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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.
References
Anderson, E.M., 1951. The Dynamics of Faulting. Oliver and
Boyd, Edinburgh.
Angelier, J. and Mechfer, P., 1977. Sur une methode graphique
de recherche des contraintes principales dgalement utilisa-
bles en tectonique et en seismologic: la methode des diedres
droits. Bull. Sot. Geol. Fr. (7), 19(6): 1309-1318.
Arthaud, F. and Choukroune, P., 1972. Methode d’analyse de
la tectonique cassante a I’aide des microstructures dans les
zones peu deformees: exemple de la plate-forme Nord-
Aquitaine. Rev. Inst. Fr. Pet., 27(5): 715-732.
Arthaud, F. and Choukroune, P.. 1976. Mise en evidence l’une
phase de compression a 35 Ma separant deux episodes
d’ouverture du rift oceanique de Tadjourah (T.F.A.I.). CR.
Acad. Sci. Paris. 283. Ser. D: 13-16.
Be&on, Y., Be&he& J. and Guiraud, R.. 1984. Mise en
Kdence de d~fo~ations d’origine compressive dans le
Continental intercalaire de la partie m&idionale du bassin
de Taoudenni (Hodh oriental, confins mauritano-mahens).
Bull. Sot. Geol. Fr., (7). 26(6): 713771147.
Bles, J.L. and Feuga, B., 1986. The Fracture of Rocks. North
Oxford Academic Publishers-B.R.G.M., London-Orleans,
132 pp.
Bib, J.L. and Gros, Y.. 1980. La fracturation du granite de
Bass& (Pyre&es ariegeoises. France). Chronologie des
phases tectoniques, evolution des diaclases et des failles.
Bull. Sot. Geol. Fr.. (7). 22(3): 3777390.
Bless, J.L., Bonijoly, D.. Castaing, C. and Gras, Y., 1989.
Successive post-Variscan stress fields in the French Massif
POST-PAN-AFRICAN TECTONIC EVOLUTION OF SOUTH MALAWI 71
Central and its borders (Western European plate): com-
parison with geodynamic data. Tectonophysics, 169: 79-
Bloomfield, K,, 1961. The age of the Chilwa Alkaline Province.
Rec. Geol. Surv. Nyasaland, 1 [for 19591: 95-100.
Bloomfield, K., 1966. Geological map of Malawi; scale
1: 1000000. Map, Geol. Surv. Malawi.
Bloomfield, K., 1968. The pre-Karroo geology of Malawi. Mem. Geol. Surv. Malawi, 5.
Bloomfield, K., 1970. Orogenic and post-erogenic plutonism in
Malawi. In: T.N. Clifford and LG. Gass (Editors), Oliver
and Boyd, Edinburgh.
Bond, G., 1952. The Karroo System of Southern Rhodesia. Int.
Geol. Congr., I9 (Symp. Gondwana): 209.
Bosellini, A., 1986. East Africa continental margins. Geology,
14(l): 76-78,
Bosworth, W., 1985. Geometry of propagating continental
rifts. Nature, 316: 625-621.
Bunce, E.T. and Molnar, P., 1977. Seismic reflection profiling
and basement topography in the Somali Basin: possible
fracture zones between Madagascar and AFrica. J. Geo-
phys. Res., 82: 5305-5311.
Cahen, L., Sneliing, N-J., Delhal, J. and Vail, J.R., 1984. The Geochronology and Evolution of Africa. Clarendon Press,
Oxford, 512 pp.
Cannon, R.T., Simiyu Siambi, W.M.N. and Karanja, F.M.,
1981. The proto-Indian Ocean and a probable Paleozoic/
Mesozoic Triradial Rift System in East Africa. Earth Planet.
Sci. Lett., 52: 419-426.
Carey, E., 1979. Recherche des directions principales de-s con-
traintes associees au jeu dune population de failles. Rev.
G&gr. Phys. Geol. Dyn., 21(l): 57-66.
Carter, G.S. and Bennet, J.D., 1973. The geology and mineral
resources of Malawi. Bull. Geol. Surv. Malawi, 6.
Chorowicz. J., 1989. Transfer and transform fault zones in
continental rifts: examples in the Afro-Arabian Rift Sys-
tem. Implications of crust breaking. J. Afr. Earth Sci., 8
(2/3/4): 203-214.
Chorowicz, J. and Mukonki, M.N.B., 1980. Lineaments anciens,
zones transformantes recentes at geotectonique des fosses
de I’Est africain, d’apr& la t&detection et la micro-
tectonique. Mm. R. Afr. Centr., Tervuren (Belg.), Dept.
G601. Min., Rapp. Ann.: 143-167.
Chorowicz, J., Le Foumier, J., Le Mut, C., Richert, J.P.,
Spy-Anderson, F.L. and Tiercehn, J.J., 1983. Observation
par teledttection et au sol de mouvements decrochants
NW-SE dextres dans le secteur transformant Tanganyika-
Rukwa-Malawi du rift est africain. C.R. Acad. Sci. Paris,
296, II: 997-1002.
Chorowicz, J., Le Fournier, J. and Vidal, G., 1987. Model of
the rift development in Eastern Africa. In: P. Bowden and
J. Kinnairds (Editors) Thematic Issue. “African Geology
Reviews”. Geol. J., 22: 495-513.
Cooper, W.G.G. and Bloomfield, K., 1961. The geology of the
Tambani-Salambidwe area. Bull. Geol. Surv. Nyasaland,
13.
Coward, M.P., 1986. Heterogeneous stretching, simple shear
and basin development. Earth Planet. Sci. Lett., 80: 325-
336.
Coward, M.P. and Daly, MC., 1984. Crustal iineaments and
shear zones in Africa: their relationships to plate move-
ments. Precamb. Res., 24: 27-45.
Cox, K.G., 1970. Tectonics and volcanism of the Karroo
Period and their bearing on the postulated fragmentation
of Gondwanaland. In: T.N. Clifford and I.G. Gass (Edi-
tors), African Magmatism and Tectonics. Oliver and Boyd,
Edinburgh.
Cox, K.G., Johnson, R.L., Monkman, L.J., Stillman, J.C., Vail,
J.R. and Wood, D.N., 1965. The geology of the Nuanetsi
Igneous Province. Philos. Trans. R. Sot. Lond., Ser. A, 257:
71.
Crossley, R. and Crow, M.J., 1980. The Malawi Rift. In:
Geodynamic Evolution of the Afro-Arabian Rift System,
Accademia Nazionale Dei Lincei, Rome, pp. 77-87.
Daly, M.C., Chorowicz, J. and Fairhead, J.D., 1989. The
reactivation of steep basement shear zones and their in-
fluence on Rift Basins in Africa. In: G. Williams and M.A.
Cooper (Editors), Inversion Tectonics. Geol. Sot. London,
Spec. Publ. Publ., 29. Blackwell, Oxford.
De Wit, M., Jeffery, M., Berg, H. and Nicolaysen, L., 1988.
Geological Map of sectors of Gondwana reconstructed to
their disposition. American Association of Petroleum Geol-
ogists and University of the Witwatersrand.
Dixey, F., 1939. The early Cretaceous valley-floor peneplain of
the Lake Nyasa Region and its relation to Tertiary rift
structures. Q-J. Geol. Sot. London, 95: 75.
Diiey, F., 1956. The East African Rift System. Bull. Colon.
Geol. Miner. Resour., Suppl. 1.
Ebinger, C.J., Crow, M.J., Rosendahl, B.R., Livingstone, D.A. and Lefournier, J., 1984. Structural evolution of Lake
Malawi, Africa. Nature, 308: 627-629.
Ebinger, C.J., Rosendabl, B.R. and Reynolds, D.J., 1987.
Tectonic model of the Malawi rift, Africa. Tectonophysics,
141: 215-235.
Fairhead, J.D. and Green, CM., 1989. Controls on rifting in
Africa and the regional tectonic model for the Nigeria and
East Niger rift basins. J. Afr. Earth Sci., 8 (2/3/4): 231-
249.
Fairhead, J.D. and Henderson, N.B., 1977. The seismicity of
southern Africa and incipient rifting. Tectonophysics, 41:
T19-T26.
Fairhead, J.D. and Stuart, G.W., 1982. The seismicity of the
East African Rift System and comparison with other con-
tinental rifts. Tectonophysics, 133: 277-285.
Garson, M.S. and Walshaw, R.D., 1969. The geology of the
Mulanje area. Bull. Geol. Surv. Malawi, 21.
Guiraud, R. and Alidou, S., 1981. La faille de Kandi (Benin),
timoin du rejeu fini-c&ace dun accident majeur a l’echelle
de la plaque africaine. C.R. Acad. Sci. Paris, 293, ser. II:
779-782.
Habgood, F., 1963. The geology of the country west of the
Shire River between Cbikwawa and Chiromo. Bull. Geol.
Surv. Nyasaland, 14.
Hallam, A., 1967. The bearing of certain palaeozoogeographic
72
data on continental drift. Palaeogeogr. Palae~limatol.
Palaeoecol., 3: 201.
Harris, P.G., 1970. Convection and magmatism with reference
to the African continent. In: T.N. Clifford and LG. Gass
(Editors), African Magmatism and Tectonics. Oliver and
Boyd, Edinburgh.
Jackson, J. and McKenzie, D., 1983. The geometrical evolution
of normal fault systems. J. Struct. Geol., S (5): 471-482.
Katz, M.B., 1987. East African rift and northeast hneaments:
continental spreading-transform system? J. Afr, Earth Sci.,
6(l): 103-107.
Kazmin, V., 1980. Transform faults in the East African Rift
System. In: Geodynamic Evolution of the Afro-Arabian
Rift System. Accademia Nazionaie Dei Lincei, Rome, pp,
67-73.
Kent, P.E., 1974. Continental margin of East Africa-a region
of vertical movements. In: C.A. Burk and C.L. Drake
(Editors), The Geology of Continental Margins. Springer,
New York, p. 1009.
King, B.C., 1970. Volcanicity and rift tectonics in East Africa.
In: T.N. Clifford and LG. Gass (Editors), African Magma-
tism and Tectonics. Oliver and Boyd, Edinburgh
King. B.C., 1978. A comparison between the older (Karroo)
rifts and the younger (Cenozoic) rifts of eastern Africa. In:
LB. Ramberg and E.R. Neumann (Editors), Tectonics and
Geophysics of Continental Rifts. Reidel, Dordrecht.
Kreuser, T. and Semkiwa, M., 1987. Geometry and deposi-
tional history of a Karroo (Permian) Coal basin (Mc-
huchuma’ Ketewaka) in SW-Tanzania. Neues Jabrb. Geoi.
Pallontol., Monatsh., 2: 69-98.
Lambiase, J.J., 1989. The framework of African rifting during
the Phanerozoic. J. Afr. Earth Sci., 8(2/3/4): 183-190.
McConnel, R.B., 1972. Geological development of the Rift
System of Eastern Africa. Geol. Sot. Am. Bull., 83: 2549-
2572.
McConnel, R.B., 1980. A resurgent taphrogenic Iineament of
Precambrian origin in eastern Africa. J. Geol. Sot. London,
137(4): 483-489.
Norton, 1.0. and Sciater, J.G., 1979. A model for the evolution
of the Indian Ocean and the break up of Gondwanafand. J.
Geopbys. Res., 84: 680336830.
Noyer, M.L., 1981. Reconstitution dun tenseur moyen des
cantraintes B partir de I’observation de plans de failles
stries: le programme CARELU. Rapp. B.R.G.M., No. 81
SGN 645 FAU, Orl&ns.
Orpen, J.L., Swain, C.J., Nugent, C. and Zhou, P.P., 1989.
Wrench-fault and half-graben tectonics in the development
of the Pateozoic Zambesi Karroo Basins in Zimbabwe --the
“ Lower Zambesi” and ” Mid-Zambesi” basins respectively
-and regional implications. J. Afr. Earth Sci., 8(2/3/4):
215-229.
Pinna, P., Marteau, P., Becq-Giraudon, J.F. and Manigault, B.,
1987. Carta geologica de Mqambique. escaia 1 : 1 WWQO. Minist~~o dos recursos minerais.
Rach, N.M. and Rosendahl, B.R., 1989. Tectonic controls on
the Speke Cuff. J. Afr. Earth Sci., 8(2/3/4): 471-488.
Rais-Assa, R., 1988. Stratigraphy and geodynamics of the
Mombasa Basin (Kenya) in relation to the genesis of the
proto-Indian Ocean. Geoi. Mag., 125(2): 141-147.
Reeves, C.V., 1978. A failed Gondwana spreading axis in
southern Africa. Nature, 273: 22-223.
Reeves, C.V., Karanja. F.M. and MacLeod, I.N., 1987. Geo-
physical Evidence for a failed Jurassic rift and triple junc-
tion in Kenya. Earth Planet. Sci. Lett., 81: 299-311.
Reyre, D.. 1984. Remarques sur I’origine et I’evolution des
bassins sedimentaires africains de la c&e atlantique. Bull.
Sot. Geol. Fr., (7), 24(6): 1041-1059.
Rosendahl, B.R., 1987. Architecture of continental rifts with
special reference to East Africa. Annu. Rev. Earth Planet.
Sci., 15: 445-503.
Rosendahl, B.R. and Livingstone. D.A., 1983. Rift lakes of
East Africa, new seismic data and implication for future
research. Episodes. 1: 14- 19.
Scott, D.L.. Rosendahl, B.R., Burgess, C.F. and Sander, S.,
1989. Comments on “Variable extension in Lake Tanga-
nyika” by C.K. Morley. Tectonics, 8(3): 647-650.
Segoufin, J.. 1978. Anomalies magnetiques mesozdiques dam le
bassin de Mozambique. C. R. Acad. Sci. Paris., Ser. D, 287:
109-112.
Segoufin, J. and Patriat, P., 1981. Reconst~ctions de I’Ocdan
Indien Occidental pour les epoques des anomalies M21, M2
et 34. Bull. Sot. Ghoi. Fr., (7), 23: 5933607.
Shudofsky, G., 1985. Source mechanisms and focal depths of
East African earthquakes using Rayleigb wave inversion
and body-wave modelling. Geophys. J.R. Astron. Sot., 83:
563-614.
Unesco, 1971. Tectonics of Africa. Unesco, Paris.
Unesco, 1975. Geological World Atlas. Unesco, Paris,
Unesco, 1978. International Tectonic Map of Africa. Unesco,
Paris.
Vail, J.R.. 1968. The southern extension of the East African
Rif! System and related igneous activity. Geot. Rundsch.,
57: 601.
Vail, J.R., 1970. Tectonic control of dykes and related irruptive
rocks in eastern Africa. In: T.N. Clifford and LG. Gass
(Editors), African Magmatism and Tectonics. Oliver and
Boyd, Edinburgh.
Villeneuve, M., 1983. Les sitlons tectoniques du Precambrien
superieur dans I’est du Zaire; comparaisons avec les direc-
tions du rift Est-Africain. Bull. Cent. Rech. Explor. Prod..
EIf-Aquitaine, 7( 1): 163- 174.
Walker, F. and Poldervaart, A., 1949. Karroo doierites of the
Union of South Africa. Buli. Geol. Sot. Am., 60: 591.
Wernicke, B. and Burchfiel, B.C., 1982. Modes of extensionat
tectonics. J. Struct. Geol., 4(2): 10% 115. Westermann, G.E.G., 1975. Bajocian Ammonoid Fauna of
Tethyan affinities from the Kamhe Limestone Series of
kenya and implications to plate tectonics. Newslett. Stra-
tigr., 4: 23-48.
Wheeler, W.H. and Karson. J.A., 1989. Structure and kine-
matics of the Livingstone Mountains border fault zone,
Nyasa (MaIaw~) Rift, southwestern Tanzania. J. Afr. Earth
Sci., 8(2/3/4f: 393-413.
Woolley, A.R. and Garson, MS., 1970. Petrochemical and
POST-PAN-AFRICAN TECTONIC EVOLUTION OF SOUTH MALAWI 73
tectonic relationship of the Malawi carbonatite-alkaline
province and the Lupata-Lebombo volcanics. In: T.N.
Clifford and LG. Gass (Editors), African Magmatism and
tectonics. Oliver and Boyd, Edinburgh.
Yairi, K. and Saka, Y., 1977. Tectonic notes on the Livings-
tonia Area, Northern Malawi, in relation to the post-Kar-
roo Rift faulting. 2nd Prelim. Rep. Afr. Stud., Nagoya
Univ., pp. 1088132.
Yemane, K., Siegenthaler, C. and Kelts, K., 1989. Lacustrine
environment during Lower Beaufort (Upper Permian) Kar-
roo deposition in Northern Malawi. Palaeogeogr., Palaeo-
climatol., Palaeoecol., 70: 165-178.
Zoback, M.L. et al., 1989. Global patterns of tectonic stress.
Nature, 341: 291-298.