19
Tectonophysics, 191 (1991) 55-73 Elsevier Science Publishers B.V., Amsterdam lost-Pan-Af~can tectonic evolution of South Malawi in relation to the Karroo and Recent East African Rift Systems C. Castaing Bureau de Recherches GMogiques et Mini&es, DEpartement G&oiogie, B. P. 6009, 45060 Ori&ans Cedex 02, France (Received March 5, 1990; revised version accepted October 5, 1990) ABSTRACT Castling, C., 1991. Post-Pan-African tectonic evolution of South Malawi in relation to the Karroo and Recent East African Rift Systems. Tectonophysics, 191: 55-73. Structural studies conducted in the Lengwe and Mwabvi Karroo basins and in the basement in South Malawi, using regional maps and published data extended to cover Southeast Africa, serve to propose a series of ge~yn~c r~onst~ctions which reveal the persistence of an extensional tectonic regime, the minimum stress us of which has varied through time. The period of Karroo rifting, and the tboleiitic and aikahne magmatism which terminated it, were controlled by NW-SE extension, which resulted in the creation of roughly NE-SW troughs articulated by the Tanganyika-Malawi and Zambesi pre-transform systems. These were NW-SE sinistral-slip systems with directions of movement dipping slightly to the Southeast, which enabled the Mwanza fault to play an important role in the evolution of the Karroo basins of the Shire Valley. The Cretaceous was a transition period between the Karroo rifting and the formation of the Recent East African Rift System. Extension was NE-SW, with some evidence for a local compressional episode in the Lengwe basin. Beginning in the Cenozoic, the extension once more became NW-SE, and controlled the evolution in transtension of the Recent East African Rift System. This history highlights the major role of transverse faults systems dominated by strike-slip motion in the evolution and perpetuation of the continental rift systems. These faults are of a greater geological persistence than the normal faults bounding the grabens, especially when they are located on major basement anisotropies. Introduction The region examined is located in the Lower Shire Valley, in South Malawi, between Blantyre and the border with Mo~mbiquc (Fig. 1). This area corresponds to the northeast contact between the Middle Zambesi Karroo volcanosedimentary basin and the basement deformed in the Pan- African orogeny, which developed in the Late Proterozoic. This contact is partly reworked by the Recent East African Rift System. The part of the Middle Zambesi Karroo volcanosedimentary basin outcropping in Malawi includes the Lengwe and Mwabvi basins, which are separated by an intervening basaltic unit (Fig. 2). The basins are bounded to the NE by the Mwanza and Namalambo faults, which bring them into contact with the Pan-African basement. They are affected by intense brittle tectonics, develop- ing two major sets of fractures trending NW-SE and NE-SW, with the formation of tilted blocks which are the major structural feature of these basins (Fig. 3). The structural analysis of the Lengwe and Mwabvi basins and their surroundings forms part of a geological survey programme carried out by the Bureau de Recherches Geologiques et Mini&es {France) and the Geological Survey Department (Malawi) on behalf of the Government of Malawi. The studies reviewed here are based on structural analysis at all scales and, more specifically, on morphological, kinematic and chronological stud- ies of fracturing on outcrop scale. Fracture trends and mo~holo~cal components enabling defini- tion of the type of fracture, the strike and the direction of displacement have been systematically 0040~1951/91/$03.50 0 1991 - Elsevier Science Publishers B.V.

Post-Pan-African tectonic evolution of South Malawi in relation to the Karroo and recent East African rift systems

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Tectonophysics, 191 (1991) 55-73 Elsevier Science Publishers B.V., Amsterdam

lost-Pan-Af~can tectonic evolution of South Malawi in relation to the Karroo and Recent East African Rift Systems

C. Castaing Bureau de Recherches GMogiques et Mini&es, DEpartement G&oiogie, B. P. 6009, 45060 Ori&ans Cedex 02, France

(Received March 5, 1990; revised version accepted October 5, 1990)

ABSTRACT

Castling, C., 1991. Post-Pan-African tectonic evolution of South Malawi in relation to the Karroo and Recent East African Rift Systems. Tectonophysics, 191: 55-73.

Structural studies conducted in the Lengwe and Mwabvi Karroo basins and in the basement in South Malawi, using regional maps and published data extended to cover Southeast Africa, serve to propose a series of ge~yn~c r~onst~ctions which reveal the persistence of an extensional tectonic regime, the minimum stress us of which has varied through time. The period of Karroo rifting, and the tboleiitic and aikahne magmatism which terminated it, were controlled by NW-SE extension, which resulted in the creation of roughly NE-SW troughs articulated by the Tanganyika-Malawi and Zambesi pre-transform systems. These were NW-SE sinistral-slip systems with directions of movement dipping slightly to the Southeast, which enabled the Mwanza fault to play an important role in the evolution of the Karroo basins of the Shire Valley. The Cretaceous was a transition period between the Karroo rifting and the formation of the Recent East African Rift System. Extension was NE-SW, with some evidence for a local compressional episode in the Lengwe basin. Beginning in the Cenozoic, the extension once more became NW-SE, and controlled the evolution in transtension of the Recent East African Rift System. This history highlights the major role of transverse faults systems dominated by strike-slip motion in the evolution and perpetuation of the continental rift systems. These faults are of a greater geological persistence than the normal faults bounding the grabens, especially when they are located on major basement anisotropies.

Introduction

The region examined is located in the Lower Shire Valley, in South Malawi, between Blantyre and the border with Mo~mbiquc (Fig. 1). This area corresponds to the northeast contact between

the Middle Zambesi Karroo volcanosedimentary basin and the basement deformed in the Pan- African orogeny, which developed in the Late Proterozoic. This contact is partly reworked by the Recent East African Rift System.

The part of the Middle Zambesi Karroo volcanosedimentary basin outcropping in Malawi includes the Lengwe and Mwabvi basins, which are separated by an intervening basaltic unit (Fig. 2). The basins are bounded to the NE by the Mwanza and Namalambo faults, which bring them into contact with the Pan-African basement. They

are affected by intense brittle tectonics, develop- ing two major sets of fractures trending NW-SE and NE-SW, with the formation of tilted blocks which are the major structural feature of these basins (Fig. 3).

The structural analysis of the Lengwe and Mwabvi basins and their surroundings forms part of a geological survey programme carried out by the Bureau de Recherches Geologiques et Mini&es {France) and the Geological Survey Department (Malawi) on behalf of the Government of Malawi. The studies reviewed here are based on structural analysis at all scales and, more specifically, on morphological, kinematic and chronological stud- ies of fracturing on outcrop scale. Fracture trends and mo~holo~cal components enabling defini- tion of the type of fracture, the strike and the direction of displacement have been systematically

0040~1951/91/$03.50 0 1991 - Elsevier Science Publishers B.V.

2 AM BIA

Fig. 1. Location map.

measured (see, for example, Arthaud and

Choukroune, 1972; Bles and Gros. 1980; Bles and

Feuga, 1986; Bl&s et al., 1989).

The relationships between fractures and other

geological structures are analyzed so as to group

structures with similar kinematic and chronologi-

cal characteristics in the same deformation system

corresponding to a particular overall state of stress.

The analysis of the reactivation of these struc-

tures, and the study of the offset of some of them

by others, serve to establish the sequence of events.

The identification of relationships between epi-

sodes of deformation, sedimentation and emplace-

ment of dated igneous rocks and veins allows

definition of the ages of the deformations. At map

scale, this dating has been checked by comparing

the movements of the regional faults and the way

in which they may have controlled pluton and

dyke swarm emplacement or the formation of

sedimentary basins. The principal stress directions

(ui and u3) in the Lengwe and Mwabvi basins are

determined either by defining the bisectrices of

the dihedra under compression or extension,

according to the methods of Anderson (1951),

Arthaud and Choukroune (1972) or Angelier and

Mechler (1977) for simple cases, or by using a

program to compute the stress tensor correspond-

ing to the associated faults in a complex system of

fracturing (Carey, 1979; Noyer, 1981). In this way,

the different stages of the tectonic evolution of the

basins and of their surrounding formations were

determined, in order to gain a close understanding

of their present geometry. The location of these

basins on a key geodynamic zone corresponding

POST-PAN-AFRICAN TECTONIC EVOLUTION OF SOUTH MALAWI

+

LENGWE_ BASIN

MWABVI / BASIN

LEGEND

Fig. 2. Structural map of the Lengwe and Mwabti Karroo basins (modified after Habgood, 1963). 1 = Recent deposits; 2 = syenite

of Saiambidwe Will; 3 = Pan-African basement; 4 = fault; 5 = main normal fault; 6 = mylonite and quartz; 7 = folded structure.

to the superposition of the Karroo and Recent Rifts led to the extension of the problems to the scale of Southeast Africa. This enables the pre- ponderant role of transverse fault systems dominated by strike-slip motions in the perpetua- tion of the rifting mechanisms over geological time scales to be recognized. These faults, which de- velop along major basement structures and con- nect rift zones, correspond to “ transforms waiting

to be born”, called “pre-~ansform faults” by Rosendahlf1987).

Stages of post-Pan-African tectonic evolution

The final phase of post-kinematic plutonic ac- tivity within the Mozambique belt, at the end of the Pan-African orogeny, is represented by the Lake Malawi granitic province, which was em-

NKOMBE~ZI PANGA FAULT FAULT

< C’AS’I 41N<i

NWANZA FAULT NE

LENGWE BASIN

SW NE

Ver!icol exoggeiotm

MWABVI BASIN

Fig. 3. Tilted blocks in the Lengwe and Mwabvi Karroo basins (inspired by T&worth, 19X5: C’oward. 1986: Jackson and McKenzie.

1983; Wernicke and Burchfiel, 1982). I = Recent deposits: ? = Mwanza Grits and Caicareous Shales; 3 = Lower Sandstones:

4 = horizon of flaggy sandstone; 5 = Coal Shales: 2, 3, 4. 5 = Karroo deposits: 6 = Pan-African basement: 7 = mylonite and quartz:

8 = listric normal fault: Y = direction of extension.

placed between 500 and 400 Ma (Bloomfield, 1966,

196X; Pinna et al., 1987). These structurally high-

level talc-alkaline plutons were preceded by basic

minor intrusions. and are locally extensively hy-

bridized. Granitic, adamelliti~ and syenitic phases

are found, with associated minor intrusions of

microgranite and microtonahte. A number of the

complexes have a distinctive ring-form, slightly

elongated N-NW. They appear to follow a proto-

rift zone developed during the closing stages of the

Pan-African orogeny (Bloomfield, 1970). This

proto-rift is the first evidence of the Middle

Palaeozoic to Recent extensional tectonic regime.

The sedimentary. tectonic and volcanic evolu-

tion of Southeastern Africa during the Karroo

period can be ascribed to extensionai tectonics,

and is correlated with a postulated convective

uplift of mantle materials responsible for the frag-

mentation of the supercontinent of Gondwana-

land (Cox, 1970).

Shire und Middle Zambesi Valieys

In the Shire Valley (Lengwe and Mwabvi

basins: Fig. 2). the base of the Karroo (Late

Carboniferous) is not exposed. The Coal Shales, a

formation of carbonaceous and coaly shales with

interbedded sandstones, are the lowest part of the

outcropping sequence {Fig. 3). The Coal Shales

have yielded a Late Ecca-Early Beaufort (Middle

Permian) flora, and are followed by the Lower

Sandstones, a thick feature-for~ng sequence of

current-bedded, pebbly grits and coarse arkoses.

This passes upward into &he Mwanza Grits and

Shales, a succession of soft-weathering arkosic grits

POST-PAN-AFRICAN TECTONIC EVOLUTION OF SOUTH MALAWI 59

with interbedded mudstones that become thicker

and more numerous towards the top. This forma-

tion in turn grades into Red Beds, which have a

Early Beaufort (Late Permian) fossil assemblage.

Middle and Late Beaufort (Early Triassic) fossil

assemblages are not represented, and the Upper

Sandstone formation, which is composed of cur-

rent-bedded grits and arkoses, has yielded a flora

with Early Stormberg (Middle Jurassic) affinities.

This formation acquires a desertic character to-

wards the top, and is followed by basalt lava flows

(Habgood, 1963). Major faulting during the Early

Jurassic coincided with the initiation of eruption

of these basaltic lava flows, the majority being of

the fissure type, formed under terrestrial condi-

tions. The basalts are mostly holocrystalline rocks

formed largely of augite and labradorite. Dolerite

dykes and sills, which are associated with the

Stormberg volcanicity and show affinities with the

basalts, form major swarms in the basement com-

plex of parts of southern Malawi. They are espe-

cially prominent to the south of Blantyre, where

they trend predominantly NE-SW (Carter and

Bennett, 1973; see Fig. 5).

The microtectonic studies presented in the In-

troduction and including analysis of the slip direc-

tion along the sets of faults were conducted on the

Lengwe and Mwabvi basins and their surround-

ings. They helped to identify a major syn-sedimen-

tary tectonics exhibited by normal faults influenc-

ing the thickness of the beds, and by tension joints

filled with remobilized sedimentary material (Fig.

4). The preferential trends of these syn-sedimen-

tary structures and of the main post-sedimentary

structures (diaclases, networks of conjugate nor-

mal faults, etc.) reveal the existence of two sub-or-

thogonal directions of extension during the filling

of the basins: a major NW-SE direction and a

less important NE-SW direction (Figs. 4 and 5).

The NW-SE extension seems to be more im-

portant than the NE-SW extension, because the

NE-SW-trending syn-sedimentary normal faults

and tension joints are the most representated on

an outcrop scale (Fig. 4). Consideration of the

network of sills and dykes which close the sedi-

mentary filling of the basins shows that the dykes

form systems that preferentially trend NE-SW.

These are particularly clear between the Karroo

Fig. 4. Microtectonic analysis of the extensional structures in

the Lengwe basin (between Knombezi fault and Panga fault,

see Fig. 2) and in the Mwabvi basin (west of the Namalambo

fault, see Fig. 2). (A) Syn-sedimentary normal fault. (B) Syn-

sedimentary tensional joint, (C) Schmidt stereo net showing

the poles of the syn-sedimentary structures and the directions

of major and minor extensions (data are plotted in lower-hemi-

sphere, equal-area projection, 120 data points).

basins and Blantyre, and are compatible with an

NW-SE direction of extension (Fig. 5). The Kar-

roo sedimentation and the Late Karroo volcanic-

ity of the Shire and the Middle Zambesi area thus

appear to be controlled by an extensional tecton-

ics regime, of which the NW-SE trend of the

minimum stress u3 is increasingly evident from the

Permian to the Early Jurassic. These results are in

perfect agreement with those of Yairi and Saka

(1977), obtained on the Livingstonia Coal Field, in

the Karroo basin of North Malawi (Yemane et al.,

1989). In this basin, the post-Karroo faulting ap-

pears to be normal, and to have occurred in two

main directions, NE-SW and NW-SE, as in the

Lengwe and Mwabvi basins. The fault system of

either trend is characterized by conjugate sets of

normal faults, which demonstrate that the NE-

SW-trending faults occurred under NW-SE hori-

zontal extension, and that the NW-SE-trending

faults occurred under NE-SW horizontal exten-

sion. The NE-SW-trending faults have originated

in an early stage of the Karroo sedimentation.

They may correspond to the main NW-SE exten-

sion defined in the Mwabvi and Lengwe basins

(Figs. 5 and 6).

60

SUCCESSIVE STRESS FIELDS

KARROO SEDIMENTARY STORMBERG DEPOSITS VOLCANICITY ‘v

,:’

MOZAMBIQUE \ M p;L A W I

PERM IAN_LOWER

17’

\

Fig. 5. Structural control of the Karroo evolution of the Lengwe and Mwabvi basins (modified after Habgood, 1963; Orpen et al.,

1989; Pinna et al., 1987). I = Malawi-Mozambique border: 2 = Pan-African basement: 3 = Karroo sedimentary deposits; I = post-

Karroo sedimentary deposits: 5 = Karroo volcanic deposits (Stormberg volcanicity): 6 = Karroo dolerite dykes (Stormberg volcanic-

ity); 7 = sinistral strike-slip faults; 8 = normal faults.

Accordingly, the NW-SE Mwanza fault, which

bounds the basins to the northeast, functions as a

strike-slip fault, with a direction of movement

dipping slightly to the southeast (Fig. 5). It can be

considered as playing the role of a sinistral pre-

transform fault, allowing the opening of the E-W

Middle Zambesi Karroo graben which develops in

Mozambique, to the west of the Lengwe and

Mwabvi basins (Fig. 6).

These basins can hence be interpreted as corre-

sponding to a transtension zone guided by sinistral

NW-SE pre-transform faults, with directions of

movement dipping slightly to the southeast. Within

the basins, this dynamics induces major exten-

sional tectonics involving variable-subsiding blocks

delimited by NW-SE and NE-SW syn- and

post-sedimentary faulting (Figs. 2 and 3). At the

end of the sedimentation, during the Stormberg

volcanic episode, the network of dolerite dykes

followed the NE-SW fractures system more easily

because of the NW-SE extension.

Southeust African geological setting

On the scale of Southeast Africa, a large part of

the troughs and of the Karroo basins trend NE-

SW; the basins of the Limpopo Valley (Cox et al..

1965), the Kariba trough (Bond, 1952), the Luanga

rift (McConnel, 1972). and the Ruhuhu and

\

POST-FAN-AFRICAN TECTONIC EVOLUTION OF SOUTH MALAWI 61

1

4

+

. .

. ..-

.:.,.

M

+2

/

i )p’ : .’ 35

k-3’ + .+

LAKE

TANGANY IKA- MALAWI

ViCTORlA .’

PRE-TRANSFORM:; ::y”: .?:, SYSTEM

,&-p $ _....._+A KARROO RiFT SYSTEM

t

1 Fig. 6. Karroo Rift System in SE Africa (Mafia after Cannon et al., 1980; Coward and Daly, X984; Daly et al., 1989; King, 1978;

Lambiase, 1989; Orpen et al., 1989; Rais-Assa, 1988; Rosendahl, 1987; Vail, 1968, 1970; Wheeler and Karson, 1989). I = Karroo

boundary normal faults; 2 = pre-transform faults; 3 = opening of the proto-Indian Ocean; 4 = Karroo deposits; 5 = Karroo dolerite

dykes; 6 = direction of extension (a = Lengwe and Mwabvi basins-present study, b = Livingstonia basin-Yairi and Saka (1977).

c = Mombasa basin-Rais-Assa (1987); 7 = general extension.

Luwegu grabens (Kent, 1974; Kreuser and

Semkiwa, 1987) (Fig. 6). Moreover, if Madagascar

is replaced in its presumed position at the time

(Bunce and Molnar, 1977; Segoufin, 1978; Segou-

fin and Patriat, 1981; Bosellini, 1986; Reeves et

al., 1987; Rais-Assa, 1988, de Wit et al., 1988), the

boundaries of the Malagasy Karroo basins and the

Luwegu graben form an entity that trends NE-

SW, and is divided by the genesis of the proto-In-

dian Ocean from the Jurassic (Norton and Sclater,

1979; Cannon et al., 1981). If we consider the

group of dolerite dykes of the Karroo system in

the basement and in the interior of the basins

(Vail, 1970), the great majority of them are in-

cluded between the NNE-SSW and ENE-WSW

directions (Fig. 6).

The microtectonic data discussed above on the

Malawi Karroo basins and their setting, as well as

the present considerations on the scale of SE

Africa, lead us to propose NW-SE extension

throughout the Karroo rifting period, from the

Late Carboniferous to the Early Jurassic, This

NW-SE extensional setting gives rise to the prin-

cipal NE--SW trend of the Karroo troughs and

grabens. However, this is not always the case.

because the emplacement of these grabens also

depends on the zones of weakness existing in the

basement. In fact, the Middle Zambesi basin

trends E-W- and then NW-SE in the Lower Shire

Valley, because it is moulded on the Zimbabwe

craton. This also applies to the Karroo deposits of

the Lake Rukwa, which are controlled by the

structural trends of the Ubendian Range (Fig.

6-see also Fig. 12). We propose that the NW-SE

transcurrent structures of the Zambesi and Shire

Valleys (including the Mwanza fault; Fig. .5), as

well as those of Lakes Tanganyika-Rukwa-

Malawi, may have begun to play the role of pre-

transform faults during the Karroo intracontinen-

tal rifts (Fig. 6).

The post-Karroo alkaline igneous activity (Middle

Jurassic to Cretaceous)

The essentially tholeiitic Karroo volcanicity

(Walker and Poldervaart, 1949; Pinna et al., 1987)

which terminates the sedimentary evolution of the

intracontinental troughs and basins of the rift

type, is followed by an alkaline magmatic episode

giving rise to granites, syenites and nepheline

syenites, together with centres of carbonatites,

nephelinites and alkaline lava flows (Woolley and

Garson, 1970). Throughout the world in general,

the Cretaceous appears to be a period of maxi-

mum marine flooding (Hallam. 1967; Harris.

1970): this is what occurred between Southeast

Africa and Madagascar (Rais-Assa, 1987).

Shire and Middle Zumbesi Valleys

In South Malawi, the alkaline igneous activity

is known as the Chilwa Alkaline Province. It con-

sists of a number of syeno-granitic and nepheline

syenite plutons, volcanic vents infilled with

carbonatite, agglomerate and feldspathic breccia,

and a varied suite of alkaline dyke rocks (Carter

and Bennett, 1973). All are of Late Jurassic to

Early Cretaceous age, yielding isotopic ages within

the range 13X-105 Ma (Bloomfield, 1961; Garson

and Walshaw, 1969). At Salambidwe, in the

Lengwe basin (Figs. 2 and 7) Cooper and Bloom-

field (1961) have described a ring structure incor-

porating both oversaturated and undersaturated

rocks. The surrounding Karroo sediments are up-

domed, indurated and locally ferriticized. Sedi-

mentary rocks of Late Jurassic and Cretaceous age

outcrop in the Middle Zambesi Valley and overlay

the eastern part of the Lengwe basin (Fig. 7).

If we consider the networks of alkaline dykes

(siilvsbergites. trachytes. microfoyaites. phonolites

and nephelinites) belonging to the Chilwa Al-

kaline Province, they are grouped in sub-orthogo-

nal systems trending NE-SW and NW--SE, com-

patible with two extensional directions. NW-SE

and NE-SW (Fig. 7). The microtectonic studies

conducted in the Lengwe and Mwabvi basins and

their surroundings help us to identify the existence

and the activity of a large system of NW-SE

normal faults which partly rework the syn-sedi-

mentary faults in the same direction (Figs. 2 and

7) and exaggerate the tilted blocks geometry ini-

tiated during the sedimentation (Fig. 3). The

Mwanza, Namalambo. Panga and Nkombedzi

faults are reactivated as normal faults by brecciat-

ing the Late Karroo dolerite dykes which they

contained, allowing the development of hydrother-

mal circulation along some of these faults, particu-

larly to the north of the Namalambo fault

(Hagbood, 1963). The throws of these faults are

approximately 1 km, and these tectonics are in-

conceivable without the existence of a strong ex-

tension trending NE--SW, which is in fact proved

by the calculation of the stress tensor taken from

the microtectonic analysis of these fault systems

(Fig. 8). Apart from the outliers of the two exten-

sional directions above (NW-SE and NE-SW). a

local outlier indicates the existence of a compres-

sional episode, which is difficult to explain only

by drag folds associated with the tilted blocks.

This is a slightly folded structure on a kilomet-

ric scale (Fig. 2). deforming the dykes and sills of

the Stormberg volcanicity from the end of the

Karroo sedimentation. The NW--SE normal faults

POST-PAN-AFRICAN TECTONIC EVOLUTION OF SOUTH MALAWI 63

SUCCESSIVE STRESS FIELDS

CHILWA ALKALINE CRETACEOUS MAGMATIC PROVINCE SEDIMENTARY OEPOSlSS

IS’ + IS

MOZAMBIQUE /

rl’ f \ ’ MIDOLE JURASSIC_CRETACE~~JS

Fig. 7. Post-Karroo and Cretaceous magmatic and tectonic evolution of South Malawi (modified after Dixey, 1939 Pinna et al., 1987;

Woohey and Garson, 1970). I = Malay-Mo~mbique border; 2 = Pan-African basement and Karroo deposits; 3 = granite, syenite

and nepheline syenite (Chilwa Alkaline Province); 4 = NE-SW alkaline dykes; 5 = Cretaceous deposits; 6 = NW-SE dykes;

7 = normal faults; 8 = carbonatite centre and nephelinite-phonolite plug.

cut this plicative structure, and their later activity

limits the Cretaceous sedimentation towards the

southwest (Habgood, 1963), which often appears

to be controlled by NW-SE structures (Figs. 7

and 9).

The following evolution of the successive stress

fields can be proposed. The NW-SE extension

which controlled the end of the Karroo sedimenta-

tion and the Stormberg volcanicity is perpetuated

in the Jurassic, and controls the emplacement of

the first systems of NE-SW alkaline dykes of the

Chilwa Alkaline Province. This is followed by an

ENE-WSW local compressional episode. The ex-

tensional tectonic regime is then established in the

NE-SW direction at the Jurassic-Cretaceous

boundary, and controls the last NW-SE dykes of

the Chilwa Alkaline Province and the Cretaceous

deposits causing the strong reactivation of the

NW-SE Karroo structures (Fig. 7).

The change in direction of the ~nimum stress

us can also explain the structuring of the Karroo

basin of Livingstonia in North Malawi. Two sets

of NE-SW and NW-SE fractures are described

by Yairi and Saka (1977). We have shown that the

first NE-SW set corresponded to an early activity

during the Karroo sedimentation compatible with

the first NW-SE-trending extension. The post-

sedimentary activity of the later NW-SE fractures

64

Fig. 8. Microtectonic analysis of the Mwanza, Panga, Knombezi

and Namalambo faults in the Mwabvi and Lengwe basins (see

Fig. 2). Schmidt stereo net showing the normal movement

striae of the post-Karroo faulting and the direction of general

extension (data are plotted in lower-hemisphere, equal-area

projection, with the striae plotted in the fault plane at the

origin of each arrow).

can be explained by the second NE-SW-trending

extension. It is possible to associate the compres-

sional episode suspected in the Lengwe basin with

the outliers of the Late Cretaceous compression

which affected the African continent (Guiraud

and Alidou, 1981; Bellion et al., 1984) but this is

possibly only an artefact connected with the slid-

ing movement of the Mwanza fault, playing the

role of a pre-transform fault, and hence possibly

being accompanied by a sligh? collision, as shown

today on the Kivu-Tanganyika fault (Chorowicz

and Mukonki, 1980).

Southeast African geological setting

Contrary to the map of the Karroo troughs

(Fig. 6), the map of the Middle Jurassic to Creta-

ceous basins tends to indicate NW-SE to NNW-

SSE boundaries, except concerning the structure

strongly inherited from the Sabi monocline (Di-

xey, 1939; Unesco, 1975, 1978; Fig. 9). The last

groups of alkaline dykes also underline the NW-

SE trend, and the fracturing data of the Karroo

Linvingstonia basin (North Malawi; Yairi and

Saka, 1977) and of the Karroo to Cretaceous

Mombasa basin (Kenya; Rais-Assa, 1988) tend to

prove the existence of an NE-SW extension since

the beginning of the Cretaceous, as in Central

Africa (Fairhead and Green, 1989). In the

Mombasa basin, post-Kimmeridgian evolution is

characterized by the presence of conjugated faults

with N-S and N140’ E strikes, which correspond

to an NE-SW extensive phase. These are normal

faults which gave rise to the downthrow of the

northeastern component.

If we combine these considerations with the

microtectonic data obtained on the Karroo basins

of Mwabvi and Lengwe in South Malawi, we can

consider the generalization of the NE-SW exten-

sion on the scale of Southeast Africa, from the

Cretaceous (Fig. 9). The extension thus changes

direction, from NW-SE during the Karroo period

and the early alkaline magmatism (Chilwa Al-

kaline Province) to NE-SW during the Creta-

ceous. This change is concomitant with the end of

the Karroo rifting phase. reflected by the estab-

lishment of a passive margin to the east of the

Southeast African block with the sedimentation of

epicontinental and lagoonal series (Montbasa

basin; Westermann. 1975; Rais-Assa, 1988). It

thus appears that the drifting of Madagascar

blocked the evolution of the Karroo proto-rifts of

the SE African continental margin, disturbing the

stress fields and possibly explaining the comprea-

sional features. In the new NE-SW extensional

setting, the transcontinental dislocations of the

Zambesi and Shire Valleys and of Lakes

Tanganyika-Rukwa-Malawi function as normal

faults rather than as strike-slip faults (Figs. 6 and

9). The Middle Jurassic to Cretaceous period thus

appears as a transition period which corresponds

to the end of the evolution of the Karroo Rift

System, the development in Central Africa of the

Cretaceous Rift System and the start of the evolu-

tion of the Recent East African Rift System.

The East African Rift System (Cenozoic to Recent)

The East African Rift System and its associated

volcanism are among the most remarkable geo-

logical phenomena in the world. The details of rift

faulting are closely determined by older, mostly

Precambrian and Karroo structures (Dixey, 1956;

POST-PAN-AFRICAN TECTONIC EVOLUTlON OF SOUTH MALAWI 65

Fig. 9. Post-Karroo and Cretaceous geological setting in SE Africa (modified after Fairhead and Green, 1989; Reeves, 1978; Unesco, 1974,1975,1978). I = Cretaceous boundary normal faults; 2 = Cretaceous deposits; 3 = igneous centres (Chilwa Alkaline Province); 4 = NW-SE dykes; 5 = direction of extension (a = Lengwe ad Mwabvi basins-present study, b = Livingstonia basin-Yairi and

Saka (1977), c = Mombasa basin-Rais-Assa (1988); 6 = general extension.

Vail, 1968; Ring, 1970; McConnel, 1972, 1980; Villeneuve, 1983; Rach and Rosendahl, 1989).

Shire and Middle Zambesi Valleys The Tertiary and post-Tertiary lacustrine and

alluvial deposits occupy a narrow belt on either side of Lake Malawi, around Lake Chilwa, in the Shire Valley, and continue to the south of the Zambesi River in the Urema Graben (Fig. 10). It

is during this period that the N-S normal faults are formed, which delimit the Lake Malawi rift (Crossley and Crow, 1980; Rosendahl and Living- stone, 1983; Ebinger et al., 1984; Rosendahl, 1987; Ebinger et al., 1987). These N-S normal faults are relayed southwards by the NW-SE transverse faults of Mwanza and Cholo which, although functioning as normal faults, also tend to transmit the rifting of Lake Malawi to the Urema Graben.

66

: (\’ +

SUCCESSIVE STRESS FIELDS ., ZAXF :

15' + t

\ MOZAMBIQUE

Fig. 10. Reactivation of the Shire Valley area by the Recent East African Rift System (modified after Habgood, 1963; Pinna et al.,

1987). I = Malawi-Mozambique border; 2 = ante-Cenozoic formations; S = Cenozoic to Recent deposits; 4 = dextral strike-slip

faults; 5 = normal faults; 6 = strike-slip fault with normal component.

Accordingly, the Mwanza and Cholo faults func-

tion with a strong dextral strike-slip movement,

and they can be considered as playing the role of

pre-transform faults, allowing the simultaneous

opening of Lake Malawi and the Urema Graben

(Fig. 10). As during the Karroo period (Fig. 5), the

Mwanza fault is active both as a strike-slip fault

and as a normal fault, and therefore influences the

sedimentation covering the Lengwe and Mwabvi

Karroo basins to the northeast (Fig. 2). The basins

are hence affected by Cenozoic to Present rifting

mechanisms, which tend to exaggerate their tilted

block geometry, due chiefly to the NW-SE fault

systems (Fig. 3), but mainly tend to reactivate the

NE-SW fault systems owing to the recurrence of

the NW-SE-trending extension (Fig. 10). This di-

rection is identified by microtectonic analyses

(Chorowicz, 1989) and by calculation of the mech-

anisms at the earthquake epicentres (Shudofsky,

1985; Fig. 11).

Southeast African geological setting

Like the map of the Karroo Rift System (Fig.

6) the map showing the Recent East African Rift

System (Fig. 11) reveals the activity of the two

major NW-SE transcurrent systems of Lakes

Tanganyika-Rukwa-Malawi and of the Zambesi

and Shire Valleys, which play the role of continen-

tal pre-transform faults, between Lake Tanganyika

and Lake Malawi, and between Lake Malawi and

POST-PAN-AFRICAN TECTONIC EVOLUTION OF SOUTH MALAWI

EAST AFRICAN RIFT SYSTEM

Fig. 11. Recent East African Rift System {modified after Chorowicz, 1989; Chorowicz and Mukonki. 1980; Chorowicz et al., 1983,

1987; Daly et al., 1989; Ebinger et al., 1987; Katz, 1987; Kazrnin, 1980; McConnel, 1972; Rach and Rosendahl, 1989; Rosendahl,

1987; Villeneuve, 1983; Wheeler and Karson, 1989). I = Rift boundary normal faults; 2 = pre-transform faults; 3 = Cenozoic and

Recent volcanics; 4 = Cenozoic granites; 5 = direction of extension (a = Lengwe and Mwabvi basins-present study and focal

mechanism solution of 6 May 1966 earthquake from Shudofsky (1985), b-h = microtectonic observations between Lake Edward and

Lake Malawi from Chorowicz (1989) and Chorowicz and Mukonki (1980)); 6 = general extension.

Urema Graben respectively (Chorowicz and Fairhead and Stuart, 1982; Shudofsky, 1985) tend Mukonki, 1980; Rosendahl, 1987). The present to prove, in agreement with Scott et al. (1989) the and already published microtectonic studies existence of a present NW-SE-trending extension. (Kazmin, 1980; Daly et al., 1987; Chorowicz, 3989; Thus the present scenario is comparable to that Wheeler and Karson, 1989; Zoback et al., 1989) of the Karroo period, also controlled by an NW- and the analysis of the mechanisms at the earth- SE extension (Figs. 6 and 11) but the transten- quake epicentres (Fairhead and Henderson, 1977; sional character of the present rifting is much

68 <‘ C‘ASTAINC;

more pronounced than during the Karroo rifting.

The trough elongations were sub-orthogonal to the

extension direction, with a sinistral action of the

two major pre-transform systems of the Zambesi

and of Lake Rukwa, whereas- today- the troughs

are oriented at 45 degrees to the extension direc-

tion, and the two transcurrent systems function as

dextral strike-slip faults.

Role of pre-existing anisotropies in the evolution

of successive riftings

During the Archaean and the Proterozoic, the

different orogenies caused the continental crust to

create a number of fundamental anisotropies rep-

resented by the boundaries between the earliest

blocks (Tanganyika Shield, Zambia Block,

Zimbabwe Craton and Transvaal Nucleus) and

the mobile zones (Ubendides, Kibarides, Irumides,

Zambesi Belt, Limpopo Belt and Mozambique

Belt), and by the fabric of the basements, associ-

ated primarily with the regional arrangement of

the foliation planes (Fig. 12).

If we consider the Karroo rifting (Figs. 6 and

12), a number of grabens can be observed to be

moulded on the earlier nuclei, and on the general

basement fabric directions. In fact, the basins of

the Limpopo Valley are controlled by the

boundaries between the Zimbabwe Craton and the

Transvaal Nucleus, the Lake Kariba trough and

the Zambesi Valley basins are moulded on the

northern margin of the Zimbabwe Craton, and the

Luanga Rift is parallel to both the structure of the

irumides Range and the SE margin of the Zambia

Block. The best evidence of the role of the in-

herited structures corresponds to the Tanganyika-

Malawi pre-transform system, which develops in

the NW-SE corridor existing between the

Tanganyika Shield and the Zambia Block, a cor-

ridor already used by the Ubendides Range. This

system controls the deposits to the west of Lake

Tanganyika and those existing between Lake

Tanganyika and Lake Malawi. However, there is

no obvious relationship between the Zambesi pre-

transform system which controls the Karroo basins

of the Shire Valley (Lengwe and Mwabvi basins}

and the structure of the Proterozoic basement,

apart from an inflection of the foliations in the

NW-SE direction at the level ot the Mwanza

fault, also striking NW-SE. The Karroo rifting,

influenced both by an NW-SE extension and the

pre-existing anisotropies of the basement on which

it develops, thus creates a new network of ani-

sotropies which is superimposed on the Protero-

zoic network.

If we consider the Recent East African Rift

System (Figs. 11 and 12) corresponding to a trans-

tension system under the effect of a distension

that is also NW-SE (Kazmin, 1980; Chorowicz,

1989; Chorowicz and Mukonki, 1980; Rosendahl,

1987), the Western Rift Branch and the Eastern

Rift Branch tend to be moulded on the Tanganyika

Shield, and the Lake Malawi rift is more or less

parallel to the Mozambique Belt structure. But

this corresponds to the re-utilization of the

Tanganyika-Malawi and Zambesi pre-transform

systems, initiated during the Karroo period. More

so than the grabens, these strike-slip systems must

have a very deep counterpart which gives them a

great geological persistence (Reyre, 1984). In fact,

if they are not situated on these pre-transform

systems, the Karroo grabens are only rarely re-

worked by the Recent East African Rift System,

which nevertheless develops under an NW-SE

extension which should exert substantial control

on the NE-SW Karroo structures.

Condusions

Structural analyses conducted in South Malawi,

with the regional maps and published data ex-

tended to cover Southeast Africa, serve to propose

a series of geodyna~c re~nstructions which re-

veal the persistence of an extensional tectonic

regime, the minimum stress of which, a,, has

varied through time (Fig. 13).

The Karroo rifting period, and the tholeiitic

and aikaline ma~atism which ter~nated it, were

controlled by NW-SE extension, which resulted

in the creation of roughly NE-SW troughs articu-

lated by the Tanganyika-Malawi and Zambesi

pre-transform systems. These were NW-SE

sinistral strike-slip systems with directions of

movement dipping slightly to the southeast, which

enabled the Mwanza fault to play an important

role in the evolution of the Karroo basins of the

POST-PAN-AFRICAN TECTONIC EVOLUTION OF SOUTH MALAWI 69

BASEMENT FABRIC AND SUPERIMPOSED RIFTS

Fig. 12. Basement structure and superimposed Karroo and Recent Rift systems in SE Africa (modified after Cahen et al., 1984; McConnel, 1972,198O; Rach and Rosendahl, 1989; Rosendahl, 1987; Scott et al., 1989; Villeneuve, 1983). I = Recent normal faults; 2 = Recent pre-transform faults; 3 = Karroo normal faults; 4 = Karroo pm-transform faults; 5 = Archaean fabric (2700-2300 Ma);

6 = Ubendian fabric (2~-14~ Ma); 7 = Kibaran fabric (1300-800 Ma); 8 = earlier structures reactivated in the Pan-African Belt (900-450 Ma); 9 = Pan-African fabric (750-500 Ma); 10 = Great Dyke; Ii = Limpopo Valley; 1.2 = Zambesi Valley; I3 = Shire Valley, I4 = Mwanza faults; 15 = Kariba trough; 16 = Luanga Rift; 27 = Luwegu Graben; 18 = West Rift Branch; 19 = East Rift

Branch.

Shire Valley. The Cretaceous was a transition period between the Karroo rifting and the forma- tion of the Recent East African Rift System. The extension was NE-SW, with some evidence for a local compressional episode in the Lengwe basin. Beginning in the Cenozoic, the extension once

more became NW-SE and controlled the evolu- tion in transtension of the Recent Rift System.

This history highlights the major role of trans- verse fault systems dominated by strike-slip mo- tions in the evolution and perpetuation of the continents rift systems. These faults are of greater

70

moior KARROO RIFTING elte”*lO” mnor

L.2

PERMIAN-TRIASSIC

CHILWA ALKALINE PROVINCE

UPPER JURASSIC LOWER CRETACEOUS

TILTED BLOCKS

7-l

CRETACEOUS

STORMBERG VULCANICITY

LOWER JURASSIC

GENTLE FOLDING

CRETACEOUS

-___ -- ,..__.. -.__ _

EAST AFRICAN ~ RIFT SYSTEM

CENOZOIC-RECENT

Fig. 13. Stereograms showing successive stress fields in South Malawi

geological persistence than the normal faults

bounding the grabens, especially when located on

major basement anisotropies such as the Tanga-

nyika--Malawi structure.

Acknowledgements

The author would like to thank the Malawi

Geological Survey Department and P. Marteau.

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