Tectonics Versus Eustasy: North African Mesozoic And Cenozoic Facies Patterns And Hydrocarbon...

127

Geologische Rundschau 81/1 ] 127-141 I Stuttgart 1992

Plate tectonics and intracratonic mountain ranges in Morocco -

The Mesozoic-Cenozoic development of the Central High Atlas

and the Middle Atlas.

By RAINALD BREDE, MANFRED HAUPTMANN, Berlin & HANS-GEORG HERBIG, Marburg*)

With 15 figures

Zusammenfassung

Die post-variskische sediment~ire und strukturelle Entwicklung der intrakontinentalen Gebirgs- ketten des Hohen und des Mittleren Atlas wurde dutch plattentektonische Prozesse im atlantischen und westmediterranen Raum gesteuert, die auf ein ererbtes panafrikanisches bzw. variskisches St6- rungsmuster einwirkten. Zu Zeiten relativer tektonischer Ruhe bestimmten eustatische Meeresspie- gelschwankungen die sediment~ire Entwicklung. Die von horizontaler und vertikaler Bruchtektonik beherrschte Atlas-Tektonik wurde durch in der Zeit wechselnde Stregfelder bestimmt. Wichtigste grof3tektonische, oft zusammenwirkende Steue- rungsfaktoren waren: die bis in die Unterkreide wirkende Taphrogenese im Bereich des NW-afrika- nischen Kontinentalrandes (triadisches Rifting und anschlieBendes Spreading im Atlantik), Transform- Bewegungen an der Neufundland-Gibraltar-St6- rungszone zwischen Lias und filtestem Eozfin sowie die Konvergenz zwischen Europa (Iberia) und Afrika, welche nach ihrem Beginn in der sp~iten Kreide ihren H6hepunkt im Neogen erreichte. Das Zusammenspiel dieser Faktoren bewirkte im zentralen Hohen Atlas und im Mittleren Atlas die Entstehung kontinentaler Riftgrfiben wfihrend der Trias, die vonder Tethys ausgehende Flutung der zusammengewachsenen Riftgr~ben im Lias und untersten Dogger (Sinemurium - Aalenium) sowie die zuerst marine (Bajocium - Bathonium), dann kontinentale Ffillung und Heraushebung der still-

gelegten Riftzone zwischen sp~item Mitteljura und sp~iter Unterkreide. Zwischen Apt und Ende des Mitteleoz~in kontrollierten vor allem eustatische Meeresspiegelschwankungen die sediment~ire Ent- wicklung. Nach ersten Hebungen im zentralen Ho- hen Atlas wfihrend des Senon begann die eigent- liche Heraushebung der intrakontinentalen Ketten an der Wende Mittel-/Obereoz~in. Die mio-/plioz~i- ne Hauptphase der Atlas-Tektogenese war zeit- gleich mit der Orogenese im Betico-Rif-Bogen.

Abstract

Plate tectonic processes in the Atlantic and western Tethyan realm directed the post-Variscan sedimentary and structural evolution of the High Atlas and Middle Atlas intracontinental mountain ranges of Morocco. Plate movements caused a re- activation of an inherited pan-African or Hercy- nian fault pattern by the variation of stress regimes through time. This resulted in strike-slip as well as vertical tectonics. During times of relative tectonic quiescence eustatic sea-level changes governed the sedimentary development. The most important, often interacting, global tectonic determinants are: taphrogenesis of the NW-African continental margin lasting until the Early Cretaceous (Triassic rifl-

i n g and subsequent mid-Atlantic spreading), strike-slip-faulting at the Newfoundland-Gibraltar fault zone (Liassic - earliest Eocene), and continental convergence between Europe (Iberia) and

*) Authors' addresses: Dr. R. BREDE, Dr. M. HAUPTMANN, Elbestral3e 15, D-1000 Berlin 44, Germany and Priv.-Doz. Dr. H.-G. HERmG, Philipps-Universitfit Marburg, Institut fiir Geologie und Pal~iontologie, Hans-Meerwein-Str., D-3550 Marburg.

Manuscript received: 2. 11. 1990; accepted: 23. 8. 1991.

128 R. BREDE, M. HAUPTMANN & H.-G. HERBIG

Africa which started in the Late Cretaceous and reached its acme in the Neogene. In the realm of the central High Atlas and the Middle Atlas the interaction of these processes triggered continental rifting (Triassic) and subsequent marine flooding of the intergrown riftgrabens prograding from the Tethys realm (Early Jurassic - earliest Middle Ju- rassic). After its abortion, the former Atlas rift was filled up with marine sediments (Bajodan - Batho- nian), followed by continental redbeds and final uplift (late Mid Jurassic - late Early Cretaceous). Eustatic sea-level changes mostly governed the sedimentary evolution from Aptian to latest Mid Eocene. After a first weak uplift of the central High Atlas during the Senonian major uplift of the intracontinental chains commenced at the Mid/ Late Eocene transition. Diastrophism of the Atlas ranges during the Miocene and Pliocene coindded with the main orogenic movements of the Betico- Rifean arc.

R6sum6

Udvolution sddimentaire et structurale post- varisque des chaines intracontinentales du Haut et du Moyen Atlas a 6t6 rdgie par les processus de la tectonique des plaques qui se d6roulaient dans 1Atlantique et dans la M6diterrannde ocddentale. Ces mouvements de plaques ont r6activ6 un en- semble de failles h6rit6es des phases pan-africaine et hercynienne, en y produisant des d6placements verticaux et horizontaux en rdponse aux fluctua- tions temporelles des champs de contraintes. Au cours des pdriodes de calme tectonique relatif, les changements eustatiques du niveau de l a m e r ont d6termin6 le rdgime de la sddimentation. Les facteurs de la tectonique les plus importants, ~i action d'ailleurs souvent combinde, ont 6t6: la traphroge- n6se, active jusqu'au Crdtac6 infdrieur, de la mar- ge nord-ouest du continent africain (rifting triasi- que et ouverture subs6quente de lAtlantique), les mouvements de d6crochement le long de la zone transformante de Terre Neuve-Gibraltar depuis le Lias jusqu'au d6but de l'Eoc6ne, ainis que la convergence entre l 'Europe (Ibdrie) et 1Afrique qui d6buta au Cr6tacd sup6rieur pour culminer au Ndogbne. Dans le Haut Atlas central et dans le Moyen Atlas, le jeu combin6 de ces facteurs a pro- voqu6 la formation de grabens continentaux (Trias) puis leur envahissement par la mer h partir de la T6thys (Lias-Dogger inf6rieur). Apr6s la fin de son activit6, le rift atlasique a 6t6 combl6 par des sddiments, d'abord marins (Bajocien-Batho-

nien), puis continentaux jusqu'au soul6vement final (fin du Jurassique moyen - fin du Crdtac6 infdrieur). De 1Aptien h la fin de l'Eoc6ne moyen, l'6volution sddimentaire a 6t6 r6gie par les changements eustatiques du niveau de lamer . Apr6s un premier soulhvement moddr6 du Haut Atlas central au cours du Sdnonien, le soulhvement majeur des chaines intracontinentales ddbuta 5 la limite Eochne moyen-Eoc6ne supdrieur. Le diastro- phisme de 1Atlas au cours du Mio-Plioc6ne a co'/n- cid6 avec les mouvements orog6niques de l'arc bd- tico-rifain.

KpaTgoe co~ep~ganne

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pa3aIOMOB, npeaonpe~xea~aHcb, H3MeH.qrOmHMI~Ca SO speMeHH noaaM~ cTpecca. Baxa~e~mHMn Kpyn- HOTeKTOHnqeCKnMH qbaKTOpaMH ynpaBaeHI~, qaCTO ReI~CTByrOn~HMn OaHOBpeMeHHO, 6bIaH: TaqbporeHe3 B ceBepo-3ana;xr~o6 o6aacTn aqbpH- KaHCKOFO MaTeprmoBoro Kpas, npo~Ba~Bmr~h CBOe BaVmHne ~o Hnxc~ero Meaa (Tpna3m,ffI pHqb- THHF H r~ocae~y~omee pacmnpeHHe ~xna ATJIaHT~- qec~oro oKeaHa), TpaHcqbopMm,m 3BmKeHrL~ B 3one Hapymemua HbroqbayHaaeH~x-Fr~6paaTap u neprIo~X Mexc~y aefiacoM n paHHnM 30r~eHOM, a TaK>Ke KormepreHmzZt M e ~ y EBponofi (H6epru 0 H Acl3p~Koh, HaqaBma~c~ B nO33HeM Me;re n Roc- TnrnyBmaa cBoero anore~ B HeoreHe. COBOKyn- HOCTB 3THX IIpoKeccoB B Tpnacce Bt, I3BaJIa nom3ae- ime B i, eHTpe S~,ICOKOrOpHoro ATaaca, a TaK>Ke B cpe~HeM ATJIace MaTepHKOm, IX prIqbTom, Ix rpa6e- HOB, KOTOpi, Ie B aeflace H HI4X~IeM ;Iorrepe (cri- HeMIopcKoe-aa:ieHcKoe BpeMa), CamBmHCb BMecTe, 3arlosmm-mcb cHa~iaaa MOpCKI~rI OTaO>KennaMia (6afIOCCKUh-6aTCKUfI seKa), nperlecerim,iMrI BO~IaMn pa33rlBmeroot TeTtlCa, 3aTeM MaTepHKO- BBIM/4 ceRHMeHTaMrI; Bce 3aBepmtiaiocb noJImtTtleM y>Ke ycIIOKOeHItOfI 3OHBI pHr B nepno3 Me>KIIy

Plate tectonics and intracratonic mountain ranges in Morocco 129

no3~IHefI UmKHefi ~opofi H HO3AHI4M HHN(HHM Me- JIOM. MexcjIy alITOM H KOIIIIOM cpe~Inoro aotieHa 3BCTaTI4qeCKI4e KO2ie6aHI4.'I ypoBH.q Mop~ ynpa- BJDIJII40TJIO)KeHHeM ce~I4MeHTOB. I-[ocyle nepBoro n O A ~ T ~ B IIeHTpe BLICOKOFOpHOFO ATJIaca BO BpeM~I ceItoI-Ia ita,-iaJioct, I4MeltltO IIO~ItNTIte I4HTpa- KOHTIatteHTaJIt)Ht)IX aenefi Ha py6e)Ke cpe~sero H BepxHero 3oIIeHa. OCHOBHa~I qba3a TeKTOHW-IeCKI/IX npo~ieccoB B ATJIace coBnajIaJIa iio BpeMem~ B Miaoi~eHe/i~Hotiene c oporeHeuOM ~yrH t~eTHKO- p.qba.

Introduction

The mountain chains of the Middle Atlas and the High Atlas (Fig. 1) represent typical intracra-

tonic ranges. Though they are not part of the Al- pine Mediterranean fold belts, their history is tightly linked to the evolution of these belts due to plate motions related at first to the Mesozoic breakup of Pangaea and secondly to the mostly Cenozoic convergence of the European (Iberia) and African plates.

The principal processes controlling the Me- sozoic - early Cenozoic evolution near today's northwestern edge of Africa were mid-Atlantic rifting and subsequent spreading. Different rifting/ spreading rates in the northern and central At- lantic (adopted from PITMAN & TALWANI, 1972; OWEN, 1983; ZlEGLER, 1988) caused a secondary feature, the Newfoundland-Gibraltar fault zone, which still crosses the Atlantic and formerly

1,0o

GeoIoqical Sketch of Morocco

6 o 2 o I I

<~~~EMEDITE RRANEAN Ta nger~"~

c) (2)

A~

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30 ~ R"

Rabal

Casablanca f

BeN NleIIa[

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/F - A T r t '~ .s t " ~ #

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Errachidia

s . o

/ F- vx

[ ~ Inliers of crystaUine rocks in the Anti-Atlas

lbo go Fig. 1. Geological sketch map of Morocco (after MICHARD, 1976).

0 100 200 I ' I I

Young basins

Young volcanoes

Research areas

SEA

io

130 R. BREDE, M. HAUPTMANN & H.-G. HERBIO

trended through the Straits of Gibraltar into the Alboran Sea. These main processes - mid-Atlantic spreading and the resulting transform faulting along the Newfoundland-Gibraltar fault zone - directed the Mesozoic - early Cenozoic development of the Middle Atlas and High Atlas mountain ranges of Morocco.-Later on, the increasing European-African convergence exerted the principal structural control.

Moreover, a tendency to reacitvate the pre- existing fault pattern already evolved in the Trias- sic and is still evident today. This fault pattern is at least of Hercynian, and in the south presumably of Precambrian origin (e.g. SAADI, 1972; MICHARD et al., 1975; MICHARD, 1976; HEINITZ, 1984; PIQUE, 1987). A sketch of the main features of this fault pattern is given in Fig. 2, showing two main directions: 30-40 ~ and 70 ~ In addition, a direction of 120 ~ has local significance. One important 30-40 ~ lineament is suggested as the southwestern prolongation of the Transalboran fault into the Tizi n'Test fault zone. This slightly arcuate lineament separated the Middle Atlas and the central High Atlas from the Moroccan Meseta and in our opinion enabled small scale rotational movements of the Meseta continental block which were in-

duced by Mesozoic-Cenozoic plate motions (Figs. 5, 9, 14-15).

According to these presuppositions, the geotec- tonic evolution of the central High Atlas and the Middle Atlas can be subdivided into four principal periods.

1. Tr iass ic and E a r l i e s t L iass ic

After its consolidation towards the end of the Palaeozoic, Pangaea began to break up again during a taphrogenetic phase in the Triassic (herein the term taphrogenesis is used as a general term for the formation of rift phenomena, i.e. for the first stage of continental rupture and plate separation, cf. BATES & JACKSON, 1987). Rift grabens subsided within the supercontinent (MANSPEIZER et al., 1978; MANSPEIZER, 1982) and its central part was split up into a mosaic of blocks. The radial pattern of Triassic-Lower Jurassic doleritic dykes with its center in the Bahama region indicates a concentric stress field, presumably derived from a partial decoupling of crust and mantle (MAY, 1971). During that time, no final continental boundaries existed between the future African, European and American continents.

D U B O U R D I E U (1962) . . . .

f v l l C H A R D (1976)

ELNI (1984) .........

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Lo 2 ~ 360

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Fig. 2. Fault pattern of Morocco after DUBOURDIEU (1962), ELMI (1984) and MICHARD (1976).


M.~.r.,,~_

I]:,;(il;.:;~] and/or halite

interbasinal areas covered by {evaporitic) mudstones

Fig. 3. Distribution of Triassic-Earliest Liassic sedimentary basins in Morocco (after CHOURERT & FAURE-MURET, 1962; LORENZ, 1988; SALVAN, 1974, 1984; VAN HOUTEN & BROWN, 1977). AE = Argana-Essaouira, Be = Berrechid, BF = Bou Fekrane, Dou = Doukkala, G = Kh6misset, Mou = Moulouya, R = Rharb, Ms = Mousa ou Salah, OuC = Ourika and Central, ZG = Ziz-Guir.

Like in eastern North America, Moroccan grabens also subsided and were filled with clastic and volcanic material. Triassic-Earliest Liassic sedimentation started with conglomerates and sandstones with thicknesses varying between tens of meters and a few hundred meters. Up-section the coarse clastics change to red silt - and mudstones with intercalated evaporites (gypsum, partly halite). The top of the section is frequently capped by tholeiitic lava flows, recently dated at 210-196 Ma (M.ANSPEIZER, 1982; FIECHTNER, 1990).

The basin axes show two main directions, i.e. 45 ~ and 70 ~ (Atlantic and Mediterranean trend, KANES et al., 1973) corresponding to the strike of the future mountain ranges of the High Atlas and the Middle Atlas (Fig. 3). Besides the Argana- Essaouira basin, the most important Triassic basins within the Atlas domain were the basins of Ourika (central High Atlas, MArnS, 1977) and of Kerrou- chen (Middle Atlas, LORENZ, 1976, 1988). A system of synsedimentary faults, also running 45 ~ and 70 ~ controlled basin development. Their synsedimentary activity can be proved most confidently by extreme and rapid changes of facies and thick-

hess within one basin. The sense of their movement cannot be clearly determined, but both fault sets show indicatons for lateral and normal movements during redbed sedimentation (MArrAUER et al., 1977; LAVILLE & PETIT, 1984; PETIT & BEAU- CHAMI', 1986; HEn'ZNANN, 1987; PETIT & LAVILEE, 1987; SCnEELE, 1990). This suggests a stress field with the extensional stress 03 in a direction around 140 ~ (Fig. 4), which allowed both fault sets to react partly as normal and partly as strike-slip faults. A movement of Africa in the direction of 03 in relation to North-America and Iberia during the Trias- sic is evident because of the rifting in the central Atlantic and the beginning opening of the Tethys in the eastern Mediterranean progressing to the west.

2. J u r a s s i c and E a r l y C r e t a c e o u s

In the early Jurassic - during the Toarcian - spreading started in the central Atlantic - between the Newfoundland-Gibraltar fault zone in the north and the Bahama-Doldrums transform fault in the south - while the northern Atlantic re-


. . . . . . . . . . 7.7.--" . ...... ~.~.-~ //""'.[...v .........

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platlorm - -

h / so, Or~

Triassic. Earliest Liossic

Fig. 4. Tectonic reaction of the Atlas domain to the beginning rifting during Triassic-Earliest Liassic. TaF = Transalboran Fault.

mained in a rifting stage (PITMANN & TALWANI,

1972; HARLAND et al., 1990). These relationships caused Africa to slide eastward in relation to Ibe- ria along the Newfoundland-Gibraltar fault zone (Fig. 5).

At the northwestern edge of Africa, the Juras- sic - Early Cretaceous period was characterized by a gradually diminishing taphrogenetic uplift. The uplift interfered with tensional features causing regional subsidence. The most important subsiding regions were the aulacogen of the western High Atlas - a failed rift originating at a triple junction of the Atlantic rift (BURKE, 1976; HOSSNER, 1985)

- - and the system of the central High Atlas and the Middle Atlas. The tectonic development of this

system during the Jurassic - Early Cretaceous period can be subdivided into three successive stages:

The first stage during Sinemurian and Pliensba- chian was characterized by the propagation of the Tethys sea along rift zones toward the west. This caused the rapid flooding of the former continental riftgrabens of the Middle Atlas and the central High Atlas. Correspondingly, deposition of thick dolomite and/or limestone sequences prevailed. Cyclic sequences dominated by dolomites are related to the evolution of large-scale tidal flats, especially around the western end of the Central High Atlas, in the Atlas of Beni-Mellal, the tabu- lar Middle Atlas and all along the Haute Mou- louya (Figs. 6, 7). In contrast, deposition of more limestone-dominated sequences and/or alterna- tions of limestones and marls is linked to hemipelagic troughs in the central High Atlas and in the northern Middle Atlas (Fig. 7). During this stage sedimentation was mainly controlled by global eustatic sea-level changes due to poorly evolved re- lief and relative tectonic quiescence. Only the evolution of small hemipelagic troughs reflects continuing tensional tectonics. Accordingly, activity of the pre-existing fault pattern was low. The 70 ~ striking faults reacted as normal faults, while negative flower structures along the 30-40 ~ striking faults indicate strike-slip movements (LAvmLE, 1977; DUEE et al., 1979; HAUVXMANN, 1990) (Fig. 9).

The second stage, from Toarcian to Aalenian, was characterized by increase of siliciclastic sediments. The widespread carbonate tidal flats of the first stage emerged and dried out completely (see Figs. 7 and 8). Only in small areas around the western end and along the southern rim of the central

YV" Fig. 5. Relative motion between Africa and Iberia from Early Jurassic to latest Early Cretaceous. Note sinistral strike-slip regime at the Newfoundland-Gibraltar fault zone (NGF) induced by high drift velocities of Africa. AMP = ALKAPEKA-Microplate, MM = Moroccan Meseta, OM = Oran Meseta (after PITMAN & TALWANh 1972; OWEN, 1983; Z~mLER, 1988).


/ . / A Khanifr" ATLAS ~ ' A ~ ~ MESET hSATLTLA ~ ="

Fig. 6. Structural subdivision of the Middle Atlas and adjoin- ing parts of the central High Atlas (central Morocco). Note the two main trends of major fault zones: Atlantic trending Ttzi n'Tretten fault (ATT = Accident du Tizi n'Tretten), Northern Middle Atlas fault (ANMA = Accident nord moyen-atlasique), Ait Oufella fault (AAO = Accident des Ait Oufella), and the Mediterranean trending border faults at the northern rim of the central High Atlas. (AA = Afourer - Arhbala fault).

High Atlas alternating shallow water carbonate sediments and variegated silt- and sandstones were deposited. At the same time the marine connec- tion between the Middle Atlas and the central

High Atlas troughs was interrupted (Fig. 8). Such regressive trends during late DomeriaNearly Toar- clan can be correlated with the global sea-level lowstand of this period (see HAQ et al., 1987). However, during the same time the central part of the High Atlas trough and the northeastern part of the Middle Atlas trough experienced an acceler- ated subsidence (HAUPTMANN, 1990; SC~IEEI~E, 1990). In these hemipelagic troughs carbonate turbidite sequences changed to siliciclastic turbidite sequences. This fits well with the eustatic lowering of the sea-level according to the model of MULLINS (1983).

Later on the paleogeographic evolution reflects the increasing tectonic activity due to the onset of spreading in the central Atlantic. This caused Af- rica to move eastward in relation to Iberia and, according to OWEN (1983) and Z~ECLER (1988), to pass a bend in the Newfoundland-Gibraltar fault zone. In our opinion, this had caused the Moroc- can Meseta to rotate slightly counterclockwise. Even almost negligible rotation rates of a few de- grees will induce the proved dextral strike-slip movements in the Middle Atlas (Fig. 9, see also Fig. 5: stages -150 Ma and -125 Ma). For the

12 11113:

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i I i I i I i I i i i I i i i I i I i i i I i I A i i i i III ~-I l l / L /~ , : ~ i i, , i ' " . . . . . . . . . . . . . . .

�9 Z U K m "

Fig. 7. The palaeogeography during Early and Middle Liassic seems to be mainly controlled by the pre-existing fault pattern. Reactivated faults were bordering deeper hemipelagic basins against extended platform areas around an island or a group of is- lands in the Haute Moulouya region. Thicknesses of marine carbonate rocks compiled after BENZAQUEN (1963), COLO (1961, 1964), FEDAN (1988), HAUerMA.NN (1990), MONBARON (1981), STUDER (1987). Question marks: no data available.


Fig. 8. Note regional regression of the widespread former platform areas (cf. Fig. 7) in spite of the rising global sea-level during late Liassic and Dogger (HAQ et al., 1987). However, near-by small local depocenters with immense rates of subsidence evolved. Additional depocenters with more than 3000 m of sediments were situated in the northeastern parts of the Middle Atlas ranges (north of sketch). Thicknesses of marine carbonates and mixed carbonate-silicMastic sequences compiled after CoLo (1961, 1964), FEDAN (1988), HAUPTMANN (1990), MONBARON (1981), STUDER (1987).

Oran Meseta a well compatible clockwise rotation has been postulated by WARME (1988).

The third stage (Bajocian to Early Cretaceous) was characterized by the final retreat of the Juras- sic sea from the central High Atlas and Middle At- las realms. Though Bajocian shallow water limestones (>>Calcaire Corniche~< and overlying Batho- nian and Callovian marginal marine siliciclastics can be correlated with the global sea-level curve, most of the period was dominated by telTigenous redbeds due to the taphrogenetic uplift of the whole region. During that time, marine sediments were restrained to northeastern-most Morocco close to the Tethys realm and to the western High Atlas close to the Atlantic.

In comparison with the second stage, during the Bajocian-Early Cretaceous period the tectonic activity decreased; the taphrogenetic uplift of Northwest Africa ceased in the early Cretaceous (see JANSA & WIEDMANN, 1982; RAD & WIss- MANN, 1982, WIEDMANN et al., 1982). In relation to Iberia the African plate continued to move eastward (Fig. 5, stage -150 Ma to -105 Ma) generat- ing sinistral wrenching at the Newfoundland-

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Fig. 9. Beginning eastward drift of Africa during late Liassic induced transform faulting along the Newfoundland-Gibraltar fault zone (NGF). Passing a bend at this fault caused a slight counterclockwise rotation of the Moroccan Meseta in the latest Liassic. Note induction of dextral strike-slip faulting along NE striking faults and normal faulting at 70 ~ striking faults.


.e

. . . . . . . . 2 - - 7"-"-" .' ~,o / - " . ..............

/ ..... .

/ ,'/.....--'" ......

/ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . j . �9 . . . . . . . - . "

/ [Dogger - Eorly Cretaceous I

Fig. 10. Eastward drift of Africa from Middle Jurassic to Early Cretaceous. Sinistral wrenching along the Newfoundland- Gibraltar fault zone caused a sinistral strike-slip system in the central High Atlas and mostly normal faults in the Middle Atlas.

Gibraltar fault zone. This strike-slip regime was also transmitted to the 70 ~ pre-existing faults of the central High Atlas (Fig. 10). At stepovers, basins and ridges were formed (MONBARON, 1981, 1982; LAVILLE, 1985, 1988; SEUFE~T, 1986, 1988). In the Middle Atlas the 45 ~ faults mostly reacted as normal faults (Fig. 10). At intersection points of the two main fault directions the tensional and trans- tensional regimes overlapped and locally created zones of extreme tension (Bm~DE 1990). There, basic magmas intruded, probably restricted to the

middle and late Middle Jurassic (JACOBSHAGEN

et al., 1988a; SCnAER, 1988). Though well known from the central High Atlas, they are also docu- mented from the Middle Atlas (FEOAN, 1988; FE- DAN et al., 1989).

3. La te C r e t a c e o u s to E a r l y E o c e n e

Around the Early/Late Cretaceous transition the geodynamic setting changed. Enforced spreading started in the northern Atlantic; Iberia slid eastward relative to Africa causing dextral wrenching at the Newfoundland-Gibraltar fault zone (Fig. 11). Since the eustatically controlled Aptian transgression (WURSTER & STETS, 1982) all further transgressions invading central Morocco originated from the Atlantic. For a last time the eustatically controlled Late Cenomanian-Turonian transgression connected the Tethys and the Atlantic across the Middle Atlas realm east of the emergent Mo- roccan Meseta (ANDREU, 1989).

In spite of global eustatic sea-level highstands the Moroccan Senonian generally displays marginal marine to non-marine sequences. This is attri- buted to an initial wide-arched updoming within the High Atlas realm and adjacent regions of the Anti-Atlas (STZTS & WURSTER, 1981; HE~IG, 1991). It should be discussed whether this uplift can be correlated with the beginning convergence of Africa and Iberia which caused compressive stresses in several regions of Iberia (ZIEGLER, 1988,

/ / / I

' - . : f J

/A,r,coo c o,~ Fig. 11. Relative motion between Afiqca and Iberia from earliest Late Cretaceous to latest Early Oligocene. Note dextral strike- slip regime at the Newfoundland-Gibraltar fault zone (NGF) induced by increased drift velocities of Iberia until earliest Eocene. AMP = ALKAPECA-Microplate (after PrrMA~ & TALWANI, 1972; OW~N, 1983; ZIEGLER, 1988).


p. 84, p. 92). Moreover, the uplift coincides with the onset of compressional deformation along the southern margin of the ALI(AWCA microplate (W~LDI, 1983; ALKAPECA = Alboran-Kabylian- Peloritanian-Calabrian Microplate, BOUILHN et al., 1986). The latter, however, happened in a position farther to the east.

From Maastrichtian to late Middle Eocene times a final transgression inundated wide parts of central Morocco. The transgression reached its acme during the early Thanetian - late Lutetian (latest Bartonian?) with the complete inundation of the central High Atlas realm and a NE trending Middle Atlas gulf extending almost to Boulemane (Fig. 12). During that time a carbonate ramp de- veloped which was inclined from near-shore sediments at the southern rim of the later central High Atlas, respectively the later Middle Atlas, toward the central Moroccan phosphate basins (Hzlmi6, 1986, 1991; TI~PPE, 1989, 1991a, b). Remarkably, the palaeogeography of this epicontinental sea did not resemble the outline of the future Atlas mountain ranges.

At the southern rim of the later central High Atlas, directly adjacent to the stable African craton, Thanetian - late Middle Eocene marine sedimentation was governed by eustatic sea level changes (HERBIG, 1986, 1987, 1991). However, in the Middle Atlas synsedimentary fault movements act- ing at least since the earliest Senonian created NE striking halfgrabens which were bound to major lineaments (Northern Middle Atlas Fault, Middle

iiiiiii l

Fig. 12. Extension of the early Thanetian - latest Middle Eocene epicontinental sea in middle Morocco (after TRAVPE, 1989, 1991a, b; HEVd3m, 1991). Outcrops in black, emergent areas stippled.

/ .......... y.:-....-.. . .....

/ : ./'/ .. .......... / /.

,. /..." o r o o / /s..,.

,/ / \ / .. .... Me~et~ .......

J , O, r ~ / ..:. ........................... :/ _0 o~ :..~.J

/ _~ o~ .:i" ~ ~ .................. / :" :.: ...... .....

/ / s~ ~'~r~

[ Late Cretaceous + Early Tertiary I

Fig. 13. Dextral wrenching along the Newfoundland-Gibraltar fault zone (cf. Fig. 11) induced a dextral strike-slip regime in the central High Atlas and mostly reverse faulting in the Middle Atlas.

Atlas Fault: Fig. 6; see FEDAN 1988, H E l m 1988). Their formation was caused by the activation of the 30-45 ~ faults as reverse faults in reaction on the dextral wrenching along the Newfoundland- Gibraltar fault zone (Fig. 13). DUEE et al. (1979), CHARRIERE (1985), and HEVd~IG (1988) demon- strated sinistral wrenching components at these faults, probably resulting from the beginning convergence between Africa and Iberia. Also induced by dextral wrenching at the Newfoundland- Gibraltar fault zone, presumably dextral strike-slip movements took place along 70 ~ faults in the central High Atlas (Fig. 13).

In spite of the >>epeirogenetic<< uplift of the central High Atlas during the Senonian and continued synsedimentary fault tectonics in the Middle Atlas, the Cretaceous and early Paleogene were a time of tectonic quiescene. It preceeded the principal tec- togenesis of the Atlas domain during Neogene time (JENNY et al., 1981; MONBARON, 1982; JACOBS- HAGEN et al. 1988b).

4. La te E o c e n e to R e c e n t

During latest Paleocene - earliest Eocene wrenching along the Newfoundland-Gibraltar fault zone ceased (ZIEGLER, 1988: 122) because Iberia and Africa drifted eastward at equal velocities (Fig. 14). During late Middle Eocene and Late Eocene overthrusting along the southern margin of ALr~PECA started (W~LDI, 1983), which eventually

Plate tectonics and intracratonic

led to the nappe structures of the Malaguides and Ghomarides during the Oligocene (BOUILLIN et al., 1986). During the same period, the emplacement of the Alboran block started, being underthrusted by continued eastward movement of Iberia and Africa (AN]:)]aEux et al., 1971; LEBLANC & OLmER, 1984; BO~ILLIY et al., 1986). These tectonic processes caused a first uplift in the High At- las realm. Local conglomerate fans containing re- worked Jurassic limestones of High Atlas origin were deposited as early as latest Middle Eocene at the southern rim of the central High Atlas. Local clastic input continued through late Eocene and Early Oligocene (HERBIG, 1991); further uplift during the Oligocene is assumed. In the Middle Atlas coarse conglomerates occur in a comparable li- thostratigraphic position south of the Northern Middle Atlas Fault. Though biostratigraphically undated, they are presumably also of Oligocene age (DRESNAY, 1969; MARne, 1981). Concurrently with this uplift, around the Eocene - Oligocene transition first alcaline volcanism occurred in the eastern prolongation of the deep-sited Afourer- Arhbala fault zone south of Midelt (Fig. 6) (HAR- MAND & CANTAGREL, 1984:35 + 3 Ma).

The uplift of the Atlas, beginning in the latest Middle Eocene, actually affected all of central Morocco south of the Rif. This is unequivocally stressed by the absence of the important Early Oli-

mountain ranges in Morocco 137

gocene eustatic transgression in the realm of the earlier Paleogene epicontinental seas.

In addition to their joint eastward drift, convergence between Iberia and Africa increased since the Miocene, thus triggering the nappe thrusting in the Betic-Rifean internal zones (DEWEY et al., 1973, MXKEL, 1985; DU~ND- DELGA & OLMER, 1988; PLATY & VISSERS, 1989). Synchronously, compressional deformation and uplift reached a first climax in the central High Atlas (e. g. G6RLER et al., 1988: gravitative emplacement of the nappe of Toundout in late Middle Miocene times) and probably in the Middle Atlas as well. There, scattered mid and late Miocene alcaline volcanism occurred between 15 and 6 Ma on 40-60 ~ faults reactivated by the Rif tectonics as sinistral strike-slip faults (HARMAND & CANTAGREL, 1984).

The stress field causing these tectonic events was mostly orientated with the compressional stress o1 in a direction around 160 ~ (LETOUZEY & TREMOL~Em~S, 1980; AIT BgA~M & CHOnN, 1984; FgAISSI~ET et al., 1988). Correspondingly, the 70 ~ faults in the central High Atlas show a strict up- thrust character (BREDE, 1988); the 30-45 ~ faults reacted as left oblique-slip reverse faults (Bm~DE & HEINrrz, 1989; see Fig. 15). The increasing shor- tening amount of the sedimentary cover in the central High Atlas from 10% in its western part to

O m ! !

an plate / -

/ ,

~ 'e75 /~_ '=J~ - ..r / /

~)5./ # / . / " /

. / j .

ix r\cO

/ /

/

/ ~ f '

Fig. 14. Relative motions between Africa and Iberia from Oligocene to Recent. Note missing strike-slip at the Newfoundland- Gibraltar fault zone because of equal drift velocities of Africa and Iberia. Intern. = Internides of the Betic-Rif orogen, MM = Moroccan Meseta, OM = Oran Meseta (after PITMAN & TALWANI, 1972; Ovcz~, 1983; ZIE~LER 1988).


15% in its middle part, and finally to 20% in its eastern segment (JACOBSHA6EN, 1986; SEUFERT, 1986; BR~OE 1987, 1988) points to a dextral rotation of the Moroccan Meseta of 3 -5 ~ (Fig. 15). This is compatible with the fault movements.

In addition to chronological coincidences the orientation of the compressional stress ~1 clearly demonstrates the strict link between the tectonic development of the Betic-Rif orogen and the intracratonic Atlas tectogen. The compressional stress seems to have been transmitted by a thin- skinned thrusting according to the model of BALLY (1984); presumably shear planes were used, which had already been activated during the formation of the Atlas guff in the Jurassic (JAcoBSHAGEN et al., 1988b).

Conclusions

The sedimentary and structural history of the central High Atlas and the Middle Atlas during the Mesozoic and Cenozoic is governed by the complex interaction of sea-level changes and plate tectonics.

Principal elements guiding the structural evolution were: 1. The waning taphrogenesis of northwestern Af-

rica following the late Variscan formation of the supercontinent Pangaea (Permian - Early Cre- taceous). It comprised central-Atlantic rifting and following spreading since the Triassic.

2. Transform faulting at the Newfoundland- Gibraltar fault zone caused by different spreading rates in the northern and central Atlantic (Liassic - earliest Eocene). Thereby, changing drift velocities between lberia and Africa governed the relative strike-slip motions and induced correspondingly changing stress regimes in the Atlas domain.

3. Convergence between Europe (Iberia) and Af- rica starting in the Late Cretaceous and culmi- nating during Miocene-Pliocene times. The resulting stress regimes, changing through

time, were overprinted on a pre-existing Panafrican-Variscan fault pattern. This resulted in an interplay of wrench tectonics and vertical tectonics depending on the orientation of the faults.

Betic- ~/ Rif orogen ~ ~}

" ' " r

/ ...'

/ ''" .oo.+o ('/ ~ 02 ~ "..."" MesetO ... J . . . . . . . . . . . . . . . . . . .

O " o 4 , . , ~ o-'- . . . . . . . . . . . . . . . . . . . . . . . . . / . . . . . . . . . . . . . .

...... ;:! ........... > .......... i I'~[;.'~i .~''#y'~'~ """ ................... iotform

k / s~ ~

[Late Eocene - recent] Fig. 15. Joint eastward drift and final convergence of iberia and Africa from Late Eocene to Recent caused south directed thrusting of the Rifean nappes. Eastward increasing shorten- ing rates in the central High Atlas indicate a slight clockwise rotation of the Moroccan Meseta.

Correspondingly, the different strike of the axes of the central High Atlas and the Middle Atlas is responsible for the different tectonic history of both ranges (see also JACOBSHAGEN et al., 1988b).

The sedimentary history of the Atlas ranges strongly reflects the action/interaction of the single structural elements. However, major global eustatic sea-level variations were able to overprint the structural evolution. They even controlled sedimentary history during times of tectonic quiescence or, temporarily as well, at the southern rim of the central High Atlas adjacent to the stable Af- rican craton.

Acknowledgements

The authors are deeply indebted to the Moroccan author- ities, especially to Dr. M. Bensaid and M. Dahmani, Minis- t~re de l 'Energie et des Mines, Rabat. Their friendly coopera- tion enabled uns to carry out the field work during 1984-1989. The study was supported by grants of the Deut- sche Forschungsgemeinschaft and the Freie Universitfit Berlin. Special thanks are due to Prof. Dr. V. Jacobshagen (Berlin) for initiation and manifold support of the investigations. Many thanks also to Prof. Dr. J. E. Warme (Colorado School of Mines, Golden) for the careful revision of an earlier version. Prof. Dr. D. Bernoulli (ZOrich) and Prof. Dr. H. Miller (M0nchen) provided critical reviews.

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