Subduction and tectonics on the continental margin off northern Spain

Marine Geology, 32 (1979) 53--70

53

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

SUBDUCTION AND TECTONICS ON THE CONTINENTAL MARGIN OFF NORTHERN SPAIN*

GILBERT BOILLOT ~, PIERRE ALAIN DUPEUBLE 2 and JACQUES MALOD I ~Laboratoire de G~ologie dynamique, Universit$ P. et M. Curie, Paris (France) 2Laboratoire de G~ologie, Universit$ de Rouen, Mont Saint Aignan (France)

(Received December 14, 1977; revised and accepted July 11, 1978)

ABSTRACT Boillot, G., Dupeuble, P.A~ and Malod, J., 1979. Subduction and tectonics on the continental margin off northern Spain. Mar. Geol., 32: 53--70. Seismic reflection profiles and numerous pre-Neogene samples obtained from the continental margin off northern Spain provide a means to reconstruct its Mesozoic and Cenozoic evolution. During the Mesozoic, this evolution can be compared to that of the passive margins of the western Iberian peninsula and France (rifting during the Late Jurassic, transgression and deposition of "black shales" during the Early Cretaceous). These series have been tectonically deformed and emerged, together with their basement, during the latest Cretaceous and Eocene when the oceanic lithosphere from the Bay of Biscay underwent a limited (southward) subduction. Then the margin of northern Spain was again submerged during the late Paleogene and Neogene.

INTRODUCTION A s h a r p c o n t r a s t can be o b s e r v e d b e t w e e n t h e m o r p h o l o g y o f t h e n o r t h e r n a n d s o u t h e r n p a r t s o f t h e c o n t i n e n t a l m a r g i n in t h e B a y o f Biscay. T h e n o r t h e r n m a r g i n , b o r d e r i n g t h e " M a s s i f A r m o r i c a i n " , is c h a r a c t e r i z e d by a wide continental shelf (70--80 km) and a m o d e r a t e l y steep slope ( 5 - 6 % declivity, 3 0 - - 4 0 k m wide) w h e r e a s t h e s o u t h e r n o n e s h o w s a n a r r o w ( 3 0 - - 4 0 k m ) shelf a n d a s t e e p slope ( 1 0 - - 1 2 % ) ( B r e n o t and Berthois, 1 9 6 2 ; L a u g h t o n et al., 1 9 7 5 ) . This c o n t r a s t can b e e x p l a i n e d b y d i f f e r e n c e s in geological h i s t o r y . T h e e v o l u t i o n o f t h e m a r g i n o f f t h e " M a s s i f A r m o r i c a i n " is t h a t o f a passive m a r g i n o f t h e " A t l a n t i c T y p e " ( M o n t a d e r t et al., 1 9 7 1 a , 1 9 7 4 ) . In c o n t r a s t , t h e n o r t h e r n Spanish m a r g i n has u n d e r g o n e d e f o r m a t i o n and s h o r t e n i n g d u r i n g t h e l a t e s t C r e t a c e o u s and E o c e n e b e c a u s e o f s u b d u c t i o n o f t h e o c e a n i c l i t h o s p h e r e f r o m t h e Bay o f Biscay u n d e r t h e I b e r i a n p e n i n s u l a (Boillot et al., 1 9 7 1 a , 1 9 7 3 a , 1 9 7 6 ; M o n t a d e r t et al., 1 9 7 1 b ) . T h e r e f o r e , this m a r g i n can b e c o n s i d e r e d as a fossil active m a r g i n , w h i c h has b e e n stable *Publication No.68 du Groupe d'Etude de la Margc Continentale (ERA No. 605).

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Fig.1. Location and b a t h y m e t r y of the "Le Danois Bank", m o d i f i e d after Brenot and Berthois (1962).

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since the Paleogene, and which was prevented from colliding with the south Armorican margin when the Iberian and European plates stopped converging at the time of folding of the Pyrenean chain (Le Pichon and Sibuet, 1971; Boillot and Capdevila, 1977). It is difficult to estimate the amount of lithosphere that has been subducted in the southern part of the Bay of Biscay. It appears that it did not exceed 100--200 km, so that significant volcanism or metamorphism were not induced, although other geological consequences of early phases of subduction such as structural deformation can be observed on that margin. In our study we have chosen the area located north of the Asturias (Fig.l) where the margin is characterized, from north to south, by the following structural units: (1) The northern Spanish marginal trench (presently filled), which corresponds with the early Cenozoic boundary between converging plates. This trench is characterized both by a thick sediment fill and by a strongly negative gravity anomaly (Sibuet et al., 1971; Montadert et al., 1971b, c; Sibuet et Le Pichon, 1971). (2) A relatively shallow (-- 500 m) marginal plateau, the "Le Danois Bank", where pre-Mesozoic basement (Capdevila et al., 1974) and Mesozoic sedimentary rocks (Boillot et al., 1971b) outcrop. (3) An "inner basin", characterized by a thick sediment cover, locally disturbed by diapirs (Lepr~tre, 1974). (4) The northern Asturian continental shelf, where numerous studies have shown the effects of pre-Late Lutetian tectonic deformation of the Cretaceous layers. Although this margin appears rather complex, its study has been possible through the dredging and coring of numerous outcrops of pre-Neogene sediments, thus providing the basis for correlation between these rocks and seismic reflectors. The result is a relatively precise geological map of the area (Fig.2) and a reconstruction of the stratigraphic sedimentary column for the Mesozoic and Cenozoic (Fig.3). Later in this paper the stratigraphic record will be compared to that of IPOD/DSDP sites 398, 400, 401 and 402 (legs 47b and 48) (Ryan et al., 1976; Montadert et al., 1976, 1977). THE PRE-MESOZOIC BASEMENT

The Hercynian basement of the Asturias disappears beneath its sediment cover in the immediate vicinity of the present shore-line. It is deeply buried beneath the continental shelf and the "inner basin". This contrasts with the occasional outcropping of a highly diffracting acoustic basement near the top of the "Le Danois Bank" as observed on seismic profiles. Abundant samples of pre-Mesozoic rock fragments, often more or less rounded, have been recovered by dredging in the area of these outcrops. Although none of these rocks has apparently been obtained directly from outcrops, we believe that they were picked up from talus accumulations nearby. Therefore our interpretation is that the acoustic basement corresponds, at least in part, with pre-Mesozoic basement rocks (Fig.7).

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Fig.2. S c h e m a t i c geological structure o f the "Le D a n o i s Bank". 1 = Pre-Mesozoic b a s e m e n t ; 2 = Jurassic and Cretaceous; 3 = l o w e r M i o c e n e ( A q u i t a n i a n ) ; 4 = upper N e o g e n e ; 5 = diapir; 6 = fault; 7 = direction o f dip o f M e s o z o i c strata. Geological structure o f the c o n t i n e n t a l shelf to the s o u t h w e s t o f the "Le D a n o i s B a n k " is after Boillot et al. ( 1 9 7 3 ) . Insert: location o f seismic profiles o f Figs.7 and 8.

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Fig.3. Location of dredge and core samples on the "Le Danois Bank" with lithologic and stratigraphic short description of the samples. Mesozoic and Cenozoic rocks outcropping on the " L e Danois Bank" are compared with those outcropping on the Asturian continental shelf, as described by Boillot et al. (1973a). Sedimentation environments for each period are indicated (simplifications used on the map to designate various samples are explained in the insert).

58 Pre-Mesozoic rocks are of two different sorts (Capdevila et al., 1974) : (1) Metamorphic rocks where the degree of metamorphism varies from the greenschist to the amphibolite facies. This first assemblage, occasionally associated with granitoid rocks, might belong to the Hercynian chain that constitutes the northern part of the Iberian Meseta (Bard et al., 1971). (2) Acidic granulites and both acidic and basic Middle Precambrian charnockites (about 2000 m.y.: Capdevila and Vidal, 1975). These rocks are absent in northwestern Spain so that they probably come from deep levels of continental crust which do not outcrop on land near the edges of the Iberian peninsula. Their submarine outcropping might be explained by erosion and extension phases during the formation of the margin. In such a case the basement o f the " L e Danois Bank" would represent thin continental crust comparable to the basement that is generally considered to underlie continental rises around passive margins. Its present location, at shallow depth, would result from Tertiary tectonic activity. This interpretation is consistent with a positive gravity anomaly observed on the " L e Danois Bank", which can be explained either b y the presence of highdensity material within the crust or by a local shoaling of the mantle (Sibuet and Le Pichon, 1971). MESOZOIC EVOLUTION OF THE MARGIN Triassic and L o w e r Jurassic rocks have not been recovered from the northern Spanish margin. Occurrence of diapirs in the "inner basin" (Fig.8), however, strongly suggests the presence of Triassic evaporites b y comparison with the Aquitaine Basin. It is therefore probable that lower Mesozoic rocks are present beneath the "inner basin" (Fig.4, I). Upper Jurassic and lowermost Cretaceous rocks outcrop on the " L e Danois Bank". They belong to two different facies: (1) a pelagic facies, rich in remains of planktonic microorganisms (calpioneUid micrite); (2) a shallow platform facies, with c o m m o n algae, foraminifera, and bivalve and echinoid debris. Precise age determination of the pelagic rocks was possible because of the occurrence of calpioneUids. Samples from the neritic facies, on the other hand, did n o t yield good age-diagnostic fossils. They contain fragments o f organisms that could be of Kimmeridgian through Berriasian or even Valanginian age. Age determinations of these rocks, therefore, are not very precise. Table I summarizes our observations. The co-occurrence o f calpionellid-rich pelagic deposits together with shallow-water bioclastic sediments is remarkable. Such a contrast between facies o f rocks sampled from outcrops lying very close to each other, sometimes found in the same dredge haul, is interpreted here as resulting from the location o f the " L e Danois Bank" during the Late Jurassic and Early Cretaceous near the b o u n d a r y between a continental shelf and an open sea basin, the latter being characterized b y pelagic sedimentation. Based on this interpretation, we have indicated in our reconstruction (Fig.4, III) a break in

59 N

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Fig.4. M e s o z o i c evolution o f the northern Spanish c o n t i n e n t a l margin. D = drifting; L = coast line; LDB = Le D a n o i s Bank; R = Upper Jurassic--Early Cretaceous rifting phase; S = c o n t i n e n t a l slope - - lithological s y m b o l s are e x p l a i n e d on Fig. 5.

the topography that could have corresponded with the continental slope during these times. Another character, specific to the Upper Jurassic, is that it is transgressive: apparently Kimmeridgian--Portlandian limestones directly overlie the preMesozoic basement o f the "Le Danois Bank". This suggests that the area was emergent and subject to erosion during the Late Jurassic (Fig. 4, H), and that basement subsidence brought it below sea level during the latest Jurassic. Such vertical motion can be explained by development of a rifting phase during the Late Jurassic (Wilson, 1975), followed by a partial collapse that caused the Tithonian transgression. Finally, calpionellids observed in our samples are of Tethyan character. Their occurrence then suggests that from the Tithonian to the Berriasian, communication was effective between the Tethys and an open sea that occupied the present location of the Bay of Biscay (Boillot et al., 1971b; Durand Delga, 1973; Pastouret et al., 1976). Such a communication was probably established through the west o f Iberia where the same calpionellid facies have been observed (Dupeuble et al., 1976). Therefore, the Atlantic margin of the Iberian peninsula was probably characterized by both continental shelves and deeper basins as far back as latest Jurassic times. Lower Cretaceous facies from the present-day continental shelf differ

60

TABLEI

Titho n ian--Berriasian H 76 09

Fine-grained, often pelletoidal, limestone with c o m m o n miliolids, Nautiloculina oolitica, Trocholina alpina, and fragments of Dasycladacea.

Late Tithonian X 301, X 302

Calpionellid micrite ( Calpionella alpina, Crassicolaria brevis, C. parvula, C. gr. intermedia, Globochaete alpina).

H 76 12

Bioclastic pebbly limestone with Nodophthalmidium, bivalve and echinoid fragments and oncholites (some with entrapped calpionellids).

Early Late Tithonian B712

Micritic limestone with rare small quartz grains and calpionellids ( Crassicolaria intermedia, Praetintinopsella andrusovi, Pr. brouweri) associated with Globochaete and Cadosina.

K immeridgian H 76 12

Pebbly limestone with nodules of Cyanophycea, Nodophthalmidium, Nautiloculines, fragments of bivalves and dasycladacean algae ; comparable to Kimmeridgian peri-reef limestones observed in Portugal.

Oxfordian--Kimmeridgian X 307

Pelagic limestone with common radiolaria, Globochaete alpina, some filaments and rare ? Spirillinidae.

from those o f the "Le Danois Bank". On the shelf are either lagoonal or shallow-water neritic facies (Boillot et al., 1971a, 1973a; Lamboy and Dupeuble, 1975; Lamboy, 1976), whereas sediments deposited contemporaneously in the "Le Danois Bank" area suggest open sea. Generally, they are marls with predominant pelagic microfauna, except for some silty limestone (see Table II). Except for the Neocomian limestone rich in annelid tubes, these samples show a definite ressemblance with the "black shale" facies sampled by the "Glomar Challenger" during legs 47b and 48 (Ryan et al., 1976; Montadert et al., 1976, 1977). In particular, the same abundance of fine-grained detrital particles, of organic matter and especially plant debris, the same rarity of microfauna and nannoflora, where components of pelagic origin predominate however over benthic ones, and the same low amounts of carbonates -- which differs strongly from the earliest Cretaceous deposits -- have been observed in various samples. The boundary between the platform areas (the present-day continental shelf) and the basin in which "black shales" accumulated, was necessarily a slope (Fig. 4, IV). It is very difficult, however, to estimate the difference in water depth on both sides of the slope, owing to the lack of precise information on the depth at which the "black shales" were deposited.

61 TABLE II

Middle--Upper A lbian B714

Grey marl, pyrite-rich, indurated, with rare very small and poorly preserved planktonic foraminifera (Hedbergella sp. planispira) and occasional benthic foraminifera (Gyroidinoides

parva ). Upper Aptian B 71 12

Indurated marl with Globigerinelloides algerianus and some agglutinated foraminifera (Spiroplectinata).

Aptian X 3 2 5 ; X 347 ?

Grey micaceous marl with ammonites (Melchiorites melchioris Tietze).

H 76 09

Black marls rich in organic matter and especially plant debris ("black shale" facies) containing Lower Cretaceous spores, rare peredinians and some basal membranes of foraminifera. Nannofossils, rare and badly preserved, suggest a probable Aptian age.

Barremian V715

Grey marl with Leticulina ouachensis and Trocholina cf.

pau cigranu la ta. Neocomian ? H 76 07

Slightly argillaceous and silty limestone with abundant annelid tubes and bivalve debris, with a facies similar to that of some layers of the Aquitaine Neocomian.

Upper Cretaceous layers have not been sampled from the " L e Danois Bank". They o u t c r o p extensively, however, on the Asturian and Galicean continental shelves where they consist of flysch on the outer part of the shelf, and limestones ("Asturian" facies) closer to the present-day coast line. We have interpreted the flysch (Fig.4, V) as continental rise deposits, and the "Asturian" limestones as slope or shelf edge deposits. The shelf break thus must have receded again toward the south since the Late Cretaceous (Fig.4, III, IV, V, S and L ). The rarity/or absence of Upper Cretaceous outcrops on the " L e Danois Bank" should be related to the hiatus observed in the same stratigraphic interval at the base of the north Portuguese and west Armorican margins (legs 47b and 48 o f the IPOD program) where the Upper Cretaceous is almost completely missing. During the Late Cretaceous the "Le Danois Bank" could have occupied a location comparable to that of the drill-sites (deep part of the margin). It appears from the observations described above that the Mesozoic

62 evolution of the northern Spanish margin has been controlled by extension accompanying the progressive opening of the Bay of Biscay. A first phase of rifting occurred during the Triassic (Pautot et al., 1970; Auffret et al., 1976). The data that we have examined here, however, do not d o c u m e n t that phase. The evolution of sedimentation during the latest Jurassic and Cretaceous appears to have been controlled by a second phase of rifting initiated during •the Late Jurassic (Fig. 4), before the creation of oceanic crust in the Bay of B i s c a y (Montadert and Winnock, 1971; Williams, 1975). CENOZOIC EVOLUTION OF THE MARGIN The effects o f subduction during the latest Cretaceous and Eocene The Mesozoic evolution was abruptly interrupted at the end of the Cretaceous by the convergence of the Iberian and European plates, which caused deformation of the Mesozoic passive margin and for a short period transformed it into an active margin. (1) The age o f the tectonic p a r o x y s m (tectonic phase 1) was deduced from the geological study of the Asturian continental shelf. There the deformation of the Cretaceous layers predates the deposition of the Upper Lutetian limestone which is transgressive, with an angular unconformity, over structures resulting from that deformation during phase 1. Eocene rocks were n o t found on the " L e Danois Bank" but a pebble of neritic limestone of Paleocene age was recovered. It contained abundant lithothamnid algae (Archaeolithothamonium ) as well as Discocyclina seunesi and Planorbulina cretae. This suggests, if the pebble comes from outcrops on the S VI Latest Cret . . . . . . .

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Fig. 5. Cenozoic e v o l u t i o n o£ the n o r t h e r n Spanish c o n t i n e n t a l margin. L D B = Le Danois B a n k ; Age an(] ]ithology: ] = Cenozoic; 2 = U p p e r Cretaceous; 3 = L o w e r Cretaceous; 4 = U p p e r Jurassic an(] l o w e r m o s t Cretaceous; 5 = L o w e r an(] M i d d l e Jurassic; 6 = Triassic; ? = pre-Mesozoic basement.

63

Fig.6. Photographs o f thin sections showing deformation o f Mesozoic sediments. Sample H 76 09. Black shales. B = bedding; S = schistosity.

" L e Danois Bank", that the deformation of the margin and the resulting paleogeographic change occurred prior to the Paleocene. The evidence is still very weak, however, in the absence of more samples of the same age. (2) The intensity of the deformation is evidenced by occasional centimetric microfolding in the uppermost Jurassic--lowermost Cretaceous limestones (H 76 12) as well as by a well-defined schistosity in some "black shale" samples (H 76 09). The Mesozoic sediments thus appear to have undergone significant stress (Fig.6). (3) Structural geometry is more difficult to reconstruct. Seismic profiles show that the Mesozoic layers outcropping at the top of the " L e Danois Bank" form a monoclinal series dipping toward the south. Anticlines and faults, with an east--west orientation, appear locally on the actual continental shelf. Everywhere else Mesozoic strata appear to correspond with a diffracting acoustic basement within which no structures are visible. In our hypothetical reconstruction (Fig. 5) we consider (a) the distribution of outcrops and faults identified and described above, (b) the E--W orientation of the observed main structures, and (c) a southward subduction of the floor of the Bay of Biscay at the time of maximum margin deformation, b y comparison with structural models proposed for active margins (Dewey and Bird, 1970; Roeder, 1973; Seely et al., 1974, etc.). In our tentative reconstruction we also take into account the evolutionary model proposed b y Choukroune (1976) for the northern Pyrenean zone, of which the northern Spanish margin is a lateral extension (Boillot et al.,

64

Fig.7. Sparker seismic profiles from the "Le Danois Bank". Location of profiles appears on Fig.2 (insert). X 307 and B 71 5 = samples (see location on Fig.3). Vertical scale is given in seconds two-way travel time. 1973b; Boillot and Capdevila, 1977). In t h at model normal faults (1', 2', 3', 4' and 5' o f Fig.4), t h a t were active during the Mesozoic tensional phase, u n d e r w e n t renewed activity during the subsequent compressive phase, but this time as inverse or thrust faults. Thus the Asturian margin appears as a fold belt, where deep-sea sediments and basement were d e f o r m e d , uplifted and b r o u g h t above sea level during the latest Cretaceous and the Eocene. U n f o r t u n a t e l y , not hi ng is at present know n a b o u t the several wedges t hat eventually form this chain; the profiles o f Fig.5 should t herefore be considered merely as a possible structural model, and not as the result of real observations -- at least for the deep structure o f the margin.

65

Fig.8. Sparker seismic profile from the "inner Basin"*. Location of profile appears on Fig.2 (insert). AD = diapir; n = Neogene; e-g (?) = possibly Paleogene; J - C = Mesozoic. Vertical scale is given in seconds two-way travel time.

The post-Eocene evolution (1) At the end of the Oligocene, an important tectonic phase (phase 2) caused the formation of a graben approximately in the middle of the modern Asturian continental shelf, and of a horst near its outer edge. This compares well with the structural relation between the "inner basin" (a graben or a half graben) and the " L e Danois Bank" (a horst or a half horst), and we believe that these various structures, which are close to each other and display comparable features, are also of the same age*. (2) The latest Oligocene and the Aquitanian sediments are transgressive, with an angular unconformity, over deformed Mesozoic layers of the " L e Danois Bank". They consist of neritic limestones containing large foraminifera (Lepidocyclina, Operculina, miogypsinids, rupertiids) c o m m o n l y associated with algae and abundant bryozoans, and occasional silty limestones and calcite-cemented sandstones**. The nature of these sediments suggests that the top of the " L e Danois B a n k " was under very shallow water during the Early Miocene. *The "inner basin" at present separating the " L e Danois Bank" from the Asturian continental shelf is filled with approximately 500 m of Neogene sediments. It is bordered by E--W or NW--SE faults which appear to have controlled the location of diapirs (Lepr~tre, 1974) (Fig.8). The age of these diapirs is difficult to determine. They are sealed by Neogene layers, whereas Mesozoic and possibly Paleogene layers have been subject to deformation. The latest halokinesis seems therefore contemporaneous with tectonic phases 1 or 2 (Early Eocene or Late Oligocene). The structures presently observed, however, probably result from the evolution of older diapirs that formed during rapid subsidence in Cretaceous times (Fig.4, V). We believe that the evaporites involved in those structures are Triassic as in the Aquitaine Basin (Anonymous, 1974). * * S o m e samples, however, show an admixture of neritic and planktonic constituents, suggesting depositon near the outer edge o f the continental shelf or on the upper part of the continental slope.

66 (3) The post-Aquitanian fraction of the Neogene sediments on the "Le Danois Bank" consists of phosphorites, glauconitic limestone, and more or less indurated marls, where microfauna are generally planktonic in origin (abundant Orbulina ). It is difficult to determine precisely the age of these sediments, although some samples yielded relatively good stratigraphic markers (see Table III). Present-day sedimentation, which is essentially pelagic, developed progressively during the Miocene, probably after subsidence of the "inner basin" which may have entrapped the sediments of continental shelf origin. The northern Spanish continental margin, briefly transformed into an active margin during the latest Cretaceous and early Cenozoic, evolved again into a passive margin, characterized essentially by subsidence and normal faulting. This is not a complete return to prior conditions, however, as Mesozoic layers, after having undergone deformation, constitute some sort of a basement which has been segmented into large blocks and formed horsts and grabens controlling Cenozoic sedimentation. SUMMARY AND CONCLUSIONS

The geological evolution of the northern Spanish margin is characterized by three successive distinct phases: (1) First phase: Mesozoic passive margin (Fig.4). After a first rifting, probably during the Late Triassic, a second one started during the Late Jurassic, causing extensive erosion and extension of the pre-Mesozoic basement which outcrops today on top of the "Le Danois Bank"..4, local transgression began with the Portlandian and the "Le Danois Bank" then marked the transition between a shallow submarine platform and a deeper basin where pelagic sediments accumulated.

T A B L E III

Pleistocene X 335

Plio-Pleistocene ?

Pelitic limestone w i t h Globorotalia truncatulinoides.

Indurated m a r l s w i t h s e c t i o n s o f Sphaeroidinella o r Globorotalia sp.

Late Pliocene H 76 05

Pelitic limestone w i t h Globorotalia puncticulata and G. miocenica.

Late Miocene (Tortonian to Messinian) Pelitic limestones w i t h Globorotalia obesa, G. merotumida, Pulleniatina primalis, Globoquadrina dehiscens.

H 76 0 8

67 This basin widened toward the south during the Early Cretaceous and accumulated first calcareous deposits, then black shales. Finally, while the Bay of Biscay fully opened, the passive margin reached maturity during the Late Cretaceous. At that time the continental rise was characterized by deposition of flysch which outcrops today on the Asturian continental shelf whereas non-deposition prevailed on the "Le Danois Bank", probably as a result of the onset of vigorous bottom circulation. The evolution of the area of the "Le Danois Bank" during the Mesozoic presents remarkable similarities with that of areas drilled by "Glomar Challenger" during legs 47b and 48 (north Portuguese and west Armorican margins). Conversely, comparison between these areas can help solve some of the problems uncovered during the drilling. To the north of the Asturias latest Jurassic neritic facies are seen to overlie directly the pre-Mesozoic basement. This suggests that uplift and erosion caused by rifting occurred during the Late Jurassic and that the latest Jurassic transgression resulted from the subsequent initial subsidence of the young margin. (2) Second phase: latest Cretaceous--Early Eocene active margin. The short-lived subduction of the floor of the Bay of Biscay caused deformation of the Mesozoic margin. Profile VI on Fig.5 shows a tentative structural reconstruction of that active margin, following folding, uplift, emergence, and erosion of Jurassic and Cretaceous sediments. (3) Third phase: return to a passive margin during the late Cenozoic. After subduction ceased, the newly formed fold belt began to subside. First the Paleogene sea covered the Eocene erosional surface, then latest Oligocene tectonic activity caused the formation of depressions parallel to the margin. Finally the formation of the inner basin during the Miocene resulted in isolation and deepening of the "Le Danois Bank". The northern Spanish continental margin offers a good opportunity to study at least three problems of prime interest: (a) the short-lived latest Mesozoic--Early Cenozoic subduction allows the observation of the transformation of a passive margin into an active one and provides information on the structure of an active margin during its early stages of evolution, before the formation of an accretion prism; (b) deformation, uplift and erosion of the Mesozoic margin during subduction are responsible for outcropping of the older Mesozoic layers, which could be sampled by dredging; (c) for the same reasons, the underlying basement probably outcrops on the presentday margin and could be considered as part of the thinned continental crust that characterizes the ocean--continent transition beneath passive margins. ACKNOWLEDGEMENTS We gratefully acknowledge the help of the following persons and institutions: The Spanish government for permitting the completion of cruises Biscaye 70 and 72 of the R/V " J o b Ha Zelian", cruises 70 and 71 of the R/V

68

"Thalassa", and cruise Hesperides 76 of the R/V "Jean Charcot" during which the data forming the basis of this paper were acquired. The crews and scientific parties of these cruises. The "Centre National pour l'Exploitation des Oceans", for providing us with the necessary shiptime on board R/V's "Job Ha Zelian" and "Jean Charc ot' '. The "Station biologique de Roscoff", which invited us to participate in dredging cruises of the R/V "Thalassa". Our colleagues, M. Gennesseaux, J. Mascle and A. Mauffret, for critically reading our manuscript. Yves Lancelot for comments and translation of the French manuscript. Financial support was provided by the "Centre National de la Recherche Scientifique" for the Hesperides 76 expedition, within the framework of ATP "Geodynamique" and "Soutien ~ IPOD" under contracts 35-11 and A 12 618. For the determination of the various microfossils we are indebted to M. Durand-Delga (calpionellids), M. Rioult (ammonites), Mrs. Taugourdeau (lower Cretaceous spores), C. Muller (nannoflora), and Mrs. G. Glaqon (foraminifera). P. Choukroune and R. Dubois helped in examining the deformation of the rock from samples H 76 12 and H 76 09. REFERENCES Anonymous (BRGM, ELF-Re, ESSO-REP, SNPA), 1974. G~ologie du bassin d'Aquitaine. Bureau de Recherche G~ologique et Mini~re, France. Auffret, G.A., Pastouret, L., Auzende, J.M. and Olivet, J.L., 1976. L'~volution du Golfe de Gascogne et de ses marges (abstract). In : 4 i~me R~union annuelle des Sciences de la Terreo Soc. G~ol. Fr., p.25. Bard, J.P., Capdevila, R. and Matte, Ph., 1971. La structure de la chafne hercynienne de la Meseta ib~rique : comparaison avec les segments voisins. In : Histoire structurale du Golfe de Gascogne. Technip, Paris, pp.I.4.1--I.4.68. Boillot, G. and Capdevila, R., 1977. The Pyrenees: subduction and collision? Earth Planet. Sci. Lett., 35: 151--160. Boillot, G., Dupeuble, P.A., Lamboy, M., d'Ozouville, L. and Sibuet, J.C., 1971a. Structure et histoire g~ologique de la marge continentale au Nord de l'Espagne (entre 4 ° et 9 ° W). In: Histoire structurale du Golfe de Gascogne. Technip, Paris, pp.V.6.1--V.6.52. Boillot, G., Dupeuble, P.A., Durand Delga, M. and d'Ozouville, L., 1971b. Age minimal de l'Atlantique nord d'apr~s la d4couverte de calcaire tithonique ~ Calpionelles dans le Golfe de Gascogne. C.R. Acad. Sci., Ser. D, 273 : 671--674. Boillot, G., Dupeuble, P.A., Hennequin-Marchand, I., Lamboy, M. and Lepr~tre, J.P., 1973a. Carte g~ologique du plateau continental nord-espagnol entre le Canyon de Capbreton et le Canyon d'Aviles. Bull. Soc. G~ol. Fr., 7, XV(3--4): 367--391. Boillot, G., Capdevila, R., Hennequin-Marchand, I., Lamboy, M. and Lepr~tre, J.P., 1973b. La zone nord-pyr~n~enne, ses prolongements sur la marge continentale nordespagnole et sa signification structurale. C.R. Acad. Sci., Ser. D, 277 : 2629--2662. Boillot, G., Dupeuble, P.A., Hennequin-Marchand, I., Lamboy, M., Lepr~tre, J.P. and Musellec, P., 1974. Le rSle des d~crochements "tardi-hercyniens" dans l'~volution structurale de la marge continentale et dans la localisation des grands canyons sousmatins ~ l'Ouest et au Nord de la p~ninsule ib~rique. Rev. G4ogr. Phys. G~ol. Dyn., V, XVI(1): 75--86.

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