Journal of African Earth Sciences, Vol. 14, No. 1, pp.57-65, 1992. 0899-5362/92 $5.00+0.00 Printed in Great Britain © 1992 Pergamon Press Lid

Notes on Neogene basin history of the Western Alboran Sea and its implications for the tectonic evolution of the Rif-Betic orogenic belt

C. K. MORmY

Amoco Production Co., 501 Westlake Park Blvd. Houston, Texas 77253

(First received and accepted for publication 21 May, 1991)

Abstract - Several recent tectonic models for the Alpine deformation in the Rif-Betic Cordillera have proposed some form of uplift of the hinterland of the thrust belt (presently the Alhoran Sea), which as it collapsed and extended, drove thrusting in the more external zones. This thrusting episode occurred predominantly during the Middle and Upper Miocene; consequently if the tectonic model is correct, basin fdl of the same age underlying the Alboren Sea should be extensively affected by large scale extensional features. This is not the case in the Western Alboren Basin where seismic reflection and well data indicate that little if any Upper Miocene extension has occurred, end deformation of the Miocene section is predominantly by shale diapirism. Although the crust under the Alboren sea is thirmed the timing of the thinning is unknown. It may either be attributed to a pre-Miocene extensional phase, or to delamination of the lower crust perhaps caused by subduction during the Oligocene.

INTRODUCTION

The Moroccan Rif is a n alpine fold and t h r u s t bel t t ha t ex tends into Spa in a s the Betic Cordillera. T h r u s t t r a n s p o r t d i rect ions swing th rough a 180 ° arc, f rom a sou the r ly direct ion in Morocco to a nor ther ly di rect ion in Spain. The h in te r land to the Rif-Betic Cordi l lera is c u r r e n t l y a d e p r e s s i o n occup ied b y t he A lbo ran Sea (Fig.l). Several tectonic hypo thes i s have b e e n advanced to l ink the de format ion in the onshore t h r u s t be l t s wi th va r ious driving m e c h a n i s m s in the Alboran Sea. These inc lude the collision o f a micropla te (Alboran plate) wi th Africa and Iberia (Andrieux et al., 1971; Wildi, 1983) and Neogene radial gravi tat ional col- lapse of an uplif ted orogenic belt. In the la t ter model c o m p r e s s i o n in the t h r u s t bel t is dr iven b y b o d y forces re la ted to the uplift. This uplift h a s b e e n a t t r i b u t e d to e i t he r a m a n t l e t h e r m a l an o ma ly (Van B e m m e l e n 1959; Loomis, 1975; Torres-Roldain, 1979; WeiJermars, 1985) or to c rus ta l th ickening (Platt e t al., 1983; Platt 1986; Platt and Vissers , 1989; Dob las and Oyarzun 1989). Dewey (1988) ques t ioned the validity of the gravitational conapse hypo thes i s b e c a u s e in order to drive the ex t ema l t h ru s t i ng the col lapsing uplift a rea m u s t r ema in topographical ly higher t h a n the t h r u s t belt. This clearly w a s not the case s ince the Alboran Bas in received mar ine s ed imen t s f rom the Middle Miocene onwards , s y n c h r o n o u s l y with the m a i n th rus t ing event. Recent pape r s (Platt and V i s s e r s , 1989; D o b l a s a n d O y a r z u n , 1989; Doglioni, 1989) p roposed tha t large a m o u n t s of

ex tens ion bo th onshore and offshore occur red dur ing the Middle-Upper Miocene. This ex tens ion shou ld be appa ren t a s no rma l faul t ing in the Wes te rn Alboran Sea Basin .

The object of th i s p a p e r is to examine the de- format ion style of the w e s t e r n Alboran bas in , a nd integrate the Alpine deformat ion h is tory onshore , with the k n o w n b a s i n h is tory offshore. The age of the Wes te rn Alboran Bas in fill is k n o w n th rough DSDP 121 and several i n d u s t r y w e n s including the El J e b h a - l wen p r e s e n t e d here. This well da t a correlate with mult ifold se ismic data. On the bas i s of t hese da ta a revised tectonic evolut ion of the Rif- Alboran Sea area is proposed .

STRUCTURE OF THE ALBORAN SEA

Se i smic r e f r ac t i on d a t a i n d i c a t e s t h a t t he Alboran Bas in is unde r l a in b y 13-20 kin th ick cont inenta l c r u s t and b y a n o m a l o u s l y low velocity (Vp = 7.6 - 7 .9 / sec ) u p p e r mant le (Banda et al., 1983). Hence c rus ta l th inning h a s apparen t ly occurred , and m o s t a u t h o r s da ted th is event a s Neogene (part icularly Middle and Uppe r Miocene). In addit ion, relatively inconclus ive se ismic re- flection da ta have b e e n u s e d to sugges t the pre- sence of hors t and g raben s t r u c t u r e s in the ba s in (Dillon e t a / . , 1980). The m o s t vital informat ion tha t could suppo r t the theor ies concern ing the Neogene gravi tat ional col lapse of wha t is now the Alboran Sea is the t iming and deformat ion style of s t r u c t u r e s in t he Neogene s e d i m e n t s of t he Alboran Basin. Yet the indus t r ia l se ismic reflection

57

58 C. K. MORLEY

da ta wh ich con ta in th is informat ion have not b e e n effectively incorpora ted into the tectonic models . S u c h mul t i - channe l reflection da ta have b e e n acqu i red in the Wes te rn Alboran Sea and provides a detai led view of the ba s in geomet ry (Mulder and Parry, 1977). Amoco r ep roces sed and re in te rpre ted th is da t a du r ing the ear ly 1980s, acqui red new da ta and drilled the El J e b h a - l well w h i c h provides some firm age control on the bas in . Age da t ing of foraminifera b y Amoco indica ted t ha t t he oldest s e d i m e n t s encoun te red b y the El J e b h a - l well, a t the final TD of 8 9 8 7 feet ( subsea 8838'), were Langhian (Fig. 2). In addit ion, a Langh ian /Aqu i t an i an un- conformi ty w a s pene t r a t ed b y the Andaluc ia G-l well drilled in the Alboran Sea off the S p a n i s h coas t (Fig. 1). Seismic reflection d a t a indicate th is unconformi ty lies below the b a s e of the sediment- a ry sec t ion in the El J e b h a - l well. Correlat ing reflectors away from the El J e b b a well demon- s t r a tes tha t m u c h of the bas in fill visible on se ismic is of Middle to U p p e r Miocene age.

Figure 3 d i sp lays th ree line drawings of regional se ismic l ines a c r o s s the Alboran Sea. The da ta are domina ted b y a s e q u e n c e of parallel and sub- parallel ref lectors t ha t d i sp lay very low dips. Line 50 is the s imples t line with two ma in fea tu res tha t in te r rup t the parallel reflectors. At the sou th - wes t e rn end of the line. the reflectors show thin- ning and te rmina t ion on to an area of higher- angle ref lect ions and diffractions. This area pro- bab ly r e p r e s e n t s Hereyn ian b a s e m e n t coming to

the nea r - su r face nea r the Moroccan coast- l ine, i.e. the b a s i n fill p inching ou t and on lapp ing t he rocks which a p p e a r as the internal zones of the Rf fonshore in Morocco. The second in te r rup t ion of the reflectors is a m o u n d a b o u t 15 krn wide t owards the nor th- ea s t e rn end of the line. The ref lectors from the b a s i n fill a p p e a r to t e rmina te wi th on lap against the m o u n d . The m o u n d h a s s loping s ides and an i rregular top which gene ra t e s diffractions. This is p robab ly a b a s e m e n t high and it d i sp lays s imilar i r regular reflector pa t t e rn to the b a s e m e n t high drilled by DSDP 121 (see line 26). The m o u n d is p robab ly a n eros ional feature , not a faul t b o u n d e d b lock b e c a u s e there L~ no a p p a r e n t ro ta t ion of the re- f lectors f rom horizontal - sub-hor izon ta l to inclined as wou ld be expec ted for a normal fault . On lines 50 and 26. t he overall s a u c e r s h a p e b a s i n geomet ry is appa ren t where the b a s i n t h i c k n e s s t owards the cen te r and on laps a t the margins . The b a s i n fill on llne 50 r eaches a t h i c k n e s s of a t leas t 4-5 k m (based on ana lys i s of se ismic interval velocities). All three seismic Ines in Figure 3 s h o w zones wi th in the bas in fill reflectors where there are f ew or no p r imary reflectors p r e sen t and diffract ions are a b u n d a n t . Commonly , diffractions occu r at the top of the no da ta zones. On ei ther s ide of the no d a t a zones sub - horizontal , parallel ref lectors are present . If the no da ta zones r ep re sen ted faul t zones tilting of the reflectors would be expected , th i s h a s no t occurred . The ref lec tor p a t t e r n is typ ica l of p i e r c e m e n t fea tures , and in th is case is p robab ly d u e to shale diapir ism (as sugges t ed previous ly b y Mulder and

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Parry, 1977). Individual diapirs appear to be about 5 km in diameter, bu t in some areas they have coalesced to form broader zones (especially on Line 17). The mos t intense diapirism appears to have occurred in the center of the basin (Fig. I).

The key points tha t arise from analysis of the seismic data are tha t the basin fill is extensively deformed by shale diapirs (as discussed by Mulder and Parry, 1977) bu t is very litUe deformed by extensional faults (Fig. 3). The diapiric shale lies below the reflectors tha t can be traced to the El Jebha-I well, and is therefore at least Middle if not Lower Miocene in age. A marked erosional event tha t is probably an unconformity at the top of Her- cynian basement is prominent on some lines (Fig. 3). In places it appears tha t basement lles very close to the sea floor. The geometry of the

59

'"oasement" event suggests tha t there is extreme erosional topography causing the variations in basement level and not normal faults (Fig. 3). Certainly large-dlsplacement normal faults are not present in the overlying Neogene sediments. Thus ffnormal faulting has affected the basement it is of pre-Middle Miocene age. The offshore Neogene sediments display very low dips. and evidence of extensional faulting is very limited. It can. there- fore. be stated most emphatically tha t there is no evidence for the large scale extension required to drive the major Middle-Upper Miocene thrust ing event seen onshore in the Rff.

In gross geometry the WesternAlboran Basin has gently dipping flanks tha t dip towards the basin center. The basin fill onlaps the basin margin and the contacts between the sediment and the "pre- basin" reflectors are strat igraphic, not fault- bounded. Hence, the geometry is essentially that of a sag basin.

THINNING OF THE JtI.~ORAN CRUST

The seismic refraction data and drilling from the Alboran Sea a rea ind ica te the p r e sence of continental c rus t tha t has been th inned to 13-20 km (Hatzfield et al., 1977; Banda et aL, 1983). Onshore a round the arc there is evidence of episodic Miocene extension (Nold etal. , 1981, Platt e t a/., 1983; Platt and Vissers 1989; Saddiqi et al., 1988). Hence it was logical to llnk this extension with the thinning of crus t in the Alboran Sea. The Miocene extension was thought to have occurred during a period of overall plate convergence (Dewey et al., 1989). Consequently, to accommodate the extension, thrus t ing has to have been generated a round the periphery of the extending Alboran region as discussed previously. In the last section, it was demonstra ted tha t major extension in the Alboran Sea is absent during the time of Middle- Upper Miocene thrusting. If extension occurred earlier in the Alboran Sea (Upper Oligocene-Lower Miocene) then it corresponds with a time of relative tectonic quiescence onshore (Fig. 4).

The sag basin geometry of the Middle-Upper Miocene basin fill is indicative of a period a relative thermal cooling of the c rus t (McKenzie 1978; White and McKenzie, 1989). Consequently, this suggests a heating event prior to deposition of the basin fill i.e. Lower Miocene or older. Such heat ing would have occurred after the collision and crustal t h i cken ing events of the Late Eocene-Ear ly 01igocene (Torres-Roldan 1979; Platt and Vissers, 1989) and would logically coincide with the high temperature-low pressure Upper Oligocene meta- morphism recorded in the Internal Rif (Felnberg et a/,, 1990).

An alternative to maj or extension as the thinning

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Neogene basin history of the Western Alboran Sea and its implications for the tectonic evolution... 61

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1988a, b, in reviev~, Leblanc, 1979. 1987; Saddiqi etoL, 1988; FeinbergetaL, 1990.

mechan i sm for the c rus t benea th the Alboran Sea is delamination and subduct ion of the lower part of the c rus t as well as the mant le li thosphere (Fig. 5). The late Neogene volcanic events (Bellon and Brousse, 1977; Lopez Ruiz and Rodrlguez Badiola, 1980) may be related to partial melting of this subducted lithosphere. This model does not seem particularly attractive due to the problems of subduct ing large areas of relatively buoyant lower c rus t into denser mantle material. However, given a sufficient volume of descending oceanic crust and mantle lithophere, some at tached continental crus t could be dragged down with it (Molnar and Gray, 1979). Apart from the subduct ion of conti- nental crust , this early history of the region is similar to tha t proposed by Platt and Vissers. 1989.

A third explanation for the th inned crust unde r the Alboran Sea is to assume tha t the Alboran plate

(Andrieux, 1971; Wildi, 1983) was composed of thinned continental c rus t prior to its collision with, and emplacement on the African and Iberian con- t inental crust. Hence, there is then no need to find a mechanism for crustal thinning during the Alpine deformation.

TIMING OF DEFORMATION AND DEPOSITION EVENTS IN THE

RIF AND ALBORAN SEA

Figures 4 and 5 contain summar ies of the de- formation and deposiUonal events in the Rlf thrust belt. It is apparent tha t there are two main com- pressional events. One occur red from latest Eocene into the Middle Ollgocene and generally affected the more internal uni ts (Nold et oL, 1981; Morley, 1988 a, b, Saddiqi etal . , 1988; Feinberg et a/., 1990). the other lasted from Middle Miocene

62 C. K. MORLEY

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Fig. 5, One possible model for the tectonic evolution of the Moroccan l~If, which involves delaminatlon and subduction of the lower crust ofa microplate after it collided with North Africa, Subsequent upweUing of hot mantle asthencmpheve first caused uplift in the Alboran Sea area, followed by down-

warping as the asthenosphere cooled. Maps showing the kinematic reconstruction of the evolution of Morocco and lberla are mostly derived from Dewey e t a/. ( 1989}, It should be noted that this Is only a partial model for the area since it considers only the timing of events in the Moroccan Rlf. to complete the model the evolution of the Betic Cordillera needs to be incorporated.

Neogene basin history of the Western Alboran Sea and its implications for the tectonic evolution... 63

into the Pliocene, affected the entire R i f th rus t belt and appears to be the m a i n period of alpine folding and t h ru s t i ng (Nold et oL, 1981; Leblanc, 1979, 1990; Suter , 1980a, b; Wfldi, 1983). Some relative- ly mild m e t a m o r p h i s m accompanied bo th de- format ion episodes. These events are separa ted by a t ime of relative tectonic quiescence or mild com- press ive events in the Upper Ollgocene-Lower Miocene, dur ing which extensive turbidi te depo- si ts (Acflah, Zoumi , Ouezzane and Numid i an Sands tones) were laid down. Thus , if any thermal upliR of the Internal Zones did occur, it occured dur ing the pre-Middle Miocene t ime as indicated by the Western Alboran Sea deposi t ional history.

CONCLUSIONS

Tectonic mode l s for the Alpine evolution of the Alboran Sea which propose major Middle-Upper Miocene extensional events, are Incorrect based on analysis of t he bas in fill u s ing industr ia l seismic and well information. These data show tha t the Western Alboran Basin h a s a sag bas in geometry which displays few extension faul ts and extensive shale diapir lsm in a bas in tha t was init iated at least in the Middle Miocene, if not older.

Major episodes of t h rus t ing can be identified onshore in the Rif be tween the latest Eocene and Middle Oligocene, and Middle Miocene-Pliocene. If an event involving uplift and extension of the Internal Zones occurred, it m o s t probably occur- red dur ing the Upper Oligocene-Lower Miocene. This uplift and extens ion occurred dur ing a period of relative t h rus t i ng quiescence, hence extension canno t be invoked to drive per ipheral compressive events, as h a s been sugges ted in m a n y models for the Rif-Betic orogenic belt (Van Bemmelen , 1959; Loomis. 1975; Torres-Roldaln, 1979; Platt and Vissers , 1989; Doblas a n d Oyarzun , 1989; Doglioni, 1989).

Th inn ing of the c rus t under ly ing the Alboran Sea could have occurred in one of two ways If the model of upliR and extension dur ing the late Tertiary is rejected: 1. de lamina t ion and s u b d u c t i o n of the lower c rus t a t t ached to subduc t i ng mant le litho- sphere , and 2. extension and th inn ing of the crust , p e rh aps dur ing the early Mesozoic, bu t at least prior to involvement with the Alpine collision belt. The pr imary point of this paper is to point to the lack of evidence for Late Tertiary extension in the Western Alboran Sea region. Unfortunately, it is not so easy to sugges t which of the remain ing models bes t explains the tectonic evolution of the Alboran Sea. To conc lude this paper, t h e possible evolution of the orogenic belt can be considered as a c rus ta l de laminaUon model, s u c h a scenario has no t been widely d i scussed in the li terature.

The leading edge of the overriding plate in some

t h r u s t bel ts {Austro-Alpine t h r u s t sheet, Alps) is composed of a n u p p e r c rus ta l s ed imen t s with no b a s e m e n t a t t ached . The b a s e m e n t tha t once under lay the t h r u s t shee t ha s now been de tached from it. The only place to move th is cor responding lower c rus t is to s u b d u c t it b e n e a t h the cont inenta l c rus t t ha t the u p p e r c rus ta l s ed imen t s are over- r iding (Oxburgh, 1972; Laubscher , 1988). The same problem arises in the Rif where low-grade to u n m e t a m o r p h o s e d Mesozoic ca rbona tes form the leading edge of the Alboran plate onshore . It would appear tha t the lower c rus t of the Alboran plate decoupled from the u p p e r crust , at least a t the leading edge. A s s u m i n g tha t oceanic c rus t was s u b d u c t e d prior to collision of the Alboran plate with North Africa, there would be a zone of oceanic l i thosphere tha t could exert sufficient slab pull force to s u b d u c t a considerable a m o u n t of delami- na ted lower cont inenta l c rus t coupled wi th mant le l i thosphere, prior to buoyancy forces completely opposing the slab-pull forces. A plate only a few h u n d r e d miles long on each side, like the proposed Alboran plate, migh t completely lose its lower c rus t t h rough delaminat ion, while s u c h a process would be unlikely w h e n applied to a major cont inenta l m a s s (Molnar and Grey, 1979).

The following evolut ion of the Rif and Alboran Sea r e tu rns to some of the earlier plate tectonic models for the region which rely on collision be- tween Iberia, Africa and the Alboran microplate (Andrieux, etal., 1971, Wfldi, 1983), see Figure 5. The a im is to p resen t a model which fits the data from the R i f th rus t belt. This mode l considers only one hal f of the t h r u s t belt. and in the fu ture if the model is shown to have any validity it needs to be reconciled with data from the Betic Cordillera.

1. Initial collision occurred dur ing the Eocene between the Alboran plate and Africa (Figs 5a, b). The m a i n t h rus t i ng direction was towards the west to west southwest . Subsequent ly , the Alboran plate and Africa collided wi th Iberia. Thrus t ing con t inued unt i l the Middle Oligocene.

2. Middle Oligocene - Lower Miocene A period of low differential s t ress in the AIboran

plate was initiated, pe rhaps because the slab-pull force ceased as a consequence of complete delami- na t ion of the lower cont inenta l c rus t and mant le l i thosphere b e n e a t h the Alboran plate (Fig. 5c). Subsequen t emplacemen t of hot a s thenospher ic mant le benea th the t h inned Alboran plate caused uplift, local extension, and h igh tempera ture- low pressure me tamorph i sm.

3. Middle Miocene-Pliocene As plate mot ions con t inued to move the African

plate towards Iberia h igh differential s t resses were renewed in the t h r u s t belt and extensive th rus t ing occurred in the Rif (Fig. 5d). Cooling of the Alboran plate c rus t after the emplacemen t of hot as theno-

64 C.K. MORLEY

s p h e r i c m a n t l e r e s u l t e d in c r e a t i o n of a t h e r m a l s a g b a s i n in t h e W e s t e r n A l b o r a n B a s i n . S o m e m i n o r e x t e n s i o n a l f a u l t i n g o n s h o r e m a y h a v e ac - c o m p a n i e d t h e s a g b a s i n f o r m a t i o n of fshore ,

M a n y t e r t i a r y - a g e t h r u s t b e l t s a r o u n d t h e w o r l d d i sp l ay h i n t e r l a n d s e d t m e n t a ~ / b a s m s . (see Dewey , 1988 fo r examples} In s o m e b a s i n s c o n s i d e r a b l e e x t e n s i o n h a s b e e n r ecogn ized . A p p l i c a t i o n of t h e e x t e n s i o n a l m o d e l to o t h e r l e s s well e x p o s e d h i n t e r l a n d b a s i n s , h a s b e e n w i d e s p r e a d , a n d in- c l u d e s t h e A l b o r a n s e a d i s c u s s e d he re . T h i s p a p e r h a s a t t e m p t e d to d e m o n s t r a t e t h a t t h e A l b o r a n S e a h i n t e r l a n d b a s i n h a s i t s o r ig in s a s a s a g b a s i n . T h e s a g b a s i n w a s p r o b a b l y f o r m e d in r e s p o n s e to t h e r m a l r e l a x a t i o n of t h e c r u s t a f t e r a t h e r m a l p l u s e , w h i c h in t u r n m a y b e r e l a t e d to a c r u s t a l d e l a m i n a t i o n e v e n t o r a n ea r l i e r r i f t ing even t . A l t h o u g h t h e o r i g i n s of t h e W e s t e r n A l b o r a n B a s i n a r e u n c e r t a i n , w h a t is c l e a r is t h a t a n e x t e n s i o n a l g e o m e t r y is n o t t h e o n l y p o s s i b l e g e o m e t r y fo r t h i s a n d p o s s i b l y o t h e r b a s i n s in t h e h i n t e r l a n d s of t h r u s t be l t s .

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