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Transfer and transform fault zones in continental rifts: examples in the Afro-Arabian Rift System. Implications of crust breaking
JournalofAfrican Earth Sciences, Vol. 8, Nos. 2/3/4, pp. 203-214, 1989
0899-5362/89 $3.00 + 0.00 © 1989 Pergamon Press plc
Printed in Great Britain
Transfer and transform fault zones in continental rifts: examples in the Afro-Arabian Rift System. Implications of crust breaking. J. CHOROWICZ D6partement de G6otectonique, Universit~ Pads 6 4, Place Jussieu, 75252 Pads Cedex 05, FRANCE
Abstract-The concepts of transfer and transform fault zones are applied to the Afro-Arabian Rift System. Transfer fault zones are described in the Gulf of Suez and in LakeMalawi,where they trendrespectivelynorthnortheastand northeast. The intracontinentaltransformfaultzonesof Tanganyika-Rukwa-Malawiand Aswaare characterizedby northwesttrending"en echelon"faults,(possiblyassociatedwithbasementfoldsand pull-apart basins) thatlink the principal armsof the EastAfricanRift System. In eachcase the typicaldirectionof extension is approximatelyparallel to the fault zones. Comparisonbetweentransferand transformfaultzones leads to the conclusion that the former are due to reactivationof local supra-crustal ancient disontinuitieswhile the latter rework steeply dipping deep lithosphericboundaries. Asymmetricpolarity of rift basins separated by transfer fault zones alternates frequentlywitha spacingof approximately70 km. Duringformationof typicalhalf-graben shaped rift-segments,dip-slipreworkingof frontal thrustramps may have linkedhigh level normal faults with lower crust detachmentzones, giving the rift asymmetry. However,the frequent change in polarity of this asymmetryshows that it is an epiphenomenonand thereforea fundamentallithosphericdiscontinuitymust have initiated rifting. Such preexisting boundaries within the lithospheremay be deep steep dipping suture zones corresponding to ancientorogenic belts.
INTRODUCTION
The c o n c e p t of t r a n s f o r m fault w a s e s t a b l i s h e d b y Wilson (1965) for t h e c r o s s - f a u l t s linking lithospheric s p r e a d i n g ridges. The t r e n d of a n oceanic t r a n s f o r m f a u l t parallels the relative m o v e m e n t of p l a t e s a n d its m o t i o n a c c o m p a n i e s oceanic floor accretion. A c h a r a c t e r i s t i c f e a t u r e is right-lateral m o t i o n c o r r e s p o n d i n g to left-lateral stepover of the oceanic ridge, a n d vice versa. The concept m a y be e x t e n d e d to i n t r a c o n t i n e n t a l f a u l t s linking two s e p a r a t e s e g m e n t s of a c o n t i n e n t a l rift. In contin e n t a l c r u s t , h e t e r o g e n e i t y i n d u c e s a complex fault a r r a n g e m e n t described as a n i n t r a c o n t i n e n tal t r a n s f o r m fault zone. T r a n s f e r f a u l t s are of s m a l l e r i m p o r t a n c e a n d c o r r e s p o n d to cross fault inside a rift segment, evincing p a r t of t h e e x t e n s i o n s y s t e m . Gibbs (1984) i n t r o d u c e d t h e t e r m to a c c o u n t for crossf a u l t s allowing linkage b e t w e e n n o r m a l t e n s i o n strike f a u l t s with differing slip rates. The t e r m of " a c c o m m o d a t i o n zone" a p p e a r s from Reynolds & R o s e n d a h l (1984). It h a s similarities with the t r a n s f e r fault concept, d i s c u s s e d below, Several p a p e r s have recently proposed m o d e l s for lithospheric t h i n n i n g a n d extension, linking n o r m a l f a u l t s in t h e brittle u p p e r c r u s t with lowangle d e t a c h m e n t s u r f a c e s at deeper d e p t h within
t h e ductile zone (Wernicke, 1981, 1985; Bally, 1982; W e m i c k e a n d Burchflel, 1982; AUmendinger et aL, 1983; A n d e r s o n et aL, 1983; Gibbs, 1984). Listric n o r m a l f a u l t s of the brittle zone are envision e d to curve d o w n to a low-crustal d e t a c h m e n t w i t h i n a zone of brittle-ductile transition. This p a p e r p r e s e n t s new d a t a c o n c e m i n g t r a n s fer a n d t r a n s f o r m fault zones in selected a r e a s of the Afro-Arabian Rift S y s t e m from t h e viewpoint of their r e l a t i o n s h i p s with a n c i e n t s t r u c t u r e s . In so doing, it a t t e m p t s to p r e s e n t a n overview of their m e c h a n i s m s a n d to explore w a y s to b e t t e r u n d e r stand how crust breaks. TRANSFER
FAULTS
In the Gulf of Suez
In t h e Gulf of Suez (Fig. 1), t h e r e are principally two fault s y s t e m s (Chorowicz et aL, 1987) t h a t have b e e n active since e x t r u s i o n of late Oligoceneearly Miocene lavas (Meneisy & Kreuzer, 1974; Steen, 1982). Normal t e n s i o n faults, b o u n d i n g tilted blocks, t r e n d n o r t h w e s t - s o u t h e a s t , parallel to the axis of t h e rift. F a u l t s striking at N010°E N020°E t r e n d at high angles to the rift, a n d are t r a n s c u r r e n t faults, as advocated b y one of t h e m , clearly observed in the Abu R u d e i s area (Fig. 1). In site 51, slickensides affecting the middle or late
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Fig. 1-Regional structural sketch map of the Gulf of Suez area, outlining the major border faults trending northwesterly, linked by transfer fault zones striklng north-northeast. 1: sedimentary cover, mainly Mesozoic to Eocene; 2: Pl'ecambrlan basement; 3: major fault zone; 4: reverse fault; 5: fold axis. 51: site along Abu Rudeis Fault, corresoponding to the faults analysis described in Fig. 2.
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ved along each side of the gulf of Suez, in the Sinai and in the Eastern Desert of Egypt. This direction seems to be related to Precambrian b a s e m e n t tectonics and infrequently appears in the trend of the Dead Sea t r a n s c u r r e n t fault system. The northwest-southeast direction is also strong on both sides of the Gulf of Suez and more particularly in the Eastern Desert of Egypt. Then, the rift of the Gulf of Suez seems to form due to a north-northeast directed relative m o v e m e n t of continental blocks (Chorowicz et al., 1987), reactivating ancient faults that reflect the grain of the basement. The rift is superimposed upon the northwestsoutheast direction and the NNE-SSW transfer .-~faults direction is closely paralleled by the trend of Fig. 2-Structural analysis of site 51, located on Fig. 1. the relative movement. Sehmldt diagram, lower hemisphere. Both compression I n L a k e 'l~alnw/ and tension axe horizontal (black arrows). Lake Malawi is divided {Ebinger et al., 1984; Miocene rocks (Garfunkel & Bartov, 1977) testify Rosendahl, 1987) into several asymmetric basins a 1eli-lateral m o v e m e n t along the fault plane (Fig. that are separated by transverse ridges {Fig. 3). 2). The Abu Rudeis fault, with leit-slip, apparen- Each basin h a s a half-graben geometry and is tly offsets in a right-lateral sense the maj or border bounded on one side by one or several large "down faults fringing the eastern wall of the rift, there- to the rift" normal faults while the opposite side is fore it acts as a transfer fault. In this location, the a flexural monocline broken by synthetic or antim a i n extensional stress is horizontal, trending thetic normal faults (Crossley & Crow, 1980; north-northeastwards, roughly parallel to the Rosendahl & Livingstone, 1983; Ebinger et al., fault. 1984; Rosendahl, 1987). The general plunge of the Cross sections through the Gulf of Suez (Shaw- basin floors alternates from one basin to another, ki 1981) clearly show a n asymmetric arrange- changing direction on opposing sides of the ridge. ment. One conjugate margin of the rift is b o u n d e d Yalri (1977) and Ebinger et al., (1984) suggested by one or two major normal strike faults with a that the basins are separated by northwestvertical throw of several kilometers. Along the southeast trending tectonic features. Both indicaopposite margin, tilted blocks, all dipping the ted a northeasterly directed regional extensional same direction towards the axis of the rift, are direction. Alternatively, the basins m a y b e separaseparated by small normal strike faults, shaping ted by northwest trending strlke-slip faults a faulted flexure. (Chorowicz et al., 1987; Rosendahl, 1987). StrucShawki (1981) h a s shown that dip azimuth tural analysis along the western b r a n c h of the East reverses from one to the following rift segments, African Rift (Chorowicz et al., 1987), especially expressing a polarity reversal. Location of the along Lake Malawi (Fig. 3) demonstrates a northmajor border fault changes from one side to the west direction of regional extension that is related other (Fig. 1). From Moustafa (1976), Thiebaud & to a NW-SE relative movement of continental blocks. Robson (1979) and Shawki (1981) and from ana- Observations along NW-SE trending rift faults lysis of Landsat-MSS images we proposed, for the have shown right-slip motions associated with the maj or NNE-SSW trending transfer faults, the pat- formation of the rift. For instance, in the tern shown on Fig. 1. Transfer faults link either Livingstonia area, NW-SE striking faults are expotwo major strike faults located along the same sed that seemingly traverse the Lake; their contiside for the rift, or two major border faults located nuations are evident on the eastern shore (Fig. 3A). along opposite sides. In the last case, there is a Field structural analysis (stations 27 and 29) reversal in the polarity of asymmetry. The spa- indicate that these Livingstonia faults are rightcing of the transfer faults is approximately 70 km. slip faults associated with a Cenozoic extension Fracture analysis of sllp indicators demonstra- that trends NW-SE (Fig. 3B). The principal directes that earliest tension is horizontal, trending tion of compression is along a horizontal plane and north-northeastwards or northeastwards, paral- not in a vertical direction as would be the case leling the transfer faults (Fig. 1). The relative mo- along tensional normal faults. These NW-SE vement of continental scale blocks responsible for faults form the limit between two m a i n basins in the rift appears to be north-northeasterly direc- the Malawi rift. Between the Nkhata Bay and the ted. Nkhotakota basins, the b a t h y m e t r y displays again This direction of faulting c a n usually be obser- a prominent NW-SE lineation, but there is no
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c o n t i n u a t i o n on l a n d except a s u b t l e NW-SE alignm e n t in M o z a m b i q u e t h a t is n o t reactivated. A NNE-SSW to ENE-WSW t r e n d is m o r e evident o n land, d u e to the M a n i a m b a Karroo t r o u g h on t h e e a s t e r n side of t h e lake a n d t h e Chamarilo lineam e n t on the w e s t e r n side. The latter w a s d e s c r i b e d as a P r e c a m b r i a n s h e a r zone b y Bloomfield (1966) a n d Kr6ner (1977). This s h e a r zone previously h a s b e e n i n t e r p r e t e d as evidence for Cenozoic ENEW S W t r a n s c u r r e n t m o v e m e n t s within t h e lake (Ebinger etal., 1984). Cenozoic tectonics, h o w e v e r h a s reactivated the Chamaliro fault, displacing t h e African erosional surface. S t r u c t u r a l a n a l y s i s in t h e field d e m o n s t r a t e s NW-SE extensional movem e n t along this fault (station 36, Fig. 3B) d u r i n g Cenozoic times. S e n s e of d i s p l a c e m e n t indicators s h o w t h a t t h e m o t i o n is n o r m a l with no or small strike-slip c o m p o n e n t . T h e s e d a t a a r g u e p e r s u a s i v e l y for the p r e s e n c e of strike-slip faults within the limits of the lake; t h e s e faults t r e n d n o r t h w e s t w a r d s a n d act as t r a n s f e r faults, as outlined in Fig. 3A. The pre-Cenozoic b a s e m e n t clearly e x p o s e s n o r t h e a s t t r e n d i n g Karroo rifts, b u t there are also s o m e n o r t h w e s t t r e n d i n g faults. The Malawi rift t r e n d s generally n o r t h - s o u t h , along the approxim a t e direction of the m a i n foliation in b a s e m e n t rocks, but its relationships with the M o z a m b i q u i a n belt are poorly u n d e r s t o o d . P r e s u mably, s o u t h e a s t w a r d motion of continental blocks reactivates discontinuities t h a t t r e n d n o r t h - s o u t h or n o r t h w e s t - s o u t h e a s t a n d t h a t are related to the M o z a m b i q u i a n orogeny. T h e s e discontinuities define the m a i n location of the rift. N o r t h w e s t trending faults, parallel to the Cenozoic m o v e m e n t are readily r e a c t i v a t e d as t r a n s f e r faults, even ff t h e y h a d b e e n initially only small faults. N o r t h e a s t trending faults, s u c h as the Chamarilo one, rework a s p u r e l y n o r m a l t e n s i o n faults. R e y n o l d s & R o s e n d a h l (1984) have i n t r o d u c e d the term of a c c o m m o d a t i o n zone in the E a s t African Rift, b u t the c o n c e p t is very similar to the t r a n s f e r fault zone one. The m a i n difference s e e m s to b e in the p r e s e n c e of a b a s e m e n t high in the a c c o m m o dation zone lying b e t w e e n two a d j a c e n t halfgrabens. Taking into a c c o u n t t h a t t h e s e crossfault zones are strike-slip s h e a r faults, it is r e a s o n able to c o n s i d e r t h a t local deviation of t h e s t r e s s field will occur. In this case, the m a i n c o m p r e s s i o n b e c o m e s horizontal, even if the e x p o s e d faults are n o r m a l a n d display an oblique-slip motion, a s is the c a s e in the Livingstonia area (Fig. 3B, s t a t i o n s 27 a n d 29).This local c o m p r e s s i v e s y s t e m is the c a u s e of a b u r i e d high s u p e r i m p o s e d u p o n the c r o s s - f a u l t zone. The a c c o m m o d a t i o n zone conc e p t c a n be regarded as a special c a s e of a transfer fault zone. B o r d e r strike-faults in t h e E a s t African Rift frequently are curvilinear in plan view, as u n d e r -
lined b y R e y n o l d s & R o s e n d a h l (1984), R o s e n d a h l e t a / . , (1986), E b i n g e r et al., (1987) a n d B o s w o r t h (1987); this f e a t u r e is well illustrated along Lake Malawi o n L a n d s a t - M S S i m a g e r y (Fig. 3A). This s h a p e s e e m s to b e related with listric n o r m a l faults of t h e s p o o n fault type (Chenet & Letouzey, 1983). General characteristics of transfer faults E x t e n d i n g Gibbs' definition, it c a n b e said t h a t t r a n s f e r faults are c r o s s - f a u l t s or fault z o n e s inside a given rift s e g m e n t . T h e s e faults t a k e p a r t of t h e e x t e n s i o n s y s t e m . T h e y have a r a t h e r small horizontal dimension, n o t exceeding 50 km. less t h a n t h e t h i c k n e s s of a c o n t i n e n t a l type lithosphere. For this r e a s o n t h e y have to be c o n s i d e r e d a s u p p e r c r u s t a l f e a t u r e s with no deep ductile c o n t i n u a t i o n . T r a n s f e r f a u l t s do n o t e x t e n d b e y o n d t h e rift system. T h e y t r e n d parallel to a n c i e n t faults, selectively reactivating s o m e of t h e older faults. T h e y generally parallel the direction of relative m o t i o n of c o n t i n e n t a l blocks. T h e y link t h e m a j o r f a u l t s bordering a s y m m e t r i c rifts. The "down to basin" polarity of n o r m a l faulting m a y c h a n g e on opp o s i n g side of t h e t r a n s f e r fault while, in o t h e r cases, t h e r e is only a n offset in t h e position of the m a i n b o r d e r f a u l t s w i t h o u t a polarity reversal. INTRACONTINENTAL T R A N S F O R M FAULT ZONES Tanganylka-Rukwa-Malawi fault zone
The n o r t h w e s t - s o u t h e a s t t r e n d i n g T a n g a n y i k a R u k w a - M a l a w i fault zone is m o r e t h a n 1,000 k m long for 40 to 100 k m wide. e x t e n d i n g from the central p a r t of Lake T a n g a n y i k a to n o r t h e r n Lake Malawi (Fig. 4). This fault zone links the n o r t h e r n s e g m e n t of the w e s t e r n b r a n c h of the E a s t African Rift, c o m p r i s i n g Lakes M o b u t u , E d o u a r d , Kivu a n d N o r t h e r n T a n g a n y i k a , with t h e s o u t h e r n segm e n t (Lake Malawi). This link c o r r e s p o n d s to a left-stepping s y s t e m of the w e s t e r n a r m of t h e rift b u t t h e Cenozoic n o r t h w e s t striking faults have dextral d i s p l a c e m e n t s a n d t h e e x t e n s i o n direction t r e n d s n o r t h w e s t e r l y (Chorowicz et al., 1983; Chorowicz et al., 1987). This s t r u c t u r e t h e n is r e g a r d e d as a n i n t r a c o n t i n e n t a l t r a n s f o r m fault zone (Chorowicz & Mukonki, 1979; Kazmin, 1980). The faults are a r r a n g e d in a n "en echelon" pattern. a n d define folded s t r i p s w h i c h form large anticlines a n d s y n c l i n e s c o r r e s p o n d i n g to hills exposing t h e P r e c a m b r i a n b a s e m e n t a n d low-lying c o u n t r i e s filled with Cenozoic s e d i m e n t s (Fig. 5). The faults a s s o c i a t e d w i t h t h e folds give t h e b a s i n s a c o m p l i c a t e d p a t t e r n as s h o w n b y R o s e n d a h l et a/., (1986) a n d R o s e n d a h l (1987) in central Lake Tanganyika. The folds allow this tectonic zone to have a n overall right-lateral t r a n s c u r r e n t motion. The t r o u g h s are elongated along t h e NW-SE direction, as s e e n in the Lake R u k w a a n d S o u t h e m
Transfer and transform fault z o n e s Rungwe
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Fig. 3A Structural sketch m a p of the Malawi rift, from Landsat-MSS imagery. Note the northeast trending dashed lines inferred to represent z o n e s of transfer faults. 27, 29, and 36: sites corresponding to structural analysis. NBB: Nkhota Bay Basin; NB: N k h o t a k o t a Basin. 1: Neogene volcanics; 2: Neogene sediments; 3: Karroo deposits; 4: Precambrlan basement; 5: stratigraphlc contact; 6: lithologic mark; 7: ring structure; 8: large fault; 9: fault; 10: inferred fault; 11: linear; 12: attitude of beds or foliation; 13 to 15: local stress field from slip indicators on fault planes; 13: trends of horizontal c o m p r e s s i o n and extension; 14: trend of horizontal extension; 15: trend of latest extension. Fig. 3B Structural analysis in sites 27, 29 and 36, in Lake Malawi area. Schmidt diagrams, lower hemisphere.
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Fig. 4- Regional s t r u c t u r a l sketch m a p of the E a s t African Rii~. 1: m a j o r active fault; 2: active fault; 3: fault; 4: m a i n dlp of the rift tilted blocks. E: Elgon volcano; I~ KflimanJaro: N: Nandi fault scarp.
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Fig. 5- Block diagram outlining the gross deformation of the Tanganyika-Rukwa-Malawi transform fault zone. Note folded strips formed between the fault segments and step faults crossing the basins at high angle of the maln faults direction at the northwestem end of Lake Rukwa and southeastern end of Lake Tanganyika. T a n g a n y i k a areas; t h e NW-SE t r o u g h s are n o t c u t b y NE-SW t r a n s f e r faults (Rosendahl etal., 1986). Lake R u k w a c a n b e regarded a s a c o m b i n a t i o n of b o t h a large syncline affecting t h e b a s e m e n t a n d of a pull-apart. S t e p f a u l t s c u t t i n g the g r a b e n at high angle c a n b e o b s e r v e d fom L a n d s a t - M S S s p a c e i m a g e r y in t h e s o u t h e a s t e r n e n d of Lake T a n g a n y i k a a n d in t h e n o r t h w e s t e r n e n d of Lake R u k w a . T h e s e faults curve in p l a n view. This Cenozoic t r a n s f o r m zone is s u p e r i m p o s e d u p o n a n U b e n d i a n - K i b a r i a n deep fault zone u n derlined b y b l a s t o m y l o n i t e s a n d ultramylonites. t r e n d i n g t h e s a m e direction (McConnel, 1972). The m a p on Fig. 6A s h o w s t h a t t h e n o r t h w e s t t r e n d i n g faults are strongly reactivated, b u t o t h e r fault directions t r e n d i n g differently a n d covered d i s c o n f o r m a b l y b y late P r e c a m b r i a n s e d i m e n t s , well e x p o s e d in t h e Kipfli area, are n o t reactivated. In s o m e areas, a s K a r e m a for i n s t a n c e , a Cenozoic Riedel-type p a t t e r n o c c u r s (Fig. 6B). A s w a fault z o n e The n o r t h w e s t t r e n d i n g A s w a i n t r a c o n t i n e n t a l t r a n s f o r m fault zone (Fig. 4) e x t e n d s along strike for s o m e 1,000 k m from t h e n o r t h of Lake M o b u t u to t h e Gregory Rift a n d c o n t i n u e s s o u t h e a s t w a r d s for a n o t h e r 1,000 k m to t h e Indian Ocean, n o r t h of t h e K e r i m b a s g r a b e n (Mougenot et al., 1986; Chorowicz etal., 1987). F a u l t s of several h u n d r e d kilometers in length d i s p l a y an, "en echelon" patt e r n inside t h e zone (Vidal, 1987).
The Aswa fault (Almond, 1969) f o r m s t h e nort h e a s t e m limit of t h e W e s t e r n Rift a n d is c o n t i n u e d along strike s o u t h e a s t e r l y b e y o n d t h e Elgonvolcano b y the Nandi fault. The A s w a fault zone c u t s t h e Gregory Rift w h i c h a b r u p t l y c h a n g e s direction in t h e c r o s s i n g area, swinging s o u t h e a s t w a r d s from a n o r t h - s o u t h strike to parallel the A s w a fault orientation. S o u t h of this area, t h e E a s t e r n swings b a c k to n o r t h - s o u t h strike a n d finally e x t e n d s s o u t h w a r d s into the n o r t h T a n z a n i a n divergence. C o n s e q u e n t l y , the A s w a fault zone m a y b e c o n s i d e r e d as a m a j o r tectonic feature of t h e Rift, allowing t h e right-lateral s t e p o v e r betw e e n the w e s t e r n a n d the e a s t e r n b r a n c h e s of the E a s t African Rift. S t r u c t u r a l a n a l y s i s along t h e fault s c a r p s in this a r e a (Chorowicz et al., 1987) i n d i c a t e s t h a t t h e m a i n Miocene e x t e n s i o n direction is horizontal, trending n o r t h w e s t w a r d s . C o m p r e s s i o n m a y b e horizontal or vertical, d e p e n d i n g on location a n d age. This fault zone h a s f u n c t i o n e d a s either a compressive (transpressional) or extensional (transtensional) s y s t e m during Miocene times. Volcanic a n d tectonic activity along t h e AswaNandi fault line h a s o c c u r r e d since early Miocene times (Baker et al., 1971) b u t seismic d a t a s h o w little activity. Along the Nandi fault scarp, one c a n observe s u b - h o r i z o n t a l striations t h a t s e e m to b e young. T h e y indicate left-lateral m o t i o n s with a small dip-
210
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Fig 6A Structural sketch map of southern Tanganyika. Note Precambrian faults disconformably overlain by Bukoban sandstone, along the eastern shore of Lake Tanganyika, in the Kipili area. F1 to F5 are Neogene faults cutting the Precambrian fault system. 1: Neogene sediments; 2: Precambrian rocks; 3: stratlgraphic contact; 4: inferred stratigraphic contact; 5: normal fault (with indication of the downthrown compartment); 6: inferred fault; 7: lithologic mark; 8: dips. PKm: middle Proterozoic; PKR: Russian; PKB: Burundian; PKt: Bukoban (terminal sandstones); PC: Permo-Carbonfferous; Ka: Karroo. Fig. 6B Riedel-type patten close to the village of Karema (Central Lake Tanganyika), comprising faults and tension Joints. slip c o m p o n e n t . The right-lateral stepover betw e e n t h e e a s t e r n a n d the w e s t e m b r a n c h e s of the E a s t African Rift, linked b y the left-lateral AswaNandi fault zone, m a y b e t h e n interpreted as a n int r a c o n t i n e n t a l t r a n s f o r m fault zone, active during the early p a r t of Neogene. This s t r u c t u r e is poorly u n d e r s t o o d owing to a lack of detailed w o r k a n d awaits f u r t h e r analysis. The Aswa-Nandi fault zone is regarded (Sanders, 1965; Vail, 1983) as a n i m p o r t a n t tectonic feature during the M o z a m b i q u i a n orogeny. It m a y corresp o n d to a lateral t h r u s t r a m p of this age, linked with a frontal t h r u s t r a m p reworked with back-slip m o v e m e n t in the n o r t h e r n Gregory Rift, along the
Elgeyo e s c a r p m e n t (Bosworth, 1987). During the early activity of t h e rift, e a s t of the Gregory Rift, the A s w a - N a n d i fault zone c o n t i n u e s along strike s o u t h e a s t w a r d s b y a set of , e n echelon, faults a s s o c i a t e d with v o l c a n o e s s u c h a s the Kilimanj aro. It c o n t i n u e s f u r t h e r s o u t h e a s t w a r d s into t h e T a n z a n i a n c o n t i n e n t a l shelf w h e r e it links several offshore g r a b e n s including t h e Cenozoic K e r i m b a s basin. Field evidence of small r e c e n t right-slip d i s p l a c e m e n t s have b e e n o b s e r v e d e a s t of t h e Gregory Rift. Large o p e n t e n s i o n g a s h e s , corresponding to lines of volcanoes, t r e n d northeasterly. T h e s e f e a t u r e s are easily s e e n on L a n d s a t - M S S satellite imagery. This fault zone m a y b e regarded
Transfer and transform fault zones as a diffuse right-lateral transform fault zone forming the left-lateral stepover which links the eastern b r a n c h of the East African Rift with the Kerimbas graben. It is roughly superimposed u p o n a Mozambiquian tectonic discontinuity. General characteristics of intracontinental t r a n s f o r m f a u l t z o n e s in t h e rift s y s t e m The above examples allow several general points to be m a d e about intracontinental transform fault zones in a rift system. They appear to be long s t r u c t u r e s extending over h u n d r e d s of kilometers. They are m a d e of a zone of faults trending at a high angle to the rift direction. This zone is almost parallel to the regional extension direction and links distinct segments of the rift. Both compressive (transpressional) and extensional (transtensional) s t r u c t u r e s m a y develop depending on the relative orientation of the faults and the direction of movement of separating continental blocks. Although not true transform faults of the oceanic type, the geometry and kinematics of these intracontinental fault zones are similar. True oceanic transform faults t h a t would appear in the future forming ocean, m a y be originated from these intracontinental transform fault zones. It is relevant t h a t they are superimposed u p o n very large ancient s t r u c t u r e s that have suffered remobilization. They correspond with very large discontinuities of the crust. Supposing that the longer the discontinuities are, the deeper they could be, several h u n d r e d s kilometers long discontinuities are likely to represent lithospheric scale steep dipping anisotropy. INTERPRETATION AND DISCUSSION R e a c t i v a t i o n o f a n c i e n t s t r u c t u r e s in t h e rift Transfer faults are relatively short faults that trend at a high angle to the orientation of the m a i n rift for some tens of kilometers. They seem to reactivate ancient faults of local importance participating the b a s e m e n t grain. Transform fault zones are m u c h longer, running over several h u n d r e d s or t h o u s a n d s of kilometers and they appear to rework deep lithospheric discontinuities. Previous a u t h o r s have emphasized the importance of ancient s t r u c t u r e s in the formation of the Afro-Arabian Rlft System (King, 1970; McConnel, 1972, 1978, 1980; Mohr, 1983; Fairhead & Anderson 1977; Rosendahl etal., 1986; Daly etal., 1987). For instance, the w e s t e m branch of the rift seems to avoid the Tanzanian shield and roughly follows the Ubendian and Kibarian orogenic belts that wrap a r o u n d the cratonic areas. But locally, it is not strictly superimposed upon the major accidents. In the n o r t h e r n segment of the w e s t e m AES 8-2/4---F
211
arm for example, the northwest regional structural trend of the Kibarian orogenic belt is often at a high angle to the rift northeast trend. In the n o r t h e m Tanganyika region, the Kibarian folds and t h r u s t s swing from east-west to the northeast trend, and are cross-cut by the n o r t h - s o u t h rift trend. In the s o u t h e r n segment, along Lake Malawi, basement control on the location a n d geometry of the major rift faults is due to small faults and to foliation and gneissbandlng trend. In the central segment ofthe Western Rift, the major faults are superimposed upon steep dipping ductile fault zones of lithospheric scale. It finally appears that the overall rift system follows ancient orogenic suture belts that correspond to zones of w e a k n e s s between the major cratonic areas, repeatedly reworked in different deformations. Several authors (Wernicke 1981; Hermance, 1982; Wernicke and Burchflel, 1982; Allmendinger et aL, 1983; Anderson et a t , 1983; B r u n & Choukroune, 1983) believe that graben-like segm e n t s of a rift m a y be reworked from frontal t h r u s t ramp with back-slip movements. The major listric faults would curve down to a zone of brittle-ductile transition and to a low-crustal d e t a c h m e n t which could be the c r u s t - m a n t l e trandition zone (Hermance, 1982). This pattern m a y be argued in the n o r t h e r n part of the Gregory Rift (Bosworth, 1987) in the Elgeyo scarp area, and in the n o r t h e m part of the Malawi rift where a similar structure occurs. The transform fault zones reactivate ancient s t r u c t u r e s supposed to be of the type of lateral t h r u s t ramps (Daly etal., 1987). Aspects of this relation and new interpretations of ancient tectonics Waft, 1983; Daly, 1986) have lead to the suggestion that the f u n d a m e n t a l s t r u c t u r e s which are reactivated could be suture zones. This interpretation could explain their great length. They could be ancient plate boundaries where collision resulted in frontal and lateral t h r u s t ramps. As ancient plate boundaries, these major discontinuities m a y be considered not only in the crust but also in the whole lithosphere. DISCUSSION
Models dealing with a dip-slip reworking of frontal t h r u s t ramps for the formation of typical rift segments have to be confronted with the transfer faults geometry. Frontal t h r u s t r a m p s imply a t h r u s t polarity. Rift a s y m m e t r y also corresponds to a polarity, the major bordering fault being generally considered as the frontal t h r u s t ramp reworked by dip-slip motion. But m a n y rift segm e n t s are cross-cut by transfer faults associated with a polarity reversal. In addition to the examples presented here, rift a s y m m e t r y has been observed to alternate along rift axes in other situations (Bally, 1982; Bosworth, 1985; Lister et al.,
212
J. CHOROWICZ
1986; Rosendahl etal., 1986). The spacing of these changes is small compared to the size of ancient suture zones. If the rift polarity easily reverses while superimposed along the frontal t h r u s t r a m p s of the suture zones, this suggests that the direction in which the underlying d e t a c h m e n t s dip is likely to alternate as well. Contrarily, it is unlikely that the polarity of a s u t u r e zone would change every 70 km; it can reverse on each end of large and several h u n d r e d s
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kilometers long crustal faults (Daly, 1986). These polarity reversals along the m a i n ~ segments on each side of transfer faults m a y be interpreted as a shallow crustal p h e n o m e n o n correponding to a rather "symmetrical" deeper discontinuity that m a y be located below the deep-crustal d e t a c h m e n t zone (Chen & Molnar, 1983). The best arrangem e n t to have a "symmetric" deep discontinuity is a steep lithospheric b o u n d a r y (Fig. 7A). Above a deep steep lithospheric ductile discontinuity rela-
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Flg. 7 - Block d i a g r a m s showing possible r e l a t i o n s h i p s between a s u t u r e zone a n d formation. I: crust; 2: u p p e r mantle, 3: tension Joint. A - Three d i m e n s i o n a l inferred o u t l i n e s of a s u t u r e belt d u e to oblique convergence of two plates. B- Rift formation reworking t h r u s t faults in the c r u s t a n d a deep d i s c o n t i n u i t y in the lower c r u s t a n d in the mantle.
Transfer and transform fault zones ted with a s u t u r e zone, t h e u p p e r c r u s t a l s t r u c t u res in t h e brittle zone c a n easily be u n d e r s t o o d as h a v i n g a n indifferent polarity t h a t reverses frequently. It m a y locally also rework frontal t h r u s t r a m p s or b a c k - t h r u s t r a m p s located above the low-crustal d e t a c h m e n t zone, in t h e brittle zone (Fig. 7B). A steep a t t i t u d e for a lithospheric d i s c o n t i n u i t y of large scale is likely to o c c u r along s u t u r e zones. Relative m o t i o n of converging plates c a n be at a n y angle, n o t p a r t i c u l a r l y at high angle to the plates b o u n d a r y . W h e n t h i s h i g h angle occurs, it is s u p p o s e d to originate collision or hypercollision of c o n t i n e n t a l lithospheres. The m o s t f r e q u e n t patt e m is oblique-slip relative m o v e m e n t along s u t u r e zones. In t h i s case, regardless of the style of c r u s t a l tectonics, c h a r a c t e r i z e d b y b o t h t h r u s t i n g a n d t r a n s c u r r e n t m o v e m e n t s , t h e deep lithospheric c o n t a c t is m o s t likely to have a steep dip (Mattauer & CoUot, 1986). The m o s t f r e q u e n t a t t i t u d e for the deep p a r t of a s u t u r e zone is t h e n to have steep dip a n d it m a y t u r n d o w n w a r d s m o r e vertical in depth. CONCLUSIONS
T r a n s f o r m fault zones, as well as rift b a s i n s , in the Afro-Arabian Rift System, were controUed during initiation of the rift by a p r o n o u n c e d m e c h a n i c a l a n i s o t r o p y of t h e b a s e m e n t . T r a n s f o r m fault z o n e s c o r r e s p o n d to a n c i e n t steep c r u s t a l lateral r a m p s c h a r a c t e r i z e d b y mylonitic rocks types. Typical graben-like rift b a s i n s are often c o n f o r m to a n c i e n t tectonic belts a n d s u t u r e zones. T r a n s f e r faults rework local f a u l t s of the u p p e r c r u s t t h a t participate t h e g r a i n of t h e b a s e m e n t . Both t r a n s f e r a n d t r a n s f o r m fault zones are b r o a d l y paralleled by t h e m a i n e x t e n s i o n direction d u r i n g the earliest stages of the rift formation. The t r e n d of t r a n s f e r f a u l t s are at high angle to t h e rift length. The c r o s s - s e c t i o n a s y m m e t r y of b a s i n s f r e q u e n t l y reverses on opposite sides of t r a n s f e r faults. This s t r u c t u r a l style is b e t t e r explained b y reactivation of steeply dipping lithospheric ductile d i s c o n t i n u i t i e s r a t h e r t h a n reactivation of dip-slip frontal t h r u s t r a m p s only. The latter m a y o c c u r b u t n o t as t h e m a i n p h e n o m e n o n . S u c h deep lithospheric d i s c o n t i n u i t i e s are inferred to be a n c i e n t s u t u r e z o n e s w h i c h s e e m to control s u b s e q u e n t c r u s t a l failure resulting in c o n t i n e n t a l breakup. REFERENCES
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Daly, M.C., Chorowicz, J. a n d Fairhead, D.K. 1987. The reactivation of steep b a s e m e n t s h e a r zones a n d their influence on rift b a s i n s in Africa. In: Williams, G. & Cook, M.A. (Eds), Inversion tectonics-Spec. Publ. geol. Soc. London, (in press). Ebinger, C.J., Crow, M.J., Rosendahl, B.R. Livingstone, D.A. & Le Fournier, J. 1984. S t r u c t u r a l evolution of Lake Malawi, Africa. Nature, 308, 627-629. Fairhead, J.D., Henderson, N.B., 1977. The seismicity o f s o u t h e m Africa and incipient rifting. Tectonophysics, 41, 19-26. Garfunkel, Z. & Bartov, Y. 1977. The tectonics of the Suez rift. Isr. Geol. Surv. Bull. 71, 44p. Gibbs, A.D. 1984. S t r u c t u r a l evolution of extensional b a s i n margins. J. Geol, Soc. London, 1 4 1 , 6 0 9 - 6 2 0 . Hermance, J.F. 1982. Magnetotelluricand geomagnetic deep s o u n d i n g studies in rifts a n d a d j a c e n t areas: c o n s t r a i n t s a n d physical p r o c e s s e s in the c r u s t a n d u p p e r mantle. In: Continental a n d oceanic rifts, G. P a l m a s o n (Eds), G e o d y n a m i c series. 8, Am. Geophys. Union, 169-172. King, B.C. 1970. Vulcanicity and rift tectonics in E a s t Africa. In: African m a g m a t i s m a n d tectonics, Clifford & G a s s (Eds), Oliver & Boyd, Edinburgh. 263-284. Kazmin, V. 1980. T r a n s f o r m faults in the E a s t African Rift System. In: G e o d y n a m i c evolution of the AfroArabic rift system., Roma: Accademia Nazionale dei Lincei (Atti dei Convegni Lincef, 47,) 65-73. Kr6ner, A. 1977. P r e c a m b r i a n mobile belts of S o u t h e r n a n d E a s t e m Africa. Ancient s t r u c t u r e s or sites of ensialic mobility? A case for crustal evolution towards plate tectonics. Tectonophysics, 40, 101135. Lister, G.S., Etheridge, M.A. and S y m o n d s , P.A. 1986. D e t a c h m e n t faulting and the evolution of passive continental margins. Geology. 14, 246-250. Mattauer, M. & Collot, B. 1986. Continental s u b d u c Uon, t h r u s t i n g a n d strike-slip faulting in the C a n a d i a n Cordillera. Bull, Soc. g~ol. France (8), II, 6, 899-9O9. McConnel, R.B. 1972. Geological Development of the Rift S y s t e m of E a s t e r n Africa. Geol. Soc. of Amer. Bull 83, 2549-2572. McConnel, R.B. 1978. Discussion on F u r t h e r d a t a on the alignment of basic igneous intrusive complexes in S o u t h e r n a n d E a s t e r n Africa by J.R. Vail. Trans. geol. Soc. S. Air., 80 (2), 87-92. Trans. geol. Soc. S. Afr. 81, 225-226. McConnel, R.B. 1980. A r e s u r g e n t taphrogenic lineam e n t of P r e c a m b r i a n origin in e a s t e r n Africa. J. Geol, Soc. Lond, 137, 4, 483-489. Meneisy, M.Y., Kreuzer, H. 1974. P o t a s s i u m - a r g o n ages of Egyptian basaltic rocks. Geol.Jb. D 99, 21-31.
Mohr, P.A. 1983. Volcanotectonic a s p e c t s of Ethiopian rift evolution. Bull, Centres Rectu Explor. Prod. ElfAquitaine. 7, 1, 175-189. Mougenot, D., Recq M., Virlogeux, P. & Lepvrier, C. 1986. S e a w a r d extension of the E a s t African Rift. Nature, 321, 599-603. Moustafa, A.M. 1976. Block faulting in the G u l f of Suez. Egyptian General P e t r o l e u m Organization, 5th Exploration Seminar, Cairo. Deminex-Cairo, 19p. Reynolds, D. & Rosendahl, B. 1984. Tectonic expressions of continental rifting. Eos. Trans. Am. Geophys. Union. 65, 1116. Rosendahl, B.R., Livingstone, D.A. 1983. Rift lakes of E a s t Africa: New seismic d a t a a n d implications for future research. Episodes. 83-1, 14-19. Rosendahl, B.R. 1987. Architecture of continental rifts with special reference to e a s t Africa. Ann. Rev. Earth Planet. Scl. 15, 445-503. Rosendahl, B.R., Reynolds, D.J., Lorber, P.M., Burgess, C.F., McGill, J., Scott, D., Lambiase, J . j . & Derksen, S.J. 1986. S t r u c t u r a l e x p r e s s i o n s of rifting: lessons from Lake Tanganyika, Africa. Frostick, L.E. et al., (eds), S e d i m e n t a t i o n in the African Rifts, Geol. Soc. Sp. Pub. 25, 29-43. Sanders, L.P. 1965. Geology of the contact between the Nyanza shield a n d the M o z a m b i q u e belt in Western Kenya. Degree S h e e t 33, NE Quarter. 45p. Shawky, A. 1981. Egypt's p e t r o l e u m geology. Good g r o u n d s for optimism. World Oil. 99-112. Steen, G. 1982. Radiometric age dating a n d tectonic significance of s o m e gulf of Suez igneous rocks. In: H a n t a r G., Ed., The Egyptian General Petroleum Corporation, 6th. Exploration Seminar, Cairo, 20p. Thiebaud, C.E., Robson, D.A. 1979. The geology of the a r e a Wadi W a r d a n a n d Wadi G h a r a n d a l . East Clysmic rift, Sinai Egypt. J. Pert. Geol., I, 63-75. Vail, J.R. 1983. Pan-African crustal accretion in northe a s t Africa. Journal o f African Earth Sciences. I, 3 / 4 , 285-294. Wernicke, B. 1981. Low-angle n o r m a l faults in the Basin a n d Range province: Nappe tectonics in a n extending orogen. Nature. 2 9 1 , 6 4 5 - 6 4 8 . Wernicke, B. & Burchfiel, B.C. 1982. Modes of extensional tectonics. Journal of Structural Geology. 4, 105-115. W e m i c k e , B. 1985. Uniform s e n s e n o r m a l simple s h e a r of the continental lithosphere. Can. J. Earth Sc. 22, 1, 108-125. Wilson, J.T. 1965. A new class of faults a n d their bearing on continental drift. Nature, 207, 343-347. Yairy, K. 1977. Fault p a t t e r n of the s o u t h e r n p a r t of the E a s t e m Rift, Tanzania. 2nd Prelim. Rep Air. Studies, Nagoya Univ., 39-50.
0899-5362/89 $3.00 + 0.00 © 1989 Pergamon Press plc
Printed in Great Britain
Transfer and transform fault zones in continental rifts: examples in the Afro-Arabian Rift System. Implications of crust breaking. J. CHOROWICZ D6partement de G6otectonique, Universit~ Pads 6 4, Place Jussieu, 75252 Pads Cedex 05, FRANCE
Abstract-The concepts of transfer and transform fault zones are applied to the Afro-Arabian Rift System. Transfer fault zones are described in the Gulf of Suez and in LakeMalawi,where they trendrespectivelynorthnortheastand northeast. The intracontinentaltransformfaultzonesof Tanganyika-Rukwa-Malawiand Aswaare characterizedby northwesttrending"en echelon"faults,(possiblyassociatedwithbasementfoldsand pull-apart basins) thatlink the principal armsof the EastAfricanRift System. In eachcase the typicaldirectionof extension is approximatelyparallel to the fault zones. Comparisonbetweentransferand transformfaultzones leads to the conclusion that the former are due to reactivationof local supra-crustal ancient disontinuitieswhile the latter rework steeply dipping deep lithosphericboundaries. Asymmetricpolarity of rift basins separated by transfer fault zones alternates frequentlywitha spacingof approximately70 km. Duringformationof typicalhalf-graben shaped rift-segments,dip-slipreworkingof frontal thrustramps may have linkedhigh level normal faults with lower crust detachmentzones, giving the rift asymmetry. However,the frequent change in polarity of this asymmetryshows that it is an epiphenomenonand thereforea fundamentallithosphericdiscontinuitymust have initiated rifting. Such preexisting boundaries within the lithospheremay be deep steep dipping suture zones corresponding to ancientorogenic belts.
INTRODUCTION
The c o n c e p t of t r a n s f o r m fault w a s e s t a b l i s h e d b y Wilson (1965) for t h e c r o s s - f a u l t s linking lithospheric s p r e a d i n g ridges. The t r e n d of a n oceanic t r a n s f o r m f a u l t parallels the relative m o v e m e n t of p l a t e s a n d its m o t i o n a c c o m p a n i e s oceanic floor accretion. A c h a r a c t e r i s t i c f e a t u r e is right-lateral m o t i o n c o r r e s p o n d i n g to left-lateral stepover of the oceanic ridge, a n d vice versa. The concept m a y be e x t e n d e d to i n t r a c o n t i n e n t a l f a u l t s linking two s e p a r a t e s e g m e n t s of a c o n t i n e n t a l rift. In contin e n t a l c r u s t , h e t e r o g e n e i t y i n d u c e s a complex fault a r r a n g e m e n t described as a n i n t r a c o n t i n e n tal t r a n s f o r m fault zone. T r a n s f e r f a u l t s are of s m a l l e r i m p o r t a n c e a n d c o r r e s p o n d to cross fault inside a rift segment, evincing p a r t of t h e e x t e n s i o n s y s t e m . Gibbs (1984) i n t r o d u c e d t h e t e r m to a c c o u n t for crossf a u l t s allowing linkage b e t w e e n n o r m a l t e n s i o n strike f a u l t s with differing slip rates. The t e r m of " a c c o m m o d a t i o n zone" a p p e a r s from Reynolds & R o s e n d a h l (1984). It h a s similarities with the t r a n s f e r fault concept, d i s c u s s e d below, Several p a p e r s have recently proposed m o d e l s for lithospheric t h i n n i n g a n d extension, linking n o r m a l f a u l t s in t h e brittle u p p e r c r u s t with lowangle d e t a c h m e n t s u r f a c e s at deeper d e p t h within
t h e ductile zone (Wernicke, 1981, 1985; Bally, 1982; W e m i c k e a n d Burchflel, 1982; AUmendinger et aL, 1983; A n d e r s o n et aL, 1983; Gibbs, 1984). Listric n o r m a l f a u l t s of the brittle zone are envision e d to curve d o w n to a low-crustal d e t a c h m e n t w i t h i n a zone of brittle-ductile transition. This p a p e r p r e s e n t s new d a t a c o n c e m i n g t r a n s fer a n d t r a n s f o r m fault zones in selected a r e a s of the Afro-Arabian Rift S y s t e m from t h e viewpoint of their r e l a t i o n s h i p s with a n c i e n t s t r u c t u r e s . In so doing, it a t t e m p t s to p r e s e n t a n overview of their m e c h a n i s m s a n d to explore w a y s to b e t t e r u n d e r stand how crust breaks. TRANSFER
FAULTS
In the Gulf of Suez
In t h e Gulf of Suez (Fig. 1), t h e r e are principally two fault s y s t e m s (Chorowicz et aL, 1987) t h a t have b e e n active since e x t r u s i o n of late Oligoceneearly Miocene lavas (Meneisy & Kreuzer, 1974; Steen, 1982). Normal t e n s i o n faults, b o u n d i n g tilted blocks, t r e n d n o r t h w e s t - s o u t h e a s t , parallel to the axis of t h e rift. F a u l t s striking at N010°E N020°E t r e n d at high angles to the rift, a n d are t r a n s c u r r e n t faults, as advocated b y one of t h e m , clearly observed in the Abu R u d e i s area (Fig. 1). In site 51, slickensides affecting the middle or late
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Fig. 1-Regional structural sketch map of the Gulf of Suez area, outlining the major border faults trending northwesterly, linked by transfer fault zones striklng north-northeast. 1: sedimentary cover, mainly Mesozoic to Eocene; 2: Pl'ecambrlan basement; 3: major fault zone; 4: reverse fault; 5: fold axis. 51: site along Abu Rudeis Fault, corresoponding to the faults analysis described in Fig. 2.
Transfer and transform fault zones
205
ved along each side of the gulf of Suez, in the Sinai and in the Eastern Desert of Egypt. This direction seems to be related to Precambrian b a s e m e n t tectonics and infrequently appears in the trend of the Dead Sea t r a n s c u r r e n t fault system. The northwest-southeast direction is also strong on both sides of the Gulf of Suez and more particularly in the Eastern Desert of Egypt. Then, the rift of the Gulf of Suez seems to form due to a north-northeast directed relative m o v e m e n t of continental blocks (Chorowicz et al., 1987), reactivating ancient faults that reflect the grain of the basement. The rift is superimposed upon the northwestsoutheast direction and the NNE-SSW transfer .-~faults direction is closely paralleled by the trend of Fig. 2-Structural analysis of site 51, located on Fig. 1. the relative movement. Sehmldt diagram, lower hemisphere. Both compression I n L a k e 'l~alnw/ and tension axe horizontal (black arrows). Lake Malawi is divided {Ebinger et al., 1984; Miocene rocks (Garfunkel & Bartov, 1977) testify Rosendahl, 1987) into several asymmetric basins a 1eli-lateral m o v e m e n t along the fault plane (Fig. that are separated by transverse ridges {Fig. 3). 2). The Abu Rudeis fault, with leit-slip, apparen- Each basin h a s a half-graben geometry and is tly offsets in a right-lateral sense the maj or border bounded on one side by one or several large "down faults fringing the eastern wall of the rift, there- to the rift" normal faults while the opposite side is fore it acts as a transfer fault. In this location, the a flexural monocline broken by synthetic or antim a i n extensional stress is horizontal, trending thetic normal faults (Crossley & Crow, 1980; north-northeastwards, roughly parallel to the Rosendahl & Livingstone, 1983; Ebinger et al., fault. 1984; Rosendahl, 1987). The general plunge of the Cross sections through the Gulf of Suez (Shaw- basin floors alternates from one basin to another, ki 1981) clearly show a n asymmetric arrange- changing direction on opposing sides of the ridge. ment. One conjugate margin of the rift is b o u n d e d Yalri (1977) and Ebinger et al., (1984) suggested by one or two major normal strike faults with a that the basins are separated by northwestvertical throw of several kilometers. Along the southeast trending tectonic features. Both indicaopposite margin, tilted blocks, all dipping the ted a northeasterly directed regional extensional same direction towards the axis of the rift, are direction. Alternatively, the basins m a y b e separaseparated by small normal strike faults, shaping ted by northwest trending strlke-slip faults a faulted flexure. (Chorowicz et al., 1987; Rosendahl, 1987). StrucShawki (1981) h a s shown that dip azimuth tural analysis along the western b r a n c h of the East reverses from one to the following rift segments, African Rift (Chorowicz et al., 1987), especially expressing a polarity reversal. Location of the along Lake Malawi (Fig. 3) demonstrates a northmajor border fault changes from one side to the west direction of regional extension that is related other (Fig. 1). From Moustafa (1976), Thiebaud & to a NW-SE relative movement of continental blocks. Robson (1979) and Shawki (1981) and from ana- Observations along NW-SE trending rift faults lysis of Landsat-MSS images we proposed, for the have shown right-slip motions associated with the maj or NNE-SSW trending transfer faults, the pat- formation of the rift. For instance, in the tern shown on Fig. 1. Transfer faults link either Livingstonia area, NW-SE striking faults are expotwo major strike faults located along the same sed that seemingly traverse the Lake; their contiside for the rift, or two major border faults located nuations are evident on the eastern shore (Fig. 3A). along opposite sides. In the last case, there is a Field structural analysis (stations 27 and 29) reversal in the polarity of asymmetry. The spa- indicate that these Livingstonia faults are rightcing of the transfer faults is approximately 70 km. slip faults associated with a Cenozoic extension Fracture analysis of sllp indicators demonstra- that trends NW-SE (Fig. 3B). The principal directes that earliest tension is horizontal, trending tion of compression is along a horizontal plane and north-northeastwards or northeastwards, paral- not in a vertical direction as would be the case leling the transfer faults (Fig. 1). The relative mo- along tensional normal faults. These NW-SE vement of continental scale blocks responsible for faults form the limit between two m a i n basins in the rift appears to be north-northeasterly direc- the Malawi rift. Between the Nkhata Bay and the ted. Nkhotakota basins, the b a t h y m e t r y displays again This direction of faulting c a n usually be obser- a prominent NW-SE lineation, but there is no
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J. CHOROW[CZ
c o n t i n u a t i o n on l a n d except a s u b t l e NW-SE alignm e n t in M o z a m b i q u e t h a t is n o t reactivated. A NNE-SSW to ENE-WSW t r e n d is m o r e evident o n land, d u e to the M a n i a m b a Karroo t r o u g h on t h e e a s t e r n side of t h e lake a n d t h e Chamarilo lineam e n t on the w e s t e r n side. The latter w a s d e s c r i b e d as a P r e c a m b r i a n s h e a r zone b y Bloomfield (1966) a n d Kr6ner (1977). This s h e a r zone previously h a s b e e n i n t e r p r e t e d as evidence for Cenozoic ENEW S W t r a n s c u r r e n t m o v e m e n t s within t h e lake (Ebinger etal., 1984). Cenozoic tectonics, h o w e v e r h a s reactivated the Chamaliro fault, displacing t h e African erosional surface. S t r u c t u r a l a n a l y s i s in t h e field d e m o n s t r a t e s NW-SE extensional movem e n t along this fault (station 36, Fig. 3B) d u r i n g Cenozoic times. S e n s e of d i s p l a c e m e n t indicators s h o w t h a t t h e m o t i o n is n o r m a l with no or small strike-slip c o m p o n e n t . T h e s e d a t a a r g u e p e r s u a s i v e l y for the p r e s e n c e of strike-slip faults within the limits of the lake; t h e s e faults t r e n d n o r t h w e s t w a r d s a n d act as t r a n s f e r faults, as outlined in Fig. 3A. The pre-Cenozoic b a s e m e n t clearly e x p o s e s n o r t h e a s t t r e n d i n g Karroo rifts, b u t there are also s o m e n o r t h w e s t t r e n d i n g faults. The Malawi rift t r e n d s generally n o r t h - s o u t h , along the approxim a t e direction of the m a i n foliation in b a s e m e n t rocks, but its relationships with the M o z a m b i q u i a n belt are poorly u n d e r s t o o d . P r e s u mably, s o u t h e a s t w a r d motion of continental blocks reactivates discontinuities t h a t t r e n d n o r t h - s o u t h or n o r t h w e s t - s o u t h e a s t a n d t h a t are related to the M o z a m b i q u i a n orogeny. T h e s e discontinuities define the m a i n location of the rift. N o r t h w e s t trending faults, parallel to the Cenozoic m o v e m e n t are readily r e a c t i v a t e d as t r a n s f e r faults, even ff t h e y h a d b e e n initially only small faults. N o r t h e a s t trending faults, s u c h as the Chamarilo one, rework a s p u r e l y n o r m a l t e n s i o n faults. R e y n o l d s & R o s e n d a h l (1984) have i n t r o d u c e d the term of a c c o m m o d a t i o n zone in the E a s t African Rift, b u t the c o n c e p t is very similar to the t r a n s f e r fault zone one. The m a i n difference s e e m s to b e in the p r e s e n c e of a b a s e m e n t high in the a c c o m m o dation zone lying b e t w e e n two a d j a c e n t halfgrabens. Taking into a c c o u n t t h a t t h e s e crossfault zones are strike-slip s h e a r faults, it is r e a s o n able to c o n s i d e r t h a t local deviation of t h e s t r e s s field will occur. In this case, the m a i n c o m p r e s s i o n b e c o m e s horizontal, even if the e x p o s e d faults are n o r m a l a n d display an oblique-slip motion, a s is the c a s e in the Livingstonia area (Fig. 3B, s t a t i o n s 27 a n d 29).This local c o m p r e s s i v e s y s t e m is the c a u s e of a b u r i e d high s u p e r i m p o s e d u p o n the c r o s s - f a u l t zone. The a c c o m m o d a t i o n zone conc e p t c a n be regarded as a special c a s e of a transfer fault zone. B o r d e r strike-faults in t h e E a s t African Rift frequently are curvilinear in plan view, as u n d e r -
lined b y R e y n o l d s & R o s e n d a h l (1984), R o s e n d a h l e t a / . , (1986), E b i n g e r et al., (1987) a n d B o s w o r t h (1987); this f e a t u r e is well illustrated along Lake Malawi o n L a n d s a t - M S S i m a g e r y (Fig. 3A). This s h a p e s e e m s to b e related with listric n o r m a l faults of t h e s p o o n fault type (Chenet & Letouzey, 1983). General characteristics of transfer faults E x t e n d i n g Gibbs' definition, it c a n b e said t h a t t r a n s f e r faults are c r o s s - f a u l t s or fault z o n e s inside a given rift s e g m e n t . T h e s e faults t a k e p a r t of t h e e x t e n s i o n s y s t e m . T h e y have a r a t h e r small horizontal dimension, n o t exceeding 50 km. less t h a n t h e t h i c k n e s s of a c o n t i n e n t a l type lithosphere. For this r e a s o n t h e y have to be c o n s i d e r e d a s u p p e r c r u s t a l f e a t u r e s with no deep ductile c o n t i n u a t i o n . T r a n s f e r f a u l t s do n o t e x t e n d b e y o n d t h e rift system. T h e y t r e n d parallel to a n c i e n t faults, selectively reactivating s o m e of t h e older faults. T h e y generally parallel the direction of relative m o t i o n of c o n t i n e n t a l blocks. T h e y link t h e m a j o r f a u l t s bordering a s y m m e t r i c rifts. The "down to basin" polarity of n o r m a l faulting m a y c h a n g e on opp o s i n g side of t h e t r a n s f e r fault while, in o t h e r cases, t h e r e is only a n offset in t h e position of the m a i n b o r d e r f a u l t s w i t h o u t a polarity reversal. INTRACONTINENTAL T R A N S F O R M FAULT ZONES Tanganylka-Rukwa-Malawi fault zone
The n o r t h w e s t - s o u t h e a s t t r e n d i n g T a n g a n y i k a R u k w a - M a l a w i fault zone is m o r e t h a n 1,000 k m long for 40 to 100 k m wide. e x t e n d i n g from the central p a r t of Lake T a n g a n y i k a to n o r t h e r n Lake Malawi (Fig. 4). This fault zone links the n o r t h e r n s e g m e n t of the w e s t e r n b r a n c h of the E a s t African Rift, c o m p r i s i n g Lakes M o b u t u , E d o u a r d , Kivu a n d N o r t h e r n T a n g a n y i k a , with t h e s o u t h e r n segm e n t (Lake Malawi). This link c o r r e s p o n d s to a left-stepping s y s t e m of the w e s t e r n a r m of t h e rift b u t t h e Cenozoic n o r t h w e s t striking faults have dextral d i s p l a c e m e n t s a n d t h e e x t e n s i o n direction t r e n d s n o r t h w e s t e r l y (Chorowicz et al., 1983; Chorowicz et al., 1987). This s t r u c t u r e t h e n is r e g a r d e d as a n i n t r a c o n t i n e n t a l t r a n s f o r m fault zone (Chorowicz & Mukonki, 1979; Kazmin, 1980). The faults are a r r a n g e d in a n "en echelon" pattern. a n d define folded s t r i p s w h i c h form large anticlines a n d s y n c l i n e s c o r r e s p o n d i n g to hills exposing t h e P r e c a m b r i a n b a s e m e n t a n d low-lying c o u n t r i e s filled with Cenozoic s e d i m e n t s (Fig. 5). The faults a s s o c i a t e d w i t h t h e folds give t h e b a s i n s a c o m p l i c a t e d p a t t e r n as s h o w n b y R o s e n d a h l et a/., (1986) a n d R o s e n d a h l (1987) in central Lake Tanganyika. The folds allow this tectonic zone to have a n overall right-lateral t r a n s c u r r e n t motion. The t r o u g h s are elongated along t h e NW-SE direction, as s e e n in the Lake R u k w a a n d S o u t h e m
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Fig. 3A Structural sketch m a p of the Malawi rift, from Landsat-MSS imagery. Note the northeast trending dashed lines inferred to represent z o n e s of transfer faults. 27, 29, and 36: sites corresponding to structural analysis. NBB: Nkhota Bay Basin; NB: N k h o t a k o t a Basin. 1: Neogene volcanics; 2: Neogene sediments; 3: Karroo deposits; 4: Precambrlan basement; 5: stratigraphlc contact; 6: lithologic mark; 7: ring structure; 8: large fault; 9: fault; 10: inferred fault; 11: linear; 12: attitude of beds or foliation; 13 to 15: local stress field from slip indicators on fault planes; 13: trends of horizontal c o m p r e s s i o n and extension; 14: trend of horizontal extension; 15: trend of latest extension. Fig. 3B Structural analysis in sites 27, 29 and 36, in Lake Malawi area. Schmidt diagrams, lower hemisphere.
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Fig. 5- Block diagram outlining the gross deformation of the Tanganyika-Rukwa-Malawi transform fault zone. Note folded strips formed between the fault segments and step faults crossing the basins at high angle of the maln faults direction at the northwestem end of Lake Rukwa and southeastern end of Lake Tanganyika. T a n g a n y i k a areas; t h e NW-SE t r o u g h s are n o t c u t b y NE-SW t r a n s f e r faults (Rosendahl etal., 1986). Lake R u k w a c a n b e regarded a s a c o m b i n a t i o n of b o t h a large syncline affecting t h e b a s e m e n t a n d of a pull-apart. S t e p f a u l t s c u t t i n g the g r a b e n at high angle c a n b e o b s e r v e d fom L a n d s a t - M S S s p a c e i m a g e r y in t h e s o u t h e a s t e r n e n d of Lake T a n g a n y i k a a n d in t h e n o r t h w e s t e r n e n d of Lake R u k w a . T h e s e faults curve in p l a n view. This Cenozoic t r a n s f o r m zone is s u p e r i m p o s e d u p o n a n U b e n d i a n - K i b a r i a n deep fault zone u n derlined b y b l a s t o m y l o n i t e s a n d ultramylonites. t r e n d i n g t h e s a m e direction (McConnel, 1972). The m a p on Fig. 6A s h o w s t h a t t h e n o r t h w e s t t r e n d i n g faults are strongly reactivated, b u t o t h e r fault directions t r e n d i n g differently a n d covered d i s c o n f o r m a b l y b y late P r e c a m b r i a n s e d i m e n t s , well e x p o s e d in t h e Kipfli area, are n o t reactivated. In s o m e areas, a s K a r e m a for i n s t a n c e , a Cenozoic Riedel-type p a t t e r n o c c u r s (Fig. 6B). A s w a fault z o n e The n o r t h w e s t t r e n d i n g A s w a i n t r a c o n t i n e n t a l t r a n s f o r m fault zone (Fig. 4) e x t e n d s along strike for s o m e 1,000 k m from t h e n o r t h of Lake M o b u t u to t h e Gregory Rift a n d c o n t i n u e s s o u t h e a s t w a r d s for a n o t h e r 1,000 k m to t h e Indian Ocean, n o r t h of t h e K e r i m b a s g r a b e n (Mougenot et al., 1986; Chorowicz etal., 1987). F a u l t s of several h u n d r e d kilometers in length d i s p l a y an, "en echelon" patt e r n inside t h e zone (Vidal, 1987).
The Aswa fault (Almond, 1969) f o r m s t h e nort h e a s t e m limit of t h e W e s t e r n Rift a n d is c o n t i n u e d along strike s o u t h e a s t e r l y b e y o n d t h e Elgonvolcano b y the Nandi fault. The A s w a fault zone c u t s t h e Gregory Rift w h i c h a b r u p t l y c h a n g e s direction in t h e c r o s s i n g area, swinging s o u t h e a s t w a r d s from a n o r t h - s o u t h strike to parallel the A s w a fault orientation. S o u t h of this area, t h e E a s t e r n swings b a c k to n o r t h - s o u t h strike a n d finally e x t e n d s s o u t h w a r d s into the n o r t h T a n z a n i a n divergence. C o n s e q u e n t l y , the A s w a fault zone m a y b e c o n s i d e r e d as a m a j o r tectonic feature of t h e Rift, allowing t h e right-lateral s t e p o v e r betw e e n the w e s t e r n a n d the e a s t e r n b r a n c h e s of the E a s t African Rift. S t r u c t u r a l a n a l y s i s along t h e fault s c a r p s in this a r e a (Chorowicz et al., 1987) i n d i c a t e s t h a t t h e m a i n Miocene e x t e n s i o n direction is horizontal, trending n o r t h w e s t w a r d s . C o m p r e s s i o n m a y b e horizontal or vertical, d e p e n d i n g on location a n d age. This fault zone h a s f u n c t i o n e d a s either a compressive (transpressional) or extensional (transtensional) s y s t e m during Miocene times. Volcanic a n d tectonic activity along t h e AswaNandi fault line h a s o c c u r r e d since early Miocene times (Baker et al., 1971) b u t seismic d a t a s h o w little activity. Along the Nandi fault scarp, one c a n observe s u b - h o r i z o n t a l striations t h a t s e e m to b e young. T h e y indicate left-lateral m o t i o n s with a small dip-
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Fig 6A Structural sketch map of southern Tanganyika. Note Precambrian faults disconformably overlain by Bukoban sandstone, along the eastern shore of Lake Tanganyika, in the Kipili area. F1 to F5 are Neogene faults cutting the Precambrian fault system. 1: Neogene sediments; 2: Precambrian rocks; 3: stratlgraphic contact; 4: inferred stratigraphic contact; 5: normal fault (with indication of the downthrown compartment); 6: inferred fault; 7: lithologic mark; 8: dips. PKm: middle Proterozoic; PKR: Russian; PKB: Burundian; PKt: Bukoban (terminal sandstones); PC: Permo-Carbonfferous; Ka: Karroo. Fig. 6B Riedel-type patten close to the village of Karema (Central Lake Tanganyika), comprising faults and tension Joints. slip c o m p o n e n t . The right-lateral stepover betw e e n t h e e a s t e r n a n d the w e s t e m b r a n c h e s of the E a s t African Rift, linked b y the left-lateral AswaNandi fault zone, m a y b e t h e n interpreted as a n int r a c o n t i n e n t a l t r a n s f o r m fault zone, active during the early p a r t of Neogene. This s t r u c t u r e is poorly u n d e r s t o o d owing to a lack of detailed w o r k a n d awaits f u r t h e r analysis. The Aswa-Nandi fault zone is regarded (Sanders, 1965; Vail, 1983) as a n i m p o r t a n t tectonic feature during the M o z a m b i q u i a n orogeny. It m a y corresp o n d to a lateral t h r u s t r a m p of this age, linked with a frontal t h r u s t r a m p reworked with back-slip m o v e m e n t in the n o r t h e r n Gregory Rift, along the
Elgeyo e s c a r p m e n t (Bosworth, 1987). During the early activity of t h e rift, e a s t of the Gregory Rift, the A s w a - N a n d i fault zone c o n t i n u e s along strike s o u t h e a s t w a r d s b y a set of , e n echelon, faults a s s o c i a t e d with v o l c a n o e s s u c h a s the Kilimanj aro. It c o n t i n u e s f u r t h e r s o u t h e a s t w a r d s into t h e T a n z a n i a n c o n t i n e n t a l shelf w h e r e it links several offshore g r a b e n s including t h e Cenozoic K e r i m b a s basin. Field evidence of small r e c e n t right-slip d i s p l a c e m e n t s have b e e n o b s e r v e d e a s t of t h e Gregory Rift. Large o p e n t e n s i o n g a s h e s , corresponding to lines of volcanoes, t r e n d northeasterly. T h e s e f e a t u r e s are easily s e e n on L a n d s a t - M S S satellite imagery. This fault zone m a y b e regarded
Transfer and transform fault zones as a diffuse right-lateral transform fault zone forming the left-lateral stepover which links the eastern b r a n c h of the East African Rift with the Kerimbas graben. It is roughly superimposed u p o n a Mozambiquian tectonic discontinuity. General characteristics of intracontinental t r a n s f o r m f a u l t z o n e s in t h e rift s y s t e m The above examples allow several general points to be m a d e about intracontinental transform fault zones in a rift system. They appear to be long s t r u c t u r e s extending over h u n d r e d s of kilometers. They are m a d e of a zone of faults trending at a high angle to the rift direction. This zone is almost parallel to the regional extension direction and links distinct segments of the rift. Both compressive (transpressional) and extensional (transtensional) s t r u c t u r e s m a y develop depending on the relative orientation of the faults and the direction of movement of separating continental blocks. Although not true transform faults of the oceanic type, the geometry and kinematics of these intracontinental fault zones are similar. True oceanic transform faults t h a t would appear in the future forming ocean, m a y be originated from these intracontinental transform fault zones. It is relevant t h a t they are superimposed u p o n very large ancient s t r u c t u r e s that have suffered remobilization. They correspond with very large discontinuities of the crust. Supposing that the longer the discontinuities are, the deeper they could be, several h u n d r e d s kilometers long discontinuities are likely to represent lithospheric scale steep dipping anisotropy. INTERPRETATION AND DISCUSSION R e a c t i v a t i o n o f a n c i e n t s t r u c t u r e s in t h e rift Transfer faults are relatively short faults that trend at a high angle to the orientation of the m a i n rift for some tens of kilometers. They seem to reactivate ancient faults of local importance participating the b a s e m e n t grain. Transform fault zones are m u c h longer, running over several h u n d r e d s or t h o u s a n d s of kilometers and they appear to rework deep lithospheric discontinuities. Previous a u t h o r s have emphasized the importance of ancient s t r u c t u r e s in the formation of the Afro-Arabian Rlft System (King, 1970; McConnel, 1972, 1978, 1980; Mohr, 1983; Fairhead & Anderson 1977; Rosendahl etal., 1986; Daly etal., 1987). For instance, the w e s t e m branch of the rift seems to avoid the Tanzanian shield and roughly follows the Ubendian and Kibarian orogenic belts that wrap a r o u n d the cratonic areas. But locally, it is not strictly superimposed upon the major accidents. In the n o r t h e r n segment of the w e s t e m AES 8-2/4---F
211
arm for example, the northwest regional structural trend of the Kibarian orogenic belt is often at a high angle to the rift northeast trend. In the n o r t h e m Tanganyika region, the Kibarian folds and t h r u s t s swing from east-west to the northeast trend, and are cross-cut by the n o r t h - s o u t h rift trend. In the s o u t h e r n segment, along Lake Malawi, basement control on the location a n d geometry of the major rift faults is due to small faults and to foliation and gneissbandlng trend. In the central segment ofthe Western Rift, the major faults are superimposed upon steep dipping ductile fault zones of lithospheric scale. It finally appears that the overall rift system follows ancient orogenic suture belts that correspond to zones of w e a k n e s s between the major cratonic areas, repeatedly reworked in different deformations. Several authors (Wernicke 1981; Hermance, 1982; Wernicke and Burchflel, 1982; Allmendinger et aL, 1983; Anderson et a t , 1983; B r u n & Choukroune, 1983) believe that graben-like segm e n t s of a rift m a y be reworked from frontal t h r u s t ramp with back-slip movements. The major listric faults would curve down to a zone of brittle-ductile transition and to a low-crustal d e t a c h m e n t which could be the c r u s t - m a n t l e trandition zone (Hermance, 1982). This pattern m a y be argued in the n o r t h e r n part of the Gregory Rift (Bosworth, 1987) in the Elgeyo scarp area, and in the n o r t h e m part of the Malawi rift where a similar structure occurs. The transform fault zones reactivate ancient s t r u c t u r e s supposed to be of the type of lateral t h r u s t ramps (Daly etal., 1987). Aspects of this relation and new interpretations of ancient tectonics Waft, 1983; Daly, 1986) have lead to the suggestion that the f u n d a m e n t a l s t r u c t u r e s which are reactivated could be suture zones. This interpretation could explain their great length. They could be ancient plate boundaries where collision resulted in frontal and lateral t h r u s t ramps. As ancient plate boundaries, these major discontinuities m a y be considered not only in the crust but also in the whole lithosphere. DISCUSSION
Models dealing with a dip-slip reworking of frontal t h r u s t ramps for the formation of typical rift segments have to be confronted with the transfer faults geometry. Frontal t h r u s t r a m p s imply a t h r u s t polarity. Rift a s y m m e t r y also corresponds to a polarity, the major bordering fault being generally considered as the frontal t h r u s t ramp reworked by dip-slip motion. But m a n y rift segm e n t s are cross-cut by transfer faults associated with a polarity reversal. In addition to the examples presented here, rift a s y m m e t r y has been observed to alternate along rift axes in other situations (Bally, 1982; Bosworth, 1985; Lister et al.,
212
J. CHOROWICZ
1986; Rosendahl etal., 1986). The spacing of these changes is small compared to the size of ancient suture zones. If the rift polarity easily reverses while superimposed along the frontal t h r u s t r a m p s of the suture zones, this suggests that the direction in which the underlying d e t a c h m e n t s dip is likely to alternate as well. Contrarily, it is unlikely that the polarity of a s u t u r e zone would change every 70 km; it can reverse on each end of large and several h u n d r e d s
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kilometers long crustal faults (Daly, 1986). These polarity reversals along the m a i n ~ segments on each side of transfer faults m a y be interpreted as a shallow crustal p h e n o m e n o n correponding to a rather "symmetrical" deeper discontinuity that m a y be located below the deep-crustal d e t a c h m e n t zone (Chen & Molnar, 1983). The best arrangem e n t to have a "symmetric" deep discontinuity is a steep lithospheric b o u n d a r y (Fig. 7A). Above a deep steep lithospheric ductile discontinuity rela-
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Transfer and transform fault zones ted with a s u t u r e zone, t h e u p p e r c r u s t a l s t r u c t u res in t h e brittle zone c a n easily be u n d e r s t o o d as h a v i n g a n indifferent polarity t h a t reverses frequently. It m a y locally also rework frontal t h r u s t r a m p s or b a c k - t h r u s t r a m p s located above the low-crustal d e t a c h m e n t zone, in t h e brittle zone (Fig. 7B). A steep a t t i t u d e for a lithospheric d i s c o n t i n u i t y of large scale is likely to o c c u r along s u t u r e zones. Relative m o t i o n of converging plates c a n be at a n y angle, n o t p a r t i c u l a r l y at high angle to the plates b o u n d a r y . W h e n t h i s h i g h angle occurs, it is s u p p o s e d to originate collision or hypercollision of c o n t i n e n t a l lithospheres. The m o s t f r e q u e n t patt e m is oblique-slip relative m o v e m e n t along s u t u r e zones. In t h i s case, regardless of the style of c r u s t a l tectonics, c h a r a c t e r i z e d b y b o t h t h r u s t i n g a n d t r a n s c u r r e n t m o v e m e n t s , t h e deep lithospheric c o n t a c t is m o s t likely to have a steep dip (Mattauer & CoUot, 1986). The m o s t f r e q u e n t a t t i t u d e for the deep p a r t of a s u t u r e zone is t h e n to have steep dip a n d it m a y t u r n d o w n w a r d s m o r e vertical in depth. CONCLUSIONS
T r a n s f o r m fault zones, as well as rift b a s i n s , in the Afro-Arabian Rift System, were controUed during initiation of the rift by a p r o n o u n c e d m e c h a n i c a l a n i s o t r o p y of t h e b a s e m e n t . T r a n s f o r m fault z o n e s c o r r e s p o n d to a n c i e n t steep c r u s t a l lateral r a m p s c h a r a c t e r i z e d b y mylonitic rocks types. Typical graben-like rift b a s i n s are often c o n f o r m to a n c i e n t tectonic belts a n d s u t u r e zones. T r a n s f e r faults rework local f a u l t s of the u p p e r c r u s t t h a t participate t h e g r a i n of t h e b a s e m e n t . Both t r a n s f e r a n d t r a n s f o r m fault zones are b r o a d l y paralleled by t h e m a i n e x t e n s i o n direction d u r i n g the earliest stages of the rift formation. The t r e n d of t r a n s f e r f a u l t s are at high angle to t h e rift length. The c r o s s - s e c t i o n a s y m m e t r y of b a s i n s f r e q u e n t l y reverses on opposite sides of t r a n s f e r faults. This s t r u c t u r a l style is b e t t e r explained b y reactivation of steeply dipping lithospheric ductile d i s c o n t i n u i t i e s r a t h e r t h a n reactivation of dip-slip frontal t h r u s t r a m p s only. The latter m a y o c c u r b u t n o t as t h e m a i n p h e n o m e n o n . S u c h deep lithospheric d i s c o n t i n u i t i e s are inferred to be a n c i e n t s u t u r e z o n e s w h i c h s e e m to control s u b s e q u e n t c r u s t a l failure resulting in c o n t i n e n t a l breakup. REFERENCES
Allmendinger, R.W., Sharp, J.W., Tish, D.V., Serpa, I., Brown, L., Kaufman, S., Oliver, J. and Smith, R.B. 1983. Cenozoic and Mesozoic structure ofthe eastern Basin and Range province, Utah, from COCORP seismic-reflectiondata. Geology. I I , 532-536.
213
Almond, D.C. 1969. Structure and metamorphism of the basement complex of North-East Uganda. Overseas Geol. Min resources. I0, 146-163. Anderson, I~F., Zoback, M.I. and "lhomson, G.A. 1983. Implications of selected subsurface data on the structural form and evolution of some basins in the northem Basin and Range province, Nevada and Utah. Geological Society of Amerlca Bull. 94, 1055-1072. Baker, B.H., Williams, L.A.J., Miller, J.A. & Fitch, F.J. 1971. Sequence and geochronology of the Kenya rift volcanics. Tectonophysics, I I , 191-215. Bally, A.W. 1982. Musing over sedimentary basin evolution. RoyaI Society ofLondon PhilosophicaI Transactlons, set. A, 305, 325-328. Bloomfield, K. 1966. AmaJor eastnortheast dislocation zone in central Malawi. Nature, 211, 612-614. Bosworth, W. 1985. Geometry of Propagating rifts. Nature. 316, 15, 625-627. Bosworth, W. 1987. Off-axis volcanism in the Gregory rift, eastAfrica: implications for models of continental rifting. Geology. 15, 397-400. Briden, J.C., Whitecombe, D.N., Stuart, G.W., Fairhead, J.D., Dorbath, C. and Dorbath, L. 1981. Depth of geological contrast across the West African craton margin. Nature. 292, 123-128. Brun. J.P., Choukroune, P. 1983. Normal faulting, block tilting and decollement in a stretched crust. Tectonics. 2, 4, 354-356. Chen, W.P., Molnar, P. 1983. Focal depths ofintracontinental and intra-plate earthquakes and their implications for the thermal and mechanical properties of the lithosphere. J. Geophys. Res. 88, 85, 4183-214. Chenet, P.Y. & Letouzey, J. 1983. Tectonique de la zone compressive entre Abu Durba et Gebel Mezzazat (Sinai, Egypte), dans le contexte de l'evolutlon du rift de Suez. Bull Centr. Rech. E.x'pl.-Prod. Elf Aquttalne. 7, 1,201-215. Chorowicz, J., Mukonkl, M. na B. 1979. Lineaments anciens, zones transformantes recentes et geotectonique des fosses de l'Est africain, d'apres la teledetection et la microtectonique. Mus. Roy. Afr. centr., Tervuren (Belg), Dept Geol. Min, Rapp. ann. 1979, 143-167. Chorowicz, J., Le Foumier, J., Le Mut, C. Richert, J,P., Spy-Anderson, F.L., Tlercelin, J.J. 1983. Observation par teledetectlon et au sol de mouvements decrochants NW-SE dextres dans le secteur transformant Tanganyika-Rukwa-Malawi du rift est africain. C.R. Acad. Sc. Paris 296, II, 997-1002. Chorowlcz, J., Le Fournier, J. Vidal, G. 1987. Model of the rift development in Eastern Africa. Geological Journal. 22, Thematic Issue,"Afrlcan Geology Reviews", P. Bowden and J. Kinnairds (eds), 495-513. Chorowicz, J., Henry, C. et Lyberis, N. 1987. Tectoniques superpos6es synsedimentaires des secteurs des golfes de Suez et d'Aqaba. Bull. Soc. g~ol. France. (8), Ill, n°2, 223-234. Crossley, R. & Crow, M.J. 1980. The Malawi Rift. In: Geodynamic evolution of the Afro Arabic Rift System. Roma: Accademia Nazionale dei Lincei (Atti dei Convegn Lincel, 47). 77-87. Daly, M.C. 1986. Crustal shear zones and thrust belts: their geometry and continuity in Central Africa. Phil. Trans. R. Soc. Loncl., A. 317, 111-128.
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