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Oriental Moroccan Neogene volcanism and strike-slip faulting
J'ouma/ofat.~am Earth Sdemces,VoL 11, No. 3/4, pp. 273280, 1990. in ~ Britain
0899-5362/90 $3.00 + 0.00 ~rg~non l ~ u pie
Oriental Moroccan Neogene volcanism and strike-slip faulting L. Art BRAmM*and P. CHOTU¢** *D(~panmnmt de G~ologie, Universitd Mohamed V, BP 1014, Rabat, Maroc **D~partement de G6otectonique, UA 215 du CNRS, Universit8 Pierre et Marie Curie, T.25-26, El, 4 place Jussieu, 75252 Paris cedex 05, France Abstract - The microtectonic analysis of faults and of volcano-phtonic associations in the Eastern Rif shows that the distribution of a large number of Miocene to (~_m_t_,~nmTvolcanic emission points is tectonically controlled. Their lecalisation, p~allel to the strike-slip faults orientated N 070-090, N 04(0050 and N-S suggests that the volcanic emissions could have occurred along tension gashes making an engle of 40-50 ° to the initial strike- slip plane. The new orientation, of the principal compressional stress to N 040-000 during upper Miocene, reactivates faults striking N 070, N 040-050 and N-S; the volcanic emissions with a NW-SE, NE-SW and NS directions, occur along gashes developed in this systmn. During the piio-Quaternary, the counterclockwise rotation of the main stress to NW-SE, initiate strike-alip movemem along the N-S, N 090 and N 120-140 fanlts; the volcanic emissions take place on ,en ~chelon~ tension gashes developed in this system. In this area, subjected since 10 M.s. to a compressional regime, the crustal mega shear zones develop scattered fissures and tension gashes which initiate partial fusion at depth by pressure release. The gaps in volca~dc activity may correspond to the changes in the stress field. R~sum~ - L'analyse microtectonique des failles et de leur relation avec l'activit~ magmatique dens le Rif oriental, montre le contr61e tectonique de la r~partition spatiale d'un grand hombre d'&iifices volcaniques. La disposition des Epp&eils volcaniques le long des d~rochements N 70-90, N 40-50 et ]q-S, s u g g ~ que les ~missions ont pu s'effecmer sur des fentes de temion faisant un angle de 40 ~t50° avec ces plans de cisaillemems initiaux. La r~orientation an Miocene supdrieur de la contrainte principale en compression vers ]q-S r6tctive les accidents de direction N 70, N 40-50 et N-S et les ~missions volcaniques de direction N'W-SE, NE-SW et NS se placent sur les fentes ddveloppees dens ce syst~me. Au Plio-Quatm'naire, la rotation anti-horaire de cette contrainte vers le NW-SE amorce le jeu dOcrochant des accidents N-S, N 90 et N 120-140, et les ~nissions volcanklues se placent aiors sur des fentes de tension on ~helon N-S et NW-SE. Dans cette r~gion soumise depuis 10 millions d'mm~es ~tun r~gime de compression, les grands cisaillements crustaux d ~ , v e l o ~ t des fissures diss~min~.es et des fentes de tension qui mnorcent la fusion par chute de pression. Les p~riodes d'arr~t de l'acfivit6 volcanique pourraient correspondre aux p(~'riodesde changement de direction des contraintes.
(1976) and Pugs (1980) as a p h e n o m e n o n linked to subduct/on. During the Neogene and Quaternary, important Hernandez (1983) h a s shown t h a t this volcartism volcanic activity (Hernandez, 1983; Berrahma, located In u p p e r Miocene sedimentary basins, 1982; H a r m a n d et Moukadiri, 1986) developed In does not display any chronological or geochemical a zone whose evolution is characterized by the polarity which permits to corroborate this hypooccurrence of major strlke-slip faults striking NE- thesis. This volcanism is thought rather to be SW. From Agadir, these faults follow the Haut and related to the working of a transverse strike-slip Moyen Arias and connect up with the faults of the system, situated on the border of the Iberian and oriental Rif (Nekor) by a n " e n 6chelon" fault system. African plates, putting In contact two different They cross the Alboran sea and llnk up with the crusts, a n d affecting the entire lithosphere (Bands oriental end of the Betic Cordillera (Hatzfeld, 1977; and Ansorge, 1980). Aft Brahim, 1983-85). The aim of this paper is to provide structural In relation to plate tectonics, the Neogene a r g u m e n t s In favour of the latter hypothesis, based m a g m a t i s m In the oriental Rif and In the East of on microtectonic analyses of fault patterns and Betic Cordillera has b e e n Interpreted b y A r a n a and associated Intrusions which have been the feeders Vegas (1974), Delarue and Brousse (1974), Bellon to the oriental Rlf volcanism. Field data (trend of INTRODUCTION
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faults, s e n s e of m o v e m e n t indicated on slickenside lineations, chronology) a n d t r e a t m e n t s for the d e t e r m i n a t i o n of m e a n principal directions of s t r e s s e s (Angelier, 1979, 1984) have b e e n described in t h e earlier p a p e r s (/kit B r a h i m , 1983; A~t B r a h i m et aL, 1983b, 1984) a n d we c o n s i d e r it proper to r e p r i n t only t h e resulting s y n t h e t i c d i a g r a m (Fig. I). MAIN TECTONIC EVENTS F R O M T O R T O N I A N 1"0 P R F ~ E N T
N o r t h - E a s t e m Morocco, s u b j e c t since t h e u p p e r C r e t a c e o u s to a compressive s t r e s s regime, displays since t h e T o r t o n l a n volcanic activity i n t i m a t e l y r e l a t e d to t h e s e d i m e n t a r y a n d tectonic evolution of t h e Neogene p o s t - n a p p e b a s i n s of B o u d i n a r , Kert a n d Guercif. These s e d i m e n t a r y a n d eruptive bodies have registered a s u c c e s s i o n of tectonic events (A~t B r a h i m et Chotin, 1983b, 1984): * The first, w h o s e s t r e s s field is c h a r a c t e r i z e d by a m a x i m u m compressional stress ~ trending N 040 (a a horizontal), is responsible for the origin a n d evolution of T o r t o n i a n b a s i n s along strike-slip f a u l t s t r e n d i n g N 070-090 (Kebdana. North Gareb, T e m s a m a n e ) N 0 4 0 - 0 5 0 (Nekor, T e m s a m a n e , C h a r r a n a ) a n d N-S (Boudinar) (Fig. 2). A c a l c - a l k a l i n e v o l c a n i s m is s u p e r i m p o s ed at Ras T a r f a n d at t h e Trois F o u r c h e s cape. TEPHROC~IRONOLOGY as
* The second, from late T o r t o n i a n a n d d u r i n g t h e Messinian, is c h a r a c t e r i z e d by a m a x i m u m compressive s t r e s s a ~, t r e n d i n g N-S: it is t h e late T o r t o n i a n event of Gufllemin (1976), also recorded by De L u c a (1978) in t h e K e b d a n a M o u n t a i n s a n d b y Hervouet (1985) in t h e Gareb area. This first c h a n g e in t h e direction of the palaeostresses i n d u c e s a reverse m o v e m e n t along the N 090 f a u l t s (Kebdana, Gareb) a n d a strikeslip m o v e m e n t on N 070 f a u l t s (Temsamane, Guercii], N 0 4 0 - 0 5 0 f a u l t s (Nekor, C h a r r a n a , Beni Bou Ifrour) a n d N-S (Boudinar, B a s Nekor). We also observed s o m e related E-W folding (Tarj at anticline). D u r i n g t h i s period, m o v e m e n t along N 070, N 0 4 0 - 0 5 0 a n d N-S f a u l t s led to the opening of the B o u d i n a r , Kert a n d G u e r c i f b a s i n s a n d controled the s e d i m e n t a t i o n (Ait Brahim, 1985). A set of N-S t r e n d i n g n o r m a l f a u l t s w a s created b y a n E-W e x t e n s i o n (a I a n d ~3 horizontal). These f a u l t s affected all the p o s t - n a p p e s b a s i n s in E a s t Morocco (Boudinar, Kert, Beni Said s u b s t r a t u m . Trois F o u r c h e s cape a n d Gu ercil]. This event is c o n t e m p o r a n e o u s with t h e s h o s h o n i t i c v o l c a n i s m in G o u r o u g o u a n d Guflliz (Hernandez, 1983). * The third tectonic event is c h a r a c t e r i z e d by a new counterclockwise r o t a t i o n of t h e s t r e s s field; the new m a x i m u m compressive s t r e s s now t r e n d s N 140-160 (Ait B r a h i m a n d Chotin, 1984). The N 090 a n d N 120-140 f a u l t s have now a dextral DIRECCION OF STRESS
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Oriental Moroccan neogene volcanism and strike-slip faulting
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Fig. 2. Structural map of Eastern Rif (Morocco). Distribution of volcanism in the Neogene basins. A, Ras Tarf; B, Trois Fourches cape; C, Gourougou, D, Beni Bou Ifrour. 1, Neogene and Quaternary; 2, Neogene and Quaternary volcanism; 3, Rif unit of Ketama; 4, Foreland; 5, Quaternary volcanism of Beni Bou Ifrour; 6, Granodiorite of Beni Bou Ifrour. strike-slip c o m p o n e n t , a n d t h e N-S a sinistral one. The N 0 4 0 - 0 5 0 striking faults are reactivated a s sinistral strike-slip faults or reverse f a u l t s with a sinistral c o m p o n e n t . We also o b s e r v e d a s s o c i a t e d N 0 7 0 to N 0 8 0 trending folds in K e b d a n a a n d G a r e b area. A p e r m u t a t i o n b e t w e e n a I a n d a 2 related to t h e opening of t h e b a s i n s c r e a t e d n o r m a l f a u l t s (Boudinar). This c o m p r e s s i v e s t r e s s (N 140-160) h a s b e e n o b s e r v e d everywhere in North Morocco: in t h e B o u d i n a r , Kert a n d N a d o r b a s i n s (Guillemin et H o u z a y , 1982; Aft Brahim, 1985), in A r b a a de Taourirt (Frizon de Lamotte, 1979), in t h e K e b d a n a (De L u c a or. com., Ikit Brahim, 1986), in t h e G a r e b (Hervouet, 1985) in t h e G u e r c f f b a s i n (CoUetta, 1977) in t h e S u b r i f m a s s i v e s (Aft B r a h i m a n d Chotin, 1983a) in t h e "sfllon s u d rifain" (/kit B r a h i m a n d Chotin 1883b, 1984) in t h e Middle Atlas (/kit B r a h i m , 1985) in t h e H a u t Atlas a n d j o i n t b a s i n s ( D u t o u r a n d Ferrandini, 1985; Aft B r a h i m , 1985). During this period the n a t u r e of t h e v o l c a n i s m w a s alkaline in t h e NE Morocco ( H e m a n d e z , 1983) a n d in the Middle Atlas ( H a r m a n d a n d Moukadiri, 1986).
To c o n c l u d e t h e a n a l y s i s of t h e g e o m e t r y a n d s u c c e s s i v e m e c h a n i s m s of fault t e c t o n i c s s h o w s the relationship t h a t exist b e t w e e n t h e field of palaeostresses and the genesis and kinematic evolution of t h e p o s t - n a p p e s b a s i n s in t h e oriental Rif. Within t h e T r a n s - N e k o r A l b o r a n m a j o r strike slip fault s y s t e m , we o b s e r v e d several p o p u l a t i o n s of faults: N 0 7 0 - 0 9 0 , N 0 4 0 - 0 5 0 , N-S a n d N 120140. The p o s t - n a p p e s b a s i n s are genetically linked to this fault s y s t e m in a c o m p r e s s i v e regime (a] a n d a 3 horizontal) with t e m p o r a r y peri~iutations b e t w e e n a~ a n d a 2. The c o u n t e r c l o c k w i s e rotation of t h e p a l a e o s t r e s s field, f r o m T o r t o n i a n to Present, g u i d e d t h e s e d i m e n t a t i o n b y moving t h e acc u m u l a t i o n z o n e s a n d b y developing a n active s u b s i d e n c e along t h e strike-slip faults (N 0 7 0 - 0 9 0 , N 0 4 0 - 0 5 0 , N-S), t h a t s u c c e s s i v e l y replayed with time displaying a n i m p o r t a n t vertical c o m p o n e n t . The n e o t e c t o n i c evolution h a s b e e n characterized b y splitting into n u m e r o u s b l o c k s showing a l t e m - a t i n g h o r s t a n d t r o u g h s , with t e m p o r a r y blocking u p a n d r o t a t i o n s in a s t r e s s regime w h e r e compression and extension are associated simultaneously.
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OUTLINE OF THE MAIN VOLCANIC CENTRES R ~ Tarf v o l c a n o : 1 3 , 0 t o 12,4 M.a. (Hernandez and Bellon, 1985). The calc-alkaline stratified breccias of Ras Taft volcano are cut b y metric dykes of similar composition (Bourdier, 1986). In J b e l Hadid area (Fig. 2A), we observed dykes trending N 0 1 0 - 0 4 0 and N 120-160, crossing the Throughout s u b s t r a t u m and the breccias. Dykes with s a m e direction outcrop in Sidi Chal'b area. In Ajdir area (Fig.2A), N-S dykes cross cut the breccia. T h u s the intrusions which have fed the breccias sits occur along two main directions: NE-SW and NW-SE. Fracturing which affects the Ras Taft volcano shows f o u r directions: N-S, N 120-140, N 0 7 0 and N 040.
and the second between the Bokoya a n d Setolazar quarries. West o f J e b e l Ouiksane, we observe a sill that trends NE-SW, several kilometres long, which h a s yielded a radiometric age of 7,3 % 1,5 M.a. (Moni6 e t al., 1984). Volcanoes in Monts Amar, Afra and Loma Mokhtar trend N-S to N 030. The granodioritic complex of Beni Bou Ifrour h a s b e e n accompanied by a g r o u p of dykes trending in two directions, N,S, NE-SW.
Guercif basin volcanism: 8 t o 4 , 9 M.a. (Hernandez and BeUon, 1985). Tufts and breccias in the Guilliz m a s s i f are cut by a group of domes and dykes trending NE-SW and NW-SE. The western flank of Guilliz el Kebir Mountain partially down-faulted along a N-S fault is m a r k e d by a series of intrusions which have fed t h e more Trois F o u r c h e s cape volcanism: 9,8 M.a. (Her- recent volcanics forming present summits. Hervouet (1985) h a s described a good example of n a n d e z and Bellon, 1985). The volcanic complex of this area shows n u m e r o u s domes and dykes injection along mega-tension g a s h e s developed in outcropping along two main trends: N 30 and N a N 150 m a x i m u m compressive stress system. It 120-140. This is well displayed on the West coast, concerns a small dyke located E a s t of S a k a (Jebel in the Ifri n'Douch Valley and at Tibouda, (Fig. 2B). Tassatfacht) within a N-S fault system; it displays Part of the s t r u c t u r e of this volcano is the result of a sigmoidal shape. Vertical joints of retreat trend successive inJections of m a g m a along open narrow N-S in the centre of the dyke and swing to N 150 at fissures following the two trends mentioned above. the tips. Gmzrougou volcanlsm: 9 to 4 , 6 M.a. (Hernandez and Bellon, 1985). The Gourougou stratovolcano (Fig. 2C) is mainly composed of tufts and breccia (Hernandez, 1983). Oulad Mimoun basin is the principal eruptive centre. N u m e r o u s dykes and domes outcrop on the northwestern flank along the NE-SW line of w e a k n e s s also indicated by an alignment of necks a n d dykes. The Beni Enzar volcanic flow (Fig. 2C) shows several points of emission along a NNE-SSW trending fissure. The Ibou J a j e n e and Jebel Zaouyet crests are formed b y a dykes swarm and intrusions more t h a n several h u n d r e d metres wide. At Azrou Idoudene the whole crest is an emissive centre with a s h zones a n d fumarole (Hernandez, 1983). Near Segangane the tufts are cut b y a trachytic dyke trending NW-SE. At Bajioua several successive intrusions trend N-S. A group of necks and dykes also form the Terehilast crest. The intrusions in Gourougou area t h u s occur along three directions: NE-SW, N-S and NW-SE. Benl Bou Ifrour m a g m a t l s m The granodioritic setting in Beni Bou Ifrour area (Fig. 2D) is accompanied b y several generations of dykes of variable size. The Ibekouyene area shows three successive injections: a main dyke thirty metres wide is cut by two other generations of dykes (Kerchaoui, 1985). Two groups of dykes trend N-S, the first, along the Oued Ouiksane fault
RELATIONSHIP BETWEEN FRACTURATION AND VOLCANISM Intrusions (dykes, alignments of eruptive centres) which feed the lava flows of Northeast Morocco occur along three main directions: NESW, NW-SE and N-S. These volcanic emissions lie parallel to faults trending N 070-090, N 040-050 and N-S that determine the s t r u c t u r e of this area; locally N 120-140 faults are associated. The m e c h a n i s m of magma injection is t h u s related to "en ~chelon" mega-tension gashes making an angle of 40 to 50 ° to the m a i n strikeslip fault planes. Some c a s e s at 90 ° to this direction are related to syn to post deformation (sigmoidal tension gashes) during tectonic reactivation of these faults. Some of these tension g a s h e s will evolve becoming normal faults on the border of the postn a p p e s basins. The direction of the mega tension g a s h e s is related to the direction of the m a x i m u m compressive stress a] and the direction of the inherited faults which replayed at this time. During the Tortonian, the m a x i m u m compressive stress trends N 040 (Fig. 3). The calc-alkaline volcanic emissions trending NE-SW (Ras Taft, Trois Fourches) are injected in "en 6chelon" megatension g a s h e s b o u n d by sinistral N 0 9 0 - 0 7 0 t r e n d ing faults. The volcanic emissions trending N-S are
Oriental Moroccan neogene volcanism and strike-slip faulting
277
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Fig. 3. Strike-slip m o v e m e n t s of faults during N 040 compressive e v e n t and associated extensive structures.
LATE-TORTONIAN,
MESSINIAN
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so..
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injected in favour of sinistral strike-slip movement of N 040-050 faults. At the e n d of the Tortonian and during the Messinian, the direction of m a x i m u m compressive stress (a i horizontal) b e c a m e N-S (Fig. 4). It closes the mega-tension gashes a n d the basins developed by m o v e m e n t of the N 090 sinistral faults; these faults become reverse. The shoshonitic volcanic emissions a n d the Beni Bou Ifrour granodiorite trending NE-SW (Gourougou, GuiUiz, Beni Bou Ifrour), N-S (Gourougou, Beni Bou Ifrour) and NWSE (Guilliz, Gourougou) were injected in megatension gashes developed during the sinistral transc u r r e n t m o v e m e n t of N 070, N 040-050 and N-S faults. From the end of Messinian to Present, there is a counterclockwise rotation of the m a i n compressive stress from N-S to N 140-160 (Fig. 5). The NWSE trending alkaline emission centers (Guercif,
Fig. 5. Strlke-slip and reverse m o v e m e n t s of faults during N 140-160 compressive event and associated extensive structures.
Gourougou) developed in tension g a s h e s created by the dextral strike-slip m o v e m e n t along N 090 faults and by the stnistral offset of N-S faults. The N-S trending volcanic events (Guilliz, Beni Bou Ifrour) were developed in tension gashes created by a dextral play of N 120-140 faults; but sometimes, the compression c a n be absorbed in blocking up zones along N 060-070 faults, by the development of open fissures (Ait Brahim a n d Chotin, 1986; H a r m a n d a n d Moukkadiri, 1986) regarded as tension gashes parallel to a i (Tapponnier et aT., 1977) (Fig. 5). This system works as long as the s l direction maintains a N 140-160 trend. This m e c h a n i s m can be extended to Spain and to the Canaries islands. Nevertheless, the volcanic intrusions are dynamically easier along the N 090, N 040-050, N-S and N 120-140 faults. THE RAS TARF AND GOUROUGOU VOLCANISM AND THE AGE OF ALPINE METAMORPHISM IN T H E EXTERNAL RIF
Ages of 12 to 13 M.a. and 9 M.a. obtained respectively in Ras Tarf and Trois Fourches cape (Hernandez and Bellon, 1985) led Gufllemin and Houzay (1982) a n d Bourdier (i 986) to reconsider the age ofvolcanism as being prior to the Boudinar basin; however, these ages are not in agreem e n t with Ar39/Ar4° dating of the last syntectonic metamorphic event affecting the Ketama and T e m s a m a n e Units which h a s yielded ages of 7,3 to 8,2 M.a. (Moni6 et aT., 1984). We consider t h a t the micropalaeontological dating (Wemli, 1982; Guillemin a n d Houzay, 1982) in the post-nappes sedimentary series of Boudinar basin (8 to 6 M.a.) a n d the age of the last metamorphic recristallisations in the T e m s a m a n e
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L. A[T BRAIIIMand P. CHOTIN
(8,2 M.a.) are in a g r e e m e n t with o u r field observations s h o w i n g t h a t the volcanic activity is s y n c h r o n o u s to the p o s t - n a p p e b a s i n opening. The Ras Tarf volcano m a y be b e t t e r related to activity in t h e Alboran Sea a n d t h u s , the pyroclastic horizons observed in B o u d i n a r b a s i n are indep e n d - a n t of t h e Ras Tarf volcanism; t h e y link up chronologically (6,4 M.a.; H e r n a n d e z , 1983) a n d petrologically with t h e s h o s h o n i t i c v o l c a n i s m of Nador basin. CONCLUSION OCEAN
F r o m u p p e r Miocene to Q u a t e r n a r y , the postn a p p e b a s i n s of t h e oriental Rif have b e e n affected b y i m p o r t a n t volcanic activity. During t h a t period, t h e whole a r e a w a s s u b m i t t e d to a compressive regime c a u s e d b y t h e c o n t i n u o u s convergence of A f r i c a n - E u r o p e a n plates. The g e o d y n a m i c model p r o p o s e d for t h e m a g m a t i c setting is t h a t of c r u s t a l s h e a r i n g developing s c a t t e r e d fissures a n d "en 6chelon" t e n s i o n g a s h e s which initiate partial fusion a t d e p t h b y p r e s s u r e release. The c r u s t a l s h e a r i n g affects t h e l i t h o s p h e r e along a strike-slip zone t h a t t r e n d s NE-SW a n d p u t s in c o n t a c t two c r u s t s of different n a t u r e a n d s t r u c t u r e (Hernandez e t a l . , 1985). The previous evolution of t h i s a r e a d u r i n g the w e s t w a r d m o v e m e n t of the Alboran block u p o n the Rffan d o m a i n , w o u l d have permited the interaction b e t w e e n f r a g m e n t s of a n old oceanic lithosphere a n d the u p p e r mantle; t h a t explains in part t h e n a t u r e of t h e volcanism. The d i s t r i b u t i o n of volcanism is guided by the reactivation of N 090-070, N 040 a n d N-S f a u l t s w h i c h create n e o r u p t u r e s m a k i n g a 4050 ° angle to t h e principal plane, w i t h i n a s t r e s s field c h a r a c t e r i z e d b y a counterclokwise r o t a t i o n of t h e m a x i m u m compressive s t r e s s ~ (~ a n d ~:~ horizontal). This explains the s y n c h r o n i s m a n d the close relationship b e t w e e n the volcanic activity a n d the opening of t h e Miocene b a s i n s which migrate w e s t w a r d s from Miocene to Q u a t e r n a r y . The v o l c a n i s m however does not s h o w the s a m e s e n s e of migration. The c o u n t e r c l o k w i s e r o t a t i o n of stress m a y explain also t h e g a p s in the volcanic activity by blocking the m o v e m e n t of faults, p e r p e n d i c u l a r to t h e compressive s t r e s s ~ inhibiting the rise of magma. This m o d e l does n o t exclude the local o c c u r r e n c e of distensive e v e n t s d u e to the relaxation of s t r e s s a n d to w h i c h c a n be linked a volcanic activity ( p e r m u t a t i o n b e t w e e n ~] a n d ~3)" Regionally, the volcanic activity is not only located in t h e Neogene b a s i n s North a n d S o u t h of the Alboran Sea, b u t m a n i f e s t s itself s o u t h w a r d s a s far as C a n a r i e s I s l a n d s (Fig. 6) in a zone
•
S
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"~ ~,oe,
0
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Fig. 6. Structural sketch map of the trans-Agadir-NekorAlboran zone. Distribution of Neogene and Quaternary volcanism and main active structures during Neogene and Quaternary (from /klt Brahlm, 1985; Hemandez et o1., 1987). l, Strike-slip faults; 2, Normal faults; 3. Alkaline volcanism; 4. Shoshonitlc volcanism: 5, Calc-alkaline volcanism; 6, Anateetic volcanism; 7, Lamproites. characterized since 10 M.a. by a c o m p r e s s i o n a b s o r b e d in strike-slip m o v e m e n t of N 040 a n d N 070 faults, from Agadir to t h e Rif. This is one of the m o s t i m p o r t a n t s t r u c t u r e s of Morocco (Fig. 6): the trans-Moroccan Agadir-Nekor fracture (/kit Brahim. 1985) w h o s e activity Is a t t e s t e d by p r e s e n t sismicity (Hatzfeld, 1978). Moreover, the s t r u c t u r a l a n d volcanic p a t t e m in S p a i n and Morocco s h o u l d be s i t u a t e d in t h e prolongation of the E u r o p e a n b a s i n opening (Bergerat, 1987) a n d s h o u l d c o r r e s p o n d to the axis of a major, b u t sporadic, t r a n s c o n t i n e n t a l m a g m a t i c anomaly. Acknowledgements - We warmly thank Dr D. Ben Sari of the CNCPRST for his help in organizing our field trip and Dr J. Hernandez of University P. et M. Curie, Paris for petrological data and advice.
REFERENCES
A~t Brahim. L. 1983. Etude de la deformation du Neog6ne & l'Actuel sur la bordure Sud-Rifaine dans le contexte du rapprochement des plaques AfriqueEurope. These 3~me Cycle. Universite Mohamed V, Rabat, Maroc, 178 p.
Oriental Moroccan neogene volcanism and strike-slip faulting Aft Brahim, L 1985a. Migration d e s aires de s u b s i d e n c e m a x i m u m et r~orientation de la direction d u c h a m p de contrainte d a n s les b a s s i n s neogenes du Maroc septentrional. Bull. Inst. ScL Rabat, 8, 55-65. Aft Brahim, L. 1985b. La zone de fracture trans-AgadirNekor : crlteres geophysiques, donnees de terrain et analyse de d o c u m e n t s ,LANDSAT~ (Maroc). Bull. Inst. ScL Rabat, 9, 1-12. Aft B r a h i m , L. 1986. Tectonique m e s s i n i e n n e et plio-quaternaire d u p i e d m o n t septentrional des Kebd a n a (Rifortental, Maroc). 4 e m e Colloque des B a s s i n s SL~limentaires, Rabat, p. 25, Livre en depSt l'Inst. Sci. Rabat. Aft Brahim, L. a n d Chotin, P. 1983a. R6sultat de r a n a l y s e de la fracturation des formations "postnappes" d u Rif central, d e p u i s Teroual ~i l'Ouest Jusqu'& Boured ~I l~Est. Mines, G~l. et Energ., Rabat, Maroc, 54, 210-225. Aft Brahim, a n d Chotin, P. 1983b. Mise en evidence d ' u n episode c o m p r e s s i f d a n s les calcaires pllo-quaternaires d u b a s s i n de Saiss, Rif, Maroc. C. R. Acad. Sc., Paris, 298, 1333-1336. Aft Brahim, L. a n d Chotin, P. 1984. Mise en evidence d ' u n c h a n g e m e n t de direction de c o m p r e s s i o n d a n s r a v a n t - p a y s rifain (Maroc) a u cours d u Tertiaire et d u Q u a t e m a i r e . Bull. Soc. G ~ l . France, 4, 681-691. Aft Brahim, L. a n d Chotin, P. 1986. R61e des cisaillem e n t s c r u s t a u x d a n s la d y n a m i q u e volcanique neogene et q u a t e r n a i r e d a n s le Maroc Nord. 1 l e m e R.S.T., Clermont Ferrand, p. 10, Livre en depSt & la Soc. Geol. France. Angeller, J. 1984. Determination of the m e a n principal directions of s t r e s s e s for a given fault population. Tectonophyslcs, 56, TI7-T26. Angelier, J. 1984. Tectonic analysis of fault slip d a t a sets. J. Geophys. Res., 89, B7, 5835-5848. Arana, V. a n d Vegas, R. 1974. Plate tectonic a n d v o l c a n i s m in the Gibraltar arc. Tectonophysics, 24, 197-212. Banda, E. a n d Ansorge, J. 1980. Crustal s t r u c t u r e u n d e r the central a n d eastern part of the betic Cordillera. J. Geophys. Res., 63, 515-532. Bellon, H. 1976. S~ries m a g m a t i q u e s neogenes et quat e m a i r e s d u p o u r t o u r de la Mediteerran6e occidentale, c o m p a l ~ e s d a n s leur cadre g6ochronologique. Implications geodynamiques. These Sci., Paris XI (Orsay n°AO 1750), 367 p. Berggren, W. A. a n d Van Couvering J. A. 1974. The late Neogene: biostratigraphy, geochronology a n d paleoclimatology of the last 15 million y e a r s in marine and continental s e q u e n c e s . Paleogeogr. Paleoclimatol. Paleoecol., 16, 1-216. B e r r a h m a , M. 1982. Volcanisme mio-pliocene de la partie NW d u Massif d u Siroua (Anti-Arias Central, Maroc), e t u d e s t r u c t u r a l e et p~trologique. These 3eme Cycle, Universit6 Paris Sud, Orsay, 152 p. Bourdier, J. L. 1986. Formation s o u s m a r i n e de breches and6sitiques d u Ras T a r f (Rif, Maroc). C. R. Acad. Sc., Paris, 303, 619-622. Bourgois, J. 1978. La t r a n s v e r s a l e de Ronda, d o n n e e s geologiques p o u r u n modele d'evolution de rArc de Gibraltar. These d'Etat, Besan~on, 445 p., 1 carte 1/100.000~me.
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CoUetta, B. 1977. Evolution neotectonique de la partie merldlonale du b a s s i n de Guercif (Maroc oriental). These 3eme Cycle, Universite de Grenoble, 150 p. Delarue, C. a n d B r o u s s e R. 1974. Volcanisme miocene en s u b d u c t i o n a u Maroc a l'extr6mite orientale de l'arc de Gibraltar. C. R. Acad. Sc., Paris, 279, 971-974. De Lucas, P. 1978. L'unite chaoUque des K e b d a n a (region de Zaio, Maroc). Relation structurale avec l'avantpays du Rif Oriental. Bull. Soc. Gcaol. France, 3, 339343. Dutour, A. a n d Ferrandini, J. 1985. Nouvelles observations neotectoniques d a n s le Haut-Atlas de M a r r a k e c h et le H a o u z central (Maroc). Rev. G~,ol. Dyn. C~ogr. Phys., 26, 285-297. Frizon De Lamotte, D. 1979. Contribution & r e t u d e de revolution s t r u c t u r a l e d u Rif Oriental. These de 3eme Cycle, Paris Sud, Orsay, 175 p. Frizon De Lamotte, D. a n d Leikine, M. 1982. Les massifs externes a schistosite d u Rif (Maroc) : u n e zone de c o u l i s s e m e n t s y n m 6 t a m o r p h e . C. R. Acad. Sc., Parts, 293, 1285-1290. Guillemin, M. 1976. Les formations n6ogenes et quaternaires de Melilla, Nador et leurs deformations. These de 3eme Cycle, Orleans, 215 p. Guillemin, H. a n d H o u z a y J. P. 1982. Le N6ogene p o s t - n a p p e s et le Q u a t e r n a i r e d u Rif nord-oriental (Maroc). Stratigraphie et tectonique des b a s s i n s de Melilla, d u Kert, de B o u d i n a r et d u p i e d m o n t des Kebdana. Notes et Mem. Serv. Geol. Maroc, 314, 7-238. H a r m a n d , C. and Moukadirl, A. 1986. Synchron i s m e e n t r e t e c t o n i q u e c o m p r e s s i v e et volcan i s m e alcalin : exemple de la province q u a t e m a i r e du Moyen-Aflas (Maroc). Bull. Soc. G~ol. France, 4, 595603. Hatzfeld, D. 1978. E t u d e sismotectonique de la zone de collision ib~ro-maghr6bine. These Sci., I.R.I.G.M., Grenoble, 281 p. H e m a n d e z , J. 1983. Le volcanisme miocene d u Rif oriental (Maroc) : geologie, p6trologie et mineralogie d ' u n e province shoshonitique. These Sci., Paris VI, n°83-25, 592 p. Hernandez, J. a n d Bellon, H. 1985. Chronologie K-Ar d u volcanisme miocene d u Rif oriental (Maroc) : implications tectoniques et magmatologiques. Rev. Geol. Dyn. G~ogr. Phys., 26, 85-94. Hernandez, J. Lahouziere, J. D., Bolze, J. a n d Bordet0 P. 1987. Le m a g m a t i s m e neogene b6tico-rifain et le couloir de d e c r o c h e m e n t trans-Alboran. Bull. Soc. G~ol. France, 2, 257-267. Hervouet, Y. 1985. G e o d y n a m i q u e alpine (Trias-Actuel) de la m a r g e septentrionale de rAfrique a u Nord du Bassin de Guercif (Maroc oriental). Thc~se Sci., Bordeaux, 5 1 , 3 6 7 p. Kerchaoui, S. 1985. E t u d e geologique et s t r u c t u r a l e d u Massif des Beni Bou Ifrour (Riforiental, Maroc). These 3eme Cycle, Paris XI, O r s a y 193 p. Monie, P.0 Frizon De Lamotte, D. a n d Lelkine, M. 1984. Etude g6ochronologique pr61iminaire p a r la m e t h o d e Ar39/Ar 4° du m e t a m o r p h l s m e alpln d a n s le Rif externe (Maroc). Precisions s u r le calendrier tectonique tertiaire. Rev. G~ol. Dyn. G~:jr. Phys., 25, 4 , 3 0 7 317.
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Puga, E. 1980. Hypotheses s ur la genese des magmas Tapponnler, P. 1977. Evolution tectonlque du syst~me calco-alcalins intra-orogeniques et post-orog~rdques alpin en M~llterran~e : poin~onnement et ~crasement alplns dans les Cordll]eres ]~tiques. Bull Soc. C~ol. riglde-plastique. Bu//. Soc. G~/. F~mce, 3, 437-460. France, 2, 243-250. Wemli, R. 1979. Le Pllocene de la Mamora (Maroc) : Septfontaine, M. 1983. La formation du Jebel Binet, Rif straUgraphie et Foramlnlferes planctoniques. ~ . externe (Maroc). Un ~ t "ant~-nappe, d'~ge Miocene Geol. Helv., 72/ 1, 119-143. sup~rleur. Implications pal~otectoniques. Ectogae G~,o/. Wemli, R. 1982. Le Messlnlen & Globorotalia conomioHeir., 7~$}, 581-610. zea (Foraminif~re planctonlque) de la c6te m~:llterran~enne marocaine. Eclogae. Geol. Helv.. 73, 71-93.
0899-5362/90 $3.00 + 0.00 ~rg~non l ~ u pie
Oriental Moroccan Neogene volcanism and strike-slip faulting L. Art BRAmM*and P. CHOTU¢** *D(~panmnmt de G~ologie, Universitd Mohamed V, BP 1014, Rabat, Maroc **D~partement de G6otectonique, UA 215 du CNRS, Universit8 Pierre et Marie Curie, T.25-26, El, 4 place Jussieu, 75252 Paris cedex 05, France Abstract - The microtectonic analysis of faults and of volcano-phtonic associations in the Eastern Rif shows that the distribution of a large number of Miocene to (~_m_t_,~nmTvolcanic emission points is tectonically controlled. Their lecalisation, p~allel to the strike-slip faults orientated N 070-090, N 04(0050 and N-S suggests that the volcanic emissions could have occurred along tension gashes making an engle of 40-50 ° to the initial strike- slip plane. The new orientation, of the principal compressional stress to N 040-000 during upper Miocene, reactivates faults striking N 070, N 040-050 and N-S; the volcanic emissions with a NW-SE, NE-SW and NS directions, occur along gashes developed in this systmn. During the piio-Quaternary, the counterclockwise rotation of the main stress to NW-SE, initiate strike-alip movemem along the N-S, N 090 and N 120-140 fanlts; the volcanic emissions take place on ,en ~chelon~ tension gashes developed in this system. In this area, subjected since 10 M.s. to a compressional regime, the crustal mega shear zones develop scattered fissures and tension gashes which initiate partial fusion at depth by pressure release. The gaps in volca~dc activity may correspond to the changes in the stress field. R~sum~ - L'analyse microtectonique des failles et de leur relation avec l'activit~ magmatique dens le Rif oriental, montre le contr61e tectonique de la r~partition spatiale d'un grand hombre d'&iifices volcaniques. La disposition des Epp&eils volcaniques le long des d~rochements N 70-90, N 40-50 et ]q-S, s u g g ~ que les ~missions ont pu s'effecmer sur des fentes de temion faisant un angle de 40 ~t50° avec ces plans de cisaillemems initiaux. La r~orientation an Miocene supdrieur de la contrainte principale en compression vers ]q-S r6tctive les accidents de direction N 70, N 40-50 et N-S et les ~missions volcaniques de direction N'W-SE, NE-SW et NS se placent sur les fentes ddveloppees dens ce syst~me. Au Plio-Quatm'naire, la rotation anti-horaire de cette contrainte vers le NW-SE amorce le jeu dOcrochant des accidents N-S, N 90 et N 120-140, et les ~nissions volcanklues se placent aiors sur des fentes de tension on ~helon N-S et NW-SE. Dans cette r~gion soumise depuis 10 millions d'mm~es ~tun r~gime de compression, les grands cisaillements crustaux d ~ , v e l o ~ t des fissures diss~min~.es et des fentes de tension qui mnorcent la fusion par chute de pression. Les p~riodes d'arr~t de l'acfivit6 volcanique pourraient correspondre aux p(~'riodesde changement de direction des contraintes.
(1976) and Pugs (1980) as a p h e n o m e n o n linked to subduct/on. During the Neogene and Quaternary, important Hernandez (1983) h a s shown t h a t this volcartism volcanic activity (Hernandez, 1983; Berrahma, located In u p p e r Miocene sedimentary basins, 1982; H a r m a n d et Moukadiri, 1986) developed In does not display any chronological or geochemical a zone whose evolution is characterized by the polarity which permits to corroborate this hypooccurrence of major strlke-slip faults striking NE- thesis. This volcanism is thought rather to be SW. From Agadir, these faults follow the Haut and related to the working of a transverse strike-slip Moyen Arias and connect up with the faults of the system, situated on the border of the Iberian and oriental Rif (Nekor) by a n " e n 6chelon" fault system. African plates, putting In contact two different They cross the Alboran sea and llnk up with the crusts, a n d affecting the entire lithosphere (Bands oriental end of the Betic Cordillera (Hatzfeld, 1977; and Ansorge, 1980). Aft Brahim, 1983-85). The aim of this paper is to provide structural In relation to plate tectonics, the Neogene a r g u m e n t s In favour of the latter hypothesis, based m a g m a t i s m In the oriental Rif and In the East of on microtectonic analyses of fault patterns and Betic Cordillera has b e e n Interpreted b y A r a n a and associated Intrusions which have been the feeders Vegas (1974), Delarue and Brousse (1974), Bellon to the oriental Rlf volcanism. Field data (trend of INTRODUCTION
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L. AIT BRAHIMand P. CHOT1N
faults, s e n s e of m o v e m e n t indicated on slickenside lineations, chronology) a n d t r e a t m e n t s for the d e t e r m i n a t i o n of m e a n principal directions of s t r e s s e s (Angelier, 1979, 1984) have b e e n described in t h e earlier p a p e r s (/kit B r a h i m , 1983; A~t B r a h i m et aL, 1983b, 1984) a n d we c o n s i d e r it proper to r e p r i n t only t h e resulting s y n t h e t i c d i a g r a m (Fig. I). MAIN TECTONIC EVENTS F R O M T O R T O N I A N 1"0 P R F ~ E N T
N o r t h - E a s t e m Morocco, s u b j e c t since t h e u p p e r C r e t a c e o u s to a compressive s t r e s s regime, displays since t h e T o r t o n l a n volcanic activity i n t i m a t e l y r e l a t e d to t h e s e d i m e n t a r y a n d tectonic evolution of t h e Neogene p o s t - n a p p e b a s i n s of B o u d i n a r , Kert a n d Guercif. These s e d i m e n t a r y a n d eruptive bodies have registered a s u c c e s s i o n of tectonic events (A~t B r a h i m et Chotin, 1983b, 1984): * The first, w h o s e s t r e s s field is c h a r a c t e r i z e d by a m a x i m u m compressional stress ~ trending N 040 (a a horizontal), is responsible for the origin a n d evolution of T o r t o n i a n b a s i n s along strike-slip f a u l t s t r e n d i n g N 070-090 (Kebdana. North Gareb, T e m s a m a n e ) N 0 4 0 - 0 5 0 (Nekor, T e m s a m a n e , C h a r r a n a ) a n d N-S (Boudinar) (Fig. 2). A c a l c - a l k a l i n e v o l c a n i s m is s u p e r i m p o s ed at Ras T a r f a n d at t h e Trois F o u r c h e s cape. TEPHROC~IRONOLOGY as
* The second, from late T o r t o n i a n a n d d u r i n g t h e Messinian, is c h a r a c t e r i z e d by a m a x i m u m compressive s t r e s s a ~, t r e n d i n g N-S: it is t h e late T o r t o n i a n event of Gufllemin (1976), also recorded by De L u c a (1978) in t h e K e b d a n a M o u n t a i n s a n d b y Hervouet (1985) in t h e Gareb area. This first c h a n g e in t h e direction of the palaeostresses i n d u c e s a reverse m o v e m e n t along the N 090 f a u l t s (Kebdana, Gareb) a n d a strikeslip m o v e m e n t on N 070 f a u l t s (Temsamane, Guercii], N 0 4 0 - 0 5 0 f a u l t s (Nekor, C h a r r a n a , Beni Bou Ifrour) a n d N-S (Boudinar, B a s Nekor). We also observed s o m e related E-W folding (Tarj at anticline). D u r i n g t h i s period, m o v e m e n t along N 070, N 0 4 0 - 0 5 0 a n d N-S f a u l t s led to the opening of the B o u d i n a r , Kert a n d G u e r c i f b a s i n s a n d controled the s e d i m e n t a t i o n (Ait Brahim, 1985). A set of N-S t r e n d i n g n o r m a l f a u l t s w a s created b y a n E-W e x t e n s i o n (a I a n d ~3 horizontal). These f a u l t s affected all the p o s t - n a p p e s b a s i n s in E a s t Morocco (Boudinar, Kert, Beni Said s u b s t r a t u m . Trois F o u r c h e s cape a n d Gu ercil]. This event is c o n t e m p o r a n e o u s with t h e s h o s h o n i t i c v o l c a n i s m in G o u r o u g o u a n d Guflliz (Hernandez, 1983). * The third tectonic event is c h a r a c t e r i z e d by a new counterclockwise r o t a t i o n of t h e s t r e s s field; the new m a x i m u m compressive s t r e s s now t r e n d s N 140-160 (Ait B r a h i m a n d Chotin, 1984). The N 090 a n d N 120-140 f a u l t s have now a dextral DIRECCION OF STRESS
MOTION OF FAULTS
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Oriental Moroccan neogene volcanism and strike-slip faulting
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Fig. 2. Structural map of Eastern Rif (Morocco). Distribution of volcanism in the Neogene basins. A, Ras Tarf; B, Trois Fourches cape; C, Gourougou, D, Beni Bou Ifrour. 1, Neogene and Quaternary; 2, Neogene and Quaternary volcanism; 3, Rif unit of Ketama; 4, Foreland; 5, Quaternary volcanism of Beni Bou Ifrour; 6, Granodiorite of Beni Bou Ifrour. strike-slip c o m p o n e n t , a n d t h e N-S a sinistral one. The N 0 4 0 - 0 5 0 striking faults are reactivated a s sinistral strike-slip faults or reverse f a u l t s with a sinistral c o m p o n e n t . We also o b s e r v e d a s s o c i a t e d N 0 7 0 to N 0 8 0 trending folds in K e b d a n a a n d G a r e b area. A p e r m u t a t i o n b e t w e e n a I a n d a 2 related to t h e opening of t h e b a s i n s c r e a t e d n o r m a l f a u l t s (Boudinar). This c o m p r e s s i v e s t r e s s (N 140-160) h a s b e e n o b s e r v e d everywhere in North Morocco: in t h e B o u d i n a r , Kert a n d N a d o r b a s i n s (Guillemin et H o u z a y , 1982; Aft Brahim, 1985), in A r b a a de Taourirt (Frizon de Lamotte, 1979), in t h e K e b d a n a (De L u c a or. com., Ikit Brahim, 1986), in t h e G a r e b (Hervouet, 1985) in t h e G u e r c f f b a s i n (CoUetta, 1977) in t h e S u b r i f m a s s i v e s (Aft B r a h i m a n d Chotin, 1983a) in t h e "sfllon s u d rifain" (/kit B r a h i m a n d Chotin 1883b, 1984) in t h e Middle Atlas (/kit B r a h i m , 1985) in t h e H a u t Atlas a n d j o i n t b a s i n s ( D u t o u r a n d Ferrandini, 1985; Aft B r a h i m , 1985). During this period the n a t u r e of t h e v o l c a n i s m w a s alkaline in t h e NE Morocco ( H e m a n d e z , 1983) a n d in the Middle Atlas ( H a r m a n d a n d Moukadiri, 1986).
To c o n c l u d e t h e a n a l y s i s of t h e g e o m e t r y a n d s u c c e s s i v e m e c h a n i s m s of fault t e c t o n i c s s h o w s the relationship t h a t exist b e t w e e n t h e field of palaeostresses and the genesis and kinematic evolution of t h e p o s t - n a p p e s b a s i n s in t h e oriental Rif. Within t h e T r a n s - N e k o r A l b o r a n m a j o r strike slip fault s y s t e m , we o b s e r v e d several p o p u l a t i o n s of faults: N 0 7 0 - 0 9 0 , N 0 4 0 - 0 5 0 , N-S a n d N 120140. The p o s t - n a p p e s b a s i n s are genetically linked to this fault s y s t e m in a c o m p r e s s i v e regime (a] a n d a 3 horizontal) with t e m p o r a r y peri~iutations b e t w e e n a~ a n d a 2. The c o u n t e r c l o c k w i s e rotation of t h e p a l a e o s t r e s s field, f r o m T o r t o n i a n to Present, g u i d e d t h e s e d i m e n t a t i o n b y moving t h e acc u m u l a t i o n z o n e s a n d b y developing a n active s u b s i d e n c e along t h e strike-slip faults (N 0 7 0 - 0 9 0 , N 0 4 0 - 0 5 0 , N-S), t h a t s u c c e s s i v e l y replayed with time displaying a n i m p o r t a n t vertical c o m p o n e n t . The n e o t e c t o n i c evolution h a s b e e n characterized b y splitting into n u m e r o u s b l o c k s showing a l t e m - a t i n g h o r s t a n d t r o u g h s , with t e m p o r a r y blocking u p a n d r o t a t i o n s in a s t r e s s regime w h e r e compression and extension are associated simultaneously.
276
L. Air BRAItIMand P. CHOTIN
OUTLINE OF THE MAIN VOLCANIC CENTRES R ~ Tarf v o l c a n o : 1 3 , 0 t o 12,4 M.a. (Hernandez and Bellon, 1985). The calc-alkaline stratified breccias of Ras Taft volcano are cut b y metric dykes of similar composition (Bourdier, 1986). In J b e l Hadid area (Fig. 2A), we observed dykes trending N 0 1 0 - 0 4 0 and N 120-160, crossing the Throughout s u b s t r a t u m and the breccias. Dykes with s a m e direction outcrop in Sidi Chal'b area. In Ajdir area (Fig.2A), N-S dykes cross cut the breccia. T h u s the intrusions which have fed the breccias sits occur along two main directions: NE-SW and NW-SE. Fracturing which affects the Ras Taft volcano shows f o u r directions: N-S, N 120-140, N 0 7 0 and N 040.
and the second between the Bokoya a n d Setolazar quarries. West o f J e b e l Ouiksane, we observe a sill that trends NE-SW, several kilometres long, which h a s yielded a radiometric age of 7,3 % 1,5 M.a. (Moni6 e t al., 1984). Volcanoes in Monts Amar, Afra and Loma Mokhtar trend N-S to N 030. The granodioritic complex of Beni Bou Ifrour h a s b e e n accompanied by a g r o u p of dykes trending in two directions, N,S, NE-SW.
Guercif basin volcanism: 8 t o 4 , 9 M.a. (Hernandez and BeUon, 1985). Tufts and breccias in the Guilliz m a s s i f are cut by a group of domes and dykes trending NE-SW and NW-SE. The western flank of Guilliz el Kebir Mountain partially down-faulted along a N-S fault is m a r k e d by a series of intrusions which have fed t h e more Trois F o u r c h e s cape volcanism: 9,8 M.a. (Her- recent volcanics forming present summits. Hervouet (1985) h a s described a good example of n a n d e z and Bellon, 1985). The volcanic complex of this area shows n u m e r o u s domes and dykes injection along mega-tension g a s h e s developed in outcropping along two main trends: N 30 and N a N 150 m a x i m u m compressive stress system. It 120-140. This is well displayed on the West coast, concerns a small dyke located E a s t of S a k a (Jebel in the Ifri n'Douch Valley and at Tibouda, (Fig. 2B). Tassatfacht) within a N-S fault system; it displays Part of the s t r u c t u r e of this volcano is the result of a sigmoidal shape. Vertical joints of retreat trend successive inJections of m a g m a along open narrow N-S in the centre of the dyke and swing to N 150 at fissures following the two trends mentioned above. the tips. Gmzrougou volcanlsm: 9 to 4 , 6 M.a. (Hernandez and Bellon, 1985). The Gourougou stratovolcano (Fig. 2C) is mainly composed of tufts and breccia (Hernandez, 1983). Oulad Mimoun basin is the principal eruptive centre. N u m e r o u s dykes and domes outcrop on the northwestern flank along the NE-SW line of w e a k n e s s also indicated by an alignment of necks a n d dykes. The Beni Enzar volcanic flow (Fig. 2C) shows several points of emission along a NNE-SSW trending fissure. The Ibou J a j e n e and Jebel Zaouyet crests are formed b y a dykes swarm and intrusions more t h a n several h u n d r e d metres wide. At Azrou Idoudene the whole crest is an emissive centre with a s h zones a n d fumarole (Hernandez, 1983). Near Segangane the tufts are cut b y a trachytic dyke trending NW-SE. At Bajioua several successive intrusions trend N-S. A group of necks and dykes also form the Terehilast crest. The intrusions in Gourougou area t h u s occur along three directions: NE-SW, N-S and NW-SE. Benl Bou Ifrour m a g m a t l s m The granodioritic setting in Beni Bou Ifrour area (Fig. 2D) is accompanied b y several generations of dykes of variable size. The Ibekouyene area shows three successive injections: a main dyke thirty metres wide is cut by two other generations of dykes (Kerchaoui, 1985). Two groups of dykes trend N-S, the first, along the Oued Ouiksane fault
RELATIONSHIP BETWEEN FRACTURATION AND VOLCANISM Intrusions (dykes, alignments of eruptive centres) which feed the lava flows of Northeast Morocco occur along three main directions: NESW, NW-SE and N-S. These volcanic emissions lie parallel to faults trending N 070-090, N 040-050 and N-S that determine the s t r u c t u r e of this area; locally N 120-140 faults are associated. The m e c h a n i s m of magma injection is t h u s related to "en ~chelon" mega-tension gashes making an angle of 40 to 50 ° to the m a i n strikeslip fault planes. Some c a s e s at 90 ° to this direction are related to syn to post deformation (sigmoidal tension gashes) during tectonic reactivation of these faults. Some of these tension g a s h e s will evolve becoming normal faults on the border of the postn a p p e s basins. The direction of the mega tension g a s h e s is related to the direction of the m a x i m u m compressive stress a] and the direction of the inherited faults which replayed at this time. During the Tortonian, the m a x i m u m compressive stress trends N 040 (Fig. 3). The calc-alkaline volcanic emissions trending NE-SW (Ras Taft, Trois Fourches) are injected in "en 6chelon" megatension g a s h e s b o u n d by sinistral N 0 9 0 - 0 7 0 t r e n d ing faults. The volcanic emissions trending N-S are
Oriental Moroccan neogene volcanism and strike-slip faulting
277
/'" ~ '
Fig. 3. Strike-slip m o v e m e n t s of faults during N 040 compressive e v e n t and associated extensive structures.
LATE-TORTONIAN,
MESSINIAN
j tBOUOINARI
#
Jt " ' f ........ [
so..
f
.-~ "- HOYEllATLAS Fig. 4. Strike-slip and reverse m o v e m e n t s of faults during N 000 compressive event and associated extensive structures.
injected in favour of sinistral strike-slip movement of N 040-050 faults. At the e n d of the Tortonian and during the Messinian, the direction of m a x i m u m compressive stress (a i horizontal) b e c a m e N-S (Fig. 4). It closes the mega-tension gashes a n d the basins developed by m o v e m e n t of the N 090 sinistral faults; these faults become reverse. The shoshonitic volcanic emissions a n d the Beni Bou Ifrour granodiorite trending NE-SW (Gourougou, GuiUiz, Beni Bou Ifrour), N-S (Gourougou, Beni Bou Ifrour) and NWSE (Guilliz, Gourougou) were injected in megatension gashes developed during the sinistral transc u r r e n t m o v e m e n t of N 070, N 040-050 and N-S faults. From the end of Messinian to Present, there is a counterclockwise rotation of the m a i n compressive stress from N-S to N 140-160 (Fig. 5). The NWSE trending alkaline emission centers (Guercif,
Fig. 5. Strlke-slip and reverse m o v e m e n t s of faults during N 140-160 compressive event and associated extensive structures.
Gourougou) developed in tension g a s h e s created by the dextral strike-slip m o v e m e n t along N 090 faults and by the stnistral offset of N-S faults. The N-S trending volcanic events (Guilliz, Beni Bou Ifrour) were developed in tension gashes created by a dextral play of N 120-140 faults; but sometimes, the compression c a n be absorbed in blocking up zones along N 060-070 faults, by the development of open fissures (Ait Brahim a n d Chotin, 1986; H a r m a n d a n d Moukkadiri, 1986) regarded as tension gashes parallel to a i (Tapponnier et aT., 1977) (Fig. 5). This system works as long as the s l direction maintains a N 140-160 trend. This m e c h a n i s m can be extended to Spain and to the Canaries islands. Nevertheless, the volcanic intrusions are dynamically easier along the N 090, N 040-050, N-S and N 120-140 faults. THE RAS TARF AND GOUROUGOU VOLCANISM AND THE AGE OF ALPINE METAMORPHISM IN T H E EXTERNAL RIF
Ages of 12 to 13 M.a. and 9 M.a. obtained respectively in Ras Tarf and Trois Fourches cape (Hernandez and Bellon, 1985) led Gufllemin and Houzay (1982) a n d Bourdier (i 986) to reconsider the age ofvolcanism as being prior to the Boudinar basin; however, these ages are not in agreem e n t with Ar39/Ar4° dating of the last syntectonic metamorphic event affecting the Ketama and T e m s a m a n e Units which h a s yielded ages of 7,3 to 8,2 M.a. (Moni6 et aT., 1984). We consider t h a t the micropalaeontological dating (Wemli, 1982; Guillemin a n d Houzay, 1982) in the post-nappes sedimentary series of Boudinar basin (8 to 6 M.a.) a n d the age of the last metamorphic recristallisations in the T e m s a m a n e
278
L. A[T BRAIIIMand P. CHOTIN
(8,2 M.a.) are in a g r e e m e n t with o u r field observations s h o w i n g t h a t the volcanic activity is s y n c h r o n o u s to the p o s t - n a p p e b a s i n opening. The Ras Tarf volcano m a y be b e t t e r related to activity in t h e Alboran Sea a n d t h u s , the pyroclastic horizons observed in B o u d i n a r b a s i n are indep e n d - a n t of t h e Ras Tarf volcanism; t h e y link up chronologically (6,4 M.a.; H e r n a n d e z , 1983) a n d petrologically with t h e s h o s h o n i t i c v o l c a n i s m of Nador basin. CONCLUSION OCEAN
F r o m u p p e r Miocene to Q u a t e r n a r y , the postn a p p e b a s i n s of t h e oriental Rif have b e e n affected b y i m p o r t a n t volcanic activity. During t h a t period, t h e whole a r e a w a s s u b m i t t e d to a compressive regime c a u s e d b y t h e c o n t i n u o u s convergence of A f r i c a n - E u r o p e a n plates. The g e o d y n a m i c model p r o p o s e d for t h e m a g m a t i c setting is t h a t of c r u s t a l s h e a r i n g developing s c a t t e r e d fissures a n d "en 6chelon" t e n s i o n g a s h e s which initiate partial fusion a t d e p t h b y p r e s s u r e release. The c r u s t a l s h e a r i n g affects t h e l i t h o s p h e r e along a strike-slip zone t h a t t r e n d s NE-SW a n d p u t s in c o n t a c t two c r u s t s of different n a t u r e a n d s t r u c t u r e (Hernandez e t a l . , 1985). The previous evolution of t h i s a r e a d u r i n g the w e s t w a r d m o v e m e n t of the Alboran block u p o n the Rffan d o m a i n , w o u l d have permited the interaction b e t w e e n f r a g m e n t s of a n old oceanic lithosphere a n d the u p p e r mantle; t h a t explains in part t h e n a t u r e of t h e volcanism. The d i s t r i b u t i o n of volcanism is guided by the reactivation of N 090-070, N 040 a n d N-S f a u l t s w h i c h create n e o r u p t u r e s m a k i n g a 4050 ° angle to t h e principal plane, w i t h i n a s t r e s s field c h a r a c t e r i z e d b y a counterclokwise r o t a t i o n of t h e m a x i m u m compressive s t r e s s ~ (~ a n d ~:~ horizontal). This explains the s y n c h r o n i s m a n d the close relationship b e t w e e n the volcanic activity a n d the opening of t h e Miocene b a s i n s which migrate w e s t w a r d s from Miocene to Q u a t e r n a r y . The v o l c a n i s m however does not s h o w the s a m e s e n s e of migration. The c o u n t e r c l o k w i s e r o t a t i o n of stress m a y explain also t h e g a p s in the volcanic activity by blocking the m o v e m e n t of faults, p e r p e n d i c u l a r to t h e compressive s t r e s s ~ inhibiting the rise of magma. This m o d e l does n o t exclude the local o c c u r r e n c e of distensive e v e n t s d u e to the relaxation of s t r e s s a n d to w h i c h c a n be linked a volcanic activity ( p e r m u t a t i o n b e t w e e n ~] a n d ~3)" Regionally, the volcanic activity is not only located in t h e Neogene b a s i n s North a n d S o u t h of the Alboran Sea, b u t m a n i f e s t s itself s o u t h w a r d s a s far as C a n a r i e s I s l a n d s (Fig. 6) in a zone
•
S
J r-~ s ~ e Va--1 ?
"~ ~,oe,
0
|00 Km
Fig. 6. Structural sketch map of the trans-Agadir-NekorAlboran zone. Distribution of Neogene and Quaternary volcanism and main active structures during Neogene and Quaternary (from /klt Brahlm, 1985; Hemandez et o1., 1987). l, Strike-slip faults; 2, Normal faults; 3. Alkaline volcanism; 4. Shoshonitlc volcanism: 5, Calc-alkaline volcanism; 6, Anateetic volcanism; 7, Lamproites. characterized since 10 M.a. by a c o m p r e s s i o n a b s o r b e d in strike-slip m o v e m e n t of N 040 a n d N 070 faults, from Agadir to t h e Rif. This is one of the m o s t i m p o r t a n t s t r u c t u r e s of Morocco (Fig. 6): the trans-Moroccan Agadir-Nekor fracture (/kit Brahim. 1985) w h o s e activity Is a t t e s t e d by p r e s e n t sismicity (Hatzfeld, 1978). Moreover, the s t r u c t u r a l a n d volcanic p a t t e m in S p a i n and Morocco s h o u l d be s i t u a t e d in t h e prolongation of the E u r o p e a n b a s i n opening (Bergerat, 1987) a n d s h o u l d c o r r e s p o n d to the axis of a major, b u t sporadic, t r a n s c o n t i n e n t a l m a g m a t i c anomaly. Acknowledgements - We warmly thank Dr D. Ben Sari of the CNCPRST for his help in organizing our field trip and Dr J. Hernandez of University P. et M. Curie, Paris for petrological data and advice.
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