Deep structure of the hoggar domal uplift (central sahara, south algeria) from gravity, thermal and petrological data

71

~ecro~~~~~~jc~,152 (1988) 71-87 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Deep structure of the Hoggar domat uplift (Central Sahara, south Algeria) from gravity, thermal and petro~ogica~ data A. LESQUER I, A. BOURMATTE

2 and J.M. DAUTRIA

1

’ Centre Geologique et G&physique, Place E. Bataillon, 34060 Montpellier Cedex {France) ’ CRAA G, Bouzareah,

A lger (Algeria]

(Received July 30,1987; revised version accepted January 4,1988)

Abstract Lesquer, A., Bourmatte, A. and Dautria, J.M., 1988. Deep structure of the Hoggar domal uplift (Central Sahara, south Algeria) from gravity, thermal and petrological data. ~ecfunup~y~ic~, 152: 71-87. Initial heat flow determinations show that no significant regional thermal disturbance is associated with the volcanic Hoggar swell. However, the large-scale negative gravity anomaly associated with this swell strongly suggests the occurrence of a low-density upper mantle beneath the Hoggar. This is inferred from the occurrence of the depleted, metasomatized and intensely recrystallized and veined peridotitic upper mantle revealed by the study of xenoliths from the Cenozoic basaltic eruptions. We suggest that the low density of the upper mantle is due to magmatic events of asthenospheric origin. From various observations including petrographic, gravimetric and thermal constraints, the age of these events is estimated to be Late Mesozoic to Early Cenozoic. The alkali Cenozoic volcanism cannot be regarded as the cause of the swelling. The occurrence of an altered lithosphere, now cooled, and presently marked by low-density magmatic materials, is consequently proposed as a possible model for the Hoggar.

1. Introduction

The Hoggar massif (Central Sahara) belongs to the system of uplifts (Hoggar, Air, Tibesti, Eghei, Darfur, Cameroon, Adamawa) that surrounds the Chad Basin (Fig. la). These domal uplifts are very similar in scale, morphology, epeirogeny, volcanic activity and gravity anomaly pattern. They are linked to subsiding sedimentary troughs (Ten&-e, Benoue, Abu Gabra, Ngaoundere rifts) that developed during the Early Cretaceous. By analogy with the structure of the East African rift, Fairhead (1976) and Brown and Girdler (1980) modelled the gravity anomalies of the swell system as a lithospheric thinning (to about 60 km). Crough (1981a,b), using free-air anomalies, proposed that the Hoggar and the Darfur domes are isostatic responses to altered lithosphere. The in0040-1951/88/$03.50

0 1988 Elsevier Science Publishers B.V.

ferred root depth (60 km) is equivalent to those calculated beneath the Kenya dome (Banks and Swain, 1978) and the oceanic swells (Crough, 1978). Crough proposed that all mid-plate swells have probably formed by similar mechanisms. In this paper we propose to use gravity data, heat flow measurements and peridotitic xenolith analysis to develop a better insight into the deep structure of the Hoggar uplift. GeologicaI setting The structure (Fig. lb) of the Hoggar shieId results from several major events which have occurred in the Central Sahara over the past 600 Ma. The main lithological and tectonic features are due to the Pan African orogeny. According to recent works (Bayer and Lesquer, 1978; Bertrand

500 km

1 --

-.--_

I

_._

I Adrar

I,.‘.

Fig. 1. Situation basement; 2 = Cenozoic

/

13

of the domaily

uplifted

2 = Cenozoic

volcanic

volcanics;

3 = Western

Iforas units (Ebumean

grant&es);

areas.

volcanic

areas of central

b. Major

and eastern

structural

Pharusian

belt;

7 = suture zone (from gravity

and Caby, 1978; Caby et al., 1981; Lesquer et al., 1984; Bertrand et al., 1986), this orogeny possibly resulted from a continental collision between two blocks, the West African craton and an East African block. This collision induced a significant crustal thickening which allowed reworking and partial melting of the crust at depth and, consequently, the emplacement of a large quantity of granitoids (nearly 50% of the Hoggar is granitic). Early megasystems of N-S trend and later vertical shear zones cross-cut the Pan African structures and separate the Hoggar basement into large, elongated and independent blocks. Four structural units can be defined: the suture zone, the Pharusian belt, the Central Hoggar and the Eastern Hoggar. The suture zone is characterized along meridian O” by a string of dense rocks only made evident by gravity surveys (Bayer and Lesquer, 1978). The Pharusian belt comprises a western and an eastern branch, both characterized by an abundance of Upper Proterozoic

western units

Africa. The box shows the Hoggar

of the Hoggar

4 = Central data);

Hoggar

shield. unit;

I = Paleozoic

S = Eastern

area.

1 = Pan African

to Quatemary

Hoggar;

6 = In-Ouzzal

cover; and

8 = heat flow sites.

volcano-detritic material. A granulitic uplift block of Eburnean age (2075 it 20 Ma; the In Ouzzal unit) separates these two branches. The Pharusian belt is bordered to the east by the 4”50’ megafault, and the adjacent Central Hoggar is largely composed of Eburnean granulites and gneisses, reactivated and injected by abundant granitoids during the Pan African orogeny (Bertrand et al., 1986). The Eastern Hoggar-Tenere domain was stabilized at an early stage of the Pan African episode around 725 Ma ago. The late-erogenic brittle deformation is characterized by a conjugate strike-slip fault system consisting of NE-SW dextral and NW-SE simstral trending sets of faults (Ball, 1980). The peneplanation of the Hoggar Pan African range occurred during Cambrian times and during the Ordovician to the Late Carboniferous a sedimentary platform developed (Fabre, 1976). The reactivation of the meridian shear zones as normal faults controlled the geometry of the sedimentary basins which

73

Eggere (28~ km’), Adrar N’Ajjer (2500 km*) and In Ezzane (500 km2). This Cenozoic activity, a typical continental intraplate alkali volcanism (Girod, 1971), is responsible for the emission of large quantities of essentially basaltic lava (for instance 250 km3 in Atakor). The volcanic activity was paroxismal during the Miocene and continued episodic~ly up to the Quatema~ with an intensity varying from district to district but everywhere decreasing with time. Local uplifts occurred at the end of the Miocene and probably also during the Pliocene. The amplitude of these Cenozoic vertical deformations is very difficult to estimate; the highest value {about 1000 m) has been reported in the Atakor district (Girod, 1971). In Adrar N’Ajjer the amplitude of this deformation is probably about 500 m.

developed on the platform. The Hercynian folds, occurring in the northern Hoggar, may be also ascribed to this reactivation. The late Variscan epeirogeny is responsible for the general uplift of the area and for the removal of its Paleozoic cover (Conrad, 1984). According to Carpena (1982), this removal probably ceased by the end of the Jurassic. The recent magmatic activity of the Hoggar began during the Late Cretaceous and Eocene with the emplacement of ring-shaped volcano-plutons (Remy, 1959; Rossi et al., 1979) of carbonatitic affinity, and with the effusion of basic lavas possibly transitional to alkaline within the Amadror depression (Eastern Hoggar) (Fig. lb). This early activity is contempor~eous with the general extension which occurred during the upper Mesozoic at the time of the initiation of South Atlantic. Since the Lower Miocene, the magmatic activity has moved from the center to the periphery of Amadror and has developed in areas where the basement is now at its highest topographic levels. Six districts were active (Fig. lb): Tahalra (1800 km*), Atakor (2150 km2), Manzaz (1500

k&‘),

Topography Topographically (Fig. 2), the Hoggar can be described as a broad (1000 km wide), almost circular, area of exposed Precambrian basement with

,,I

3"

5"

c

(n

7'

I

\

9'

Fig. 2. Simplified topographic map and geothermal measurement sites.

\I-

11'

altitude posed

ranging

from

2000 m) of Atakor to major volcanic topographic length

500 to 1000 m. Superim-

on this broad

the high massifs

and Adrar districts.

trends

altitude

dome,

N-Ajjer

correspond

variation

to

small

of basement

structural

The gravity data distribution (Fig. 3A) over the Hoggar swell has improved but remains uneven 111 the eastern

The N-S to NNW-SSE

correspond

with the Pan African

(>

However,

wave-

consistent

trends.

compiled

Bouguer

values)

features

of the gravity surveys

(Lagrula,

1959)

have and

Heat flow determinations

have been carried

out

Central

Sahara

(Algeria,

Niger). Characteristics

these heat flow sites and data processing cluded

in the Appendix.

The average

of

are invalue

[53

mW rn-‘) is comparable to results obtained from other Precambrian belts. No significant regional thermal disturbance Cenozoic intraplate

is associated with the Hoggar volcanism, whereas heat flow

by adjustment. was

determined

maximum parison

been

The height

uncertainty

the

main

tied

to

J.

(Duclaux

et al.,

were minimized

of the gravity

barometric

Lagrula

stations

levelling

with

of + 5 m. To facilitate works.

Bouguer

a

com-

anomalies

have been calculated for density reduction 2670 kg m -’ and the international Postdam 1931 reference ellipsoid. Terrain corrections have not been applied due to the lack of detailed topographic maps. Terrain corrections should be significant (> 8

anomalies greater than 80 mW mm2 are generally observed in rift zones and uplift areas (Lucazeau

mgal) only for stations within the volcanic vinces where major relief occurs; elsewhere

and Bayer, 1982). The measured heat flow is normal for a crust of that age and involves a litho-

are negligible.

sphere more than ductive geotherm

Gravity map unu~sis

100 km thick, assuming a conand steady state conditions.

3B)

data (7000

to show

J. Martin

with previous

area.

(Fig.

field.

discrepancies

by

map

published

is sufficient

1954) base systems; at nine widely spaced sites (Figs. lb and 2) in the

part of the studied anomaly

from all available

gravity The

Heat flow data

and northern the

prothey

Furthermore, Fig. 2 shows that there is no correlation between heat flow and basement elevation that is inconsistent with a thermal uplift or a

On the gravity map of Fig. 3B we can distinguish features related to the geological structures and to the basement elevation. The anoma-

recent erosion. Data repartition does not allow a description of heat flow variation in the northern

lies related wavelength,

part of the Hoggar and particularly

Pharusian belt and NNW-SSE within and the eastern Hoggar. These trends

in the volcanic

high massifs. However, we think that our heat flow data strongly suggests that there is no thermal anomaly

associated

to the Hoggar

swell.

Gravity data Gravity

data

from

U.S.

Defence

Mapping

Agency used by Crough (1981a) and Brown and Girdler (1980) do not include data from various gravity surveys performed in the Central Sahara (Louis, 1970; Lagrula et al., 1973; Bourmatte, 1977; Ly et al., 1980). In 1983 and 1984 the Centre de Recherche en Astronomie. Astrophysique et Geophysique (CRAAG, Alger) in collaboration with the CNRS (France), carried out a gravity survey in the Atakor volcanic district that completes the coverage of measurements.

the main

to the basement structure are of small and are elongated N-S within the

Pan African

structural

the central agree with

pattern.

Further

west detailed analysis of these anomalies has been made in previous papers (Bourmatte, 1977; Ly et In the southeastern Hoggar, the al., 1980). NNW-SSE treading anomalies correlate with the Cretaceous tectonic trends evidenced in the Ten&C (Louis, 1970). The relationship with basement elevation can be readily seen using the E-W profiles of Fig. 4 which show a very long wavelength anomaly, correlated with the broad basement swell, and pronounced negative anomalies, associated with the high volcanic massif of Atakor. The gravity data within the Adrar N’Ajjer massif are too few to provide a good definition of the gravity anomalies. The well defined negative

22”

76

GRAVITY 0 ____-._____

ANOMALY

(in,Zr, ! ..-. ---_.“--.-

1 ANOMALY

j ELEVATION

cmeteri ATAKOR

2000

4

1

1’ ,

1

I

1

2’

3’

4’

100 km

I

I

,

I

t

I

5’

6’

7’

8’

9’

10’

,

--‘1 11.

LONGITUDE

Fig. 4. Relationships between elevation and gravity anomalies along latitude 23ON.

anomaly on the Atakor district is perfectly correlated with the shape of the volcanic fields. Its western high gradient is roughly superimposed onto the N-S trending shear zone of 4”50’. Figures Sa and 5b show the relations~ps between gravity and elevation. We can distinguish two sets of data: for surface elevations lower than 800 m, neither the Bouguer nor the free-air anomaly is well correlated with elevation. For stations within the high volcanic massifs (> 1000 m) the correlation is obvious. The gravity anomaly variations, away from the volcanic areas, are mainly related to superficial density contrasts. Regionally, the crust is in isostatic equilibrium (mean elevation 600 m; mean Bouguer anomaly -65 mGal; mean free-air anomaly 10 mGa1). Within the volcanic districts, high negative Bouguer anomalies and high free-air values agree with local isostatic equilibrium. The Airy model of local isostatic compensation fits reasonably well with the observed Bouguer anomaly variation. The E-W trending isostatic profiles along latitude 23” (Fig. 4) clearly show that, due to the flat relief, the crustal thickness of the Pharusian belts have little effect on the calculated root attraction. The sm~l-wavelength isostatic anomalies (i.e. the difference between

Bouguer anomalies and Airy models) are thus probably related to density contrasts between crustal blocks of the Pan African belt. Over the zone of highest relief, for a density contrast between crust and mantle of 450 kg rn- ‘. the isostatic correction for 30-40 km crustal thickness fits the ahead profile best. A deeper crustal root does not lead to suitable gradients and amplitudes. Eastwards of the Atakor, the gravity field is not well documented, but not all of the gravity field can be explained by simple local isostatic root attraction. The positive anomalies are not obviously correlated with specific geological structure, and suggest the presence of dense bodies at depth. Transfer function

To extract the part of gravity field correlated with the topography we used the cross-spectral analysis of gravity and elevation data proposed by various authors (e.g. Dorman and Lewis, 1970; Watts, 1978; Karner, 1984). The relationship between Bouguer gravity anomaly, g, and topography, h, can be written: g=q*h+n

FREE

- 50I

Free

Au Anomaly

0’ 1

AIR

ANOMALY

+5b

f mGoi

(meter)

Elevation b) BOUGUER

ANOMALlES

1 2500

1

Fig. 5. Relations~ps

between gravity and elevation. a. Free-air

anomalies. b. Bouguer anomalies.

in which q is called the Transfer function, n is the noise (i.e. the part of the Bouguer anomaly uncorrelated with elevation) and * is the convolution operator. In the wavenumber domain the observed transfer function is defined as (Dorman and Lewis, 1970): Q

i=

For the Fourier analysis, gravity and elevation were calculated on grids. A constant value has been removed from each set of data (40 mGa1 for gravity and 600 m for topo~aphy}; these are then multiplied by a taper function to bring them gently towards zero at the region’s borders. The transfer function and the coherence which measures the correlation between the two sets of data are calculated for different areas and various values of grid interval. The two examples of Fig. 6 show that all patterns are similar, regardless of the area; transfer function values are higher for larger wavelength and become smaller at shorter wavelength. The transfer function values for wavelengths smaller than 200 km are higher for area B than for area A (Fig. 6). A probable explanation is the presence east and west of zone A of high gravity anomalies unrelated to the topography. This is in agreement with the very low values of coherence for wavelengths smaller than 320 km. When passing the topography through the Q filter for wavelengths greater than 320 km, we obtain the part of the Bouguer gravity field related to the compensating masses of long-wavelength topography. Figure 7 is a map of this regional anomaly which shows that a broad low-density body roughly elongated NE-SW is associated with the Hoggar swell. It is worth noting that this anomaly is centered on the Upper Mesozoic to Eocene magmatic fields of South Amadror, while the more recent volcanic districts are situated on its periphery. Removing this anomaly from the Bouguer field, we obtain the residual gravity anomalies (Fig. 8) which are uncorrelated with the topography and related to the crustal features with no relief expression. West of the 4”50’ megashear zone, the relations~p with the Pharusian belt is readily seen (Bourmatte, 1978). Within the Pan African Central Hoggar unit small-wavelength anomalies are not well defined because of data distribution. Nevertheless, the relationships between the negative anomalies and the granitoid-rich area (anomalies A, B and C of Fig. 8) or the sedimentary cover (D and E) are evident. The positive anomalies F, G, H, I and J are not related to dense rock outcrops. The weak and

(~4Gff)) (HZ-f*)

Re denotes the real part of a complex number; G and H the Fourier transforms of topography and gravity respectively, * indicates the complex conjugate, and the angle brackets represent azimutal averaging over a ring centered on a given wavenumber.

78

7

E

i

0.1 1

1’

Zone A

N

940

km

11’

modei 1 0.05

-

_

I tic

Qo 500

125

59

Am

model 2

Zone I3

960

320

190

137

167

itm

I

i

model

3

i

-$60

320

190

137

107

Am

---

._ ---

HP

-

50 Km

Hp

-

10ClKm

Fig. 6. Calculated values of transfer function (dashed line) and coherence (thick line) for zones A and B. a. Thin line-predicted transfer function for model 1 and 20 < H, -z 120 km. b. Thin line-predicted transfer function for model 2 and 40 <: H, < 60 km. c. Thin line-predicted transfer function for model 3; for each value of flexural rigidity two responses are given, corresponding to H,=50kmorH,-1OOkm.

poorly documented anomalies F, G, and H are located between the volcanic districts on granulites and gneisses. The well controlled positive

anomalies 1 and J are associated with Paleozoic and Mesozoic sedimentary cover (Erg Admer and Tafassasset depressed area). The northeastern limit

A38 A60

f

s*

f

/,-40-\ I

3

I

I

5’

I

I 7’

\ \ I

I

9

I

11’

Fig. 7. Gravity field component correlated with elevation for wavelengths greater than 320 km, Open triangte-heat dashed area--6enozoic volcanic massif; asterisks-Am&or Cretaceous magmatism.

Fig. 8. Residual gravity map for regional Mornay of Fig. 7: shaded areas correspond to positive anomalies,

flow sites:

of the anomaly

I corresponds

to a NW---SE-trend-

ing fault system. Using the reasonable low-density isostatic

root.

compensation mental

assumption

body corresponds we have models.

transfer

that the main

to the Hoggar

tested

possible

We compared

function.

swell

isostatic the experi-

for wavelengths

longer

gar, the first model involves material at the crust/mantle ond model

suggests

approximately Airy-type between

isostasy

transfer

func-

assuming

com-

Fig. hc) the relation

pensation

mechanisms.

model

(model

function pc’ is:

Q = -27iGpf

transfer

Hc and a crustal

density

,-2xkfl~

where k = (k: + k z)‘/’ frequencies

compensation

1; Fig. 6) the theoretical

for a root depth

A. The crust

is assumed

to have a

isostatic profiles of Fig. 4. Another possible model (Model 2. Fig. 6), previously used by Crough (1981a). suggests that the topography and the gravity fieid result from depth variation of two layers of different densities pc pm,

the crust

deflection

and the upward

(model

3,

h of the

mass deficiency 1984):

D 04h + p,,,gh = P The theoretical

transfer

function:

(p,,,-p,.)e-‘“““* i

and the upper

mantle

respec-

H, and H,, overlying

the

is calculated

Q = 2nG [ ( p,,, - p, ) eCznkHe - p,,,e-2TkHt]

The best fit with the observed data is obtained for 60 < H, < 40. This result is similar to Crough’s solution (1981a) calculated for a larger zone (1500 * 2000 km) and a grid of lo * 1’ averages of free-air gravity and topo~aphy. The comparison with the

observed

transfer

function (Figs. 6a and 6b) shows that gravity and elevation are consistent with a local and shallow Airy compensative mechanism for the Hoggar with the uplift history

of Hog-

of less dense

rigid-

astheno-

rigidity is rather locally compensated and must have undergone a high thermal regime to explain the low elastic plate thickness. This conclusion is tions

function for W, = 30 km, p,, - p, = 450 kg me3 and different values of Hj are plotted on Fig. 6b.

values of flexural

A good fit with the observed transfer function requires value for D( < 1022N m-‘) significantly lower than the estimated value for Proterozoic terranes (1024-1025 N ml’> and involves an elastic plate less than 35 km thick. Consequently, the Hoggar with a very low

in agreement

and has been calculated for various values of HI and P,,- pc. Examples of this predicted transfer

for different

ity D and average depth sphere H, (Fig. 6~).

asthenosphere of density pa. The transfer function is given by:

swell. To be consistent

approximation

between

direction.

density of 2700 kg m-3. The best fit is obtained for 20 < H < 30 km, which is consistent with the

tively, with mean depth

plate”

and uplift of a

D = 0. For D f 0 and

P is given by (Karner,

and k,l and k , are spatial

Q curves for several values of H, are shown in Fig. 6a, together with the observed transfer func-

and

pressure

Z(k)=2nG

in the horizontal

tion for area

a “thin

elastic lithosphere

With a simple Airy type isostatic

lithosphere

a local relationship

at depth

rigidity

regional

and

involves

mass deficiency

plate with flexural

tions

to local

a zone of altered

t)t’ crustal ‘The bet-

50 km below the surface.

than 320 km, with some theoretical corresponding

accretion boundary.

estimated

with

the P-T

by Girod

eq~~bration et al. (1981)

condifor

the

peridotite xenoliths entrained by the basaltic eruptions: P-T values are distributed close to the “high temperature geotherms” which characterize active continental and oceanic extension zones. In conclusion, regardless of the exact compensating mechanism, the low-density body is necessarily situated at a relatively small depth (50 km), but gravity data do not appreciably constrain the possible solution and do not allow any choice between a locally altered zone or an upper mantle with a lowered density over its whole thickness. Petrological evidence for a modified upper mantle The petro~ap~cal implications of this broad low-density body, represented by the long-wave-

81

length Bouguer anomaly of Fig. 7, is a matter of debate and the info~ation given by the xenoliths entrained by the recent basaltic eruptions are essential to help resolve the ~~ntrovcrsy. Mafic and ultramafic xenoliths abundamly occur within the Plio-Villafranc~an and Quate~a~ alkali basahs. Recent petrologic, geochemical and isotopic investigations (Girod et al., 1981; Leblanc et at., 1982; Dupuy et al., 1986; Dautria et al,, 1988) have demonstrated heterogeneities of various types and various scales within the Hoggar upper mantle. The small-scale heterogeneities (such as ~p~bole-~ch and cliuopyroxene-~ch veining associated with local partial melting and contact meta~~matism), have been att~b~ted to the successive Cenozoic to Quate~a~ volcanic events. On the contrary, the large-scale heterogeneities cannot easily be related to the Cenozoic activity. For instance, the progressive chemical variation of peridotitic xenoliths observed from the West (the Tahalra and Manzaz peridotitic xenoliths are essentially lherzolites and their partial melting percentages range near 1%) to the East {The Eggere and Adrar N’Ajjer samples are harzburgitic to dunitic, and their partial melting percentages are near 10%) and corresponding to an increase of depletion associated with an incompatible element enrichment, is probably a pre-Cenozoic heritage. Because all the volcanic districts have had Cenozoic activity of similar nature, intensity and duration, only partial melting and metasomatism which developed at scale of the swell, and which happened before Cenozoic times, can account for such an evolution (Dautria and Girod, f988). Moreover, the peridotitic xenoliths from the Eastern districts are texturally different: they display highly recrystallized fabrics while the samples from Tahalra are exclusively tectonized, The highly recrystallized textures of Eggere and Adrar N’Ajjer have been produced by regional stresses ranging from 4 to 5 MPa, for depths varying from 35 to 60 km (estimation from Mercier’s method, 1980). According to Mercier, these conditions characterize highly extensional continental zones such as rifts. Depletion has been produced by a partial melting event: metasomatism and re~~stal~zatio~ has been produced by intensive fluid percolation As suggested by the supe~osition of their effects

on the same samples, these events are possibly related. Moreover, geochemical inv~tigations on the Hoggar basalts (Dautria et al., 1988) have shown that the progressive enrichment in incompatible elements (such as K, LREE, etc.) with time that they exhibit, may result from two associated phenomena; a progressive deepening of the basaltic liquid source from the Miocene to the Quate~a~ and an upper mantle carbonatitic to kimberiitic melt impregnation, increasing with depth. As suggested by, the occurrence of cm-size specific megacrysts (alkali c~n~pyroxene, Mg-ilme~te~, this impregnation is probably expressed at various levels as a differentiated ma~atic dyke swarm. The injection and migration of CO,-H,Q fluids issued from these melts may also be responsible for the general met~omatism described in the Hoggar upper mantle (Dupuy et al., 1986; Dautria et al., 1988). On the other hand, magmatic complexes have been recently evidenced beneath Eggerc (Kornprobst et al., 1987). They are constitute of rocks ranging from gabbro to ortho- and clinopyroxenite, of probably th~le~itic affinity and of unknown age. This suggests the occurrence of layered intrusions equilibrated under a deep crustal layer, i.e, at depths near 30-35 km. Such a rock suite is known in Adrar N’Ajjer and Manzaz but unknown in Tahalra and Atakor and, therefore, these complexes constitute one more characteristic of the Eastern Iioggar. In conclusion, it appears that the upper mantle, corresponding to the low-density body of Eastern Hoggar, is anomalous and displays petrographical characteristics which can be properly inte~retated in terms of density reduction. These characteristics (depletion, recrystallization, metasomatism and magmatic veining) are the results of fluid percolation, magmatic injection and partial melting. The possible occurrence of highly alkali and carbonated veins within the underlying Ievets (> 100 km), probably issued from the degassing and decarbonation of the asthe~osphere, agree with the hypothesis advocating a deep origin for the upper mantle modification associated with asthenospheric material transfer,

Discussion Regional

negative

lithospheric observed

a compensation occurring at small depths km), which would imply an asthenospheric anomalies,

domes

similar

in the Hoggar,

several other localities. presence

of low-density

sphere.

The

classically

modifications, and/or its

have been materials

consecutive

described

within

density

replacement

by

and,

by the

that no thermal p-wave

processes

anomalously

thinning

the available

and

delay

thin-

stretching

on African

flow data

is associated

zone evidenced

inshow

with the

by gravity.

time at Tamanrasset

observatory

low ( -0.46

very

from the positive

asthenospheric

heat

disturbance

deep low-density

of in-situ

a crustal

with the field observations.

Moreover,

is

if there is lithospheric

consequently,

compatible

the litho-

and magmatic

of lithosphere

ing to the deep crust ning

in

reduction

to be the result

by heating

as the result

with to that

They are interpreted

lithospheric

considered

associated

in intensity

f - 50 upris-

s) and

delay time measured

rift zones (Dorbath.

The is

different at stations

1984). These con-

material. Diapiric mechanisms are invoked to explain this. being induced either by mantle convec-

siderations suggest that the Hoggar swell is not presently caused by a reheating of the lithosphere,

tion {active process)

and that the low-density zone cannot be regarded as an upweiling of the lithosphere/ asthenosphere boundary. The xenoliths entrained by the recent

or by instability

caused

tensional stress within the lithosphere process) (Turcotte and Emerman, 1983). The size of the Hoggar

region

by

(passive

basaltic eruptions imply that the Hoggar upper mantle is petrographically highly modified; reheating, partial melting, recrystallization, metasoma-

of anomalous

mantle (400 * 200 km), deduced from gravity is considerably smaller than those associated

data, with

tism and magmatic veining the modification processes.

the swells (as the Kenya Dome) where extensive lithosphe~c thinning has been evidenced (Searfe, 1970; Fairhead, hand,

1976; Girdler,

the transfer

function

HEAT

flow is normal,

3983). On the other

interpretation

must presently

suggests

have been identified as But as the present heat

the modified be regarded

upper

mantle

zone

as cooled off.

FLOW

,o.~;“--::--.:-l:I. *

1

20

0

60

40

_1-

80

100 TIME

Fig. 9. Evolution numbers

of heat

flow density

next to each curve indicate

cases. A 20 finite element technique temperature Boundary m-‘°C-‘,

pc = 3.9

was used to compute

are T= O°C x

age after an instantaneous

the top and the bottom

field and then dissipated conditions

versus

lo6 J rnw3 C-’

by thermal at the surface

and

A thermal

Constant

T= 1000 “C

and A = $J FW rnm3.

of magma

of the heat body. The lateral

this evolution.

conduction.

intrusion

thermal

at a depth

anomaly

iMa}

at a temperature

extension

of the structure

is superimposed

conductivity

of 1000°C.

The

is 200 km in all

on an initial equilibrium

and heat production

of z = 100 km. The parameters

was assumed. are K = 2.5 W

83

The low-density body is elongated ENE-WSW. A simplified two-dimensional approach permits the calculation of the cooling time by conduction in a vertical prism with an almost uniform initial temperature of 1000°C. This assumed temperature is in agreement with the thermal conditions (1000” C at 50 km} recorded by the peridotitic xenoliths (Girod et al., 1981). According to twodimensional gravity modelling of the regional anomaly (Fig. 7), this prism must be about 200 km wide. Figure 9 shows the evolution, as a function of time, of heat flow associated with an instantaneous intrusion of magma at temperature 1000 ’ C. The hot body is situated at a mean depth of 50 km according to transfer function analysis, and its vertical extent varies between 20 and 70 km. To be consistent with the present normal heat flow, it appears that the age of empla~ment must be older than 60 Ma and that the thickness must be less than 30 km. As the anomalous zone is centred on the Upper Mesozoic-Lower Cenozoic magmatic fields of south Amadror, it is tempting to relate the upper mantle modification to early magmatism. The relatively low vertical extension requires that the body corresponds to low-density (3000 kg mw3) material and consequently to very extensive upper mantle modifications. Thus we propose that the upper mantle modifications have been produced by asthenosphe~c material transfer, resulting in the development of a kind of “Rift Cushion” (as defined by IIlies, 1969; Gass, 1970; Bailey, 1972) just below the crust. This hypothesis accounts for the observed gravity field and the associated magmatism, and satisfies the requirement that, despite the present low heat flow, the uplift has probably persisted since Late Mesozoic-Early Cenozoic times. Compared with the extensive modifications of the upper mantle attributed to the Late Mesozoic-Early Cenozoic events, the recent volcanism (Mio-Plio-Quaternary) appears to be a minor event. Despite the associated amphibole and clinopyroxene-rich veining, this volcanism does not induce appreciable upper mantle density reduction, except possibly on a local scale. Given the lack of geochronological information, it is impossible to say at present whether the recent volcanism represents a new magmatic event, unre-

lated to the former one, or whether it is the late consequence of the Late Mesozoic-Early Cenozoic history. Acknowledgements

Fieldwork has been supported by CRAAG (Centre de Recherche Astronomie, Astrophysique et Geophysique, Alger, Algeria), and a joint project between ONRS (Algeria)-CNRS (France) and ASP. Afrique (CNRS, France). The authors are grateful to F. Lucazeau for help with heat flow modelling and to M.H.P. Bott, Y. Mart, G. Vasseur and M. Mareschal for their constructive comments. Heat flow measurements were greatly facilitated by the active co-operation of the following organizations or companies: EREM (Entreprise de Recherche et sexploitation Mini&e, Alger, Algeria); IRSH (Institut de Recherche en Sciences Humaines, Niamey, Niger); Mini&e des Mines (Niamey, Niger); SOMA’iR (SociCtt des Mines de l’&r, Niamey, Niger); and COGEMA (Compagnie G&r&ale des Mat&es Nucleaires, Paris, France). Appendix Heat flow data processing

The temperatures have been measured (in water) in shallow (100-150 m) and sometimes tilted boreholes (mainly drilled for mineral exploration), at thermal equi~b~um, at intervals of 5 or 10 m, with a thermistor probe equipment (relative accuracy 0.01’ C). The temperature profiles are presented in Fig. 10; Table 1 shows the main data for each heat flow computation. The lack of precise information on paleoclimatic and erosional history precludes the calculation of correction for these effects, which are probably not negligible, especially for the uplifted and eroded area of the Hoggar. Most of the sites are located within flat areas where the disturbance due to the topography is negligible. The boreholes (l-20 years old) are probably near to perfect thermal ~~~b~urn and without any evident hydrological perturbation, The departure from a straight line observed on several

_-

_

.-

_

_

on surface

me~urements

-

conductivity

_

_

_

samples

quartzy/sandstones

100-180

55

90-250

95-170

80-150

loo-190

55-195

30-

100-160

95-200

(b) conductivity

_

. .” ,,/

measurements

- ,,j _j . . II .._

or mine samples.

on core

_

.

(c) conductivity

.-^“-

samples;

~-.

,.

measurements

10.05

Conductivity

_-

+O.l

+0.3

+0.05

f0.05

+0.2

f0.2

+0.3

+OS

f0.3

f0.3

f0.2

f0.2

f0.2

+0.2

f0.2

4.1

I_. -I

(c)

(c)

(c)

(d)

(d)

(a)

(a)

(a)

(a)

(d)

(d)

(c)

(a)

(b)

(b)

(b)

(a)

(b)

(b)

(b)

(b)

(b)

(b)

(b)

(b)

(b)

(b)

(a)

(a)

.

\I ,,,

flOW

Heat

;

.

41+

6

6

6

6

I

7

7

6

I

6

8

4

4 4

4 6

8

8

4016

64i13

50*7

36+4

39*5

65*7

*,

(d) conductivity

39+

39+

41*

67+12

54*

61f13

68&20

45*

65+13

68+14

63*12

50+

56&10

43+

44+

59&

52&

45rt:

38i: 351:

38, 39+

63rt

64+

24+6

53&16

mem2)

651t: 7

;:f

56+16

51 I16

(mW

boreholes;

**

from nearby

f0.2

3.9 +0.2

3.9

3.9

1.4 +0.2

4.5

4.3

2.8

4.5

4.6

1.4

4.5

5.5

1.6 kO.3

5.3 i0.2

5.5

5.9 50.2

3.5 +0.3

5.3 f0.2

3.5 iO.10

3.5 f0.05

2.8

2.8

3.0 f0.05

3.1 fO.l

3.4 i0.2

2.15 f 0.1

2.3

3.3

3.6 f0.4

(W m-loC-‘)

on core samples

1.0

10.0+ 1.0

10.5 f 1.0

volcanoschists

48.0 & 2.0

shales

10.0+ 1.0

12.0*1.0

detritic

14.0 + 2.0

24.0 f 3.0

10.0* 1.0

14.0 + 2.0

48.0 + 3.0

14.Ok2.0

9.0 + 1.0

35.0 * 2.0

8.0* 1.0

8.0+ 1.0

10.0*1.0

15.0+ 1.0

8.0* 1.0

10.0+ 1.0

11.0* 1.0

14.0+ 2.0

13.5 f 1.0

21.Ok2.0

20.5 + 2.0

19.0% 1.0

10.5 + 2.0

ll.Of2.0

17.0* 3.0

sandstones

Precambrian

Carboniferous

Ordovician

Precambrian

Precambrian

Precambrian

Precambrian

Precambrian

Upper Cretaceous

14.0+0.3

sandstones

~____

sandstones/shales

sandstones

sandstones

95

335-460 70-

sandstones shales

171-335

quartay/sandstones

shales

quartzy/sandstones

117-171

170-230

40-170

80-220

quartzy/sandstones

shaIes/sandstones

70-200

sandstone

granite

granite

50-100

85

40-185

16-

siltstones

to the heat flow sites on Figs. 2 and 7.

from Iithology;

correspond

Timou 10

850

850

Tiiou

3

850 850

Timou 2

435

MaIa 544ter

Timou 5

430

Arh 348bis

550

1342

430

610

F104

Somair 2079

590

FlOl

430

614

F106

Arli 536

555

FlO5

in parentheses

(39)

(40,

* * (a) estimated

* Numbers

9 Timouletine

8 Arlit

41)

7 Tin-Seririne

6 Bachir (36)

(38)

16-130

30-180 30-230

500

500

5 In-Abepgui

900

50-115

1100

4 Tin Amzi (63)

(60)

900

gabbro-diorite

55-100

550

550

3 Nahoa

s 42

gabbro-diorite

45-100

580

S180

S66

gneiss

30-120 40-140

580

S219

2 Tirek (23)

70-250

450

S352

sandstone/

90-290

450

(53)

s35.5

(“C/km-‘)

(m)

Thermal gradient

Age

Depth range

Lithology

of heat flow for each borehole

(m)

and estimation

Altitude

of conductivity

1 Tanezrouft

estimation

Borehole

gradients,

Thermal

Site *

1

TABLE T

85

TEMPERATURE 1

0

DEPTH (ml

2

I

6

B .

. .

l

:

;. .

.

.

l

l l l l

l

l l

S352

s

. . . . . . . . . . .

.

l

9

,

: 160

:

. . . . . . .

.

l

.

.

.

.a*..*

. .

. .

:si3e:

. . . . . . . .

;

.

;

.

.

.

S42

.

:

. .

l

.

.

l

.

.

l

. l I

.

l

..

.

F 105

. .

:

:

;

:

.

.

-

:::

. . . .

. .

.

l . . . . . . .

. . F 104

. .

.

. .

.

.* .

‘.

l

MALI

.

. . .

’

l

.

TIMOU 3

l l l

.

’ l

: TIMOU 2

. . . .

2079

TIMOU

.

10

. .

s 355

l

. TIM. ._

l. .

l

*.

. .*.

544

somdir4

.

13H2

l

:

l.

348 bis . .

.

-0.

.

.

ARLI

l

.

. l

_*.

. . . .

.

:

:*

. .

l

.

l

.

. . .

. . .

‘.

l

F&3 ; FlOl

.

.*

.

.

a. -.

l

l

.

*..

l

.

l

.

.*. . .

.

.

.

.

. .*.

.

.

-

.

l

1

.

.

. l

I

. . .

.

l

.a...*

,

l

.

.

.**..

.

l

.

;

.*

.

.

l

s219

. .

30( I-

.

l

l

.

.

.

. . . . .

.

.

8

.

l

:*

.

l

7

( ‘C)

.

0. l .* 0.0 l .* 0. .

2oc )-

54

,.

l

. . 1oc I-

3

-II-

SCALE

ARBITRARY

.

. . . . . . . . . . . . . . . . . . . .

4oc )-

ARLI

50( I-

Fig. 10. Temperature

profiles

for each borehole.

Numbers

indicate

sites, as shown

in Table

536

1. Borehole

names

are given at the

bottom.

geotherms can be easily correlated with variations in lithology (i.e. conductivity). The effect of perturbations resulting from other causes, such as lateral variations in thermal conductivity, are more difficult to evaluate, especially within the basement where complex geological structures perturb the underground temperature distribution. Except for some sites at which conductivity was estimated from lithology, conductivity was measured on cores or on surface or mine samples. Conductivity has been measured with a transient method both on dry and water-saturated samples. In the sedimentary cover, for the boreholes without cores the results of Brigaud (1986, unpubl.) were used to estimate the bulk conductivity from the lithological description. To estimate the porosity change with depth (negligible for a shallow borehole), a theoretical exponential law was used. For simple lithology we estimate heat flow by multiplying the thermal gradient by the measured

or estimated conductivity. For complex lithology, integrated thermal resistance is used and heat flow is calculated from the Bullard linear relation (Bullard, 1940). Unfortunately, no radiogenic heat production measurements are yet available, and to estimate near-surface source contributions we have to use heat production data from other Precambrian areas.

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