- Email: [email protected]
First heat flow density determinations from Southeastern Zaïre (Central Africa)
Journal of African Earth Sciences, Vol. 16, No. 4, pp. 413-423, 1993.
0899-5362/93 $6.00 + 0.00 Pergamon Press
Printed in Great Britain
First heat flow density determinations from Southeastern Zaire (Central Africa) M. N. Sm3A~'mcn1~, G. VASSm-rR~and P. Lot~s I ICentre G6ologique et G(mphysique, Universit~de Montpellier II 34095 Montpellier Cedex 5, France
~,aboratoire de G&~hysique, Universit~de LubumbashiB. P. 1825 Lubumbashi,Zaire (First received 15th February, 1993; revised version received 30th April, 1993) Absla'aet - First beat flow density determinations from southeastern Za~re are presented. Sites are located in the late Proterozoic metasedimentary cover of the Pan-African belt (600 Me.). For each individual boreholes, heat flow ranges between 48 and 72 mW m2. The average value of 62 mW m"2for the sites is similar to that of 66 mW m2 obseawedin Zambia. Both values are higher than what is expected for P~m-Africanterranes. Theseheat flow determinations in Shabaprovinceof southeasternZa~e, togetherwith gravity and seismological observations, support the hypothesis of lithospheric thinning for this area. As already suggested for Zambia, this lithospberic thinning may be associatedwith a southwestern extension of the East African Rift System from Tanganyika across the central African plateau.
INTRODUCTION
During t h e last t w e n t y y e a r s , a significant n u m b e r of t e m p e r a t u r e m e a s u r e m e n t s h a s b e e n o b t a i n e d in Africa for h e a t flow d e n s i t y d e t e r m i n a t i o n s (HFD). T h e v a l u e s of HFD s h o w a s u b s t a n t i a l c o n t r a s t In h e a t flow b e t w e e n A r c h a e a n c r a t o n s a n d y o u n g e r s u r r o u n d i n g t e r r a n e s In w h i c h HFD is typically a b o u t 50 % g r e a t e r t h a n In t h e c r a t o n s (e.g. Ballard e t a / . , 1987; Nyblade et al., 1990; L e s q u e r a n d V a s s e u r , 1992). In Z a m b i a w h e r e C h a p m a n a n d Pollack (1977) p e r f o r m e d t h e first g e o t h e r m a l s t u d i e s , t h e high h e a t flow v a l u e a v e r a g e of a b o u t 6 6 m W m -2 o n P r o t e r o z o i c t e r r a n e s , w a s Interpreted a s a n evidence, along with seismicity a n d p l a t e a u uplift, of a t h i n n e d l i t h o s p h e r e a n d incipient rifting e x t e n d i n g from Lake Tanganyika southwestward across the central African p l a t e a u . S o u t h e a s t e r n Zaire is located In s o u t h w e s t of Lake T a n g a n y i k a (Fig. 1), w h e r e geological a n d seismological o b s e r v a t i o n s h a v e clearly evidenced WNW-ESE t e n s i o n a l pattern, a s s o c i a t e d with t h e E a s t African Rift S y s t e m (Brain, 1972; M a a s h a a n d Molnar, 1972). It b e l o n g s to t h e s a m e Proterozoic tectonic province a s Z a m b i a a n d is also c h a r acterized b y distensive s t r u c t u r e s , s u c h a s t h e y o u n g U p e m b a a n d Mwero g r a b e n s t r e n d i n g NESW (Fig. 1 a n d Fig. 5). A c c o r d i n g to F a i r h e a d a n d G i r d l e r (1969), F a i r h e a d a n d H e n d e r s o n (1977) a s s u m p t i o n a b o u t E a s t Africa Rift extension, v a r i o u s geophysical o b s e r v a t i o n s s u c h a s t r e n d of seismicity (Bram,
1972; Grlmison a n d Chen, 1988) or HFD d a t a ( C h a p m a n a n d Ponack, 1977; Nyblade etaL, 1990) have s u g g e s t e d t h e existence of o n e or several possible incipient rifting arms from Lake T a n g a n y i k a c o n t i n u i n g t o w a r d s SW, related to a n a s t h e n o s p h e r e uplift. In o r d e r to d i s c u s s t h e rifting a s s u m p t i o n in S o u t h e a s t e r n Zaire, f u r t h e r geological a n d geophysical data, s u c h a s g e o t h e r m a l a n d gravity m e a s u r e m e n t s , are needed. T h u s , t e m p e r a t u r e s u r v e y s h a v e b e e n c o n d u c t e d in mining exploration boreholes, while gravity coverage h a s b e e n e x t e n d e d on S h a b a province of Zaire in 1990 b y t h e g e o p h y s i c s t e a m of t h e University of Lubumbashi. In this paper, we describe t h e r e s u l t s of the p r o g r a m ZAIFLUX a b o u t t h e t e m p e r a t u r e a n d h e a t flow m e a s u r e m e n t s o b t a i n e d d u r i n g t h e last t h r e e y e a r s in K a t a n g a n C o p p e r Arc of Zah'e. T h e s e r e s u l t s with t h o s e from Z a m b i a provide s u b s t a n tial c o n s t r a i n t s on g e o d y n a m i c m o d e l s of a s t h e n o s p h e r i c uplift in S o u t h e a s t Zaire a n d North Zambia. GEOLOGICAL SETTING
In S o u t h e a s t e m Zaire, t h r e e wide P r e c a m b r i a n b e l t s are k n o w n (Fig. 1). In t h e n o r t h w e s t , t h e U b e n d i a n (Lower Proterozoic) a n d t h e Kibara (Middle Proterozoic) b e l t s w h i c h e n t e n d to Z a m b i a a n d to t h e e a s t e m Africa are i n t e r p r e t e d a s t h e result of a c o n t i n e n t a l collision (e.g. Daly, 1983; K a m p u n z u et al., 1985; T s h i m a n g a et al., 1988; Kabengele et aL, 1991). T h e y c o m p r i s e a variety of
413
M . N.
414
SEBAGENZI, G. VASSEUR and P. Louis 30*
2S o + , . . . .
~Qyl..'
,
-
/
x ,-"/'~4-
/
%.>
/
/
--
'
'
.'.,.
/
k
,/~
__
,-
/
/
o,+
--L __j
.10 o
iLikasi
\
/ " e . ==-o ~, e,m~
~(.,e
. _
•
• •
o
/
~'~'2~--
/ •
----.--"
*\.
z /A.
L
,.~
,
+ . .'.
.','.. _.'~
.
%
/
I
o
2noK,. [
•
/
" +"
\\~.
•
"
~"Y~""
:
.-
~-
25 °
30 °
Fig. I. Simplified gcologlcal m a p of Southeast of ZaIre and North of Zambia indicating the locations of heat flow sites, solid trlanglcsfor Zambia {Chapman and Pollack, 1977}, and barbed circlesfor ZaIrc {thisstudy}, l: Archaean and lower Proterozolc termncs: 2: middle Protcrozolc (KNxtra be/t);3, 4: late Proterozolc (Kamngan be/t);5, 6: Phancrozoic sediments.
First heat flow density determinations from Southern Zaire (Central Africa) granites, mlgmatites, gneisses, and metasediments which c o n s t i t u t e the p r e - K a t a n g a n b a s e m e n t complex. In the southeast, the Katangan ('late Proterozoic) is considered as a mobile belt formed during the so-called Lufilian orogeny of PanAfrican age (600 Ma.). All the sites of t e m p e r a t u r e m e a s u r e m e n t s are located in the Katangan metasediments. Many p a p e r s on stratigraphy of this belt have b e e n published (e.g. Francois, 1974; Lefebvre, 1978). The K a t a n g a n b e l t c o n s i s t s of c l a s t i c a n d calcareous s e d i m e n t s of the Roan supergroup, s h a l e s a n d c a l c a r e o u s s h a l e s of t h e lower Kundelungu supergroup. During the Pan-African orogeny, the Katangan a n d its b a s e m e n t have b e e n deformed simultaneously. Their relationships and the overall tectonic c h a r a c t e r of the orogeny are clearly s h o w n In open pits a n d mines of the Katangan Copper Arc. The Roan rocks are Intensively deformed a n d during the orogeny, t h e y formed northwards t h r u s t over the rocks of the Kundelungu platform, which extends to LakeTanganyika In the north (Fig. 1). In the Katangan Arc, the m a i n features of deformation are foldings, reverse faults with overthrusting a n d t h r u s t faults w h e r e a s In the Kundelungu platform the rocks are horizontal with mInor local deformations. The Katangan Arc is rich in polymetallic mineralizations and several copper, cobalt, lead, zinc and u r a n i u m mining districts are located within the fold belt In the s o u t h e a s t of Zaire and in the north of Zambia.
T E C H m g U E OF M
SUm
NTS
Temperature m e a s u r e m e n t s have b e e n obtained In 1988 and 1990, In exploration boreholes drilled b y GECAMINES and SODIMIZAmining companies of S h a b a Province. S u r v e y s were c o n d u c t e d several years after drilling h a d ceased. They were m a d e In water, m o s t of the time below 50 m, the expected depth of the water table during the dry s e a s o n , with a t h e r m i s t o r p r o b e e q u i p m e n t (relative accuracy: 0.01) combined with a Metrix MX 575 multimeter, with sampling each 10 m. In order to reach the proper steady-state during logging, it w a s n e c e s s a r y to wait a b o u t five m i n u t e s after the probe had b e e n submerged. For m o s t of the boreholes, t e m p e r a t u r e w a s m e a s u r e d only in the range of 50 to 200 m, with the exception of Kinsenda site, not far from the Zambia border, where boreholes are deeper (300-450 m). Thermal conductivity of solid rock samples were m e a s u r e d using a t r a n s i e n t line h e a t s o u r c e method. Samples of drill core were cut parallel to the origInal core axes. For m o s t boreholes, cores were not available, t h e n surface rocks were collect-
415
ed to complete samples from holes. All rocks were s a t u r a t e d with w a t e r Into a v a c u u m a p p a r a t u s before m e a s u r e m e n t . For the m a i n lithologies, m e a s u r e d thermal conductivity values of samples are: 3.56 W m q K-1 for arkose, 4.20 W m q for dolomite, 2.65 W m ~ K" for s a n d s t o n e a n d 2.15 W m q Kq for shale. TEMPERATURE DATA
Temperature-depth profiles m e a s u r e d In different sites are s h o w n In Fig. 2. In general the holes are located in a fiat c o u n t r y a n d no topographic corrections have b e e n made. All curves exhibit a significant negative t e m p e r a t u r e gradient In the first h u n d r e d meters. The relative m i n i m u m temperature is found at a depth ranging from 100 to 200 m. Examining the t e m p e r a t u r e profiles from Kinsenda site, it appears that below this depth the t e m p e r a t u r e gradient Increases until it reaches a c o n s t a n t value. As it cannot be due to conductivity c o n t r a s t s or variation of the water table, the a n o m a l o u s curvature which m a y be s e e n as a characteristic feature of the m e a s u r e m e n t s from the s o u t h e a s t of Zaire a n d p e r h a p s from the north of Zambia (Chapman, 1983), is interpreted as the effect of variations In surface conditions, i.e. of surface temperature (Sebagenzi e t al., 1992). The analysis of this p h e n o m e n o n w a s done with an inversion technique of t e m p e r a t u r e data b a s e d on the theory of heat conduction in a semi-infinite homogeneous medium. This process a s s u m e s that ground temperature variations c a n be modelled as a J u m p of temperature, starting at a n u n k n o w n time and contInuIng until present time. These variations are c o m p u t e d b y boreholes temperature data Inversion with algorithm of Tarantola and Valette (1982). One of the p a r a m e t e r s of the ground temperature model is the u n d i s t u r b e d temperature gradient, u s e d for heat flow determination. The Inversion algorithm r e t u r n s the optimal values of the parameters ( d T / d z the u n d i s t u r b e d temperature gradient, T o the original u n d i s t u r b e d surface temperature, T the present surface temperature, a T t h e disturbance between the inltial tlme and the present time, t* the time Interval during which this d i s t u r b a n c e occurred) and a poster/or/standard deviations. The development of this Inversion technique w a s recently performed (e.g. Cermak et al., 1992; Sebagenzi et al., 1992). As shown In Sebagenzi et al., the a n o m a l o u s curvature c a n be Interpreted as the result of a warming of the ground surface t e m p e r a t u r e by 3-4°C during the last 40-100 years. This warming can be associated with the environmental changes In connection with mIning activities a n d urbanization.
416
M. N. SBAG'EN~, G. VASSEUR and P. Louxs
Temperoture( "C) o
21
o
2p
Temperoture( "C) 27 2p ~
50. 100.
50-
150. 200.
100" A
~,~ 250.
E
e.
e-
4,* O, 150-
O. `500m `550-
£:3 LIKASI-KAMOYA SITE
MBUJIMAYI SITE
200
400
Temperoture( "C)
Temperoture( "C)
02 2,~ 2,4 2p 2p 27 2,e 2,9 ~
02
2`5
24
2p
50. 100.
50-
150. 200.
100-
250
e-
e-
~oo. ¢'~ ,550400-
Z. 150¢'~ 200
KIPUSHI SITE
Fig. 2. Temperature profiles for measurements, t~reholes are gathered according to the measurements sites. HEAT FLOW DETERMINATION AND D E E P S T R U C T U R E
The HFD have b e e n c o m p u t e d a s the product q = k d T / d z , where k is the average conductivity on
the depth of m e a s u r e m e n t and d T / d z the undist u r b e d t e m p e r a t u r e gradient. The value of K h a s b e e n estimated in each borehole as the harmonic average of conductivity for each lithology weighted b y their thickness. The uncertainty on this average of conductivity is estimated to 10 %. The distribution of heat flow for both Zambia a n d Zaire sites, together with regional gravity anomaly (Sebagenzi, 1993) are depicted in Fig. 4, where some HFD results from Lakes Kivu, Tanganyika, Malawi and from Tanzania reported b y Nyblade e t a/. (1990), are plotted for comparison purpose. In Table 1 where the results obtained from Zaire are s u m marized, we give for each borehole, elevation, un-
disturbed temperature gradient, harmonic average conductivity and heat flow value. In addition for each site, an average heat flow is shown in last column. In S o u t h e a s t Zaire, h e a t flow values for each individual borehole range b e t w e e n 48 a n d 72 mW m 2 w h e r e a s site averages vary from 53 to 69 m W m -2. The e s t i m a t e d error is large, b u t the various values s e e m consistent. The average for the various sites is 62 mW m 2. This value is similar to t h a t of 66 mW m 2 observed from Zambia by C h a p m a n a n d Pollack (1977). Both values give an average of a b o u t 64 m W m 2 which might be representative of the Zaire a n d Zambia heat flow values. This average value which is similar to those obtained in the Mozambique belt on the flanks of the Rift east b r a n c h (Nyblade e t a / . , 1990), is higher b y a b o u t 9 mW m 2 t h a n the 55 mW m 2 value calculated for the Hoggar Pan-African Province (Lesquer e t o L , 1989). Thus, HFD from Zambia and
417
First heat flow density determinations from Southern ZaIre (Central Africa) Zaire Proterozoic t e r r a n e s lies within the regional heat flow pattern of 68 ± 4 mW m -2average value for rifted continental m a r g i n in East Africa (Nyblade et oL, 1990). MbuJtmayi site HFD (Fig. 1 a n d T a b l e 1) which is p r e s e n t e d in the west of Fig. 4 where regional gravity is not av_afiable, is not considered in our discussion b e c a u s e this site belongs to different tectonic province of KasaI craton. However, this HFD value would be t a k e n into a c c o u n t in the interpretation of the HFD contrast between A r c h a e a n craton a n d mobile belts area~. HFD average in Proterozoic t e r r a n e s of Zambia a n d Zalre is higher by 20 mW m -2 t h a n the 44 mW m "a obtained from MbuJimayi site of Kasai A r c h a e a n craton which agrees with Tanzania a n d Zimbabwe cratons HFD (Nyblade et oL, 1990).
made. For this estimation, a two layers c r u s t with different h e a t production, h a s b e e n assumed. An u p p e r c r u s t formed b y granite gneisses, biotite gneisses, migmatltes a n d amphibolites in the amphibolite facies, and a lower c r u s t is a s s u m e d to be formed by granulite facies rocks (Chapman a n d Pollack, 1977). We have no heat production determlnation from heat flow sites of Southeast Zaire. However in Zambia, m e a s u r e m e n t s obtained by C h a p m a n and Pollack (1977) on some samples of granite gnelsses, biotite gnelsses, mlcaschists and amphibolites, belonging to the pre-Katangan basem e n t a n d Katangan metasediments, revealed a n average h e a t production of 2 . 4 ~W m a which c a n also be u s e d for Southeast Zaire. According to Hadiouche a n d Jobert (1988), the average crustal thickness was estimated to be 36 k m for this area. Crustal contribution to surface heat flow Then, the u p p e r c r u s t with average heat producIn order to discriminate the crustal component of tion of 2.4 ttW m -a, would have a thickness of 5.5 heat flow a n d t h a t from deeper origin in the mantle, km. The lower c r u s t would be 30.5 k m thick; its Birch et oL (1968) proposed a linear relationship heat production value c a n be estimated to be 0.4 between heat flow qo a n d h e a t production Ao, I~W m a as other granulite facies t e r r a n e s throughexpressed as qo = q* + bAo, where q ' i s r e d u c e d heat out the world (Pinet a n d J a u p a r t , 1987). For this flow which r e p r e s e n t s heat flow component at model, the values of integrated heat production are lower crustal level in a province and bAo the 13.2 mW m "a for u p p e r c r u s t a n d 12.2 mW m "~for integrated heat production. The p a r a m e t e r b is a lower crust, which lead to a mantle heat flow of lengh which characterizes the vertical distribution 38.6 mW m 2. of h e a t generation in the crust. This relation h a s However, the validity of this result depends on been applied by Ballard et oL (1987) to heat flow the reality of the h e a t production model a n d in a n d h e a t production d a t a from Bostwana-Namibia particular on the t h i c k n e s s of the enriched crust. a n d Zambia which belong to the same tectonic It h a s b e e n shown t h a t lateral h e a t transfer m a y province, to obtain a r e d u c e d heat flow q* of 50 + 3 significantly affect the slope of the h e a t flow-heat mW m "2a n d a slope b of 5.5 _+ 1.1 kin. p r o d u c t i o n r e l a t i o n s h i p (e.g. J a u p a r t , 1983; Under the a s s u m p t i o n s t h a t heat production is Vasseur and Singh, 1986; Nielsen, 1987; Furlong c o n s t a n t for depth range 0 < z < b with b = 5.5 ± a n d Chapman, 1987). This effect has been model1.1 k m a n d t h a t r e d u c e d h e a t flow q* of 50 ± 3 mW led quantitatively by V a s s e u r a n d Singh (1986), m -2calculated by Ballard et ol. corresponds to heat a s s u m i n g t h a t the heat generation distribution flow at u p p e r crustal level, a n estimation of the c a n be represented horizontany by a n isotropic crustal contribution to surface heat flow h a s been stationary function of the horizontal distance. Table 1. Thermal gradient, estimated conductivity a n d heat flow density for each borehole. SITES
BOREHOLES
ELEVATION (m)
KINSENDA
KIN 80B KIN 122 KIN 144 KIN 149
1295 1285 1271 1274
24.0 ± 21.3 + 24.0± 19.4 ±
1.3 2.7 1.8 2.0
2.94 ± 0.3 3.03 ± 0.3 3.01 ±0.3 2.97 ± 0.3
71 ± 65 ± 72± 58 ±
KIPUStll
MW2
1302
21.0 + 6.0
3.30 ± 0.3
69 + 26
69.0 ± 26
I.IKASI
IJKASI3 I.IKASI4 KAMOYA
1320 1320 1490
17.0 + 10.0 23.0 ± 8.0 17.0+ 9.0
2.82 ± 0.28 2.82 ± 0.28 2.801+ 0.28
48 + 32 65+29 48+ 30
53.0 + 30
18 KI0
595 590
15.0 + 6.0 18.0+ 5.0
2.67 + 0.3 2.69+0.3
40 + 20 48+19
MBUJIMAYI
TIIERMAL GRAD. ('Ckm "1)
CONDUCTIVITY (Wm"'C I)
IIEAT FLOW DENSITY (mWm"2) 11 14 12 12
66.5 ± 12
44.0+20
418
M. N. S~^G~czl, G. VASSARand P. Lotns
As s h o w n in the appendix, the a p p a r e n t thickn e s s Da of the radioactive layer d e d u c e d from h e a t flow-heat p r o d u c t i o n relation is r e d u c e d with respect to its actual t h i c k n e s s D b y a value which d e p e n d s u p o n k / D ratio, k being the characteristic horizontal scale length of h e a t production variations. For reasonable values of the ratio g / D , the reduction factor lies b e t w e e n 1 a n d 0.5. Returning to the estimation of crustal radioactive source thickness, the actual t h i c k n e s s D would t h e n range b e t w e e n 5.5 and I I kin. The latter value would imply a crustal h e a t flow contrib u t i o n of 36.2 m W m 2 a n d therefore, a mantle heat flow of 27.8 m W m a instead of 38.6 mW m a as obtained with the former assumption. Recent geothermal s t u d i e s a r o u n d the world have s h o w n t h a t m a n t l e h e a t flow in stable Precambrian shields is close to 13 mW m "2 (e.g. Pinet a n d J a u p a r t , 1987; Pinet et aL, 1991). In comparison with this result, it a p p e a r s that mantle heat flow b e n e a t h S o u t h e a s t Zaire and North Z a m b i a Proterozoic t e r r a n e s m a y p r e s e n t a n excess heat flow of 14.8 mW m a. Geothermal investigations from Zambia (Chapman and Pollack, 1977) led to estimate similar
excess mantle heat flow {mean value of 15 mW ma}. This thermal excess h a s already b e e n noted a n d interpreted as indicator of a flux from the asthenosphere, in relation with lithospherlc thinning, p e r h a p s associated with a s o u t h w e s t e r n extension of E a s t African Rift S y s t e m {Chapman a n d Pollack, 1977). As S o u t h e a s t ZaZre and North Zambia belong to the s a m e major tectonic province, we believe t h a t o u r heat flow determinations support, for the S o u t h e a s t Zaire, this hypothesis which m a y be considered as confirmed since S o u t h e a s t Zaire and Zambia HFD agrees with the regional heat flow p a t t e r n of Kenya a n d Tanzania (Nyblade etaL, 1990}, where continental rift struct u r e s are well developed. b e n e a t h S o u t h e a s t ZaIre An estimation of t e m p e r a t u r e as a function of depth for S o u t h e a s t Zaire a n d North Zambia h a s b e e n attempted. Geotherms have b e e n calculated for heat flow of 64 mW m-% the m e a n surface h e a t flow value for Zaire and Zambia terranes. Calculations were r u n for the two models corresponding to the extreme a s s u m p t i o n s on t h e v a l u e olD {5.5 and 11 kin). For the first model, the following paraGeotherms
Temperature(°(]) 0
0
I~5 0 I
500 J
750 I
1000
1250
L
1
1500 I
1750 1
2000 I
20-
40.
80-
•8 0
100
120 140
-
160 -
180 -
\\\\\\
~
200 -
Fig. 3. Geothermscomputed with mean surface heat flowof 64 mWm-LSolidline corresponds to model for apparent depth Da of 5.5 km and dashed line to modelfor actual depth of 11 km.
419
First heat flow density determinations from Southera 7-.Yre(Central Africa) m e t e r s were assumed: m e a n radiogenic heat for u p p e r and lower c r u s t are 2.4 and 0.4 ~tW m -s, respectively; thickness of 5.5 k m for the u p p e r a n d 30.5 k m for the lower c r u s t and no mantle heat production contribution; uniform thermal conductivity of 3 W m ~ K'L For the second model, all p a r a m e t e r s values r e m a i n the same except the u p p e r a n d lower c r u s t thickness values which are fixed to 11 k m for the u p p e r a n d 25 k m for the lower 25
/,#
/
319
, /
#
I
/.,/., I
/ ,' / , , " / t /
,,'/
,,'
'
,
, .,,t vi,-, ~
I I
,
l
tFI /t
!
,
i I
I/
i
i
'
,
i
I I
t,
i t i
A /\
,I
~
\
I
"
',,/,.,,\
',
I
,
l
,
Rr
'~ ,-%/' :'.
I
,"
35
~
~ f
,'
,,#
7,
/.
crust. Both geotherms presented on Fig. 3, indicate quite t e m p e r a t u r e s at depth. A t e m p e r a t u r e of 1250°C, which could correspond to the beginning of partial melting, is r e a c h e d at depth of about 85 to 110 kin. The predicted t e m p e r a t u r e s m u s t be t a k e n with caution b e c a u s e of uncertainties about heat generation and c r u s t layer thickness estimations, a n d of no sufficient constraints on thermal p a r a m e t e r s
\ ~
,..
,% ~
I
%'
-,~,-,il U .
"-.
~_l.
I ~L-~ ....
20 ~.
_.
:/(/,'"\.---___-_;I'---/---
,
.,~\'/" ,: V
7".
"it .' i
'.
\
146_L....-j
".
I"l
-'----"-'--
_-'"
44 5
O
.%
,
,#
~,.#
~
/
•
#1#
I\ I
\ k
~ ~
~
# i #f
,'~
/P/
,"
### ##fX
# f# /
i / , , ," l, i ~l \ ~\ ~\ ~ . .\. , ,,'x,/ " / ldl ,I,'' " /:/ i
"- . . . . -'" zAI~i," 10
_~.-'Y/
-'- ;-'"
\
,,--L~ a
,:~\"x,'l
/ 1o
._//
f
,~69
,
~,~,~/
~o
' I/ I/ tx,_ :51)(
--~_..~.,..'k
,,,' AUd/-,, / )klY //_ 7,,' (z,l:---'~ .I/.77// ,, \"A( . , " / 5 / / , ,," " XT).,// II ,," Z73// ,,~ / ill~xa~ //
. 15
'=
71
..............
..... ~
......
"'°" I,
~
........ -~.._. \ I ,
\
• 66---/~"~:.-'~/.~--
~AT~~~.~ -_,,.~:.,.
x/f
.,_ ....
--,_~,,,,
IS
. , ,\
Fig. 4. Regional gravity field, HFD values at the sites shown in Fig. I and at some sites reported from Kenya and Tanzania by Nyblade e t al. (1990), main faults of east central Africa associated with the East African RIll system. Gmvlty curves are in regal and HFD in mWm'~ enriched star symbol corresponds to new HFD sites reported In this study, solid triangles for Zambia (Chapman and Pollack, 1977) and solid squares for Kenya, Tanzania and Zambia HFD sites (Nyblade eta/., 1990). Thick lines display major normal faults related to main rifts l ~ means rills).
420
M. N. Sm3^o~rn, G. V ^ s s ~ and P. Louxs
assumption. However, t h e y provide a first attempt to estimate lithospheric t e m p e r a t u r e variations with depth in S o u t h e a s t Zaire. DISCUSSION
AND
tween 85 a n d 110 kin. High heat flow in S o u t h e a s t Zaire and North Zambia related to asthenospherlc uplift is supported b y other indications, s u c h as gravity a n d seismological results. Regional gravity field of S o u t h e a s t Zaire and North Zambia w a s derived from B o u g u e r anomaly m a p obtained using Zambia data compiled b y Cowan a n d Pollack (1977), combined with Southeast Zaire data recently acquired (Sebagenzl, 1993). Regional gravity low (amplitude < 100 rngal) trending NE-SW (Fig. 4) is in agreement with Girdler predictions {Girdler, 1975}, which suggested t h a t the regional gravity anomaly in s o u t h e a s t e r n Africa could continue the long wavelength anomaly associated with the E a s t African Rift System. Fig. 4 p r e s e n t s this regional gravity field, HFD value and principal sites location, tectonics of e a s t a n d
CONCLUSIONS
In spite of their limited n u m b e r and low accuracy, h e a t flow v a l u e s in the Proterozoic belts of S h a b a Province of Zaire (62 m W m 2) are consistent with the results from Zambia (66mW m2). These HFD values are higher t h a n w h a t is expected in this area of Proterozoic age and are rather similar to those observed in Rift east branch. The predicted high t e m p e r a t u r e s of 1250°C at shanow level would suggest t h a t mantle b e n e a t h Proterozoic terranes of S o u t h e a s t Zaire a n d North Zambia is a n o m a l o u s l y hot and would indicate the presence of a s t h e n o s p h e r i c uplift at a depth ranging be-
•
O
"0
'1' J
-
,
.
_
Q
°~1 I° IL'~ 0%1 I d ' ~ f o .~ ~ . ~ I'.qk]P I . ~:,~,,Z~ - * " .~t'~" 3 ° ,rir-*:-ah- ~. f~-': I • / ,.o
,,~,-~,
.o.
o
,.
o
o
.
:
(
0
•
g\ -
35
..,.?.. ;J \
+
r
A
oo I
"
o
/ . +
-s -~'
15
20
25
30
35
40
I
I
i
I
I
I
Flg. 5. Se/sm/c/ty map of southern Africa for earthquakes with magnitudes mb > 4.0 for the period 1950-1992, with available focal mechanism studies (modifiedfrom Fa/rhead and Henderson, 1977; Grhn/son and Chen, 1988). Note the accordance between focal mechan/sm deduced from 1992 september 11 earthquake and availahle previous focal mechanisms.
First heat flow density determinations from Southern Zaire (CentralAfrica)
421
extends southwestward from Lake Tanganytka into Southeast Zaire and Zambia where events have occurred in recent y e a r s (Fairhead a n d Henderson, 1977; Fairhead a n d Stuart, 1982; Maasha and Molnar, 1972; Grimison a n d Chen, 1988} a n d are nowadays occurring (e.g. Lombe and Mubu, 1992}. Seismicity NE-SW t r e n d is parallel to regional gravity a n o m a l y NE-SW axis a n d to Upemba, Mwem, Luangwa, L u k u s a s h i and Luano rifts (Fig. 5 and Fig. 6) a n d earthquake-source m e c h a n i s m s indicate a NW-SE tensional stress associated with normal faulting (Dorbath, personal communication 1992). In a recent analysis of Zambia seismicity, Lombe a n d Mubu (i 992) suggest to consider the Luangwa and Mwero rifts as already existing NE-SW trending reactivated fault zones which could indicate some w e a k n e s s in the crust. In this way, the seismicity in this zone might indicate that a n incipient b r a n c h of the western a r m of the East African Rift System extends into Southeast Zaire a n d Zambia and t h a t this b r a n c h could be traced across Lake Tanganyika a n d connects with the m o s t active part of the Rift System (Grimison and Chen, 1988). We note that this b r a n c h coincides largely with b a s e m e n t s t r u c t u r e s associated with the Proterozoic mobile belts which separate A r c h a e a n craton areas. It appears that the Proterozoic crustal s t r u c t u r e s have a major influence on the recent crustal movements.
central Africa a n d somefauRs associated with the East African Rift System. These features show clearly a close correlation between high h e a t flow a n d regional gravity anomaly over this area. This suggests t h a t the regional gravity low m a y be associated with a t h e r m a l anomaly within the u p p e r mantle. An important a r g u m e n t to discuss the existence of incipient riRing in this area, is the estimate of lithospheric thickness from teleseismic delay times a n d gravity correlation published by Fairhead a n d Reeves (1977}. Their p a t t e r n for Southeast Zaire and North Zambia t e r r a n e s of Proterozoic age, predicted a t h i n n e d lithosphere which agrees with regional gravity trending NE-SW and with high heat flow. Seismic activity is r e p r e s e n t e d by the epicenters distribution of e a r t h q u a k e s with n% _>4.0 reported by the International Seismological Center between 1965 to 1970 a n d some fault plane solutions are shown (Fig. 5}. The focal m e c h a n i s m of 1992 S e p t e m b e r 11 e a r t h q u a k e , S o u t h e a s t Zaire (Dorbath, personal c o m m u n i c a t i o n 1992) which Just supports our previous observations, has also been plotted. Both results could be u s e d to conswain the present day deformation of this zone and its relationship with the East African RIR System. Present seismic activity in Southeast Zaire and in North Zambia reveals t h a t a broad area of activity
1.0
0.8
0.6 a
0.4
0.2
0.0 I-" 0.01
l
!
i
l
!
i
|i
I
0.1
!
i
i
!
;
i
i;
I
1
i
I
i
!
I
I
Ii
i
10
Fig. 6. Variation of D J D as a function of k/D.
I
l
!
i
l
X/D
i
il
i
tO0
M. N. SEBAG$~¢n,G. VASSEURand P. Louis
422
A c k n o w l a d g e m e n t s - Field works were supported by the U n e s c o Major Project "Geology for E c o n o m i c Development" Contract S C / R P 264061.8 a n d S C / R P 2030360.0. This project was facilitated b y the following mining companies a n d their staff: GECAMINES (C~n~ral des Carrt~res et des Mines}, SODIMIZA (Soci~t~ de Developpement Industrtel et Mlnl~res d u Zalre), MiBA (Mlnl~re de Bakwanga). H. N. Pollack of the University of Michigan (USA) and M. Q. W. J o n e s of the Bernard Price Institute of Geophysical Research, University of the Witwatersrand (RSA), are t h a n k e d for their interest in this study. We are grateful toA. Lesquer and F. Lucazeau of Centre G ~ l o g i q u e et C,~'ophysique CNRS, Montpellier for valuable reviews and c o m m e n t s on the manuscript. REFERENCES
Ballard, S., Pollack, H. N. and Skinner, N. J. 1987. Terrestrial Heat Flow in Botswana and Namlbia. J. Geophys. Res. 92, B7, 6291-6300. Birch, F., Roy, R. F. and Decker, E. R. 1968. Heat flow a n d thermal history in New England and New York. In: Studies of App~_ !~chlan Geology: Northern and Maritime. E. ZenW. S. Ed. Intersclence NewYork, 437-451. Bram, IC 1972. Selsmicity of Katanga and Western Zambia, southwest of E a s t Africa Rift Systems, from 1960 to 1971. Bull. Seismol. Soc. Am. 6 2 , 1211-1216. Cermak, V., Bodrt, L. and Safanda, J. 1992. Underground t e m p e r a t u r e fields a n d changing climate: evidence from Cuba. Pa/eogeogr., Pa/eoecoL (Global Planet. Change Sect.) 97, 325-337. Chapman, D. S. 1983. Thermal regime of the L u a n s h y a Mine, Republic of Zambia. Geoexploratfon 21, 264281. Chapman, D. S. and Pollack, H. N. 1977. Heat flow and heat production in Zambia: Evidence for llthospherlc thinning in central Africa. Tectonophyslcs 41, 79- 100. Cowan, I. M. a n d Pollack, H. N. 1977. Gravity in Zambia. In: Macmillan Journals Ltd., 1977. Reprinted from Nature 2 6 6 . N ° 5603, 651-617. DaLly, M. C. 1983. The Irumide belt of Zambia and its bearing on collision orogeny in the Proterozoic of Afrlca. Geo/. Soc. Spec. PubL London, CoUtston tectonic vo/ume. Falrhead, d. D. a n d Girdler, IL W. 1969. How far does the RIR System extend through Africa ? Nature 221, 10181020. Fairhead, J. D. a n d Henderson, N. B. 1977. The seismlclty o f s o u t h e r n Africa a n d i n c i p i e n t rifting. Tectonophyslcs 41, 19-26. Falrhead, J. D. and Reeves, C. V. 1977. Teleseismic delay times, B o u g u e r anomalies and inferred thickness of the African lithosphere. Earth and Planetary ScL Letters 3 6 , 63-76. Fa/rhead, J. D. a n d Stuart, G. W. 1982. The seismicity of the E a s t African rift system and comparison with o t h e r continental rifts, in Cont/nental a n d Oceanic R/fls, Geody. Set. 8, edited b y G. Palmason, 41-61, Washington D. C. Francois, A. 1974. Stratlgraphie, tectonlque et min6ralisations de l'Arc cuprlf~re Shablen (R~publique d u Zalre). Cent. Soc. G~l. Belg., Gtsements stratlformes et provinces c u p r i ~ s , Li6ge, 79-101.
Furlong, K. P. and Chapman, D. S. 1987. Crustal heterogeneltles and the thermal s t r u c t u r e of the conUnental crust. Geophys. Res. Left. 14, 314-317. Girdler, R. W. 1975. The great Bouguer a n o m a l y over Africa. EOS, Trans., Am. Geophys. Un. 56, 516-519. Grlmison, N. L. and Chen, W-P. 1988. E a r t h q u a k e s in Davie Ridge-Madagascar Region and the S o u t h e r n Nubian-Somalian Plate Boundary. J. Geophys. Res. 9 3 , B9, 10439-1 0450. Hadiouche, O. a n d Jobert, N. 1988. Geographlcal distrlbution of surface-wave velocities a n d 3-D uppermantle structure in Africa. Geophys. J. 95, 85-109. J a u p a r t , C. 1983. Horizontal heat transfer due to radioactivity contrasts: c a u s e s and consequences of linear heat flow relation. Geophys. J. R. Astnm. Soc. 75, 411435.
Kabengele, M., Lubala, R. T. et Cabanis, B. 1991. Caracterisation l~trologique et g~ochimique d u magmatisme ubendien d u secteur de Pepa-Lubumba, s u r le plateau des Marungu (Nord-Est d u Shaba, Zalre). Signification g ~ d y n a m i q u e darts l'~'volution de la chalne ubendienne. J. Aft. Earth ScL 1S, 2, 243-265. Kampunzu, A. B., Rumvegeri, B. T., Kapenda, D., Lubala, R. T. et Caron, J . P . 1985. Les Klbartdes d'Afrtque centrale et ortentale: u n e chaine de collision intercontinentale. BUlL Inform. "La ~ i e au service du/~ve/oppemen~' U~O 5. Lefebvre, J . J . 1978. Le groupe de Mwashya, m~gacycloth~me terminal d u Roan (Shaba, Zalre sud-ortental). Approche lithostratigraphique et ~tude de l'envtronnem e n t s~ltmentaire. Ann. Soc. Gt~. Bel. 101, 209-225. L ~ q u e r , A., Bourmatte, A., LY, S. a n d Dautrla, J. M. 1989. First heat flow determination from the central Sahara: relationship with the Pan-African belt and Hoggar domal uplift. J. Aft. Earth ScL 9, 1, 41-48. Lesquer, A. and Vasseur, G. 1992. Heat-Flow constraints on the West African lithosphere structure. Geophys. Res. Lett. 9, 6, 561-564. Lombe, K. D. and Mubu, M. S. 1992. Instrumentation and seismlcity in Zambia. Tectonophysb~s 209, 31-33. Maasha, N. and Molnar, P. 1972. E a r t h q u a k e fault parameters and tectonics in Africa. J. Geophys. Res. 77, 5731-5743. Nielsen, S. B. 1987. Steady state heat flow in r a n d o m medium and the linear heat flow-heat production relationship. J. Geophys. Res. Lett. 14, 318-321. Nyblade, A. A., Pollack, H. N., Jones, D. L., Podmore, F. and Mushayandebvu, M. 1990. Terrestrial Heat Flow in East and S o u t h e r n Africa. J. Geophys. Res. 95, B 1 I, 17, 371-17, 384. Plnet, C. and J a u p a r t , C. 1987. The vertical distribution of radiogenic heat production in the Precambrlan crust of Norway and Sweden: Geothermal implications. Geophys. Res. Lett. 14, 260-263. Pinet, C., J a u p a r t , C., Mareschal, J. C., Garlepy C., Bienfait, G. and Lapointe R. 1991. Heat flow and s t r u c t u r e of the lithosphere in the E a s t e r n Canadian Shield. J. Geophys. Res. 96, BI2, 19, 941-19, 963. Sebagenzi, M. N., Vasseur, G. a n d Louls, P. 1992. Recent warming in s o u t h e a s t e r n Zah'e (Central Africa) inferred from disturbed geothermal gradients. Palecx3eogr., Paieocllmatol., Paleoecol. (Global Planet. Change Sect.) 9 8 , 209-217.
First heat flow density determinations from Southern 7ai're (Central Africa) Sebagenzi, M. N. 1993. Etude gravim~trique et g~othermique d u S u d - E s t d u Zalre et d u Nord de la Zamble (Afrique centrale). Phd Thes/s Univ. Parts V//. Tarantola, A. and Valette, B. 1982. Generalized nonlinear inverse problems solved using the least squares criterion. Rev. Geophys. Space Phys. 20(2}, 219-232. T s h i m a n g a , K., Ll~geols, J. P., Lubala, R. T. et Kampunzu, A. B. 1988. La zonaflon inverse d a n s le
eomplexe concentrique calco-alealln potasslque ubendlen (ProtC~rozoique lnf~rieur} de Lumono-Zalre: Structure, P~trologie, et C~ochimte. 12~ R~un. Sci. Terre, Lille, Avril 1988, Soc. gg,o/. Ft., g~L p. 127. Vasseur, G. and Stngh, R. N. 1986. Effects of r a n d o m horizontal variations in radiogenic heat source distribution on its relationship with heat flow. J. Geophys. Res. 91, B10, 10397-10404.
Appendix Following Vasseur and Singh (1986), two assumptions on the vertical distribution of heat production of the surface layer can be made : either an exponential decrease with depth as e-~° or a c.onstant heat production for depth range 0 < z < D. In both cases the autocorrelation function of heat production which characterizes its horizontal variations, is assumed to vary as e '"x, where ~. is the length giving the scale of horizontal heat production variations and r is the horizontal distance. With such hypotheses, the apparent thickness D, of heat production layer, as deduced from heat flow-heat production relationship, is obtained in the following closed form:
D,,
423
:"
k'dk'
"It=Jo (k'2+ l)3a(1+13k') with 13= L/D for constant heat production for 0 < z < D depth ; D, 1 : " 1 - e -~' , D Iz=~J0 ( k ' 2 + l ) 3adk for exponential heat production decrease with depth. These results are plotted on Fig.6 as a function of the parameter L/D. From this figure, one can see that, for a given D value, the smaller is ~., the smaller the apparent thickness D, deduced from the. heat flow-heat production plot. Only for 13< 1 (i.e. lateral heterogeneities of very large horizontal extension), the apparent Do value reach the true thickness. This corresponds to ~. values range of 1 < X < 0-.
0899-5362/93 $6.00 + 0.00 Pergamon Press
Printed in Great Britain
First heat flow density determinations from Southeastern Zaire (Central Africa) M. N. Sm3A~'mcn1~, G. VASSm-rR~and P. Lot~s I ICentre G6ologique et G(mphysique, Universit~de Montpellier II 34095 Montpellier Cedex 5, France
~,aboratoire de G&~hysique, Universit~de LubumbashiB. P. 1825 Lubumbashi,Zaire (First received 15th February, 1993; revised version received 30th April, 1993) Absla'aet - First beat flow density determinations from southeastern Za~re are presented. Sites are located in the late Proterozoic metasedimentary cover of the Pan-African belt (600 Me.). For each individual boreholes, heat flow ranges between 48 and 72 mW m2. The average value of 62 mW m"2for the sites is similar to that of 66 mW m2 obseawedin Zambia. Both values are higher than what is expected for P~m-Africanterranes. Theseheat flow determinations in Shabaprovinceof southeasternZa~e, togetherwith gravity and seismological observations, support the hypothesis of lithospheric thinning for this area. As already suggested for Zambia, this lithospberic thinning may be associatedwith a southwestern extension of the East African Rift System from Tanganyika across the central African plateau.
INTRODUCTION
During t h e last t w e n t y y e a r s , a significant n u m b e r of t e m p e r a t u r e m e a s u r e m e n t s h a s b e e n o b t a i n e d in Africa for h e a t flow d e n s i t y d e t e r m i n a t i o n s (HFD). T h e v a l u e s of HFD s h o w a s u b s t a n t i a l c o n t r a s t In h e a t flow b e t w e e n A r c h a e a n c r a t o n s a n d y o u n g e r s u r r o u n d i n g t e r r a n e s In w h i c h HFD is typically a b o u t 50 % g r e a t e r t h a n In t h e c r a t o n s (e.g. Ballard e t a / . , 1987; Nyblade et al., 1990; L e s q u e r a n d V a s s e u r , 1992). In Z a m b i a w h e r e C h a p m a n a n d Pollack (1977) p e r f o r m e d t h e first g e o t h e r m a l s t u d i e s , t h e high h e a t flow v a l u e a v e r a g e of a b o u t 6 6 m W m -2 o n P r o t e r o z o i c t e r r a n e s , w a s Interpreted a s a n evidence, along with seismicity a n d p l a t e a u uplift, of a t h i n n e d l i t h o s p h e r e a n d incipient rifting e x t e n d i n g from Lake Tanganyika southwestward across the central African p l a t e a u . S o u t h e a s t e r n Zaire is located In s o u t h w e s t of Lake T a n g a n y i k a (Fig. 1), w h e r e geological a n d seismological o b s e r v a t i o n s h a v e clearly evidenced WNW-ESE t e n s i o n a l pattern, a s s o c i a t e d with t h e E a s t African Rift S y s t e m (Brain, 1972; M a a s h a a n d Molnar, 1972). It b e l o n g s to t h e s a m e Proterozoic tectonic province a s Z a m b i a a n d is also c h a r acterized b y distensive s t r u c t u r e s , s u c h a s t h e y o u n g U p e m b a a n d Mwero g r a b e n s t r e n d i n g NESW (Fig. 1 a n d Fig. 5). A c c o r d i n g to F a i r h e a d a n d G i r d l e r (1969), F a i r h e a d a n d H e n d e r s o n (1977) a s s u m p t i o n a b o u t E a s t Africa Rift extension, v a r i o u s geophysical o b s e r v a t i o n s s u c h a s t r e n d of seismicity (Bram,
1972; Grlmison a n d Chen, 1988) or HFD d a t a ( C h a p m a n a n d Ponack, 1977; Nyblade etaL, 1990) have s u g g e s t e d t h e existence of o n e or several possible incipient rifting arms from Lake T a n g a n y i k a c o n t i n u i n g t o w a r d s SW, related to a n a s t h e n o s p h e r e uplift. In o r d e r to d i s c u s s t h e rifting a s s u m p t i o n in S o u t h e a s t e r n Zaire, f u r t h e r geological a n d geophysical data, s u c h a s g e o t h e r m a l a n d gravity m e a s u r e m e n t s , are needed. T h u s , t e m p e r a t u r e s u r v e y s h a v e b e e n c o n d u c t e d in mining exploration boreholes, while gravity coverage h a s b e e n e x t e n d e d on S h a b a province of Zaire in 1990 b y t h e g e o p h y s i c s t e a m of t h e University of Lubumbashi. In this paper, we describe t h e r e s u l t s of the p r o g r a m ZAIFLUX a b o u t t h e t e m p e r a t u r e a n d h e a t flow m e a s u r e m e n t s o b t a i n e d d u r i n g t h e last t h r e e y e a r s in K a t a n g a n C o p p e r Arc of Zah'e. T h e s e r e s u l t s with t h o s e from Z a m b i a provide s u b s t a n tial c o n s t r a i n t s on g e o d y n a m i c m o d e l s of a s t h e n o s p h e r i c uplift in S o u t h e a s t Zaire a n d North Zambia. GEOLOGICAL SETTING
In S o u t h e a s t e m Zaire, t h r e e wide P r e c a m b r i a n b e l t s are k n o w n (Fig. 1). In t h e n o r t h w e s t , t h e U b e n d i a n (Lower Proterozoic) a n d t h e Kibara (Middle Proterozoic) b e l t s w h i c h e n t e n d to Z a m b i a a n d to t h e e a s t e m Africa are i n t e r p r e t e d a s t h e result of a c o n t i n e n t a l collision (e.g. Daly, 1983; K a m p u n z u et al., 1985; T s h i m a n g a et al., 1988; Kabengele et aL, 1991). T h e y c o m p r i s e a variety of
413
M . N.
414
SEBAGENZI, G. VASSEUR and P. Louis 30*
2S o + , . . . .
~Qyl..'
,
-
/
x ,-"/'~4-
/
%.>
/
/
--
'
'
.'.,.
/
k
,/~
__
,-
/
/
o,+
--L __j
.10 o
iLikasi
\
/ " e . ==-o ~, e,m~
~(.,e
. _
•
• •
o
/
~'~'2~--
/ •
----.--"
*\.
z /A.
L
,.~
,
+ . .'.
.','.. _.'~
.
%
/
I
o
2noK,. [
•
/
" +"
\\~.
•
"
~"Y~""
:
.-
~-
25 °
30 °
Fig. I. Simplified gcologlcal m a p of Southeast of ZaIre and North of Zambia indicating the locations of heat flow sites, solid trlanglcsfor Zambia {Chapman and Pollack, 1977}, and barbed circlesfor ZaIrc {thisstudy}, l: Archaean and lower Proterozolc termncs: 2: middle Protcrozolc (KNxtra be/t);3, 4: late Proterozolc (Kamngan be/t);5, 6: Phancrozoic sediments.
First heat flow density determinations from Southern Zaire (Central Africa) granites, mlgmatites, gneisses, and metasediments which c o n s t i t u t e the p r e - K a t a n g a n b a s e m e n t complex. In the southeast, the Katangan ('late Proterozoic) is considered as a mobile belt formed during the so-called Lufilian orogeny of PanAfrican age (600 Ma.). All the sites of t e m p e r a t u r e m e a s u r e m e n t s are located in the Katangan metasediments. Many p a p e r s on stratigraphy of this belt have b e e n published (e.g. Francois, 1974; Lefebvre, 1978). The K a t a n g a n b e l t c o n s i s t s of c l a s t i c a n d calcareous s e d i m e n t s of the Roan supergroup, s h a l e s a n d c a l c a r e o u s s h a l e s of t h e lower Kundelungu supergroup. During the Pan-African orogeny, the Katangan a n d its b a s e m e n t have b e e n deformed simultaneously. Their relationships and the overall tectonic c h a r a c t e r of the orogeny are clearly s h o w n In open pits a n d mines of the Katangan Copper Arc. The Roan rocks are Intensively deformed a n d during the orogeny, t h e y formed northwards t h r u s t over the rocks of the Kundelungu platform, which extends to LakeTanganyika In the north (Fig. 1). In the Katangan Arc, the m a i n features of deformation are foldings, reverse faults with overthrusting a n d t h r u s t faults w h e r e a s In the Kundelungu platform the rocks are horizontal with mInor local deformations. The Katangan Arc is rich in polymetallic mineralizations and several copper, cobalt, lead, zinc and u r a n i u m mining districts are located within the fold belt In the s o u t h e a s t of Zaire and in the north of Zambia.
T E C H m g U E OF M
SUm
NTS
Temperature m e a s u r e m e n t s have b e e n obtained In 1988 and 1990, In exploration boreholes drilled b y GECAMINES and SODIMIZAmining companies of S h a b a Province. S u r v e y s were c o n d u c t e d several years after drilling h a d ceased. They were m a d e In water, m o s t of the time below 50 m, the expected depth of the water table during the dry s e a s o n , with a t h e r m i s t o r p r o b e e q u i p m e n t (relative accuracy: 0.01) combined with a Metrix MX 575 multimeter, with sampling each 10 m. In order to reach the proper steady-state during logging, it w a s n e c e s s a r y to wait a b o u t five m i n u t e s after the probe had b e e n submerged. For m o s t of the boreholes, t e m p e r a t u r e w a s m e a s u r e d only in the range of 50 to 200 m, with the exception of Kinsenda site, not far from the Zambia border, where boreholes are deeper (300-450 m). Thermal conductivity of solid rock samples were m e a s u r e d using a t r a n s i e n t line h e a t s o u r c e method. Samples of drill core were cut parallel to the origInal core axes. For m o s t boreholes, cores were not available, t h e n surface rocks were collect-
415
ed to complete samples from holes. All rocks were s a t u r a t e d with w a t e r Into a v a c u u m a p p a r a t u s before m e a s u r e m e n t . For the m a i n lithologies, m e a s u r e d thermal conductivity values of samples are: 3.56 W m q K-1 for arkose, 4.20 W m q for dolomite, 2.65 W m ~ K" for s a n d s t o n e a n d 2.15 W m q Kq for shale. TEMPERATURE DATA
Temperature-depth profiles m e a s u r e d In different sites are s h o w n In Fig. 2. In general the holes are located in a fiat c o u n t r y a n d no topographic corrections have b e e n made. All curves exhibit a significant negative t e m p e r a t u r e gradient In the first h u n d r e d meters. The relative m i n i m u m temperature is found at a depth ranging from 100 to 200 m. Examining the t e m p e r a t u r e profiles from Kinsenda site, it appears that below this depth the t e m p e r a t u r e gradient Increases until it reaches a c o n s t a n t value. As it cannot be due to conductivity c o n t r a s t s or variation of the water table, the a n o m a l o u s curvature which m a y be s e e n as a characteristic feature of the m e a s u r e m e n t s from the s o u t h e a s t of Zaire a n d p e r h a p s from the north of Zambia (Chapman, 1983), is interpreted as the effect of variations In surface conditions, i.e. of surface temperature (Sebagenzi e t al., 1992). The analysis of this p h e n o m e n o n w a s done with an inversion technique of t e m p e r a t u r e data b a s e d on the theory of heat conduction in a semi-infinite homogeneous medium. This process a s s u m e s that ground temperature variations c a n be modelled as a J u m p of temperature, starting at a n u n k n o w n time and contInuIng until present time. These variations are c o m p u t e d b y boreholes temperature data Inversion with algorithm of Tarantola and Valette (1982). One of the p a r a m e t e r s of the ground temperature model is the u n d i s t u r b e d temperature gradient, u s e d for heat flow determination. The Inversion algorithm r e t u r n s the optimal values of the parameters ( d T / d z the u n d i s t u r b e d temperature gradient, T o the original u n d i s t u r b e d surface temperature, T the present surface temperature, a T t h e disturbance between the inltial tlme and the present time, t* the time Interval during which this d i s t u r b a n c e occurred) and a poster/or/standard deviations. The development of this Inversion technique w a s recently performed (e.g. Cermak et al., 1992; Sebagenzi et al., 1992). As shown In Sebagenzi et al., the a n o m a l o u s curvature c a n be Interpreted as the result of a warming of the ground surface t e m p e r a t u r e by 3-4°C during the last 40-100 years. This warming can be associated with the environmental changes In connection with mIning activities a n d urbanization.
416
M. N. SBAG'EN~, G. VASSEUR and P. Louxs
Temperoture( "C) o
21
o
2p
Temperoture( "C) 27 2p ~
50. 100.
50-
150. 200.
100" A
~,~ 250.
E
e.
e-
4,* O, 150-
O. `500m `550-
£:3 LIKASI-KAMOYA SITE
MBUJIMAYI SITE
200
400
Temperoture( "C)
Temperoture( "C)
02 2,~ 2,4 2p 2p 27 2,e 2,9 ~
02
2`5
24
2p
50. 100.
50-
150. 200.
100-
250
e-
e-
~oo. ¢'~ ,550400-
Z. 150¢'~ 200
KIPUSHI SITE
Fig. 2. Temperature profiles for measurements, t~reholes are gathered according to the measurements sites. HEAT FLOW DETERMINATION AND D E E P S T R U C T U R E
The HFD have b e e n c o m p u t e d a s the product q = k d T / d z , where k is the average conductivity on
the depth of m e a s u r e m e n t and d T / d z the undist u r b e d t e m p e r a t u r e gradient. The value of K h a s b e e n estimated in each borehole as the harmonic average of conductivity for each lithology weighted b y their thickness. The uncertainty on this average of conductivity is estimated to 10 %. The distribution of heat flow for both Zambia a n d Zaire sites, together with regional gravity anomaly (Sebagenzi, 1993) are depicted in Fig. 4, where some HFD results from Lakes Kivu, Tanganyika, Malawi and from Tanzania reported b y Nyblade e t a/. (1990), are plotted for comparison purpose. In Table 1 where the results obtained from Zaire are s u m marized, we give for each borehole, elevation, un-
disturbed temperature gradient, harmonic average conductivity and heat flow value. In addition for each site, an average heat flow is shown in last column. In S o u t h e a s t Zaire, h e a t flow values for each individual borehole range b e t w e e n 48 a n d 72 mW m 2 w h e r e a s site averages vary from 53 to 69 m W m -2. The e s t i m a t e d error is large, b u t the various values s e e m consistent. The average for the various sites is 62 mW m 2. This value is similar to t h a t of 66 mW m 2 observed from Zambia by C h a p m a n a n d Pollack (1977). Both values give an average of a b o u t 64 m W m 2 which might be representative of the Zaire a n d Zambia heat flow values. This average value which is similar to those obtained in the Mozambique belt on the flanks of the Rift east b r a n c h (Nyblade e t a / . , 1990), is higher b y a b o u t 9 mW m 2 t h a n the 55 mW m 2 value calculated for the Hoggar Pan-African Province (Lesquer e t o L , 1989). Thus, HFD from Zambia and
417
First heat flow density determinations from Southern ZaIre (Central Africa) Zaire Proterozoic t e r r a n e s lies within the regional heat flow pattern of 68 ± 4 mW m -2average value for rifted continental m a r g i n in East Africa (Nyblade et oL, 1990). MbuJtmayi site HFD (Fig. 1 a n d T a b l e 1) which is p r e s e n t e d in the west of Fig. 4 where regional gravity is not av_afiable, is not considered in our discussion b e c a u s e this site belongs to different tectonic province of KasaI craton. However, this HFD value would be t a k e n into a c c o u n t in the interpretation of the HFD contrast between A r c h a e a n craton a n d mobile belts area~. HFD average in Proterozoic t e r r a n e s of Zambia a n d Zalre is higher by 20 mW m -2 t h a n the 44 mW m "a obtained from MbuJimayi site of Kasai A r c h a e a n craton which agrees with Tanzania a n d Zimbabwe cratons HFD (Nyblade et oL, 1990).
made. For this estimation, a two layers c r u s t with different h e a t production, h a s b e e n assumed. An u p p e r c r u s t formed b y granite gneisses, biotite gneisses, migmatltes a n d amphibolites in the amphibolite facies, and a lower c r u s t is a s s u m e d to be formed by granulite facies rocks (Chapman a n d Pollack, 1977). We have no heat production determlnation from heat flow sites of Southeast Zaire. However in Zambia, m e a s u r e m e n t s obtained by C h a p m a n and Pollack (1977) on some samples of granite gnelsses, biotite gnelsses, mlcaschists and amphibolites, belonging to the pre-Katangan basem e n t a n d Katangan metasediments, revealed a n average h e a t production of 2 . 4 ~W m a which c a n also be u s e d for Southeast Zaire. According to Hadiouche a n d Jobert (1988), the average crustal thickness was estimated to be 36 k m for this area. Crustal contribution to surface heat flow Then, the u p p e r c r u s t with average heat producIn order to discriminate the crustal component of tion of 2.4 ttW m -a, would have a thickness of 5.5 heat flow a n d t h a t from deeper origin in the mantle, km. The lower c r u s t would be 30.5 k m thick; its Birch et oL (1968) proposed a linear relationship heat production value c a n be estimated to be 0.4 between heat flow qo a n d h e a t production Ao, I~W m a as other granulite facies t e r r a n e s throughexpressed as qo = q* + bAo, where q ' i s r e d u c e d heat out the world (Pinet a n d J a u p a r t , 1987). For this flow which r e p r e s e n t s heat flow component at model, the values of integrated heat production are lower crustal level in a province and bAo the 13.2 mW m "a for u p p e r c r u s t a n d 12.2 mW m "~for integrated heat production. The p a r a m e t e r b is a lower crust, which lead to a mantle heat flow of lengh which characterizes the vertical distribution 38.6 mW m 2. of h e a t generation in the crust. This relation h a s However, the validity of this result depends on been applied by Ballard et oL (1987) to heat flow the reality of the h e a t production model a n d in a n d h e a t production d a t a from Bostwana-Namibia particular on the t h i c k n e s s of the enriched crust. a n d Zambia which belong to the same tectonic It h a s b e e n shown t h a t lateral h e a t transfer m a y province, to obtain a r e d u c e d heat flow q* of 50 + 3 significantly affect the slope of the h e a t flow-heat mW m "2a n d a slope b of 5.5 _+ 1.1 kin. p r o d u c t i o n r e l a t i o n s h i p (e.g. J a u p a r t , 1983; Under the a s s u m p t i o n s t h a t heat production is Vasseur and Singh, 1986; Nielsen, 1987; Furlong c o n s t a n t for depth range 0 < z < b with b = 5.5 ± a n d Chapman, 1987). This effect has been model1.1 k m a n d t h a t r e d u c e d h e a t flow q* of 50 ± 3 mW led quantitatively by V a s s e u r a n d Singh (1986), m -2calculated by Ballard et ol. corresponds to heat a s s u m i n g t h a t the heat generation distribution flow at u p p e r crustal level, a n estimation of the c a n be represented horizontany by a n isotropic crustal contribution to surface heat flow h a s been stationary function of the horizontal distance. Table 1. Thermal gradient, estimated conductivity a n d heat flow density for each borehole. SITES
BOREHOLES
ELEVATION (m)
KINSENDA
KIN 80B KIN 122 KIN 144 KIN 149
1295 1285 1271 1274
24.0 ± 21.3 + 24.0± 19.4 ±
1.3 2.7 1.8 2.0
2.94 ± 0.3 3.03 ± 0.3 3.01 ±0.3 2.97 ± 0.3
71 ± 65 ± 72± 58 ±
KIPUStll
MW2
1302
21.0 + 6.0
3.30 ± 0.3
69 + 26
69.0 ± 26
I.IKASI
IJKASI3 I.IKASI4 KAMOYA
1320 1320 1490
17.0 + 10.0 23.0 ± 8.0 17.0+ 9.0
2.82 ± 0.28 2.82 ± 0.28 2.801+ 0.28
48 + 32 65+29 48+ 30
53.0 + 30
18 KI0
595 590
15.0 + 6.0 18.0+ 5.0
2.67 + 0.3 2.69+0.3
40 + 20 48+19
MBUJIMAYI
TIIERMAL GRAD. ('Ckm "1)
CONDUCTIVITY (Wm"'C I)
IIEAT FLOW DENSITY (mWm"2) 11 14 12 12
66.5 ± 12
44.0+20
418
M. N. S~^G~czl, G. VASSARand P. Lotns
As s h o w n in the appendix, the a p p a r e n t thickn e s s Da of the radioactive layer d e d u c e d from h e a t flow-heat p r o d u c t i o n relation is r e d u c e d with respect to its actual t h i c k n e s s D b y a value which d e p e n d s u p o n k / D ratio, k being the characteristic horizontal scale length of h e a t production variations. For reasonable values of the ratio g / D , the reduction factor lies b e t w e e n 1 a n d 0.5. Returning to the estimation of crustal radioactive source thickness, the actual t h i c k n e s s D would t h e n range b e t w e e n 5.5 and I I kin. The latter value would imply a crustal h e a t flow contrib u t i o n of 36.2 m W m 2 a n d therefore, a mantle heat flow of 27.8 m W m a instead of 38.6 mW m a as obtained with the former assumption. Recent geothermal s t u d i e s a r o u n d the world have s h o w n t h a t m a n t l e h e a t flow in stable Precambrian shields is close to 13 mW m "2 (e.g. Pinet a n d J a u p a r t , 1987; Pinet et aL, 1991). In comparison with this result, it a p p e a r s that mantle heat flow b e n e a t h S o u t h e a s t Zaire and North Z a m b i a Proterozoic t e r r a n e s m a y p r e s e n t a n excess heat flow of 14.8 mW m a. Geothermal investigations from Zambia (Chapman and Pollack, 1977) led to estimate similar
excess mantle heat flow {mean value of 15 mW ma}. This thermal excess h a s already b e e n noted a n d interpreted as indicator of a flux from the asthenosphere, in relation with lithospherlc thinning, p e r h a p s associated with a s o u t h w e s t e r n extension of E a s t African Rift S y s t e m {Chapman a n d Pollack, 1977). As S o u t h e a s t ZaZre and North Zambia belong to the s a m e major tectonic province, we believe t h a t o u r heat flow determinations support, for the S o u t h e a s t Zaire, this hypothesis which m a y be considered as confirmed since S o u t h e a s t Zaire and Zambia HFD agrees with the regional heat flow p a t t e r n of Kenya a n d Tanzania (Nyblade etaL, 1990}, where continental rift struct u r e s are well developed. b e n e a t h S o u t h e a s t ZaIre An estimation of t e m p e r a t u r e as a function of depth for S o u t h e a s t Zaire a n d North Zambia h a s b e e n attempted. Geotherms have b e e n calculated for heat flow of 64 mW m-% the m e a n surface h e a t flow value for Zaire and Zambia terranes. Calculations were r u n for the two models corresponding to the extreme a s s u m p t i o n s on t h e v a l u e olD {5.5 and 11 kin). For the first model, the following paraGeotherms
Temperature(°(]) 0
0
I~5 0 I
500 J
750 I
1000
1250
L
1
1500 I
1750 1
2000 I
20-
40.
80-
•8 0
100
120 140
-
160 -
180 -
\\\\\\
~
200 -
Fig. 3. Geothermscomputed with mean surface heat flowof 64 mWm-LSolidline corresponds to model for apparent depth Da of 5.5 km and dashed line to modelfor actual depth of 11 km.
419
First heat flow density determinations from Southera 7-.Yre(Central Africa) m e t e r s were assumed: m e a n radiogenic heat for u p p e r and lower c r u s t are 2.4 and 0.4 ~tW m -s, respectively; thickness of 5.5 k m for the u p p e r a n d 30.5 k m for the lower c r u s t and no mantle heat production contribution; uniform thermal conductivity of 3 W m ~ K'L For the second model, all p a r a m e t e r s values r e m a i n the same except the u p p e r a n d lower c r u s t thickness values which are fixed to 11 k m for the u p p e r a n d 25 k m for the lower 25
/,#
/
319
, /
#
I
/.,/., I
/ ,' / , , " / t /
,,'/
,,'
'
,
, .,,t vi,-, ~
I I
,
l
tFI /t
!
,
i I
I/
i
i
'
,
i
I I
t,
i t i
A /\
,I
~
\
I
"
',,/,.,,\
',
I
,
l
,
Rr
'~ ,-%/' :'.
I
,"
35
~
~ f
,'
,,#
7,
/.
crust. Both geotherms presented on Fig. 3, indicate quite t e m p e r a t u r e s at depth. A t e m p e r a t u r e of 1250°C, which could correspond to the beginning of partial melting, is r e a c h e d at depth of about 85 to 110 kin. The predicted t e m p e r a t u r e s m u s t be t a k e n with caution b e c a u s e of uncertainties about heat generation and c r u s t layer thickness estimations, a n d of no sufficient constraints on thermal p a r a m e t e r s
\ ~
,..
,% ~
I
%'
-,~,-,il U .
"-.
~_l.
I ~L-~ ....
20 ~.
_.
:/(/,'"\.---___-_;I'---/---
,
.,~\'/" ,: V
7".
"it .' i
'.
\
146_L....-j
".
I"l
-'----"-'--
_-'"
44 5
O
.%
,
,#
~,.#
~
/
•
#1#
I\ I
\ k
~ ~
~
# i #f
,'~
/P/
,"
### ##fX
# f# /
i / , , ," l, i ~l \ ~\ ~\ ~ . .\. , ,,'x,/ " / ldl ,I,'' " /:/ i
"- . . . . -'" zAI~i," 10
_~.-'Y/
-'- ;-'"
\
,,--L~ a
,:~\"x,'l
/ 1o
._//
f
,~69
,
~,~,~/
~o
' I/ I/ tx,_ :51)(
--~_..~.,..'k
,,,' AUd/-,, / )klY //_ 7,,' (z,l:---'~ .I/.77// ,, \"A( . , " / 5 / / , ,," " XT).,// II ,," Z73// ,,~ / ill~xa~ //
. 15
'=
71
..............
..... ~
......
"'°" I,
~
........ -~.._. \ I ,
\
• 66---/~"~:.-'~/.~--
~AT~~~.~ -_,,.~:.,.
x/f
.,_ ....
--,_~,,,,
IS
. , ,\
Fig. 4. Regional gravity field, HFD values at the sites shown in Fig. I and at some sites reported from Kenya and Tanzania by Nyblade e t al. (1990), main faults of east central Africa associated with the East African RIll system. Gmvlty curves are in regal and HFD in mWm'~ enriched star symbol corresponds to new HFD sites reported In this study, solid triangles for Zambia (Chapman and Pollack, 1977) and solid squares for Kenya, Tanzania and Zambia HFD sites (Nyblade eta/., 1990). Thick lines display major normal faults related to main rifts l ~ means rills).
420
M. N. Sm3^o~rn, G. V ^ s s ~ and P. Louxs
assumption. However, t h e y provide a first attempt to estimate lithospheric t e m p e r a t u r e variations with depth in S o u t h e a s t Zaire. DISCUSSION
AND
tween 85 a n d 110 kin. High heat flow in S o u t h e a s t Zaire and North Zambia related to asthenospherlc uplift is supported b y other indications, s u c h as gravity a n d seismological results. Regional gravity field of S o u t h e a s t Zaire and North Zambia w a s derived from B o u g u e r anomaly m a p obtained using Zambia data compiled b y Cowan a n d Pollack (1977), combined with Southeast Zaire data recently acquired (Sebagenzl, 1993). Regional gravity low (amplitude < 100 rngal) trending NE-SW (Fig. 4) is in agreement with Girdler predictions {Girdler, 1975}, which suggested t h a t the regional gravity anomaly in s o u t h e a s t e r n Africa could continue the long wavelength anomaly associated with the E a s t African Rift System. Fig. 4 p r e s e n t s this regional gravity field, HFD value and principal sites location, tectonics of e a s t a n d
CONCLUSIONS
In spite of their limited n u m b e r and low accuracy, h e a t flow v a l u e s in the Proterozoic belts of S h a b a Province of Zaire (62 m W m 2) are consistent with the results from Zambia (66mW m2). These HFD values are higher t h a n w h a t is expected in this area of Proterozoic age and are rather similar to those observed in Rift east branch. The predicted high t e m p e r a t u r e s of 1250°C at shanow level would suggest t h a t mantle b e n e a t h Proterozoic terranes of S o u t h e a s t Zaire a n d North Zambia is a n o m a l o u s l y hot and would indicate the presence of a s t h e n o s p h e r i c uplift at a depth ranging be-
•
O
"0
'1' J
-
,
.
_
Q
°~1 I° IL'~ 0%1 I d ' ~ f o .~ ~ . ~ I'.qk]P I . ~:,~,,Z~ - * " .~t'~" 3 ° ,rir-*:-ah- ~. f~-': I • / ,.o
,,~,-~,
.o.
o
,.
o
o
.
:
(
0
•
g\ -
35
..,.?.. ;J \
+
r
A
oo I
"
o
/ . +
-s -~'
15
20
25
30
35
40
I
I
i
I
I
I
Flg. 5. Se/sm/c/ty map of southern Africa for earthquakes with magnitudes mb > 4.0 for the period 1950-1992, with available focal mechanism studies (modifiedfrom Fa/rhead and Henderson, 1977; Grhn/son and Chen, 1988). Note the accordance between focal mechan/sm deduced from 1992 september 11 earthquake and availahle previous focal mechanisms.
First heat flow density determinations from Southern Zaire (CentralAfrica)
421
extends southwestward from Lake Tanganytka into Southeast Zaire and Zambia where events have occurred in recent y e a r s (Fairhead a n d Henderson, 1977; Fairhead a n d Stuart, 1982; Maasha and Molnar, 1972; Grimison a n d Chen, 1988} a n d are nowadays occurring (e.g. Lombe and Mubu, 1992}. Seismicity NE-SW t r e n d is parallel to regional gravity a n o m a l y NE-SW axis a n d to Upemba, Mwem, Luangwa, L u k u s a s h i and Luano rifts (Fig. 5 and Fig. 6) a n d earthquake-source m e c h a n i s m s indicate a NW-SE tensional stress associated with normal faulting (Dorbath, personal communication 1992). In a recent analysis of Zambia seismicity, Lombe a n d Mubu (i 992) suggest to consider the Luangwa and Mwero rifts as already existing NE-SW trending reactivated fault zones which could indicate some w e a k n e s s in the crust. In this way, the seismicity in this zone might indicate that a n incipient b r a n c h of the western a r m of the East African Rift System extends into Southeast Zaire a n d Zambia and t h a t this b r a n c h could be traced across Lake Tanganyika a n d connects with the m o s t active part of the Rift System (Grimison and Chen, 1988). We note that this b r a n c h coincides largely with b a s e m e n t s t r u c t u r e s associated with the Proterozoic mobile belts which separate A r c h a e a n craton areas. It appears that the Proterozoic crustal s t r u c t u r e s have a major influence on the recent crustal movements.
central Africa a n d somefauRs associated with the East African Rift System. These features show clearly a close correlation between high h e a t flow a n d regional gravity anomaly over this area. This suggests t h a t the regional gravity low m a y be associated with a t h e r m a l anomaly within the u p p e r mantle. An important a r g u m e n t to discuss the existence of incipient riRing in this area, is the estimate of lithospheric thickness from teleseismic delay times a n d gravity correlation published by Fairhead a n d Reeves (1977}. Their p a t t e r n for Southeast Zaire and North Zambia t e r r a n e s of Proterozoic age, predicted a t h i n n e d lithosphere which agrees with regional gravity trending NE-SW and with high heat flow. Seismic activity is r e p r e s e n t e d by the epicenters distribution of e a r t h q u a k e s with n% _>4.0 reported by the International Seismological Center between 1965 to 1970 a n d some fault plane solutions are shown (Fig. 5}. The focal m e c h a n i s m of 1992 S e p t e m b e r 11 e a r t h q u a k e , S o u t h e a s t Zaire (Dorbath, personal c o m m u n i c a t i o n 1992) which Just supports our previous observations, has also been plotted. Both results could be u s e d to conswain the present day deformation of this zone and its relationship with the East African RIR System. Present seismic activity in Southeast Zaire and in North Zambia reveals t h a t a broad area of activity
1.0
0.8
0.6 a
0.4
0.2
0.0 I-" 0.01
l
!
i
l
!
i
|i
I
0.1
!
i
i
!
;
i
i;
I
1
i
I
i
!
I
I
Ii
i
10
Fig. 6. Variation of D J D as a function of k/D.
I
l
!
i
l
X/D
i
il
i
tO0
M. N. SEBAG$~¢n,G. VASSEURand P. Louis
422
A c k n o w l a d g e m e n t s - Field works were supported by the U n e s c o Major Project "Geology for E c o n o m i c Development" Contract S C / R P 264061.8 a n d S C / R P 2030360.0. This project was facilitated b y the following mining companies a n d their staff: GECAMINES (C~n~ral des Carrt~res et des Mines}, SODIMIZA (Soci~t~ de Developpement Industrtel et Mlnl~res d u Zalre), MiBA (Mlnl~re de Bakwanga). H. N. Pollack of the University of Michigan (USA) and M. Q. W. J o n e s of the Bernard Price Institute of Geophysical Research, University of the Witwatersrand (RSA), are t h a n k e d for their interest in this study. We are grateful toA. Lesquer and F. Lucazeau of Centre G ~ l o g i q u e et C,~'ophysique CNRS, Montpellier for valuable reviews and c o m m e n t s on the manuscript. REFERENCES
Ballard, S., Pollack, H. N. and Skinner, N. J. 1987. Terrestrial Heat Flow in Botswana and Namlbia. J. Geophys. Res. 92, B7, 6291-6300. Birch, F., Roy, R. F. and Decker, E. R. 1968. Heat flow a n d thermal history in New England and New York. In: Studies of App~_ !~chlan Geology: Northern and Maritime. E. ZenW. S. Ed. Intersclence NewYork, 437-451. Bram, IC 1972. Selsmicity of Katanga and Western Zambia, southwest of E a s t Africa Rift Systems, from 1960 to 1971. Bull. Seismol. Soc. Am. 6 2 , 1211-1216. Cermak, V., Bodrt, L. and Safanda, J. 1992. Underground t e m p e r a t u r e fields a n d changing climate: evidence from Cuba. Pa/eogeogr., Pa/eoecoL (Global Planet. Change Sect.) 97, 325-337. Chapman, D. S. 1983. Thermal regime of the L u a n s h y a Mine, Republic of Zambia. Geoexploratfon 21, 264281. Chapman, D. S. and Pollack, H. N. 1977. Heat flow and heat production in Zambia: Evidence for llthospherlc thinning in central Africa. Tectonophyslcs 41, 79- 100. Cowan, I. M. a n d Pollack, H. N. 1977. Gravity in Zambia. In: Macmillan Journals Ltd., 1977. Reprinted from Nature 2 6 6 . N ° 5603, 651-617. DaLly, M. C. 1983. The Irumide belt of Zambia and its bearing on collision orogeny in the Proterozoic of Afrlca. Geo/. Soc. Spec. PubL London, CoUtston tectonic vo/ume. Falrhead, d. D. a n d Girdler, IL W. 1969. How far does the RIR System extend through Africa ? Nature 221, 10181020. Fairhead, J. D. a n d Henderson, N. B. 1977. The seismlclty o f s o u t h e r n Africa a n d i n c i p i e n t rifting. Tectonophyslcs 41, 19-26. Falrhead, J. D. and Reeves, C. V. 1977. Teleseismic delay times, B o u g u e r anomalies and inferred thickness of the African lithosphere. Earth and Planetary ScL Letters 3 6 , 63-76. Fa/rhead, J. D. a n d Stuart, G. W. 1982. The seismicity of the E a s t African rift system and comparison with o t h e r continental rifts, in Cont/nental a n d Oceanic R/fls, Geody. Set. 8, edited b y G. Palmason, 41-61, Washington D. C. Francois, A. 1974. Stratlgraphie, tectonlque et min6ralisations de l'Arc cuprlf~re Shablen (R~publique d u Zalre). Cent. Soc. G~l. Belg., Gtsements stratlformes et provinces c u p r i ~ s , Li6ge, 79-101.
Furlong, K. P. and Chapman, D. S. 1987. Crustal heterogeneltles and the thermal s t r u c t u r e of the conUnental crust. Geophys. Res. Left. 14, 314-317. Girdler, R. W. 1975. The great Bouguer a n o m a l y over Africa. EOS, Trans., Am. Geophys. Un. 56, 516-519. Grlmison, N. L. and Chen, W-P. 1988. E a r t h q u a k e s in Davie Ridge-Madagascar Region and the S o u t h e r n Nubian-Somalian Plate Boundary. J. Geophys. Res. 9 3 , B9, 10439-1 0450. Hadiouche, O. a n d Jobert, N. 1988. Geographlcal distrlbution of surface-wave velocities a n d 3-D uppermantle structure in Africa. Geophys. J. 95, 85-109. J a u p a r t , C. 1983. Horizontal heat transfer due to radioactivity contrasts: c a u s e s and consequences of linear heat flow relation. Geophys. J. R. Astnm. Soc. 75, 411435.
Kabengele, M., Lubala, R. T. et Cabanis, B. 1991. Caracterisation l~trologique et g~ochimique d u magmatisme ubendien d u secteur de Pepa-Lubumba, s u r le plateau des Marungu (Nord-Est d u Shaba, Zalre). Signification g ~ d y n a m i q u e darts l'~'volution de la chalne ubendienne. J. Aft. Earth ScL 1S, 2, 243-265. Kampunzu, A. B., Rumvegeri, B. T., Kapenda, D., Lubala, R. T. et Caron, J . P . 1985. Les Klbartdes d'Afrtque centrale et ortentale: u n e chaine de collision intercontinentale. BUlL Inform. "La ~ i e au service du/~ve/oppemen~' U~O 5. Lefebvre, J . J . 1978. Le groupe de Mwashya, m~gacycloth~me terminal d u Roan (Shaba, Zalre sud-ortental). Approche lithostratigraphique et ~tude de l'envtronnem e n t s~ltmentaire. Ann. Soc. Gt~. Bel. 101, 209-225. L ~ q u e r , A., Bourmatte, A., LY, S. a n d Dautrla, J. M. 1989. First heat flow determination from the central Sahara: relationship with the Pan-African belt and Hoggar domal uplift. J. Aft. Earth ScL 9, 1, 41-48. Lesquer, A. and Vasseur, G. 1992. Heat-Flow constraints on the West African lithosphere structure. Geophys. Res. Lett. 9, 6, 561-564. Lombe, K. D. and Mubu, M. S. 1992. Instrumentation and seismlcity in Zambia. Tectonophysb~s 209, 31-33. Maasha, N. and Molnar, P. 1972. E a r t h q u a k e fault parameters and tectonics in Africa. J. Geophys. Res. 77, 5731-5743. Nielsen, S. B. 1987. Steady state heat flow in r a n d o m medium and the linear heat flow-heat production relationship. J. Geophys. Res. Lett. 14, 318-321. Nyblade, A. A., Pollack, H. N., Jones, D. L., Podmore, F. and Mushayandebvu, M. 1990. Terrestrial Heat Flow in East and S o u t h e r n Africa. J. Geophys. Res. 95, B 1 I, 17, 371-17, 384. Plnet, C. and J a u p a r t , C. 1987. The vertical distribution of radiogenic heat production in the Precambrlan crust of Norway and Sweden: Geothermal implications. Geophys. Res. Lett. 14, 260-263. Pinet, C., J a u p a r t , C., Mareschal, J. C., Garlepy C., Bienfait, G. and Lapointe R. 1991. Heat flow and s t r u c t u r e of the lithosphere in the E a s t e r n Canadian Shield. J. Geophys. Res. 96, BI2, 19, 941-19, 963. Sebagenzi, M. N., Vasseur, G. a n d Louls, P. 1992. Recent warming in s o u t h e a s t e r n Zah'e (Central Africa) inferred from disturbed geothermal gradients. Palecx3eogr., Paieocllmatol., Paleoecol. (Global Planet. Change Sect.) 9 8 , 209-217.
First heat flow density determinations from Southern 7ai're (Central Africa) Sebagenzi, M. N. 1993. Etude gravim~trique et g~othermique d u S u d - E s t d u Zalre et d u Nord de la Zamble (Afrique centrale). Phd Thes/s Univ. Parts V//. Tarantola, A. and Valette, B. 1982. Generalized nonlinear inverse problems solved using the least squares criterion. Rev. Geophys. Space Phys. 20(2}, 219-232. T s h i m a n g a , K., Ll~geols, J. P., Lubala, R. T. et Kampunzu, A. B. 1988. La zonaflon inverse d a n s le
eomplexe concentrique calco-alealln potasslque ubendlen (ProtC~rozoique lnf~rieur} de Lumono-Zalre: Structure, P~trologie, et C~ochimte. 12~ R~un. Sci. Terre, Lille, Avril 1988, Soc. gg,o/. Ft., g~L p. 127. Vasseur, G. and Stngh, R. N. 1986. Effects of r a n d o m horizontal variations in radiogenic heat source distribution on its relationship with heat flow. J. Geophys. Res. 91, B10, 10397-10404.
Appendix Following Vasseur and Singh (1986), two assumptions on the vertical distribution of heat production of the surface layer can be made : either an exponential decrease with depth as e-~° or a c.onstant heat production for depth range 0 < z < D. In both cases the autocorrelation function of heat production which characterizes its horizontal variations, is assumed to vary as e '"x, where ~. is the length giving the scale of horizontal heat production variations and r is the horizontal distance. With such hypotheses, the apparent thickness D, of heat production layer, as deduced from heat flow-heat production relationship, is obtained in the following closed form:
D,,
423
:"
k'dk'
"It=Jo (k'2+ l)3a(1+13k') with 13= L/D for constant heat production for 0 < z < D depth ; D, 1 : " 1 - e -~' , D Iz=~J0 ( k ' 2 + l ) 3adk for exponential heat production decrease with depth. These results are plotted on Fig.6 as a function of the parameter L/D. From this figure, one can see that, for a given D value, the smaller is ~., the smaller the apparent thickness D, deduced from the. heat flow-heat production plot. Only for 13< 1 (i.e. lateral heterogeneities of very large horizontal extension), the apparent Do value reach the true thickness. This corresponds to ~. values range of 1 < X < 0-.