An example of the relationship between rift and dome: recent geodynamic evolution of the Hoggar swell and of its nearby regions (Central Sahara, Southern Algeria and Eastern Niger)

Tectonophysics,

45

163 (1989) 45-61

Elsevier Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

An example of the relationship between rift and dome: recent geodynamic evolution of the Hoggar swell and of its nearby regions (Central Sahara, Southern Algeria and Eastern Niger) J.M. DAUTRIA 132and A. LESQUER ’ ’ Laboratoire de PPtrologie, Universite des Sciences et Techniques du Ianguedoc, Place E. Bataitlon, 34060 Montpellier (France) ’ Centre Gk’ogique et Gkophysique, UniversitP des Sciences et Techniques du Lmguedoc. (Received

March 61988;

revised version accepted

Place E. Bataillon, 34060 Montpeilier (France)

September

26,1988)

Abstract Dautria,J.M. evolution

and

Lesquer,

A., 1989. An example

of the Hoggar

of the relationship

swell and of its nearby

regions

between

(Central

Sahara,

rift and

Southern

dome: Algeria

recent and

geodynamic

Eastern

Niger).

Tectonophysics, 163: 45-61. From

essential

volcanic

Hoggar

correspond

gravimetrical

to a single distensive

NW-SE-trending opening.

Pan African

Hoggar,

stretching) Hoggar

an extensive

structure

lineament

sub~quently

mantle

modifications

spheric

material

lineament

may indicate

the Tenere troughs,

which developed

in response

beneath

lineament,

the Hoggarian oriented

reheating,

partial

(gas, fluid, kimberlitic of the peridotitic

that this lineament Thus, in the Hoggar,

doming

volcanism

Amded),

melting,

metasomatism

inclusions

of the Cenozoic

has also controlled it appears

The African plate has been stable for the past 600 m.y. Nevertheless, very important cracks of lithospheric scale happened during Late Mesozoic and Cenozoic times. In the eastern part, these cracks resulted in the development of the East African Rift System. Several pertinent geophysical, structural and petrological works (e.g., Hiolmes, 1944; Baker, 1970; Morgan, 1971; Burke and Whiteman, 1973; Gass et al., 1978; Le Bas, 1980; B.V

and magmatic resulting

was probably

districts, veining)

in a density

alkali basalts

by the intersection

On the other

emplaced

of small-sized preceded

hand,

structure.

produced

of the

with crustal

and possibly

reduction.

and

Atlantic

eruptions

associated

the distensive

volcanic

the emplacement

that doming

controlled

0 1989 Elsevier Science Publishers

magmas),

structure.

troughs

of N-S-

with the Central

alkali basaltic

for the

Hoggar

by reactivation

and probably

cross-cuts

of the Mio-Pro-Quatema~

Introduction

0040-1951/89/$03.50

of this distensive

(Oued

to carbonatitic

were possibly

affinity

evolution

and the North

associated

by the Cenozoic

(of tholeiitic

part

of geodynamic

the Late Mesozoic,

field probably

entrained

NE-SW

the localization

during

to the stress

by the xenoliths

into the upper mantle. and Cenozoic

a new model

Hoggar,

basic magmatism,

(through

hand, the high deformation

we propose

as suggested

controlled

transfer

data,

The Eastern

lower crustal

may have developed level, a transverse

regions.

faults,

Contemporaneously,

Eastern

and petrological

swell and its nearby

at the This upper

by astenoOn the other

along the Oued Amded asthenospheric

diapirs

by a rifting phase, and that

of this rift by a transverse

wrench

fault.

Chorowicz, 1983; Kampunzu et al., 1983; Mohr, 1983; Girdler, 1983; Lloyd et al., 1986; Fairhead, 1986) have been carried out on this rift system, and its superficial and deep structure are now well documented. In particular, lithospheric thinning associated with extensive modification of the upper mantle has been described. In the centralwestern part of the plate (Fig. lA), these cracks were responsible for the formation of strongly subsident basins (Eastern Niger troughs, Benoue trough, Chad and Soudan troughs) and of domed,

46

meLI’\, CoNGCj/ ,CFiATON,

424

L-P 5 l.zzYI

6 7

IzI

B Pig. 1. A. Tectonic and the subsident volcanic basement;

areas.

setting troughs

B. Schematic

2 = Paleozoic

Late Mesozoic

of the Hoggar

swell and of the Tenere trough

of the Central-Western geological

sedimentary

to Mi~ne-Quatem~

Africa.

I = Pan African

map of the Hoggar cover;

3 = Mesozoic

age (the Upper fault;

system in relation basement;

and of its nearby deposits;

Mesozoic

7 = Cretaceous

uplifted volcanic swells (Hoggar, Air, Tibesti, Darfur, Cameroon Line, Adamawa and Jos). The initiation of this system is contemporaneous with the Central Atlantic opening (Burke and Dewey, 1974; Thiessen et al., 1979; Benkhelil and Robineau, 1983). The local lithospheric mechanisms responsible for this swell and trough system are still a matter of debate. Some mechanisms, such as upper mantle density reduction beneath the volcanic swells, and crustal thinning beneath

with the domaiy

2 = Mesozoic-Cenozoic

regions.

1 = Pan African

4 = Cenozoic-Quatemary

to Lower Cenozoic (to Cenozoic)

districts

deposits; are grouped

uplifted

volcanic

swells

troughs;

3 = Cenozoic

metamorphic

and plutonic

5 = volcanic within a star);

districts

of

6 = major

troughs.

the troughs (Crough, 1981a, b; Brown and Fairhead, 1983; Dorbath and Dorbath, 1984; Dautria and Girod, 1986; Fairhead, 1986) are similar to those of the East African rifts, but their intensity, their evolutionary stage (and perhaps also their origin) are different. For instance, the spatial relationships between swell and trough have not been clearly established, except for the Dar-fur (Bermingham et al., 1983). According to these authors, the Darfur swell constitutes the third

branch of an incipient intraplate triple junction, the two other arms being the Ngaoundere and Abu Gabra rifts. In this paper, we propose to clarify the structure of another swell of the Central-Western Africa, the Hoggar. Using our recent interpretation concerning the deep structure of the Hoggar swell (Lesquer et al., 1988), we examine the possible relationships between this swell and the Cretaceous-Cenozoic trough systems of the nearby region. New hypotheses concerning the genesis of this intracontinental swell will be proposed. Geological setting

The Hoggar swell ~~o~thern Algeria) The Hoggar area is a very large Precambrian basement swell ( = 1000 km wide, 300,000 km2) which is covered northward, southward and eastward by Paleozoic sediments (Tassilis N’Ajjer) and westward by Cretaceous and Quaternary deposits (Fig. 1B). Topographically, the Hoggar can be described as a regular but asymmetrical dome with a more gentle slope to the west than in any other directions. Altitude ranges from 1000 to 1500 m but, in the central and eastern parts, several high massifs (> 2000 m), trending in two directions (NE-SW and NNE-SSW), are superimposed on the general topography: the first direction is a recent volcanic axis constituted by the Atakor, Tellerteba and Adrar N’Ajjer districts (Fig. 1B); the second is a succession of granitic massifs (Tefedest) corresponding to a horst. This horst continues southward with the uplifted volcanic fields of Atakor and northward with the Amguid uplift block (its post-Early Cretaceous vertical throw is more than 1000 m; see Fabre, 1976). Moreover, it is bounded eastward by one of the major meridian fault systems of the Hoggar, the Amguid system, which was active (as a normal fault) up to Quatemary times (Conrad, 1969). The structure of the Hoggar shield results from several major events which have taken place in the Central Sahara since 600 Ma. The main lithological and tectonic features are due to the Pan African orogeny. According to recent works (Bertrand and Caby, 1978; Bayer and Lesquer, 1978; Caby

et al., 1981; Lesquer et al., 1984), 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 a partial melting of crust at depth and, consequently, the emplacement of a large quantity of granite (nearly 50% of the belt is granitic). Early shear-zone megasystems, trending N-S and lately becoming vertical, cross-cut the Pan African structures and presently separate the Hoggar basement into large, elongated and independent blocks. The late-erogenic brittle deformation is characterized by a conjugate strike-slip fault system consisting of NE-SW dextral and NW-SE sinistral trending sets of faults (Ball, 1980). Geophysical {Bourmatte, 1977) and structural evidence, and Landsat imagery interpretation (Ball. 1980) suggests that the NE-SW-trending Oued Amded (Fig. 1B) corresponds to a major tectonic lineament. This lineament should have controlled the emplacement of some Late Pan African granitoids in the Western Hoggar (Boissonnas, 1973) the development of a little Cretaceous trough (50 km wide, deposit thickness near 800 m) along its southwestem prolongation within the Tanezrouft Basin (Fig. I), and even the location of the major Hoggarian Cenozoic volcanic fields, as we shall see. The peneplanation of the Hoggar Pan African range occurred during Cambrian times. From the Ordovician to the Upper Carboniferous a sedimentary platform developed (Fabre, 1976). The reactivation of the meridian shear zones as normal faults controlled the geometry of the sedimentary basins. The Hercynian folds, occurring in the Northern Hoggar, may also be ascribed to this reactivation. The late Hercynian epirogeny 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 was probably finished by the end of the Jurassic. In the Eastern Hoggar, residual hills of fluviolacustrine deposits (maximum 300 m thick) can be found lying on the levelled Pan African basement. Their age is Cretaceous (Mid-Cretaceous according to Bordet, 1955), and they are grouped within three low areas; the southern part of the Amadror depression, the plain of Serouenout and the pre-

48

Tassilian ward

hollow

of Djanet

geographical

(Rognon,

Oued,

(Fig.

their

either

present

Pliocene.

The amplitude

is very

to relics of a continental

sedi-

local

uplifts

these

over the whole

1967) or to basins restricted

of small

to the

Hoggar

The

outcrop

Manzaz ocene

magmatism

of the Hoggar

It was initiated

of basic lavas (possibly

tholeiitic

to the analyses

ceous continental

interbedded sedimentary

are

also

during

of the vertical to estimate,

superimposed districts

the

deforma-

because on

of

and Adrar N’Ajjer magmatic

aligned

the

these general

fields

eastward,

Amded

lineament

of southern

outside

Atakor,

Amadror

continuation

(Fig.

the small volcanic

lB), located

Tahalra,

and the Cretaceous-E-

along the northeastern

Oued

by

probably

difficult

Cenozoic

by the

published

and

of the Hoggar.

or sub-

Remy, 1959), within the southern part of the Amadror depression and within the Plain of Serouenout (Fig. 1B). Given that some volcanic flows are locally

swelling

extension,

actual

Hoggar.

discontinuous. according

phase

tion

The Meso-Cenozoic

alkaline,

Miocean

may

zones in the Eastern

effusion

south-

formations

cover extending

approximately

has been

1B). Given

distribution,

correspond mentary

of the Tafassasset

1B). More

district

the Hoggar

are of the

north-

of Illizi (Fig.

basement

area, is

of special interest because of the high alkalinity of its volcanics and the peculiar nature (highly metasomatized)

of inclusions

(Megartsi,

1972).

with the Mid-Cretadeposits,

this activ-

ity began during the Mid-Cretaceous (Remy, 1959). The magmatism continued in the same region with the emplacement of ring-shaped intrusions of carbonatitic affinity (including rocks such as shonkinite, essexite, monchiquite, trachyte and phonolite cross-cut by carbonatite veins). The lack of absolute dating (only one syenitic ring massif has been dated by the K-Ar method by Rossi et al. (1979) and was found to be of Eocene age) prevents a definition of the initiation age for this

The Eastern Niger trough system The Tenere area (Eastern Niger) topographically corresponds to a large depression which developed between two high zones, the Precambrian Air swell (basement elevated by = 0.5 km) and the Paleozoic Djado Basin (Fig. 1B). The major structural features of this area were determined from the gravity data and electrical sounding of Louis (1970). They correspond to an almost 400 km long trough system constituted by three narrow troughs (Fig. 1B): an eastern NNW-SSE-

second phase of activity. Field observations (Remy, 1959) suggest that some intrusions may be of Late Cretaceous age. Starting again in the Miocene, the magmatic activity moves from the centre to the

trending one, known as the Kafra branch (400 km long, 40-60 km wide, 3000 m deep); a central one,

periphery of the Amadror depression and of the plain of Serouenout, and becomes localized in six

(250 km long, 50 km wide, 3000 m deep); and a western NW-SE-trending one, the Tefidet branch,

districts (Fig. 1); Tahalra (1800 km2), Atakor (2150 km’), Manzaz (1500 km*), Eggere (2800 km2), Adrar N’ajjer (2500 km2), and In Ezzane (500 km2). This Cenozoic activity is responsible for the emission of a large quantity of essentially alkali

corresponding to a half graben (200 km long, 30-50 km wide), which is particularly interesting because of its sedimentary fill outcrops. The de-

basaltic lava (= 250 km3 for Atakor, according to Girod, 1971). The volcanic activity was paroxismal during the Miocene and continued episodically up to the Quaternary, with an intensity varying from district to district, but always decreasing with time. During the Miocene and Pliocene, the volcanism was of a maximum within the Atakor district, but the Villafranchian and Quaternary phases were more intensive in Tahalra and Adrar N’ajjer. Local uplifts occurred at the end of the

parallel

to the former,

called

the Tenere

branch

posits (2 1000 m), which are essentially elastic and partly marine, have ages ranging from Aptian to Cenomanian (Faure, 1966). Southward, this distensive structure extends to Lake Chad (Fig. 1A) by the Ter~t-Agadem trough (600 km long, 300 km wide, 7000 m deep), which is bounded by NW-SEto N-S-trending normal faults (Faure, 1966; Bellion et al., 1983; Mathieu, 1983; Guiraud et al., 1987). The history and the evolution of all these troughs were probably similar (Guiraud et al., 1987). Their opening was contemporaneous with

49

that of the Benoue system; it happened during Aptian times and is correlated with the Central Atlantic opening. The subsidence was intensive during the Early Cretaceous. The distension continued episodically up to the present time, as

shown by the Eocene-Oligocene thick deposits and by the Miocene to Recent volcanic activity of Southern AYir, Northern Tefidet and Northern Termit (Pouclet and Durand, 1983).

8”

4’

\I

/ Fig. 2. A. Schematic uplifted

volcanic

topographical

zones;

map of the HoBar

2 = depressed

zones;

\

lb

8’ and Tenere

3 = basement-high Cenozoic

areas.

zones.

magmatic

/

B. Interpretative The

fields.

lk

12” star

corresponds

topographical

sketch.

to the Upper

1 = Cenozoic

Mesozoic-Lower

50

Evidence for a northward extension of the Tenere distensive system through the Hoggar basement ~op~~~~p~~e~land structural evidence

As suggested in Fig. lB, the NW-SE- to NNE-SSW-trending structural directions, which control the Tenere trough system, extend north-

ward through the Hoggar basement. The distensive structure can afso be located topographically within the Hoggar basement (Fig. 2A). While the Western Hoggar slopes regularly to the west, the Eastern Hoggar on the whote corresponds to a depressed area, 250 km wide, between the uplifted block of Atakor-Tefedest and the Paleozoic Tassili N’Ajjer Plateaux. More accurately, this de-

24”

20’

18”

8”

lb”

Fig. 3. A. Simplified&zavitymap of tht: Hoggar(from Lesquer et al., 1988) and corresponds to Bouguer anomalies higher than - 60 mGal. B. Interpretative 2 = Miocene-Quaternary volcanic district; 3 = sedimentary troughs characterized zones corresponding to a thinned or an intruded tower crust. The star represents

ti*

14”

of the Tenere (from Louis, 1970). The dotted area gravimetrical sketch. I =i Pan African Basement; by negative gravity anomaIy; 4 = positive anoma& the early amid-Creta~o~ to Eocene) magma&s.

51 5’ 26’

6”

E’

,-___--_----_---+

~.

I

5'

6~

8'

10‘ . _+--1(

26.

10'

11'

Fig. 4. Residual gravity map of the Hoggar (simplified from Lesquer et al., 1988). The hatched areas correspond to positive anomalies.

pressed area is formed by a series of shallow basins (some of them displaying traces of Cretaceous ~u~olacust~ne deposits), partly covered with Quate~a~ continental deposits (the Amadror Basin, the Plain of Serouenout and the North Tintarabine and Tafassasset Oued depressions) and separated by high zones. Furthermore, these depressions have a localization and a submeridian elongation suggesting that they may represent the possible topographic northern continuation of the Tenere trough system (Fig. 2A). In this hypothesis, the NNW-SSE-trending pre-Tassilian depression of the Tafassasset Oued would be the topographic continuation of the Tenerean Kafra branch (Figs. 2A and B), an idea which is supported by the gravity data. At latitude 22’N, the Kafra trough is almost 2000 m deep (Louis, 1970), and the negative gravity anomaly associated with this low-density sedimentary fill shows that the trough extends northward for more than 80 km. The low negative anomaly, which appears to the northwest, at latitude 23O30’N on the Hoggar gravity map of Lesquer et al. (1988) (Figs. 3 and 4), could indicate the presence of a small sedimentary basin, covered by Quaternary deposits and continuing the Kafra structure.

North of Hoggar, a Cretaceous, N-S-trending, subsident trough and uplift block system is apparent between the 6 and 8 meridians (Fig. 1B) (Fabre, 1976; Takherist, 1986). The troughs (30-50 km wide, = 2000 m of Mid-Cretaceous continental filling) correspond to narrow half grabens. The N-S-trending structural directions of this northern distensive system continue southward, through the Hoggar basement. The southward continuation of the western uplifted block!(Amguid block) is quite noticeable, since it corresponds to the Tefedest-Atakor horst (Figs. 1B and 2B). Thus it appears that the morphological and structural features of the southern part of East Hoggar can be interpretated as the northwestern extension of the NW-SE Tenere trough system, while the northern part would correspond to the southern extension of a meridian trough system which develops northward. The Eastern Hoggar would constitute the link between these two distensive systems. Gravity evidence The Tenere and the Hoggar display drastically different gravity fields (Fig. 3A). The profiles of Fig. 5 (established from the gravity map of Louis, 1970) show that the shortwavelength negative Bouguer anomalies, related to the various Tenerean troughs, are superimposed on a long-wavelength (= 300 km) positive anomaly, associated with the whole Tenere depression. The short-wavelength negative anomalies are obviously related to the low-density sedimentary fill of the troughs. We have calculated the MesoCenozoic sediment thickness neglecting the possible influence of Paleozoic sediments and assuming a homogeneous density of 2.4 g/cm3 for sediments. The results are in agreement with the electrical soundings of Louis (1970) and the available oil exploration boreholes (Guiraud et al., 1987). Although simplistic, this two-dimensional approach shows that the large positive anomaly could correspond to a deep structure which may be interpreted as crustal thinning. Assuming an initial normal crust thickness of 35 km and a crustmantle density contrast of 0.45 g/cm3, our gravity models suggest that the crustal tinning beneath

52

0

00 ‘r-4

0

m

53

the Tenere is close to 3-4 km (profiles I and 2, Fig. 5). In the northern part of the Tenere, this thinning is essentially expressed beneath the Kafra branch. At the Hoggar level, the positive anomaly disappears and is replaced by a large-scale negative gravity anomaly (= 400 km, up to - 120 mGa1) centred on the Cretaceous-Eocene volcanic fields of South Amadror. Given this spatial relationship as well as petrological evidence, this zone has recently been interpretated by Lesquer et al. (1988) as being due to permanent upper mantle modifications (through reheating, partial melting, metasomatism and ma~atic veining) induced by the transfer of deep hot material (gas, fluid and magma). As suggested by the recent geochemical data of Dautria (1988) and Dautria et al. (1988) the nature of this magma would be of kimberlitic to carbonatitic affinity. The age of this magmatic event is probably Late Cretaceous to Early Cenozoic, and consequently the modified upper mantle has now cooled off. This is consistent with the present particularly low heat flow measured in the Hoggar by Lesquer et al. (1988). The residual gravity map of Fig. 4 indicates that short-wavelength positive anomalies are locally superimposed on the general negative anomaly. The most important (40 mGa1) is centred south of Djanet, within the Oued Tafassasset depression. As mentioned above, this depression probably corresponds to the northern continuation of the Tenerean Kafra trough. Since this anomaly is similar in wavelength and amplitude (profile 4, Fig. 5) to that of the Kafra trough, it may be interpreted in terms of local deep crustal modification (thinning, basic intrusions) also. Other positive anomalies occur between the Atakor-Tefedest horst and the Oued Tafassasset depression (Figs. 4 and 5), but they are less important and less well defined (because of data repartition). They are systematically elongated NNW-SSE to N-S and associated with the depressed areas (Amadror, Serouenout and North Tintarabine) which we consider as the topographical continuation of the Tenere distensive system. Thus, it is tempting to relate these positive Hoggarian anomalies to deep crustal modifications also. This interpretation is in agreement with the

recent work of Kornprobst et al. (1987) on the xenoliths enclosed in the Quaternary basalts of the Eggere volcanic district (North Amadror), which suggests the occurrence of dense-layered basic magmatic bodies of tholeiitic affinity, emplaced beneath this district at the crust-mantle boundary. This suite of rocks is also known in the Adrar N’Ajjer and Manzaz volcanic districts, but the Tahalra volcanoes do not contain such inclusions. The age of this rock suite is unknown but its petrological affinity is similar to that of the early volcanics of the Eastern Hoggar. Thus it appears that the northern continuation of the Tenere system through the Hoggar basement, suggested by topographical and structural data, is also supported by gravity data, indicating local deep-crustal modifications. Volcanic impli~~ons

The major N-S structural directions of the North Hoggar system and the NNW-SSE to NW-SE directions of the Tenere, approximately cross-cut in South Amadror (Fig. 1B). As we already noted, this region is of special interest, since it corresponds to: (1) the top of the highly negative Bouguer anomaly, associated with the Hoggar swell; (2) the centre of the early (Mid-Cretaceous to Eocene) magmatic activity, partly of carbonatitic affinity; and (3) the point of intersection between the Cenozoic volcanic axis (Tahalra-Atakor-Manzaz-Adrar N’Ajjer) and the depressed area system of East Hoggar considered as the northern continuation of the Tenere trough system. The volcanic axis is superimposed on the northeastern extension of the Oued Amded lineament (Fig. 1). Structurally, two types of Miocene-Quaternary volcanic districts can be defined in the Hoggar: the first includes the western districts (Tahalra, Atakor and Manzaz), situated outside the trough zone; the second includes the eastern districts (Eggere and Adrar N’Ajjer) located within the trough zone (Figs. lB, 2B and 3B). Each type displays noteworthy characteristics concerning the structural control of volcanism. Regardless of the district and of the age of activity, the magma used NE-SW- and NW-SE-trending faults to rise up

54

to the surface. those

These

two trends

of the conjugate

African

strike-slip

system

faults.

to Pan

eament

most of

occurs

In the Atakor,

the dykes which supplied the Mio-Pliocene basaltic emissions are oriented NE-SW (Girod, 1971). In the Tahalra

district,

the alignment

cene-Villafranchian to N80”

strombolian

trends.

and Adrar

vary between

structures

alignment, within

ring-shaped

also trend according and each

the

intrusion

dan, tites

The Up-

casual.

of Uganda

Chorowicz sents

of ring

also displays

a

along a lin-

associated

also

Rift, where the Ugan-

have been emplaced

transverse

corresponds

to a major

an

Kenyan and Tanzanian Cenozoic carbonaand the high-alkali basic lavas (including

rushayite)

(1980)

inclusions

Such

in the East African

NW-SE

intrusions

migration

upper mantle

is not

structure

tectonically and

a Precambrian

reactivated

during

of the rift.

Mukonki

this lineament

along a

(Le Bas, 1980) which

to a large lineament

ing the two branches

alignments

and NNW-SSE.

to Eocene

of South Amadror NW-SE

cones have N55 o

the major volcano

NW-SE

per Cretaceous

of the Plio-

On the other hand, in the Eggere

N’Ajjer,

phlogopitized

correspond of the late

(1979)

(lineament shear

serves as intracontinental

and

to

Kazmin

of Aswa) repre-

zone which

the Cenozoic

link-

According

rifting

transform

has been and which

fault along its

NW-SE trend (Remy, 1959). The magma injection within these Pan African faults is the result of

segment between the two branches. This lineament has apparently controlled an intensive de-

their reactivation in an extensional stress regime, but because of their pre-existence to volcanism, it

gassing

is impossible to define precisely the direction of extension at the time of eruption. However, it is clear that the direction of extension was different in the district of the first type (near NW-SE) and in those of the second type (near NE-SW). The distribution of the major volcanic districts

at depth,

which

is considered

by Bailey

(1974) to be responsible for metasomatism (hydration, enrichment in alkaline and in other incompatible elements) of the upper mantle, and consequently for the peculiar nature of xenoliths (the peridotites are either highly phlogopitized as in Tanzania (Pike et al., 1980) or wholly transformed to phlogopite

and

pargasite-rich

clinopyroxenite,

(including the earliest carbonatitic fields) along a Pan African lineament shows that this lineament has been reactivated since the Mesozoic. The Miocene-Quaternary alkali volcanism of the Hog-

Amded

gar represents typical continental intraplate magmatic activity (Girod, 1971), related to a distensive regime. The carbonatitic magmatism is also classi-

lineament may have been reactivated as a wrench fault in a manner and with results similar to that of the East African one.

cally considered (LeBas,

as a marker

of a distensive

1977, 1980). The small volcanic

as in Uganda (Lloyd, 1981)) and for the high alkalinity of their host lavas. The petrographical similarities between the Aswa and the Oued lineaments

suggest

that the Oued Amded

regime

district

of

Illizi (Fig. 1B) constitutes the possible northeastern continuation of the Hoggar volcanic axis. Twenty circular structures, aligned NW-SE and of the order of 1 km in diameter, interpreted as probably Cenozoic explosion craters (Megartsi, 1972) contain lava fragments with a composition of melilite-rich ankaratrite (rushayite) and xenoliths of phlogopite-rich clinopyroxenite (Boss&-es and Megarstsi, 1982) and of highly phlogopitized garnet lherzolite. Lava and inclusions are perfectly similar to those observed in some volcanoes of the western branch of the East African Rift system and in the Rhine Graben (Bailey, 1974; Lloyd, 1972, 1981; Dautria and Girod, 1987). The association of carbonatite, high-alkali lavas and highly

Upper mantle implications Ultramafic and mafic xenoliths are widespread within the Hoggarian Pliocene-Villafranchian and Quatemary basalts, while they are absent within the Miocene ones. Recent work on the peridotitic inclusions has led to the recognition of wide variations in texture, reflecting a diversity of deformation and recrystallization which originated within the upper mantle (Girod et al., 1981) and allowing an estimation of its stress regime. The distribution of textures in the various volcanic districts is illustrated in Fig. 6. This diagram shows that the western districts (first type) are essentially characterized by highly deformed peridotite xenoliths (porphyroclastic and granuloblastic textures),

55

while the eastern ones (second type) are characterized by highly recrystallized ones (secondary coarse equant texture). The porphyroclastic type develops by stretching within upper-mantle shear zones, while the granuloblastic one indicates re~~stal~~ation from strongly deformed rocks. The secondary coarse equant characterizes very extensive recrystallization, classically associated with high temperature, fluid percolation and a highly distensive regime (Boulder and Nicolas, 1975; Mercier and Nicolas, 1975; Harte, 1977). Moreover, in the Hoggar as well as in the French Massif Central (Nicolas et al., 1987) and the West Eifel (Witt and Seek, 1987), the shear process responsible for the deformation of the porphyroelastic xenoliths is associated with a temperature decrease (from almost 1OOO’C to 8OO’C) (Girod et al., 1981; Dautria, 1988; Lesquer et al., 1988). Regional and differential stress conditions have been estimated from statistical grain-size data on deformed samples, according to the method proposed by Mercier (1980b). In the Tahalra and Manzaz districts, the regional stress values range from 6 to 16 MPa, for depths varying between 55 and 70 km (estimation for the geobarometer of Mercier, 1980a, in Girod et al., 1981). They are in good agreement with those calculated for typical continental extension zones such as the Basin and Range Province of the U.S.A. (Mercier, 1980b). The local differential stress has values ranging from 65 to 100 MPa: similar values have also been estimated for the Basin and Range Provinces. The highly recrystallized textures from the Eggere and Adrar N’Ajjer volcanic fields have been produced by regional stresses, ranging from 4 to 6 MPa, definitely lower than those of the western districts

TAHALRA

and similar to those estimated for the Benoue Trough (Dautria et al., 1983). According to Mercier (198Ob), these conditions characterize highly distensive continental zones, such as rifts. The interpretation of the texture distribution throu~out the Hoggar is problematic. As proposed, for instance, by Coisy and Nicolas (1978a, b) for the French Massif Central, the highly recrystallized textures may correspond to the core of a large mantle diapir, while the deformed ones would mark its highly sloped shearing edges. However, (1) the linear shape and the length (= 300 km) of the zone in which the deformed samples have been collected, (2) its superposition with the major late Pan African Oued Amded lineament which probably has been reactivated since the Late Mesozoic, and (3) the fact that the shear process responsible for the deformation of the peridotitic xenoliths is associated with a temperature decrease, suggest another possible interpretation: the deformation of the peridotites from the western districts would simply result from emplacement, beneath each volcanic district, of small-sized hot asthenospheric diapirs (such as those proposed by Nicolas et al. (1987) for the French Massif Central and by Witt and Seek (1987) for the Eifel) into a cooler lithosphere mantle, along a major discontinuity. The high recrystallization of peridotites from the Eastern Hoggar would originate in the distensive regime prevailing in this region up to the present times (as inferred by the Quaternary activity of the Amguid fault). On the other hand, the Hoggarian peridotitic inclusions vary over a wide compositional range, from spine1 lherzolite to dunite, reflecting various

ADRAR

EGCERE

MANZAZ

N’

AJJER 86%

P ( 80

G samples

P

)

G

P

CE

( 33 samples)

Fig. 6. Distributionof the peridotiticxenolithsin G-granuioblastic

the Hoggar,

( 40 depending

type; CE-secondary

G

CE

P

( 17

samples)

to textural coarse equant

G

type and locality. type.

CE

samples

)

P-porphyroclastictype;

56

fMg/Mg+Fel

Cl/ .v ..,

._

,,__--;.92_

l.’

t

/

_’

Sp

/ Cr/CrtAl) I

. .I

I I

I

I

,;I

j

.4

I

/

.5

.6

I

1

Fig. 7. Mg/(Mg+ Fe) in olivine vs. Cr/(Cr+ Al) in spine1 diagram, illustrating the variation of the degree of depletion in the Hoggarian peridotite xenoliths depending on locality. Full square-Tahalra; open circle-Manzaz; full triangle-Adrar N’Ajjer; full circle-Eggere.

degrees of partial melting (Dautria, 1988). As illustrated by the diagram of Fig. 7, referring to the olivine-spine1 compositions, the xenoliths of peridotite from Tahalra and Manzaz are systematically less depleted in incompatible elements than those from Eggere and Adrar N’Ajjer. Such a distribution indicates that the partial melting processes have induced a very large scale chemical heterogeneity within the Hoggar upper mantle. Calculations of partial melting percentages have been carried out by Dupuy et al. (1986) from REE trends for samples from Tahalra: they range from 1% up to 10%. For the harzburgitic samples from Adrar N’Ajjer, they exceed 20%. As the basaltic liquids cannot pull out fragments of their own source, the partial melting event responsible for the chemical variations of peridotite xenoliths throughout the Hoggar is evidently unrelated to the Pliocene-Quatemary magmatism which generated the xenolith host lavas. Consequently, the melting event must be older. Thus, in consideration of the regional volcanic history, a Miocene age can be proposed. But, as the Miocene basalts in all the volcanic districts have chemistries varying over a very narrow range (Girod, 1971; Dautria et al., 1988) and consequently result from very

similar and low degrees (~6%) of partial melting, it is improbable that only the Miocene magmatism has been responsible for the upper mantle depletion gradient evident in the Hoggar. A pre-Miocene partial melting event applying only to the Eastern Hoggar upper mantle may be envisaged. As the most depleted peridotites have been observed in the eastern volcanic districts close to the MidCretaceous-Eocene magmatic fields, the melting event can be tentatively related to this early magmatism. Thus, it would be associated with the Late Mesozoic distension. This is in agreement with the textural characteristics of these depleted peridotites. Conclusions

Gravity anomalies, as well as the topographical, structural and petrological features of the Hoggar swell and its nearby regions (Tenere, North Hoggar), clearly result from distensive events, In the Hoggar, the distension has been associated with: (1) an extensive magmatism of Late Mesozoic to Quatemary age> the magmatic composition evolving from tholeiitic or sub-alkali basalt (Mid-~retaceous) to a c~bonatitic association (Late Cretaceous-Early Cenozoic) and to an alkali basaltic suite (Mi~ene-Quatern~); (2) a basement uplift (almost 1500 m). In a recent paper (Lesquer et al., 1988), we have shown that this uplift is, the result of upper mantle density reduction, induced by transfer of gas, fluids and magmas that are higNy undersaturated in SiO, and highly enriched in incompatible elements. At the Hoggar level, a three-branch distensive structure, developed by reactivation of Pan African pre-existing faults, has been identified. The southeastern branch corresponds to the northern extension of the highly subsident trough system of Tenere, and the northern branch to the southern extension of the half graben system of North Hoggar. The NE-SW-trending third branch has been less distensive and is superimposed on a tectonic lineament which controls the subsequent magmatism. Furthermore, the peridotitic xenoliths entrained by the Cenozoic eruptions show that this branch displays a specific upper mantle signature (high deformation associated with tempera-

51

ture

decrease),

indicating

asthenospheric diapirs)

material

the (probably

along this branch.

summarized proposed

in Fig.

emplacement

This structural

the genesis

according

and Whiteman always

early

response melting

uprising

generated

level.

controlled

Grande

and

uplift

associated

tinuities. faults,

as the isostatic

Rift

and

to this author,

faults located

is

lineament.

This

by the

the

an “rrr”

Oued Amded

Hoggar

been

by Masse (1983) for

Rio

domings

This hypotheRhine

Graben.

the initiation

of grabens

is controlled

by wrench

along main inherited

crustal

Rifts may open and propagate mainly

produced

by partial

the stress

at the base of the lithosphere.

It is nor-

particularly

to mass deficiencies

the

the

be

to the classical model of Burke is interpreted

have

sis is akin to those proposed

(1973). For these authors, and

at

may

may

of the Hoggar

According

(1) The basement

uplifted

transverse

structure: structure,

later

emplacement

model is

8. Two hypotheses

to explain

ture,

of

as small-sized

from their extremities,

field.

Lithospheric

strong

according

to

would

be

thinning

at the intersection

discon-

from these

of the ini-

mally followed by alkaline volcanism and crustal rift development. The rift usually bifurcates along

tiated rift and main transverse wrench faults. Consequently, mantle uprising would occur at the

its length, forming triple evolution of such a junction

intersection age.

“rrr” junctions. The depends on how and

whether motion across its branches can be accommodated by the plate driving mechanism. (2) The Tenere, North

Hoggar

the Eastern

belong

Hoggar

and

to a single distensive

points

The geometry

and would thus be of a younger of the distensive

Hoggar (Fig. 8) is in agreement with model. But, given the lack of information

the

struc-

ing the age of the various

epirogenic,

-\ ( -J

4

e

Ii

*

15"+ Fig. 8. Structural

model for the Hoggar

4 = anomalous

upper

mantle;

of the

the first concern-

tectono-sedi-

10' +

D

30”:

zone:

structure

+

+ and the Tenere.

I = Pan African

5 = Miocene-Quaternary

6

volcanic fields.

basement; districts:

2 = sedimentary 6 = Early

cover;

(Mid-Cretaceous

3 = Cretaceous to Eocene)

distensive

magmatic

58

mentary and magmatic events, it is very difficult to know if the Hoggar doming happened before or after the Cretaceous rifting phase and, consequently, to choose definitively between the two solutions. However, the few available dates (Lower Cretaceous for the ma~mum subsidence of troughs on the north and south of the Hoggar, Mid-Cretaceous to Eocene for the early magmatism, and Miocene for the paroxismal volcanism of the Hoggar swell) suggest that the rifting was probably earlier than the magmatic events related to the upper mantle density reduction (and consequently earlier than its topographical consequence expressed as doming), which would agree with the second solution. The probable recent reactivation of the lineament which controls the recent volcanism as a wrench fault is also in agreement with this hypothesis. This second solution also agrees with the structural setting of Tenere. At the Central-Western Africa scale, the three narrow grabens of Tenere (and their Hoggarian and North Hoggarian extensions) actually constitute the northern ter~nation of a larger and more subsident trough system (Termit trough, Chad basin; Fig. 1A). Thus, the Hoggar doming cannot be considered as responsible for such a large-sized structure expanding southwards. On the other hand, as noted below, this structure belongs to a complex trough network (including the Benoue and the Soudan trough systems) which developed during Early Cretaceous times, in connection with the opening of the Gulf of Guinea. At the Tenere level, gravity data show that the troughs are associated with a dense deep crust which extends northwards under the Eastern Hoggar. As inferred from the study of the xenoliths entrained by the Cenozoic eruptions, the upper mantle and the deep crust beneath the Eastern Hoggar are particular: the peridotites are highly recrystallized and depleted, and are associated with a suite of basic and ultrabasic rocks, of tholeiitic affinity and equilibrated under deep-crust conditions. This suggests the presence of dense magmatic bodies emplaced at the crust-mantle boundary beneath the Eastern Hoggar, and probably also beneath the Tenerean troughs. In the Tenere and the North Hoggar, the occur-

rence of half grabens allows to propose a non-uniform superficial brittle extension. In these two regions, the lack of uplift in connection with extension suggests that the crust thinned independently of the underlying mantle. At the Hoggar level, the Pan African crust is characterized by flat-lying thick mylonitic belts, interpretated as major deep-seated low-angle faults (Bertand et al., 1986). One of these major accidents bounds our distensive structure westward, and we have reported that its northern extension has been reactivated as a normal fault since Cretaceous times (up to 1000 m of vertical throw). These observations suggest that the extensional mechanism could be similar to those proposed by Wemicke (1985) for the Basin and Range Province and the Red Sea. But more structural and geophysical information is necessary before a conclusion can be reached. More geochronological data are also needed, but with the information presently at hand, we propose a simplistic two-stage history for recent evolution of the Hoggar swell: (1) An exclusively distensive first stage, of Early and Mid-Cretaceous age, which would have generated a N-S to NNW-SSE distensive structure (Fig. 8) by the reactivation of Pan African faults. The distension would be expressed as a series of subsident troughs. This first stage should be related to the crustal stretching which happened during the Late Mesozoic in Centr~-Western Africa, as a consequence of the stress field induced by the Central Atlantic opening ( CI~= E-W). In the Eastern Hoggar, basaltic volcanism (of tholeiitic to sub-alkali affinity) is associated with this distensive event. Note that this early magmatism may also correspond to an intensive depletion of the upper mantle and also to the emplacement of deep-crustal basic layered bodies (of tholeiitic affinity). (2) A second stage, of Late Cretaceous and Cenozoic age, probably corresponding to the change of stress field (as = ENE-WSW) induced by the Africa-Europe co&ion (Dorbath et al., 1985; Guiraud et al., 1987) and resulting in the reactivation of a NE-SW-trending lineament as a wrench fault. Upper mantle modifications induced by gas, fluid and magma transfer from the deep

59

mantle, and resulting in density reduction, extensive alkali magmatism (carbonatite to alkali basalt) and consecutive doming may be associated with this second stage, and was probably controlled by the transverse wrench fault. This doming resulted in the removal of the Eastern Hoggar sedimentary Cretaceous cover. The residual hills of Cretaceous flu~olacust~ne deposits observed in the Amadror and Serouenout depressions are the last relics of this cover. The Miocene-Quaternary volcanism may be associated with the emplacement of small-sized asthenospheric diapirs into the upper mantle along the transverse fault, and may be the late consequence of this second stage.

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