y s before o u r m e a s u r e m e n t . The d e p a r t u r e f r o m a s t r a i g h t line observed in several g e o t h e r m s c a n be easily correlated to v a r i a t i o n in lithology (i.e. conductivity). The effect of p e r t u r b a t i o n s , r e s u l t i n g from o t h e r c a u s e s s u c h a s lateral v a r i a t i o n s in t h e r m a l conductivity, are m o r e difficult to evaluate, especially w i t h i n t h e b a s e m e n t w h e r e complex geological s t r u c t u r e s p e r t u r b t h e u n d e r g r o u n d t e m p e r a ture distribution. Conductivity
ARLI 536
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Except for s o m e sites w h e r e c o n d u c t i v i t y w a s e s t i m a t e d f r o m lithology (sites 1,3, 5), t h e c o n d u c tivity w a s m e a s u r e d o n cores (sites 2, 4, 6, 7, 8, 9, 13) or o n s u r f a c e or m i n e s a m p l e s (sites 10, 11, 12). Conductivity has been measured by a transient m e t h o d b o t h o n d r y a n d w a t e r s a t u r a t e d samples. The b a s e m e n t core s a m p l e s are r e p r e s e n t a t i v e of lithological c o l u m n generally h o m o g e n e o u s a n d
t h e conductivities v a r y f r o m 2.3 W m -~° C -t (gabbros) to 4.2 W m ~° C "1 ( m e t a m o r p h i c rocks). For s e d i m e n t a r y rocks, t h e r m a l c o n d u c t i v i t y m a i n l y d e p e n d s o n t h e c o n d u c t i v i t y of e a c h c o n s t i t u e n t of t h e r o c k a n d of t h e fluid w h i c h fills t h e pores. For t h e b o r e h o l e s w i t h o u t core t h e r e s u l t s of B r i g a u d (1986, u n p u b l i s h e d ) were u s e d to e s t i m a t e t h e b u l k c o n d u c t i v i t y f r o m lithological description. To estimate porosity c h a n g e with d e p t h (negligible for shallow boreholes), a theoretical exponential law w a s u s e d . T h e c o n d u c t i v i t i e s v a r y from 1.4 W m 1° C "~ for C a r b o n i f e r o u s s h a l e s to 5.9 W m ~° C 1 for D e v o n i a n quartzic s a n d s t o n e s . Estimation
of heat
flow
For u n i f o r m borehole lithology we e s t i m a t e h e a t flow b y m u l t i p l y i n g t h e r m a l g r a d i e n t b y m e a s u r e d or e s t i m a t e d conductivity. For c o m p l e x lithology (sites 1 a n d 5), i n t e g r a t e d t h e r m a l r e s i s t a n c e is u s e d a n d h e a t flow is c a l c u l a t e d f r o m t h e B u l l a r d linear relation (Bullard 1940). DISCUSSION
Heat flow r a n g e s f r o m 24 m W m -2 (site 2) to 75 m W m -2 (site 5) w i t h a n average of 53 + 13 m W m 2. This m e a n v a l u e c o m p a r a b l e with t h a t of o t h e r late P r e c a m b r i a n b e l t s is g r e a t e r t h a n t h a t from t h e W e s t African c r a t o n (37 + 8 mWm-2; B r i g a u d et aL 1985). In spite of a poor sampling, d i s t r i b u t i o n t h i s difference a p p e a r s
A. LESQUER,A. BOURMATI~,S. LY and J.M. DAUIRIA
44
Table 1. Thermal gradients, estimation of conductivity and estimation of heat flow for each borehole. Solid circles : estimated conductivity from llthology; open squares : conductivity measurements on core samples; solid squares : conductivity measurements on core sample from nearby boreholes; solid triangles: conductivity measurements on surface samples or mining samples. ~ite
A l t i t u d e Range Depth
Borehole
(m)
Age
Lithology
(m)
Thermal gradient
CowJuctivity
('C/l~)
(iCm:l°C -1 )
1 TANEZROU~T
$355 $352
450 450
90-290 70-250
Sandstone/ Siltstones
2 TIREg
5219 5180
580 580
30-120 40-140
Gabbro
3
MW5
440
20- 95
Sandstone
(RETACEOUS14,0
4 NAIi~A
550 550
45-100 55-100
Granite Granite
5 OMALBAZA
300
50-460
Shales/ Standstones
1100
50-115
Gneiss
7
500
30-180
2.8
± 0.05[]
500
30-230
G a b b r o diorite 13.5 Gabbro d i o r i t e PRECAHBRIEN 1 4 . 0
1,0
IN-ABEGGIII
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2~8
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16-130 1 6 - 85
Granite Granite
11.0 10.0
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TIN AMZI
8 BACHIR
S 42 S 66
900 900
0 TIN SERIRINE
F105 F106
555 614
F101 FI04 1342
590 610 550
10
Arli
ARLIT
Somair 2079 430
11
536
13
TIHOIILET [NE
430 435
117-171 171-335 335-&60
Sandstones Shales Sandstones
70- 95 95-200 100-160 3 0 - 55 55-195
Sandstones S a n d st/S hal es Sandstones Sandstones Shales
348bis
PNC
138
510
70-170
Sandst/Shales
1,3
510
50190
saodst,sha os
PNC G 132
510
50-100
Sandstones
500
20-195
Sandstone
050 850 850 850
100-190 80-150 95-170 90-250
TA CAHP TIMOU TIHOtJ 71HOU TIHOU
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significant, a n d the general trend of average heat flow decreasing with tectonic age is similar to that observed worldwide (Vitorello a n d Pollack, 1980). The relation b e t w e e n heat flow value, rock type and tectonic setting h a s b e e n e x a m i n e d for each site from w e s t (West African Craton) to east through the Pan-African belt. Unfortunately no radiogenic heat production m e a s u r e m e n t s are yet available. To estimate near surface source contributions, the a u t h o r s h a d to u s e heat production data from other Precambrian areas. Heat flow and Pan-African belt structure Data plotted on Fig. 1 s h o w that on the eastern margin of the craton the heat flow v a l u e s range
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b e t w e e n 2 0 m W m -~ in granulitic b a s e m e n t to 68 m W m 2 within the reactivated Pan-African s u t u r e zone of Adrar des Iforas (Mall). Eastward within the Pan-African belt, the two sites of Tanezrouft (site 1) a n d Tirek (site 2) which are 50 km apart b u t situated in very different geological setting display different values. The first one (53 m W m -2} c o r r e s p o n d s to two boreholes drilled for water in a little Cretaceous trough (50 kin wide, deposit t h i c k n e s s near 8 0 0 m) trending NE-SW t h r o u g h the P h a r u s i a n b a s e m e n t of the w e s t e r n branch (gneiss, granites, greywackes). The s e c o n d one (24 m W m "2) i n c l u d e s three shallow boreholes drilled in g a b b r o s on the eastern side of the E b u r n e a n In-Ouzzal granulitic
45 First heat flow determination from the central Sahara unit. Three h u n d r e d kilometers southwards, east Heat flow a n d s e d i m e n t a r y c o v e r The average heat flow value for the sites of the "Adrar des floras" massif, the single shallow borehole of In-Tamat (site 3) exhibits a heat flow (3,5,7,9,10,11,12) within the s e d i m e n t a r y cover, value of 53 mWm "2. This borehole was drilled in a s u r r o u n d i n g the Hoggar-Air m a s s i f is slightly hi500 m thick Cretaceous s a n d s t o n e which overlies gher (58 + I0 mWm 2) t h a n the m e a n for the granitoids a n d schists of the E a s t e r n P h a r u s i a n P r e c a m b r i a n f o r m a t i o n s (42 + 14 m W m -2) considered as the basins basement. branch. The borehole of Malbaza (site 5;73 mWm 2) is The value at Tlrek (site 2) agrees with w h a t is expected for the tectonic age of the In-OuT:,~al unit situated within the central part of a basin (2150-2050 Ma) a n d the well k n o w n low (Iullemedem, south of the Hoggar). All the other radiogenic value of granulite, i.e. from 0.2 to m e a s u r e m e n t s were performed on basin margins. 0.4 ~tWm-3 (Pinet a n d J a u p a r t , 1987). This heat flow The data coverage is too u n e v e n to state that the value is similar to t h a t m e a s u r e d (20 mWm 2) by higher heat flow values are indicative either of a C h a p m a n a n d Pollack (1974) in E b u r n e a n West regional trend (associated with basin tectonic evoAfrican shield a n d corresponds to the b a c k g r o u n d lution) or of local variations (associated with shalheat flow from deep source (reduced heat flow) low Pan-African b a s e m e n t structure). We shall see calculated for the Precambrian shields. The further other possible relationships with erosion. In-OuT~al unit being only 50 k m wide, it is not It is interesting to note t h a t the reported m e a n realistic to associate this very low heat flow value values from other areas ofpost-Precambrian cover to a m a n t l e t h e r m a l anomaly. We m u s t consider are systematically higher t h a n in the exposed this unit as a crustal block representing lower Precambrian crystalline rocks (Rao a n d Rao 1983). crustal t e r r a n e s strongly depleted in radiogenic For the Indian shield, the higher heat flow observed in the Gondwana basin agrees with a high h e a t isotopes. The boreholes are very shallow and the generation from the Precambrian b a s e m e n t u n d e r complexity of local geological structure probably the basins (Rao and Rao 1983). complicates the u n d e r g r o u n d temperature distribution. Although the value at Tirek is consis- Heat flow a n d m i d p l a t e v o l c a n i s m According to the hot spot tectonic hypothesis, tent with the age a n d the tectonic setting of the In-O11~al granulitic unit, this value m u s t be used the Hoggar swell is regarded by m a n y a u t h o r s as of with caution for speculation on deep lithospheric thermal origin. The long wavelength negative Bouguer anomaly connected with Hoggar is similar structure. Ten heat flow sites within the Central Hoggar to those associated with other domal upliils or rift b a s e m e n t a n d its Paleozoic cover yield values systems. Such anomalies are classically consideranging from 36 m W m -2 to 66 mWm 2 with a n red as resulting from a t h i n n e d lithosphere (Brown average of 55 m W m 2. These values are comparable and Girdler 1980). to those observed in most Precambrian belts. In Investigations on u p p e r mantle inclusions in the the b a s e m e n t , the lowest values (site 7) are obtain- recent basalts (Girod et aL 1981; Leblanc et aL ed within gabbros, generally considered as poorly 1982; D u p u y etal. 1986; Dautria etal. 1987) and radioactive material. The highest values (site 4, 6) a gravity interpretation (Crough 1981) support the correspond to boreholes crosscutting at shallow occurence of a low density body ofmagmatic origin depth the U-rich 'Taourlxt granite" (Boissonnas, at less t h a n 60 k m below the surface. The domal 1973). These granitic intrusions belong to a Late uplift could be the isostatic response to this body, Precambrian to Lower Cambrian (560 + 40 Ma) the emplacement of which probably induced a magmatic episode. reheating of the s u r r o u n d i n g u p p e r mantle. Within the Paleozoic cover, the highest values (60 We have shown t h a t no thermally perturbed to 66 m W m a) are o b t a i n e d in Triasic to lithosphere is evidenced by heat flow data, Carboniferous sediments (sandstones, shales) along whereas heat flow anomalies greater t h a n 80 m W m the w e s t e m side of the Air m a s s i f (site 10, 11, 12). 2are generally observed in rift zone a n d uplift area Northward, the heat flow at site 9 within Devonian (Lucazeau a n d Bayer 1982). The m e a s u r e d heat q u a r t z - s a n d s t o n e s is lower with a n avgrage of 49 flow is normal for a c r u s t of that age a n d involves mWm -2. a lithosphere more t h a n 100 k m thick, assuming Within the E a s t e r n Hoggar domain, stabilized at a conductive geotherm a n d steady state condia n early stage of Pan-African orogeny (725 Ma), the tions. So Crough's low density zone c a n n o t be four boreholes of Tlmouletine (13) drilled within regarded as a n upwelling of the lithosphere/asthe Tiririne formations, exhibit a lower m e a n value t h e n o s p h e r e boundary. F u r t h e r m o r e decrease of (41 mWm-2). heat flow with b a s e m e n t elevation (Fig. 3a) is
46
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inconsistent with thermal uplift or recent erosion. The site (6) of Tin Amzi (the only one within the Cenozoic volcanic) area (Fig. 3b), deviates significantly from the general trend. This fact m a y indicate a local increase of heat flow with altitude i n t h e Atakor massif, which m a y be related with a small size thermal s t r u c t u r e linked to recent volcanic activity. Data distribution does not allow a description of heat flow variation in the n o r t h e m part of the Hoggar a n d particularly in the volcanic high massffs. However we t h i n k t h a t heat flow data strongly suggests that the Hoggar subcrustal mantle is not anomalously hot and t h a t the swell is not caused by a reheating of the lithosphere. Recent estimates of the equilibrium conditions of the peridotite xenoliths e n t r a i n e d by the Plio-Quatemary basaltic eruptions have been carried out. The pyroxene phases of all the samples, whatever the volcanic district, were
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equilibrated between 900 and 1100°C (Fig. 4) along an "oceanic geotherrn" (Girod et al., 1981). This accounts for a n uprising of the isotherms beneath the volcanic Hoggar, corresponding to a thermal event evidently related to the intense recent
First heat flow determination from the central Sahara 47 magmatism. On the other h a n d the systematical- Paleozoic a n d Mesozoic cover where the b a s e m e n t ly lower t e m p e r a t u r e s (near 8 0 0 - 8 5 0 ° C) estimated remains unaffected by erosion. from olivine spinel pairs in the same samples, CONCLUSION suggest a n equilibrium along a "shield geotherm" (Fig. 4). Because of the easy reset of the olivineIn spite of their limited n u m b e r , these data allow spinel t h e r m o m e t e r u p o n c h a n g e s in the equilia first insight into the t h e r m a l r e g i m e of the Central b r i u m conditions (FabriCs 1979), these later values should reflect the conditions at the time of Sahara. T h u s we have now some indications to the xenolith e n t r a p m e n t by the ascending m a g m a support the hypothesis that the major structural (therefore during the Plio-Quaternary). This units of the Pan-African belt have different average accounts for a recent cooling of the u p p e r mantle, heat flow values which c a n be explained by corresthat is in agreement with the very low Q u a t e r n a r y ponding variations of crustal heat production. This volcanism, and, consequently the thermal pertur- hypothesis agrees with tectonic models that consibance m u s t be ante-Pliocene. der the belt to be composed of a n amalgam of Considering t h e recent magmatlc history of the crustal blocks with different tectonic histories. Hoggar two hypotheses c a n be proposed : the ther- Heat flow values correlate negatively with topogramal perturbation is either Miocene (associated phy, suggesting a possible variation in crustal heat with the paroxysmal volcanism) or Upper- production which m a y be due to the long and Cretaceous to Eocene {associated to the early mag- complicated multistage erosional history of the m a t i s m described in the E a s t e m Hoggar). As- Hoggar region. In order to test this hypothesis, suming t h a t h e a t is transported by conduction, a heat production m e a s u r e m e n t s will be carried out time of 25 Ma is n e c e s s a r y for a thermal pertur- in the n e a r future. bance occurring at 60 k m depth to be felt at the The heat flow values from sites within the surface. For a kinematic model with transport of Hoggar are not anomalously high. This implies hot material t h r o u g h o u t the u p p e r mantle, heat that the Hoggar swell is not due to a large scale transport is more efficient. If the t h e r m a l perturba- uplift of hot material in the upper mantle. tion is Miocene, we m u s t accept a shorter thermal Nevertheless small size thermal disturbances u n d e r relaxation period (20 Ma), that c a n only be explain- the various volcanic districts should generate local ed ff we a s s u m e small size for the low density as- anomalies which we did not demonstrate possibly thenospheric intrusions responsible for reheating. because of our sampling distribution. Asthenospheric diaptric intrusions of ten kiloAcknowledgement - Field work has been supported meters, s u c h as t h a t proposed by Nicolas et oL (1987) for the French Massif Central, m a y be a by the A.S.P. Afrique (CNRS, France) and the cooperapossible model. If s u c h small s t r u c t u r e s really tion project ONRS (Algeria) - CNRS (France). This project was greatly facilited by the active cooperaoccur b e n e a t h the Hoggar, the possible remaining tion and assistance provided by the personnel of the heat perturbation m a y not have b e e n m a d e evident following o r g a n i s a t i o n s or c o m p a n i e s : E R E M by our m e a s u r e m e n t s of samples because of our (Entreprise de Recherche et d'Exploitatlon Mini~re, site distribution. Nevertheless this model does not Alger, Alg6rie); CRAAG (Centre de Recherche a c c o u n t for the large scale swell and for the large Astronomie, Astrophysique et Geophysique, Alger, volume of low density material m a d e evident by Algerie); DNGM (Direction Nationale de la Geologie et gravity (Crough 1981). An Upper Cretaceous to des Mines, Bamako, Mali); IRSH (Institut de Recherche Eocene t h e r m a l event associated with a n extensive en Sciences Humaines, Niamey, Niger); Facult6 des u p p e r mantle modification, would better explain Sciences de l'Universit6 de Niamey (Niger), Minist~re the present lack of large wavelength heat flow ano- des Mines (Niamey, Niger), SOMAIR (Societe des Mines de l'AIr, Niamey, Niger); COMINAK (Compagnie Mini6re maly. The p u ~ l t n g decrease of h e a t flow with b a s e m e n t d'Akouta. Niamey, Niger); PNC (Power Nuclear elevation (Fig. 3a) c a n be tentatively modeled as a Companie, Niamey, Niger); COGEMA (Compagnie G~nerale des Mati~res Nucleaires, Paris, France); consequence of erosion downcutting through the ORSTOM (Institut Fran~als de Recherche Scientiflque crustal heat source distribution. This relationship pour le Developpement en Cooperation, Paris, France). involves a variation of heat production rate or a variation of thickness of the heat productive zone REFERENCES (5 to 10 km) inconsistent with recent erosional processes. Bayer, R. and Lesquer, A. 1978. Les anomalies gravim6The uplifted Hoggar corresponds to a c r u s t which triques de la bordure orientale du craton ouesth a s been already uplifted a n d drastically eroded africain: g6om6trie d'une suture pan-africaine. Bu/l. during the Mesozoic a n d Cenozoic times. ConseS o c . Geol. f r a n c e , X X , 863-876. quently we c a n a s s u m e a productive c r u s t t h i n n e r Bertrand, J.M., Michard, A., Boullier, A.M., and Dautel, in the central part of the swell t h a n b e n e a t h the D. 1986. Structure and U/Pb geochronology of central
A. LESQUER,A. BOURMATrE,S. LY and J.M. DAUTPaA 48 Petrol., 6 9 : 329-336. Hoggar {Algeria): a reappraisal of its Pan-African Girod, M. 1971. Le massif volcanique de L'Atakor (Hogevolution. Tectonophysics, B, 7, 955-972. Black, R., Caby, R., Moussine- Pouchklne, A., Bayer, 1~, gar, SaharaAlg~rten).Mg~m.CRZA. 12, Ed. CN'RS Paris. Bertrand, J.M., Boulller, A.M., Fabre, J., and Lesquer, Girod, M. Dautria, J.M. and De Giovanni, R.,1981. A A. 1979. Evidence for late Precambrlan plate tectonics first insight into the constltuUon of the upper-mantle in West Africa. Nature, 250, 477-478. u n d e r the Hoggar area (Southern Algeria) : the lherzolite xenoliths in the alkali-basalts. Contrlb. Mineral Boissonnas, J. 1973. Les granites & structures PetroL, 77, 66-73. concentriques et quelques a u t r e s granites tardifs de la chalne Pan-Afrlcaine en Ahaggar {Sahara central, Leblanc, M., Dautria, J.M. and Girod, M. 1982. Magnesian ilmenite xenoliths in a basanite from TaAlg6rie). Mem CRZA (CNRS), Ser. Geol., N°18, 662p. halra, Ahaggar (Southern Algeria). Contr/b. Mineral. Brlgaud, F., Lucazeau, F., Ly, S. and Sauvage, J.F. 1985. Petrol 79, 347-354. Heat flow from the West African shield. Geophgs. Res. Lucazeau, F. and Bayer, R. 1982. Evolution geodynamiLett., 12, 549-552. Brigaud, F. 1986. Etude m*thodologique de la que d u Massif central Fran~ais depuis l'Oligoc~ne. conductivite thermique des roches sedimentaires. Ann. G~ophys., 38, 405-429. SNEAP report EP/S/F_,XP/DYN 86-003. Mercier, J.C. 1980. M a g n i t u d e of c o n t i n e n t a l Brown, C. and Girdler, R.W. 1980. Interpretation of lithospheric stresses inferred from rheomorphlc petrology. J. Geophys. Res., 8B, 6293-6303. African gravity and its implication for the break-up of the continents. J. Geophys. Res., 85, 6443-6455. Nicolas, A., Lucazeau, F. and Bayer, R. 1987. Peridotite xenoliths in Massif Central basalts, France: textural Bullard, E.C. 1940. The disturbance of the temperature a n d g e o p h y s i c a l e v i d e n c e for a s t h e n o s p h e r i c gradient in the earth crust by inequalities of height. diapirism., In "Mantle xenoliths", Ed P.H. Nixon. J o h n Mont. Nat. Astr. Soc. Geophys. Suppl., 4, 300Wiley and Sons, London. 566-573. 362. Caby, R., Bertrand, J.ML. and Black, R. 1981. Pan- Pinet, C. and J a u p a r t , C. 1987. The vertical distribution of radiogenic heat production in the Precambrian crust African ocean closure and continental collision in the of Norway and Sweden: Geothermal implications. H o g g a r - l f o r a s s e g m e n t , c e n t r a l S a h a r a . In: Geophys. Res. Lett., 14, 260-263. Precambrian Plate Tectonics (Exl.A.Kroner), Elsevier. Rao, G.V. and Rao, R.U.M. 1983. Heat flow in Indian Amsterdam, 407-434. Gondwana basins and heat production of their Chapman, D.S. and Pollack, H.N. 1987. "Cold spot" in b a s e m e n t rocks. Tectonophysics, 91, 105-117. West Africa anchoring the African plate, Nature, 250, Remy, J.M. 1959. Etude g*ologique et volcanologique du 477-478 sud-est de l'Amadror en Ahaggar (Sahara central). Crough, S.T. 1981. Free-air gravity over the Hoggar Th~se Doctorat, Universite de Paris. massif, northwest Africa: Evidence for alteration of Rossi, P.L., Lucchinl, F. and SaveUi, C. 1979. Donn*es lithosphere. Tectonophysics, 77, 189-202. geologiques et radiom6triques s u r la raise en place de Dautria, J.M., Liotard, J.M., Cabanes,N., Girod,M. and la Tellerteba (Hoggar). 10th colloque G6ologie Briqueu, L. 1987. Amphibole-rich xenoliths and host Africaine. Montpellier, 143. alkali b a s a l t s : P e t r o g e n e t i c c o n s t r a i n t s a n d implications on the recent evolution of the upper Vasseur, G. and Lucazeau, F. 1982. Some aspects of heat flow in France. in :Geothermlcs and Geothermal mantle beneath Ahaggar (central Sahara, southern energy (Budapest paper), V.Cermak and R. Haenel (Eds.). Algeria). Contrlb. Mineral. Petrol., 95, 133-144. Schweizerbart'scheVerlagsbuchhandlung, Stuttgart, Dupuy, C., Dostal, J., Dautria, J.M. and Girod, M. 1986. 79-90. Geochemistry of spinel peridotite inclusions in basalts from Hoggar, Algeria., J. African. Earth. ScL 5, Vitorello, I. and Pollack, H.N. 1980. On the variation of continental heat flow with age and the thermal 209-215. evolution of continents. J. Geophys. Res., 8B, 983-995. FabriCs, J. 1979. Spinel-olivine geothermometry in peridotites from ultramaflc complexes. Contrib. Mineral.
0899-5362/89 $3.00 + 0.00 (~) 1989 Pergamon Press plc
Printed in Great Britain.
First heat flow determination from the central Sahara : relationship with the Pan-African belt and Hoggar domal uplift A. LESQUER* , A. BOURMATrE**,S. LY*** and J.M. DAU'rRIA*
* Centre G(mlogiqueet G~,ophysique,Place E. Bataillon, 34060 - MontpelIier Ceclex, France ** Craag, Bouzareah - Alger, Alg&ie *** Ecole Nationale d'Ing(mieurs, B.P. 242, Bamako, Mall Abstract - The first heat flow determinations in the Sahara (Algeria, Mall, Niger) at 13 widely spaced sites are described.Six sites are locatedin the Late Precambrianbasementof the Pan-Africanbelt (600 Ma) and seven sites in the Late Paleozoic and Mesozoic sedimentary cover. The average value 53 mWm2 is comparable to results obtained from other Precambrian belts. The dislrihution of heat flow data according to the main geologicalunits suggests a possible relation with crustal structure and tectonichistory of the major traits of the Pan-Africanbelt. No significantregional thermal disturbance is associatedwith the Hoggar Cenozoicintraplate volcanism. The likelihood that large scale sweUingof the Precambrian basement is of thermal origin appears questionable. We propose a hypothesis involving variation of the thickness of the heat productive upper crust linked to a continuous erosional process during Cenozoic and Mesozoic times.
INTRODUCTION
The long w a v e l e n g t h negative gravity a n o m a l y a s s o c i a t e d with t h e Hoggar volcanic province s u g g e s t s t h a t a t h e r m a l l y p e r t u r b e d lithosphere is a s s o c i a t e d w i t h b a s e m e n t uplift. To t e s t this hypothesis, h e a t flow m e a s u r e m e n t s were carried o u t in 1984, 1985, 1986, s o m e t i m e s with g r e a t difficulties, m a i n l y in b o r e h o l e s drilled for m i n e r a l exploration. Finding 2 0 y e a r s old b o r e h o l e s in t h e S a h a r a w a s a l w a y s a difficult Job a n d often u n s u c c e s s f u l . The t e m p e r a t u r e survey, a t t e m p t e d in m o r e t h a n one h u n d r e d boreholes, yielded 3 0 g e o t h e r m a l gradients, w h i c h c a n b e g r o u p e d in 13 widely s p a c e d sites. GEOLOGY
D u r i n g t h e Pan-African orogeny, a wide mobile belt w a s f o r m e d e a s t w a r d of t h e W e s t Pan-African craton. F r o m v a r i o u s s o u r c e s of evidence, a complete Wilson cycle h a s b e e n p r o p o s e d ; initiated a b o u t 8 0 0 Ma, it e n d e d a b o u t 6 0 0 Ma ago b y t h e collision b e t w e e n t h e p a s s i v e m a r g i n of t h e c r a t o n a n d t h e active m a r g i n of t h e Pan-African mobile belt (Bayer a n d L e s q u e r 1978; B l a c k et a t 1979; C a b y et a t 1981). In t h e Hoggar, f o u r s t r u c t u r a l d o m a i n s have b e e n defined, s e p a r a t e d b y m a j o r N-S l i n e a m e n t s . T h e y are from w e s t to e a s t : t h e s u t u r e zone, t h e P h a r u s i a n belt, the Central Hoggar, t h e E a s t e m H o g g a r (Fig. 1). The s u t u r e zone is c h a r a c t e r i z e d b y island arc a n d m a r g i n a l t r o u g h volcano-clastic a s s e m b l a g e s . 41
High p r e s s u r e - low t e m p e r a t u r e internal n a p p e s are t r a n s l a t e d into t h e c r a t o n margin. A string of u l t r a b a s i c to b a s i c r o c k s w a s e v i d e n c e d b y gravity surveys. The P h a r u s i a n belt c o m p r i s e s a w e s t e m a n d a n eastern branch both characterized by an a b u n d a n c e of U p p e r Proterozoic volcano-detritic m a t e r i a l s ('S6rie verte"). The In OuTJal a n d t h e Iforas u n i t s s e p a r a t e t h e s e two b r a n c h e s . The InOuzzal u n i t w h i c h m o s t l y i n c l u d e s g r a n u l i t e s of E b u m e a n age ( 2 0 0 0 Ma) h a s b e e n u n a f f e c t e d during t h e Pan-African o r o g e n y a n d c o n t r a s t s tectonically with t h e highly d e f o r m e d a d j a c e n t P h a r u s i a n belt. The P h a r u s i a n belt is b o r d e r e d e a s t w a r d b y t h e 4 ° 50 m e g a fault. The Central Hoggar is largely c o m p o s e d of g r a n u l i t e s a n d gneisses, E b u r n e a n in age (2075 + 2 0 Ma), reactivated a n d injected b y a b u n d a n t granitoids during t h e Pan-African o r o g e n y (Bertrand et a t , 1984, 1986). The Eastern Hoggar-T6n6r6 domain was stabilized at a n early s t a g e of t h e Pan-African episode a r o u n d 725 Ma ago. It i n c l u d e s a Late Pan-African ensialic linear belt (Tlririne belt) along its w e s t e r n m a r g i n (8 ° 50 m e g a fault). The collision induced intense intraplate d e f o r m a t i o n in N-S linear belt a c c o m p a n i e d b y c r u s t a l thickening, g e n e r a t i o n of granite a n d m a j o r lateral d i s p l a c e m e n t along m e g a s h e a r zones. After p e n e p l a n a t i o n d u r i n g C a m b r i a n times, t h e belt h a s b e e n b u r i e d u n d e r 3 to 7 k m of Paleozoic s e d i m e n t s . T h e s e s e d i m e n t s are n o w completely
42
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Fig. I : Simplified geological m a p of Central S a h a r a s h o w i n g g e o t h e r m a l m e a s u r e m e n t s sites a n d c o r r e s p o n d i n g h e a t flow v a l u e s (in miUiwatts p e r squaremeter). 1 : Paleozoic a n d Mesozoic cover; 2 : Recent volcanics; 3 : n a p p e s ; 4 : W e s t e m a n d e a s t e r n P h a r u s i a n belt; 5 : Central H o g g a r unit; 6 E a s t e r n H o g g a r ( m e t a m o r p h i s m at a b o u t 7 2 5 Ma); 7 : I n - O u z z a l a n d Iforas Units ( E b u r n e a n granulites); 8 : P r e c a m b r i a n s e d i m e n t a r y cover o f W e s t African craton; 9 : W e s t A f r l c a n c r a t o n b a s e m e n t ; 10 : S u t u r e zone (from gravity data); 11 : Heat flow d a t a within cratonic e a s t e r n m a r g i n from B r i g a u d et aL, 1985; 12 : New h e a t flow data.
eroded in the Hoggar area and the scarce continental Cretaceous deposits are found directly on the Precambrian basement. This feature implies important pre-Cretaceous uplift and intensive erosion. The recent magmatic activity of the Hoggar b e g a n d u r i n g the U p p e r C r e t a c e o u s a n d Eocene (Remy, 1959; Rossi et al., 1979). The Miocene and Pliocene to Q u a t e m a r y volcanic activity is typical of intraplate alkali volcanism (Girod, 1971) and occurred in five m a i n districts (Tahalra, Atakor, Manzaz, Egg~r6, Adrar N'Ajjer). These d i s t r i c t s c o r r e s p o n d to z o n e s w h e r e the Precambrian b a s e m e n t has been uplifted (up to 2600 m in the Atakor) during the Miocene and Pliocene. These domes (150 k m wide) are superimposed on a very large b a s e m e n t swell (1000 k m wide) where altitude ranges from 350-400 m on the
outer part, to 1000-1200 m in the central part. DATA P R O C E S S I N G Temperature gradient
The t e m p e r a t u r e s have been m e a s u r e d (in water) in shallow (100-150 m) a n d sometimes tilted boreholes, at thermal equilibrium, at intervals of 5 or 10 meters, with a thermistor probe equipment (relative a c c u r a c y : 0.OI°C). The heat flow sites are plotted on the simplified tectonic m a p s of Fig. 1 and the t e m p e r a t u r e s profiles are presented in Fig. 2. Table 1 shows the data for each heat flow m e a s u r e m e n t . The lack of precise information on paleoclimatic and erosional history precludes the calculation of corrections for these effects, which are probably not negligible, especially for the uplifted and erod-
First heat flow determination from the central Sahara TEMPERATURE 2
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p r o f i l e s for e a c h b o r e h o l e s . N u m b e r s i n d i c a t e s i t e s a s s h o w n i n t a b l e 1 a n d Pig. I. Boreholes names are given at bottom.
ed a r e a of t h e Hoggar. For e x a m p l e w i t h i n F r e n c h M a s s i f C e n t r a l p a l e • c l i m a t i c c o r r e c t i o n s is e s t i m a t e d to 8 - 1 5 m W m -2 n e a r t h e s u r f a c e (Vasseur a n d L u c a z e a u , 1982). Most of t h e sites are located w i t h i n fiat a r e a s w h e r e t h e d i s t u r b a n c e d u e to t o p o g r a p h y is negligible. The b o r e h o l e s (1 to 20 y e a r s old) are p r o b a b l y n e a r to perfect t h e r m a l e q u i l i b r i u m a n d w i t h o u t a n y evident hydrological p e r t u r b a t i o n , except for t h e site of Malbaza (5) drilled for hydrological p u r p o s e a few d a y s before o u r m e a s u r e m e n t . The d e p a r t u r e f r o m a s t r a i g h t line observed in several g e o t h e r m s c a n be easily correlated to v a r i a t i o n in lithology (i.e. conductivity). The effect of p e r t u r b a t i o n s , r e s u l t i n g from o t h e r c a u s e s s u c h a s lateral v a r i a t i o n s in t h e r m a l conductivity, are m o r e difficult to evaluate, especially w i t h i n t h e b a s e m e n t w h e r e complex geological s t r u c t u r e s p e r t u r b t h e u n d e r g r o u n d t e m p e r a ture distribution. Conductivity
ARLI 536
determination
Except for s o m e sites w h e r e c o n d u c t i v i t y w a s e s t i m a t e d f r o m lithology (sites 1,3, 5), t h e c o n d u c tivity w a s m e a s u r e d o n cores (sites 2, 4, 6, 7, 8, 9, 13) or o n s u r f a c e or m i n e s a m p l e s (sites 10, 11, 12). Conductivity has been measured by a transient m e t h o d b o t h o n d r y a n d w a t e r s a t u r a t e d samples. The b a s e m e n t core s a m p l e s are r e p r e s e n t a t i v e of lithological c o l u m n generally h o m o g e n e o u s a n d
t h e conductivities v a r y f r o m 2.3 W m -~° C -t (gabbros) to 4.2 W m ~° C "1 ( m e t a m o r p h i c rocks). For s e d i m e n t a r y rocks, t h e r m a l c o n d u c t i v i t y m a i n l y d e p e n d s o n t h e c o n d u c t i v i t y of e a c h c o n s t i t u e n t of t h e r o c k a n d of t h e fluid w h i c h fills t h e pores. For t h e b o r e h o l e s w i t h o u t core t h e r e s u l t s of B r i g a u d (1986, u n p u b l i s h e d ) were u s e d to e s t i m a t e t h e b u l k c o n d u c t i v i t y f r o m lithological description. To estimate porosity c h a n g e with d e p t h (negligible for shallow boreholes), a theoretical exponential law w a s u s e d . T h e c o n d u c t i v i t i e s v a r y from 1.4 W m 1° C "~ for C a r b o n i f e r o u s s h a l e s to 5.9 W m ~° C 1 for D e v o n i a n quartzic s a n d s t o n e s . Estimation
of heat
flow
For u n i f o r m borehole lithology we e s t i m a t e h e a t flow b y m u l t i p l y i n g t h e r m a l g r a d i e n t b y m e a s u r e d or e s t i m a t e d conductivity. For c o m p l e x lithology (sites 1 a n d 5), i n t e g r a t e d t h e r m a l r e s i s t a n c e is u s e d a n d h e a t flow is c a l c u l a t e d f r o m t h e B u l l a r d linear relation (Bullard 1940). DISCUSSION
Heat flow r a n g e s f r o m 24 m W m -2 (site 2) to 75 m W m -2 (site 5) w i t h a n average of 53 + 13 m W m 2. This m e a n v a l u e c o m p a r a b l e with t h a t of o t h e r late P r e c a m b r i a n b e l t s is g r e a t e r t h a n t h a t from t h e W e s t African c r a t o n (37 + 8 mWm-2; B r i g a u d et aL 1985). In spite of a poor sampling, d i s t r i b u t i o n t h i s difference a p p e a r s
A. LESQUER,A. BOURMATI~,S. LY and J.M. DAUIRIA
44
Table 1. Thermal gradients, estimation of conductivity and estimation of heat flow for each borehole. Solid circles : estimated conductivity from llthology; open squares : conductivity measurements on core samples; solid squares : conductivity measurements on core sample from nearby boreholes; solid triangles: conductivity measurements on surface samples or mining samples. ~ite
A l t i t u d e Range Depth
Borehole
(m)
Age
Lithology
(m)
Thermal gradient
CowJuctivity
('C/l~)
(iCm:l°C -1 )
1 TANEZROU~T
$355 $352
450 450
90-290 70-250
Sandstone/ Siltstones
2 TIREg
5219 5180
580 580
30-120 40-140
Gabbro
3
MW5
440
20- 95
Sandstone
(RETACEOUS14,0
4 NAIi~A
550 550
45-100 55-100
Granite Granite
5 OMALBAZA
300
50-460
Shales/ Standstones
1100
50-115
Gneiss
7
500
30-180
2.8
± 0.05[]
500
30-230
G a b b r o diorite 13.5 Gabbro d i o r i t e PRECAHBRIEN 1 4 . 0
1,0
IN-ABEGGIII
± 20
2~8
t
16-130 1 6 - 85
Granite Granite
11.0 10.0
t t
8.0 15.o 10.o ORDOVICIAN 8.0 8.0 35.0 9.0
± t t ~ ± ± ~
6
IN TAHAT
TIN AMZI
8 BACHIR
S 42 S 66
900 900
0 TIN SERIRINE
F105 F106
555 614
F101 FI04 1342
590 610 550
10
Arli
ARLIT
Somair 2079 430
11
536
13
TIHOIILET [NE
430 435
117-171 171-335 335-&60
Sandstones Shales Sandstones
70- 95 95-200 100-160 3 0 - 55 55-195
Sandstones S a n d st/S hal es Sandstones Sandstones Shales
348bis
PNC
138
510
70-170
Sandst/Shales
1,3
510
50190
saodst,sha os
PNC G 132
510
50-100
Sandstones
500
20-195
Sandstone
050 850 850 850
100-190 80-150 95-170 90-250
TA CAHP TIMOU TIHOtJ 71HOU TIHOU
2 5 3 I0
1
Volcanodetrztlc
schlsts
significant, a n d the general trend of average heat flow decreasing with tectonic age is similar to that observed worldwide (Vitorello a n d Pollack, 1980). The relation b e t w e e n heat flow value, rock type and tectonic setting h a s b e e n e x a m i n e d for each site from w e s t (West African Craton) to east through the Pan-African belt. Unfortunately no radiogenic heat production m e a s u r e m e n t s are yet available. To estimate near surface source contributions, the a u t h o r s h a d to u s e heat production data from other Precambrian areas. Heat flow and Pan-African belt structure Data plotted on Fig. 1 s h o w that on the eastern margin of the craton the heat flow v a l u e s range
51 t 55 *
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0.1 ~ 0.1D
25 ± 23 ~
~ 2~
~ fl t
0.5~
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].4 31
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65 ~ 64 t
UPPER CRETACEOUS 25,0 ± J ( i
C.8 ~ 0 . 2 ~
PRECAMBRIEN
0uartzy/Sandst.
5441et
3,6 t 0 . 4 ~ 3.3 + 0 . 3 ~
PRECAHBRIEN 21.0 ~ P . i ;
Sandstone Shales/Sandst, Ouartzy/Sandst. Ouartzy/Sandst. Quartzy/Sandst. Shales
Arla
± 0.3 t 3 (I
PRECAMBRIAN11.0 10.5
40-185 50-I00 100-180 70-200 80-220 40-170 170-130
Mala
P.C 12 ANOU ARAREN
430
UPPER 14.0 CRETACEOUS 1 7 . 0
Heat fzo,,~
CARBONIFEROUS
PER.,A T RIAS TRIAS )URASSIC
PRECAHBRIEN
t
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t t
16 16
G I 5 2&t 6
8
7
65±7
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73 ~ 14
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8
38
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6
3915
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3.5 ± 0 . 0 5 ~ 38 t 3 . 5 ~ O . 1 0 ~ 35 ~
4 4
36~4
!0 10 1 0 ~ 0 1,0 20 10
5.3 115 5.9 5 5 53 1 G 5.5
~ t t ~
0.2 [] 0.3 • 0.2 D 9.2 ~ t 0.2 [ ] ~ 0.3~ ~ 0.2 •
4 5 ~ 02 • 1.4 ~ 0.2 •
45
~ t 59 t 4& ±
8 G ? 43 t G 56 ~ 10 50 ~ 7 52
1 4 , 0 t 2.0 48.0 t 30 1 4,0 t 2.9 I0.0 t I 0 24.0 t 3.0 140 ? 2.0 12.0 ~ 10 &8.0 ~ 20
~5 2.8 ~ 3 4.5 !,4
t 0.3~ 65 t 03~[45)t t 0 . 5 ~ 68 t 0.3~61 t 0.2 • 54 t 0.2 • 67
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± 04
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t 20
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39
41 39 39
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20,8 140
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b e t w e e n 2 0 m W m -~ in granulitic b a s e m e n t to 68 m W m 2 within the reactivated Pan-African s u t u r e zone of Adrar des Iforas (Mall). Eastward within the Pan-African belt, the two sites of Tanezrouft (site 1) a n d Tirek (site 2) which are 50 km apart b u t situated in very different geological setting display different values. The first one (53 m W m -2} c o r r e s p o n d s to two boreholes drilled for water in a little Cretaceous trough (50 kin wide, deposit t h i c k n e s s near 8 0 0 m) trending NE-SW t h r o u g h the P h a r u s i a n b a s e m e n t of the w e s t e r n branch (gneiss, granites, greywackes). The s e c o n d one (24 m W m "2) i n c l u d e s three shallow boreholes drilled in g a b b r o s on the eastern side of the E b u r n e a n In-Ouzzal granulitic
45 First heat flow determination from the central Sahara unit. Three h u n d r e d kilometers southwards, east Heat flow a n d s e d i m e n t a r y c o v e r The average heat flow value for the sites of the "Adrar des floras" massif, the single shallow borehole of In-Tamat (site 3) exhibits a heat flow (3,5,7,9,10,11,12) within the s e d i m e n t a r y cover, value of 53 mWm "2. This borehole was drilled in a s u r r o u n d i n g the Hoggar-Air m a s s i f is slightly hi500 m thick Cretaceous s a n d s t o n e which overlies gher (58 + I0 mWm 2) t h a n the m e a n for the granitoids a n d schists of the E a s t e r n P h a r u s i a n P r e c a m b r i a n f o r m a t i o n s (42 + 14 m W m -2) considered as the basins basement. branch. The borehole of Malbaza (site 5;73 mWm 2) is The value at Tlrek (site 2) agrees with w h a t is expected for the tectonic age of the In-OuT:,~al unit situated within the central part of a basin (2150-2050 Ma) a n d the well k n o w n low (Iullemedem, south of the Hoggar). All the other radiogenic value of granulite, i.e. from 0.2 to m e a s u r e m e n t s were performed on basin margins. 0.4 ~tWm-3 (Pinet a n d J a u p a r t , 1987). This heat flow The data coverage is too u n e v e n to state that the value is similar to t h a t m e a s u r e d (20 mWm 2) by higher heat flow values are indicative either of a C h a p m a n a n d Pollack (1974) in E b u r n e a n West regional trend (associated with basin tectonic evoAfrican shield a n d corresponds to the b a c k g r o u n d lution) or of local variations (associated with shalheat flow from deep source (reduced heat flow) low Pan-African b a s e m e n t structure). We shall see calculated for the Precambrian shields. The further other possible relationships with erosion. In-OuT~al unit being only 50 k m wide, it is not It is interesting to note t h a t the reported m e a n realistic to associate this very low heat flow value values from other areas ofpost-Precambrian cover to a m a n t l e t h e r m a l anomaly. We m u s t consider are systematically higher t h a n in the exposed this unit as a crustal block representing lower Precambrian crystalline rocks (Rao a n d Rao 1983). crustal t e r r a n e s strongly depleted in radiogenic For the Indian shield, the higher heat flow observed in the Gondwana basin agrees with a high h e a t isotopes. The boreholes are very shallow and the generation from the Precambrian b a s e m e n t u n d e r complexity of local geological structure probably the basins (Rao and Rao 1983). complicates the u n d e r g r o u n d temperature distribution. Although the value at Tirek is consis- Heat flow a n d m i d p l a t e v o l c a n i s m According to the hot spot tectonic hypothesis, tent with the age a n d the tectonic setting of the In-O11~al granulitic unit, this value m u s t be used the Hoggar swell is regarded by m a n y a u t h o r s as of with caution for speculation on deep lithospheric thermal origin. The long wavelength negative Bouguer anomaly connected with Hoggar is similar structure. Ten heat flow sites within the Central Hoggar to those associated with other domal upliils or rift b a s e m e n t a n d its Paleozoic cover yield values systems. Such anomalies are classically consideranging from 36 m W m -2 to 66 mWm 2 with a n red as resulting from a t h i n n e d lithosphere (Brown average of 55 m W m 2. These values are comparable and Girdler 1980). to those observed in most Precambrian belts. In Investigations on u p p e r mantle inclusions in the the b a s e m e n t , the lowest values (site 7) are obtain- recent basalts (Girod et aL 1981; Leblanc et aL ed within gabbros, generally considered as poorly 1982; D u p u y etal. 1986; Dautria etal. 1987) and radioactive material. The highest values (site 4, 6) a gravity interpretation (Crough 1981) support the correspond to boreholes crosscutting at shallow occurence of a low density body ofmagmatic origin depth the U-rich 'Taourlxt granite" (Boissonnas, at less t h a n 60 k m below the surface. The domal 1973). These granitic intrusions belong to a Late uplift could be the isostatic response to this body, Precambrian to Lower Cambrian (560 + 40 Ma) the emplacement of which probably induced a magmatic episode. reheating of the s u r r o u n d i n g u p p e r mantle. Within the Paleozoic cover, the highest values (60 We have shown t h a t no thermally perturbed to 66 m W m a) are o b t a i n e d in Triasic to lithosphere is evidenced by heat flow data, Carboniferous sediments (sandstones, shales) along whereas heat flow anomalies greater t h a n 80 m W m the w e s t e m side of the Air m a s s i f (site 10, 11, 12). 2are generally observed in rift zone a n d uplift area Northward, the heat flow at site 9 within Devonian (Lucazeau a n d Bayer 1982). The m e a s u r e d heat q u a r t z - s a n d s t o n e s is lower with a n avgrage of 49 flow is normal for a c r u s t of that age a n d involves mWm -2. a lithosphere more t h a n 100 k m thick, assuming Within the E a s t e r n Hoggar domain, stabilized at a conductive geotherm a n d steady state condia n early stage of Pan-African orogeny (725 Ma), the tions. So Crough's low density zone c a n n o t be four boreholes of Tlmouletine (13) drilled within regarded as a n upwelling of the lithosphere/asthe Tiririne formations, exhibit a lower m e a n value t h e n o s p h e r e boundary. F u r t h e r m o r e decrease of (41 mWm-2). heat flow with b a s e m e n t elevation (Fig. 3a) is
46
A. LESQUER,A. BOURMATrE,S. LY and J.M. DAUTRIA O
10 °
5 < basement
elevation motor
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~7
\
6
//;\
\\
-
-
,/_D~
25'
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-2"
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4
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Fig. 3 : a) maps showing the geographic relationship between heat flow sites and topography.
\
500,
\
0
0,~0~,
T("C)
,5 \
- 1000
km
t400
I
I
I
I
I
20
40
60
80
100
,
t
-
200-
Fig. 3 : Heat flow versus Precambrian basement elevation. between heat flow sites and topography.
inconsistent with thermal uplift or recent erosion. The site (6) of Tin Amzi (the only one within the Cenozoic volcanic) area (Fig. 3b), deviates significantly from the general trend. This fact m a y indicate a local increase of heat flow with altitude i n t h e Atakor massif, which m a y be related with a small size thermal s t r u c t u r e linked to recent volcanic activity. Data distribution does not allow a description of heat flow variation in the n o r t h e m part of the Hoggar a n d particularly in the volcanic high massffs. However we t h i n k t h a t heat flow data strongly suggests that the Hoggar subcrustal mantle is not anomalously hot and t h a t the swell is not caused by a reheating of the lithosphere. Recent estimates of the equilibrium conditions of the peridotite xenoliths e n t r a i n e d by the Plio-Quatemary basaltic eruptions have been carried out. The pyroxene phases of all the samples, whatever the volcanic district, were
e
O(' OO li..o~O
Heatflow
m w / m "2
/
£'~
^ Z°°@o
oo o d/i ' /^" O,
1000-
900-
~
,
i
'
',
/~A
~e°' °
/0
.
700_
i
K
I
t
t P (Kb)
Fig. 4: Plots of PTconditions of the spine] ]herzolite inclusions from the recent basaJts of the volcanic Hoggar swell. The open circles correspond to the conditions estimated from the pyroxenic t h e m - b a r o m e t e r of Mercier (1980) (in Girod et ol, 1981), and the solid circles to the condlUorm from the olivinespinel thermometer of Fabrl~s (1979), assuming an isobaric evolution for these materials (as suggested by the doted lines joining the PT conditions estimated with the two methods in the same samples). "Oceanic" (high temperature) and "continental" geotherms are from Mercier (1980).
equilibrated between 900 and 1100°C (Fig. 4) along an "oceanic geotherrn" (Girod et al., 1981). This accounts for a n uprising of the isotherms beneath the volcanic Hoggar, corresponding to a thermal event evidently related to the intense recent
First heat flow determination from the central Sahara 47 magmatism. On the other h a n d the systematical- Paleozoic a n d Mesozoic cover where the b a s e m e n t ly lower t e m p e r a t u r e s (near 8 0 0 - 8 5 0 ° C) estimated remains unaffected by erosion. from olivine spinel pairs in the same samples, CONCLUSION suggest a n equilibrium along a "shield geotherm" (Fig. 4). Because of the easy reset of the olivineIn spite of their limited n u m b e r , these data allow spinel t h e r m o m e t e r u p o n c h a n g e s in the equilia first insight into the t h e r m a l r e g i m e of the Central b r i u m conditions (FabriCs 1979), these later values should reflect the conditions at the time of Sahara. T h u s we have now some indications to the xenolith e n t r a p m e n t by the ascending m a g m a support the hypothesis that the major structural (therefore during the Plio-Quaternary). This units of the Pan-African belt have different average accounts for a recent cooling of the u p p e r mantle, heat flow values which c a n be explained by corresthat is in agreement with the very low Q u a t e r n a r y ponding variations of crustal heat production. This volcanism, and, consequently the thermal pertur- hypothesis agrees with tectonic models that consibance m u s t be ante-Pliocene. der the belt to be composed of a n amalgam of Considering t h e recent magmatlc history of the crustal blocks with different tectonic histories. Hoggar two hypotheses c a n be proposed : the ther- Heat flow values correlate negatively with topogramal perturbation is either Miocene (associated phy, suggesting a possible variation in crustal heat with the paroxysmal volcanism) or Upper- production which m a y be due to the long and Cretaceous to Eocene {associated to the early mag- complicated multistage erosional history of the m a t i s m described in the E a s t e m Hoggar). As- Hoggar region. In order to test this hypothesis, suming t h a t h e a t is transported by conduction, a heat production m e a s u r e m e n t s will be carried out time of 25 Ma is n e c e s s a r y for a thermal pertur- in the n e a r future. bance occurring at 60 k m depth to be felt at the The heat flow values from sites within the surface. For a kinematic model with transport of Hoggar are not anomalously high. This implies hot material t h r o u g h o u t the u p p e r mantle, heat that the Hoggar swell is not due to a large scale transport is more efficient. If the t h e r m a l perturba- uplift of hot material in the upper mantle. tion is Miocene, we m u s t accept a shorter thermal Nevertheless small size thermal disturbances u n d e r relaxation period (20 Ma), that c a n only be explain- the various volcanic districts should generate local ed ff we a s s u m e small size for the low density as- anomalies which we did not demonstrate possibly thenospheric intrusions responsible for reheating. because of our sampling distribution. Asthenospheric diaptric intrusions of ten kiloAcknowledgement - Field work has been supported meters, s u c h as t h a t proposed by Nicolas et oL (1987) for the French Massif Central, m a y be a by the A.S.P. Afrique (CNRS, France) and the cooperapossible model. If s u c h small s t r u c t u r e s really tion project ONRS (Algeria) - CNRS (France). This project was greatly facilited by the active cooperaoccur b e n e a t h the Hoggar, the possible remaining tion and assistance provided by the personnel of the heat perturbation m a y not have b e e n m a d e evident following o r g a n i s a t i o n s or c o m p a n i e s : E R E M by our m e a s u r e m e n t s of samples because of our (Entreprise de Recherche et d'Exploitatlon Mini~re, site distribution. Nevertheless this model does not Alger, Alg6rie); CRAAG (Centre de Recherche a c c o u n t for the large scale swell and for the large Astronomie, Astrophysique et Geophysique, Alger, volume of low density material m a d e evident by Algerie); DNGM (Direction Nationale de la Geologie et gravity (Crough 1981). An Upper Cretaceous to des Mines, Bamako, Mali); IRSH (Institut de Recherche Eocene t h e r m a l event associated with a n extensive en Sciences Humaines, Niamey, Niger); Facult6 des u p p e r mantle modification, would better explain Sciences de l'Universit6 de Niamey (Niger), Minist~re the present lack of large wavelength heat flow ano- des Mines (Niamey, Niger), SOMAIR (Societe des Mines de l'AIr, Niamey, Niger); COMINAK (Compagnie Mini6re maly. The p u ~ l t n g decrease of h e a t flow with b a s e m e n t d'Akouta. Niamey, Niger); PNC (Power Nuclear elevation (Fig. 3a) c a n be tentatively modeled as a Companie, Niamey, Niger); COGEMA (Compagnie G~nerale des Mati~res Nucleaires, Paris, France); consequence of erosion downcutting through the ORSTOM (Institut Fran~als de Recherche Scientiflque crustal heat source distribution. 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