E L S E V I E R Tectonophysics 264 (1996) 153-174

TECTONOPHYSICS

Seismic structure of the northern continental margin of Spain from ESCIN deep seismic profiles

- a,*,l - - b - "" b a J. Alvarez-Marron , A. Perez-Estaun , J.J. Danobelt la , J.A. Pulgar , J.R. Martfnez Catalfin c, A. Marcos a, E Bastida a, R Ayarza Arribas c, j. Aller a, A. Gallart b,

E Gonzalez-Lodeiro f, E. Banda b, M.C. Comas e, D. C6rdoba d

a Departamento de Geolog{a, Universidad de Oviedo, c/Arias de Velasco s/n, 33005, Oviedo, Spain b Instituto de Ciencias de la Tierra, Jaume Almera, CSIC, c/Lluis Sole i Sabaris s/n, 08028, Barcelona, Spain c Departamento de Geologia, Universidad de Salamanca, Plaza de la Merced s/n, 37008, Salamanca, Spain

d Departamento de Geoffsica, Universidad Complutense, 28040, Madrid, Spain e lnstituto Andaluz de Ciencias de la Tierra, CSIC, Universidad de Granada, c/Fuentenueva s/n, 18002, Granada, Spain

f Departamento de Geodindmica, Universidad de Granada, c/ Fuentenueva s/n, 18002, Granada, Spain

Received 1 March 1995; accepted 30 November 1995

Abstract

By the end of the Carboniferous, the crust of the continental shelf in northwestern Spain was made up of deeply rooted structures related to the Variscan collision. From Permian to Triassic times the tectonic setting had changed to mainly extensional and the northern Iberian continental margin underwent rifting during Late Jurassic-Early Cretaceous times, along with sea-floor spreading and the opening of the Bay of Biscay until the Late Cretaceous. Subsequently, the northern Iberian margin was active during the north-south convergence of Eurasia and Iberia in the Tertiary.

A multichannel seismic experiment, consisting of two profiles, one north-south (ESCIN-4) crossing the platform margin offshore Asturias, and another (ESCIN-3) crossing the platform margin to the northwest of Galicia, was designed to study the structure of the northern Iberian margin. The ESC1N-4 stacked section reveals inverted structures in the upper crust within the Le Danois Basin. North of the steep continental slope, ESCIN-4 shows a thick sedimentary package from 6 to 9.5 s, two-way travel time (TWT). Within this latter package, a 40-km-long, north-tapering wedge of inclined, mainly south-dipping reflections is thought to represent a buried, Alpine-age accretionary prism. In the north western part of the ESCIN-3 (ESCIN-3-1) stacked section, horizontal reflections from 6.5 to 8.5 s correspond to an undisturbed package of sediments lying above oceanic-type basement. In this part of the line, a few kilometres long, strong horizontal reflection at 11.2 s within the basement may represent an oceanic Moho reflection. Also, a band of reflections dips gently towards the southeast, from the base of the gently dipping continental slope. The part of ESCIN-3 line that runs parallel to the NW-Galicia coast (ESCIN-3-2), is characterized by bright, continuous lower crustal reflections from 8 to 10 s. Beneath the lower crustal reflectivity, a band of strong reflections dips gently toward the southwest from 10 to 13.5 s. The part of ESCIN-3 that parallels the northern margin (ESCIN 3-3), shows good reflectivity in all levels. Upper crustal reflections image the sedimentary fill of probable Mesozoic to recent basins. Mid-crustal reflectivity is characterized by dipping

* Corresponding author. Fax: +34 93 4110-012. 1 Current address: Instituto de Ciencias de la Tierra, Jaume Almera, CSIC, c/Lluis Sole i Sabaris s/n, 08028 Barcelona, Spain.

0040-1951/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 4 0 - 1 95 1 ( 9 6 ) 0 0 1 24-2

154 J. A lvarez-Marrrn et al. / Tectonophysics 264 (1996) 153-174

reflections until 8 s that are probably related to compressional Variscan features. The lower crustal level shows 'layered' reflections between 8 and 12 s. Dipping reflections are found below the continental Moho.

Kevwords: deep reflection seismics; North Iberian Margin; crustal structure; Alpine deformation

1. Introduction

The continental margin of northern Spain trends E-W, from the inner Bay of Biscay in the east (Lan- des Plateau) to the Ortegal Spur off the north coast of Galicia (Fig. 1). The margin bends slightly south- westward from the Ortegal Spur to join the Iberian Atlantic Margin in the Galicia Bank, constituting the southern border of the Bay of Biscay. The geology of the Bay of Biscay has been largely investigated by French researchers up until the beginning of the 70's (see compilation by Debysier et al., 1971), dur- ing which time geophysical data, including reflection seismic, magnetic and gravity data were acquired. The exploration of the continental slope by direct sampling with a submersible was undertaken during two campaigns: the Cyadanois sampled the conti- nental slope off Le Danois Bank in 1979 (Malod and Boillot, 1980), and Cybere sampled the continental slope off Ortegal Spur in 1982 (Malod et al., 1984). Much of this research was focused towards elucidat- ing the relative movements of the Iberian plate with respect to Europe during the Mesozoic, and several hypotheses for the opening of the Bay of Biscay have been proposed (synthesized in Sibuet, 1989).

The research program for the Seismic Study of the Iberian Crust (ESCI) started in Spain in 1991. The program included the acquisition of several mul- tichannel deep seismic profiles throughout mainland Spain and its continental margins. Four profiles were planned to study the crustal structure of northwestern Iberia onland and the northern continental margin. These are grouped in the ESCI-North survey (ES- CIN) (Fig. 1). The initial results from the land data, ESCIN-1, acquired in 1991, and ESCIN-2, acquired in 1993, are in Prrez-Estafin et al. (1994) and Pul- gar et al. (1996), respectively. The ESCI-N marine experiment was designed to study the deep structure of the hinterland areas of the Variscan orogen off the north coast of Galicia, the Mesozoic-Cenozoic structure and evolution of the north Iberian conti-

nental margin, and the nature of the ocean-continent transition in the southern part of the Bay of Biscay. The marine profiles (ESCIN-3 and 4) were acquired in 1993 and total about 500 km (Fig. 1). The aim of this paper is to describe the major features imaged in these two marine profiles and suggest their cor- relation with major structures related to the various tectonic events comprising the geological history of the area.

2. Geological framework and tectonic evolution

The section of the Variscan orogen that crops out in northwestern Spain largely corresponds to one of the continental margins involved in the Variscan col- lision, probably the margin of Gondwana, and forms the footwall to the Variscan suture (Prrez-Estatln et al., 1991). It consists mostly of east-verging struc- tures grouped into four major zones that are based on differences in deformation style, palaeogeogra- phy and metamorphism (Lotze, 1945; Julivert et al., 1974; Julivert, 1987). The hinterland areas to the west have been subdivided into three zones (Fig. 1) that consist mostly of deformed, Lower Palaeozoic and some Upper Proterozoic rocks that have a main NW-SE structural grain. Granitic rocks are common in the Central Iberian Zone where pre-, syn- and post-tectonic plutonism was widespread (Mart/nez et al., 1990). East of the hinterland, the Cantabrian Zone forms the foreland thrust and fold belt, and contains a well-developed, Palaeozoic sedimentary succession with a typical thin-skinned deformation style (Julivert, 1971; Prrez-Estafin et al., 1988; Prrez-Estatln et al., 1994). The youngest Variscan structures in the foreland area are of Upper Car- boniferous age (Julivert, 1987).

The deep-rooted Variscan structures of the hinter- land areas and the upper crustal level structures of the foreland thrust and fold belt were therefore devel- oped in the northwestern Iberian crust when exten- sional tectonics began in Permian and Triassic times.

J. Alvarez-Marrrn et al. / Tectonophysics 264 (1996) 153-174 155

Variscan Chain

Cantabrian Zone

West Asturian-Leonese Zone Central Iberian Zone Galicia-tras os Montes Zone Complexes with Mafic and

Ultramafic rocks

Alpine Chains ~ M e s o z o i c and Cenozoic Rocks

Paleozoic Rocks

Tertiary Basins Mainly Udeformed

Tertiary Rocks

0 250 km I I

tectonic accretionary prism

Fig. 1. Geological map of northwestern Iberia with geological zones (Lotze, 1945; Julivert et al., 1974). Offshore geology is adapted from Boillot and Malod (1988). Thick black lines are the ESCIN seismic profiles.

Onland Permo--Triassic extensional activity resulted in the formation of minor fault-bounded basins and related alkaline volcanism (Martfnez-Garcfa, 1983; Martfnez-Garcfa and Tejerina, 1985; Garcfa- Mondejar et al., 1986; Su~irez-Rodrfguez, 1988). The onshore sedimentary record consists mainly of con- tinental facies rocks of varying thickness, typified by frequent red beds.

The Bay of Biscay basin formed as a subsidiary rifted arm of the north Atlantic ocean (Williams, 1975) during the break-up between Europe and North America in the Mesozoic (Le Pichon et al., 1971; Verhoef and Srivastava, 1989), and sea-floor spreading continued in the Bay of Biscay until the Late Cretaceous (Williams, 1975). The north Iberian continental margin evolved during Late Jurassic- Early Cretaceous times by oblique separation of

Iberia from Eurasia that included an anti-clockwise rotation of the Iberian plate relative to stable Eu- rope (Van der Voo, 1969; Sibuet, 1989). Syn-rift sequences, related to pull-apart basins, are found in the Basque-Cantabrian Basin onshore (Garcfa- Mondejar, 1989) and the initiation of sea-floor spreading in the Bay of Biscay is marked by an Aptian-Albian age break-up unconformity on the platform (Le Pichon et al., 1971; Montadert et al., 1979). The Mesozoic sedimentary cover on the plat- form margin is thin, ranging from 100 to 1500 m (Malod and Boillot, 1980; Malod et al., 1984).

Tertiary convergence of Iberia and Eurasia acti- vated the continental margin, and a marginal trench developed (Sibuet and Le Pichon, 1971; Sibuet et al., 1971; Boillot et al., 1979; Grimaud et al., 1982). This trench is characterized by a belt of negative

156 J. Alvarez-Marr6n et al. / Tectonophysics 264 (1996) 153-174

. . . . - 50 . . . . . . . . . . - . - . . . . . . . -

i "- , ° , * "

ao

43 °

: o o \ =o 8,' 1,o o =,o ,0o = Fig. 2. Map of offshore gravity anomalies redrafted from Lalault et al. (1981), contours in mGal.

gravity anomalies (Fig. 2, Lalault et al., 1981). The gravity anomaly is about 100 km wide and 700 km long, following the E-W trend of the margin from the Galicia Bank to the N-S French continental margin, where it splits into two branches bounding the northern and the southern sides of the Landes Plateau. The activation of the north Iberian Margin produced inversion of extensional basins, both in the continental shelf and onland (i.e. Boillot et al., 1971; Garcfa-Mondejar, 1989), and reactivated extensional faults as reverse faults (Temine, 1984).

The offshore Cenozoic structures on the north Iberian margin (Fig. 1) include an accretionary prism (Der6ngnaucourt and Boillot, 1982) that is inter- preted to be related to the subduction of the Bay of Biscay ocean floor under the continental margin of Iberia during Palaeocene-Eocene times (Boillot et al., 1979; Boillot and Malod, 1988). This subduc- tion may have started in the Late Cretaceous and probably continued episodically until after Eocene times, possibly during Oligocene and even during the Neogene. The timing of subduction is based on seismic correlation with holes DSDP 118 and 119 by Laughton et al. (1972).

The post-Variscan evolution of the continental margin from extension through subsequent inversion has likely reworked the previous Variscan structure. Reworking of Variscan structures by the latest Alpine deformation during the Tertiary is reflected onshore in uplift of the Variscan basement, in the reactiva-

tion of previous extensional faults and in shortening along a gentle north-dipping, major reverse fault located at the northern edge of the Duero Basin (Alonso et al., 1996).

3. Seismic data acquisition and processing

The ESCI-N deep seismic marine survey was ac- quired during February 1993 by the MV SeisQuest. Acquisition and processing were done commercially by Schlumberger GECO-PRAKLA,

Profile ESCIN-4, which has a total length of 144.75 kin, started near the coast line at 43°35'N, 5°10'W and finished in the inner Biscay Bay at 44°53'N, 5°4.96'W. Profile ESCIN-3 is composed of three segments of different azimuths. The ESCIN-3- 1 is the westernmost segment, and has a length of 141 km. It crosses the continental slope from 44°N, 10°30'W to 43°35.57'N, 8°50.25'W. The ESCIN-3- 2 central segment is 98 km long, and runs near the coast from 43°35.98'N, 8°51.99'W to 43°57'N, 7°45'W. The ESCIN-3-3 is the eastern-most segment, with a length of 141 kin from 43°36.99'N, 7°47'W to 43°40.73'N, 6°4.27'W (Fig. 1).

The acquisition configuration and processing se- quence are presented in Figs. 3 and 4, respectively. The data were resampled at 8 ms and had a nom- inal coverage of 30-fold. Although the sequence is conventional, some of the parameters used were only chosen after extensive testing primarily directed to-

J. Alvarez-Marrdn et al./Tectonophysics 264 (1996) 153-174

O F F S E T

157

GUN A R R A Y

17.5 m

Fig. 3. Acquisition configuration. The vessel towed a 4500 m long analog streamer at a main depth of 12 m, that included 360 groups of hydrophones with a group interval of 12.5 m. To avoid cavitation noise from the ship an offset of 240 m was decided. The shooting was performed with a wide tuned array (80 m) configured in 6 strings of 17.5 m long each in order to reduce out-of-plane energy (Hobbs and Snyder, 1992). A mixed configuration of sleeve and g.i. guns increases the nominal power up to 25% more than the classical standard array. The data were shot at 75 m pop-rate using this large array of 5490 in 3 (90 1) at 2000 psi (13.8 MPa) nominal pressure with a record length of 20.48 s (two-way traveltime) and a sampling interval of 4 ms. Upper figure, configuration in section view. Lower figure, airgun array configuration in plan view. A total of 60 guns where under water, shaded blocks correspond to the active guns totaling 51, white blocks correspond to spare guns.

1. RESAMPLE (8 ms)

2. ADJACENT TRACE SUMMATION

3. SPHERICAL DIVERGENCE COMPENSATION

4. COMMON MIDPOINT GATHER (30fold, 12,5 m interval)

5. PRESTACK DECONVOLUTION (operator length 200 ms, predictive gap 32 ms)

6. FKDEMULTIPLE (Fk 12ms/tr)

7. VELOCITY ANALYSIS (every 3 kin)

8. NMO CORRECTION

9. PRE STACK INNER AND OUTER MUTES

10. STACK

11. NOISE ATI'ENUATION FILTER

12. POST STACK DECONVOLUTION (operator length 300 ms, predictive gap 60 ms)

13.TIMEVARIANTFILTER 0 - 2 s 4 - 50 Hz 2 - 6 s 4 - 40 Hz 6-10 s 4 - 30 Hz

10-16 s 4 - 20 HZ

14. GUN AND STREAMER STATIC CORRECTION ( + 15 ms)

Fig. 4. Processing sequence.

ward ob ta in ing a good image at great depth. Tes t ing was also required because the l ines cross diverse geological provinces and therefore o p t i m u m pro- cess ing parameters for each zone can vary dramat i - cal ly a long one line. To enhance the s ignal - to-noise ratio we have appl ied a lateral s emblance -based co- herent operator in the t ime d o m a i n (Milkerei t and Spencer, 1989). Thus, phase-coheren t s ignals can be separated f rom background noise on the basis of coherency est imates, as (a) background noise has no spatial coherency, and (b) coherent noise can be separated from the s ignal depend ing on the different dips (s lowness) (Neidel l and Taner, 1971). We have used a filter that band-passes mos t o f the veloci ty ranges with window lengths vary ing be tween 750 m and 1750 m depend ing on the profile.

158 J. A lvarez-Marr6n et al. / Tectonophysics 264 (1996) 153-174

4. Reflectivity patterns of ESCIN marine profiles

The seismic profiles are displayed in Figs. 5-8. These figures include the corresponding coherency filtered image, in which the most important features described in the following text are marked.

4.1. Profile E S C I N - 4

The southern part of this profile images the con- tinental shelf with water depths ranging from less than 200 m up to 1500 m in the area of Le Danois Basin (Fig. 5). The continental break is marked by a positive relief feature in Le Danois Bank with depths of ca. 1000 m and where a steep continental slope (more than 13 ° ) marks the transition to open sea. The oceanic area in the north of the section shows a quite flat sea bed at 4500 m depth.

The region of the continental platform shows high and coherent reflectivity in the upper 4 s in the areas that correspond to Ribadesella and Le Danois Basins (Fig. 5). A package with a fan-like geometry of sub- parallel reflections can be seen to a travel time of about 4 s in the area of the Ribadesella Basin. A thin band of strong reflections dip gently towards the south beneath the mainly curved reflections of Le Danois Basin (A in Fig. 5). Below 4 s, the reflec- tivity diminishes, probably due to interference with multiple energy. Short and discontinuous, layered reflections might correspond to primary energy at a depth corresponding to 9 s beneath shot point 150 (B in Fig. 5).

To the north of the steep continental slope, the sea floor appears at 6 s and deepens slightly towards the north (less that 1 ° slope). The seismic image is of very good quality until 12 s. Beneath this travel time, the image is hampered by the arrival of the first water bottom multiple. Three packages of different reflection fabrics can be distinguished in this deep water part of the profile. A 70-kin-long, north-tapering, wedge-shaped package of reflections with frequent diffractions can be seen from 6 to 11.5 s in the south (shot point 1100) and 8 to 9 s in the north (shot point 1800). The main attitude of the reflections in this area is south-dipping, also with disrupted inclined reflections. Above it, an upper package of well-layered, sub-horizontal reflections ranges from very thin in the south (from 6 to 6.5 s,

shot point 1100), thickening towards the north from 6 to 9.2 s at the northern end of the profile. In this northern part, the lowest reflections of this package are gently inclined to the south and directly overlie the third and lowest package. This deepest package is composed mainly of out-of-plane diffractions and disrupted reflections below the gently southward- dipping reflections at 9 to 10 s (C in Fig. 5) that can be followed southward until shot point 1700.

4.2. Profile ESCIN-3-1

Profile ESCIN-3-1 images the 110-km-long con- tinental slope that, in this section, has a gentle slope (about 2 °) starting from 200 m depth and has no sharp shelf break. The deep sea the northwestern end with water (Fig. 6).

At the northwestern end of the

area is covered at depths of 4500 m

profile, in the open sea area (west of shot point 300), two reflection fab- rics are distinguished. An upper panel of horizontal reflections from 6.5 to 8.2 s and a lower panel with mainly out-of-plane diffractions. These two panels are separated by a strong reflection at about 8.2 s that can be followed from the edge of the section until shotpoint 400. A few kilometres long, bright, horizontal reflection at 11.2 s can also be seen (D in Fig. 6). In the area of the transition to the continent, above the strong reflection at 8.2 s, a small wedge of disturbed reflections, commonly with diffractions, is seen between shot points 300 and 500. A band of two reflections (E in Fig. 6) dips towards the southeast from the foot of the continental slope, and can be followed until depths of 13 s.

Primary reflections are clear in a very thin band (less than 1 s thick) in the upper part of the crust of the continental slope where reflections frequently terminate laterally by truncation, and by onlap and downlap relationships, In deeper parts of the profile, a band of layered, bright reflections that deepens gently from west to east is present at about 8 s to 9 s (F in Fig. 6). This band of reflections joins band E near shot point 700 that merges from the base of the continental slope. On the right part of the profile, a band of WNW-dipping reflections are present at mid-crustal levels, between 6 and 8 s (G in Fig. 6).

S 1 5 0

J. Alvarez-Marr6n et al. / Tectonophysics 264 (1996) 153-174

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pp. 1 5 9 - 1 6 0

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pp. 161-162

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J. Alvarez-Marr6n et a l . / Tectonophysics 264 (1996) 153-174

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. . . . . ~ - v - - . - ~ r ~ - ' ~ - . - b - , ~ . . . - _ ' ' ~ - .::-- . . ~ . . - ~ _ : - ~ - - - . - ~ j -

i£~: " " ~ - ~ ~ " - - ~ " T ~ . :~-~.-- ~ : - -.~'~-----~-'-- ~ ' ~ -

. L ~ .:...::_~-~.~..~----__~- ~:_ ~ - _ - . ~ _ 4 _ .~.77. ~ - " i - - ~ _ - ~ - - ? ~ " ~ z

> . . : _ . ,~ ~ - . - - ~ , , -..~ ? ~ . . ~ -~J~ - . . ~ - - -~ -~ . - -

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irr6n et al./Tectonophysics 264 (1996) 153-174

- 3 - 3 1100 1300 1400 1500 1800

ESE

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J. Alvarez-Marrrn et al./Tectonophysics 264 (1996) 153-174 167

4.3. Profile ESCIN-3-2

Profile ESCIN-3-2 images a section of the con- tinental platform in a profile close to the coastline and at shallow water depths (less than 200 m). The deep part of this profile (below 5 s) shows more co- herent reflectivity than the shallow part, and primary reflections can be distinguished until 14 s (Fig. 7). It is characterized by bright, layered continuous re- flections from 8 to 10 s across the whole line, and by diffractions below shot point 1100 at about 9 s. Above the band of layered reflections, at mid-crustal levels, some inclined reflections dipping in both di- rections are also imaged. The most continuous and largest, located between shot points 900 and 1100 (H in Fig. 7), dips gently towards the southwest from 5 to 7 s. Below the sub-horizontal layered band of reflections, some dipping reflections are imaged. A band dipping to the WSW from 9 s can be followed until 14 s (I in Fig. 7). This band contains bright continuous reflections with a clear bend around shot point 1200, changing from a steeper to a gentler dip downslope.

4.4. Profile ESCIN-3-3

Profile ESCIN-3-3 images a section of the plat- form with water depths shallower than 200 m. The reflectivity pattern in this profile is good at most levels. In the upper 2 s, undisturbed sub-horizontal reflections are present across most of the profile (Fig. 8). There is an area between shot points 1100 and 1300 with a quite transparent character, where no discrete reflection can be recognized. Underneath this level, between 2 and 8 s, the reflectivity is dis- tributed in several weak and discontinuous, dipping packages of reflections and also diffractions. The zone between shot points 1100 and 1400 contains a stack of three bands of reflections and/or diffractions with dips alternating (J in Fig. 8) from northwest in the upper part, to southeast in the central part and northwest again in the lower part. These reflections fade out upwards and merge into a lower crustal reflective zone downwards. The zone between 8 and 12 s is highly reflective, with layered sub-horizontal reflections that are grouped in two horizontal layers, about 2 s thick each (K and L in Fig. 8). The two bands are more evident between shot point 1100 and

the northwestern edge of the section. In the lowest part of the profile, two groups of dipping reflections and/or diffractions can be distinguished. One is dip- ping towards the NW from 13 to 16 s between shot points 1500 and 1800 (N in Fig. 8). The other di- verges from the sub-horizontal band of reflections at 11 s and dips towards the SE until 16 s (O in Fig. 8).

5. Data interpretation and discussion

5.1. Profile ESCIN-4

Upper crustal reflectivity in Le Danois Basin de- lineates the sedimentary succession of this Mesozoic basin that has been inverted during the Eocene, with reactivation of previous basement involved exten- sional faults (Boillot et al., 1979; Temine, 1984). Based on dredge samples from the Cyadanois cam- paign (Malod and Boillot, 1980), pre-Mesozoic base- ment rocks are thought to crop out on Le Danois Bank (Boillot et al., 1979). Anticlinal folds with north-verging thrusts in their cores form growth an- ticlines with syn-tectonic, probably Tertiary sedi- mentation in their limbs (Fig. 9). The syn-tectonic sediments are imaged by the fan-like geometry of reflections in the synformal areas. Reflection termi- nations include onlap onto the limbs of the growing anticlines. The basin has not been totally inverted, but the sedimentary pile appears to be detached over the basement with probably listric, northward- verging thrusts in the cores of associated growth folds (Fig. 9).

The Cenozoic overprinting of the previous struc- ture of the basin has not been very important and therefore the Mesozoic configuration may still be deduced. From the present architecture it can be in- ferred that the basin was probably asymmetric, with thicker sedimentary fill to the south (Fig. 5). Re- flection A could correspond to the basement-cover contact that still preserves a southward inclination (Figs. 5 and 9). In the deeper parts of the continental crust the image is of poorer quality, and the few reflections seen (B in Fig. 5) could correspond to the layered lower crust (LLC).

To the north of the continental slope, the ocean- continent transition is marked by an accretionary prism that is at least 40 km long in this section. Reflection C beneath a very thick sedimentary cover

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J. Alvarez-Marrdn et al./Tectonophysics 264 (1996) 153-174 169

may be interpreted as the top of the Bay of Biscay oceanic basement. At the northern end of the profile, the thickness of the sediments may reach 3.5 km (assuming a constant velocity of 2 kin/s). The inter- preted oceanic crust dips gently landwards towards the deformation front with a corresponding thick- ening of the incoming sediment section. The onlap towards the south of the upper sedimentary package onto the accretionary prism suggests the cessation of the tectonic activity in the prism. The subduction has been interpreted to have stopped by Late Eocene times (Derrngnaucourt and Boillot, 1982).

5.2. Profile ESCIN-3-1

A striking feature of this profile is the gentle to- pographic profile of the continental slope in contrast to that of ESCIN-4, even taking into account the slight obliquity of profile ESCIN-3-1 with respect to the dip slope direction. Also, the geometry of reflective patterns in the thinned continental crust are remarkably different in both profiles.

The thin reflective band along the upper part of the crust in the continental slope in profile ESCIN- 3-1 may be related to recent slope-drape and slope basin sediments. Data from dredges of the Cybere campaign (Malod et al., 1984) on the Ortegal Spur continental slope give a Late Eocene-Neogene age to sediments with similar disposition, located above a Middle Eocene unconformity (Temine, 1984). Be- low the slope basin sediments, some reflections can be attributed to older sediments that could corre- spond to syn-rift sequences. In the Ortegal Spur area and below the unconformity, syn-rift sediments of Cretaceous age have also been found by Cybere. However, the basement-cover contact can not be clearly determined by this profile.

At mid-crustal levels, the existence of extensional faults deforming the basement can be inferred, as supported by previously collected reflection data to the south of this profile (Sibuet et al., 1987). Sibuet et al. (1987) have interpreted the structure below the continental shelf off the west coast of Galicia as tilted, fault-bounded basement blocks that pro- gressively thinned the continental crust towards the ocean-continent boundary.

Based on onshore refraction data in the Galicia area, C6rdoba et al. (1987) postulate the seaward

shallowing of the Moho discontinuity from 32 km onland to 29 km on the northern coast. According to this, the reflection F is situated at Moho depth and probably corresponds to the LLC of the continental crust stretched during the Mesozoic. In this profile, the continental crust thins progressively seawards and probably ends near shot point 700, where the LLC intercepts reflection E at a travel time of 8 s (Fig. 6).

At the western end of this profile an oceanic-type crust is imaged and reflection D (Fig. 6) probably corresponds to the oceanic Moho. The oceanic base- ment is here exceptionally deep, below 8.2 s and has an approximate thickness of 6 km (assuming a constant velocity of 6 km/s). This oceanic crust is covered by sub-horizontal undeformed sediments along the northwestern end of the profile.

With respect to the ocean-continent transition in this profile, the seismic image can be interpreted as that of a compressional ocean-continent boundary. However, accretion of ocean sediments in this sec- tion seems to have been much less important than in ESCIN-4. The wedge-shaped package of reflections between shot points 300 and 500 may correspond to deformed sediments in a poorly developed accre- tionary prism, less than 20 km long, at the foot of the continental slope (Fig. 6). The strong reflective band of reflections at 8.2 s is beneath the accretionary prism and probably corresponds to the top of the oceanic crust that is being subducted under the con- tinental margin. Reflection E may correspond to a major landward-dipping thrust that separates the ac- cretionary prism from a landward-dipping backstop formed by the thinned continental crust (Fig. 6).

Judging from the overall architecture of upper crustal basins, the extensional structures of the thinned continental crust in the area of the con- tinental shelf, along the western end of the north Iberian margin, do not seem to have been inverted or affected by any compressional deformation. This is in contrast with what has been found to the east of the margin, in ESCIN-4.

5.3. Profiles ESCIN-3-2 and ESCIN-3-3

These profiles run almost parallel to the continen- tal margin, with two orientations that follow the bend of the margin in the west, and are sub-perpendicular

170 J. Alvarez-Marr6n et al. / Tectonophysics 264 (!996) 153-174

/ ~ - i ~ . - - _ - - -- -~ z - ~ 4 ° 0

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to the onland strike of Variscan structures. They show a similar distribution of reflection patterns. In the upper part of the crust, they show clear primary reflectivity that is related to the sedimentary infill of probably recent sag basins. In profile ESCIN-3- 3, where a deeper basin appears (Fig. 8), Mesozoic syn-rift sediments may also be present. From what is seen in these profiles, the configuration of the basins appears undisturbed and seems not to have been af- fected by any subsequent compressional tectonism (Figs. 7 and 8).

At mid-crustal levels, above 8 s, there are dip- ping events in both profiles. The most striking one is the antiformal-stack feature in profile ESCIN-3-3 (Fig. 10). Onland, and a few kilometres to the south of profile ESCIN-3-3, Aller et al. (1994) propose the existence of basic rocks thrust at mid-crustal level (about 10-15 km) during Variscan orogeny to be the cause of an important magnetic anomaly in the west- ernmost part of the West Asturian-Leonese Zone. Refraction data (C6rdoba et al., 1987) point towards the existence of anomalous rocks at those levels, which produce a high compressional velocity win- dow (over 6.5 km/s) in crustal models. In agreement with these data, and with geological cross sections along the coastline onshore that show deep-rooted Variscan structures in this area (Pdrez-Esta6n et al., 1991), the dipping reflections at mid-crustal levels in these profiles may correspond to Variscan features if we assume a NNE-SSW-trend for Variscan features offshore.

The lower crust is highly reflective in both profiles, and the reflection Moho is interpreted to be located at the base of the LLC. The upper limit of the LLC has a very shallow topography, and is located at 8 s throughout most of the profiles. This LLC shallows gently towards the ENE in profile ESCIN-3-2. The lower limit has stronger topographic changes from 9 s at the ENE end of profile ESCIN-3-2 to 12 s in the central part of profile ESCIN-3-3. It is in this cen- tral part of profile ESCIN-3-3 that the LLC is con- siderably thicker (about 4 s) and comprises two re- flective packages. The origin of this two-band lower crust that is seen only in profile ESCIN-3-3, is at the moment unclear. A two-band reflective lower crust between 9 and 12 s found at the western side of pro- file ESCIN-1 onland (Pdrez-Estadn et al., 1994) has been interpreted as imaging a compressional Variscan feature. The two-band lower crust can also be inter- preted as a duplication of the LLC due to subduction processes imaged in a profile oriented along strike of the subduction direction. The LLC reflectivity in profile ESCIN-3-1 can be related to shear structures developed during the extensional phase of formation of the margin. It could also be the remnant, after ex- tension, of an originally thicker reflective lower crust. This latest interpretation has been suggested for the Goban Spur area in the southwestern continental mar- gin of Britain (Peddy et al., 1989) which has also been formed from stretching of a previous Variscan crust. The same interpretation has also been applied to the Valencia Trough (Watts et al., 1990).

J. A lvarez-Marrrn et al. I Tectonophysics 264 (1996) 153-174 171

5o0 .. ~. " ~... .. " = . . . . . i~.~] - ---.-~ -~- 5,0

. - - - _ ~ . t " . ~ . - ~ j . . ~ . - - - ~ J ~ . ~ . : ~ ~ _ - - - _ - . : . - . - - ' ~ ~ ~ - - . . . . . . _ . . .

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Fig. 11. Window of migrated profile ESCIN-3-2 coherence-filtered section with a - 0 . 4 to 0.4 s km -~ slowness passband applied at a window length of 875 m. The LLC shows a gentle topography with broad curvature in this part of the profile between 7 and 9 s. Below the reflective Moho, at the base of the LLC, a bright reflection shows a smooth staircase geometry reaching traveltimes of 13 s.

Below the Moho, dipping reflections are present along both profiles (Figs. 7, 8 and 11). Their nature and origin is for the moment unclear. Further inter- pretation of the data and correlation with other on- going geophysical analysis are needed. However, it may be speculated that they are related to subduction processes and could be produced by a downgoing slab. If this is true, then the different dipping direc- tions suggest a complex geometry of the subducted slab, perhaps it is folded or broken into different pieces.

6 . C o n c l u s i o n s

The marine ESCI-N near-vertical reflection pro- files (ESCIN-3 and ESCIN 4) cross the entire con- tinental margin of northern Spain and provide im- ages of the structure beneath the continental shelf and the transition to the Bay of Biscay oceanic crust. These profiles show very good reflectivity distributed throughout the upper 50 km of the litho- sphere (Fig. 12). Reflectivity is seen at upper, middle and lower crustal levels, and also in the upper man- tle. Old features have not been totally overprinted by younger ones and therefore the seismic image constitutes a record of the geological history of this region. This history includes at least three major tec- tonic events: the Variscan collision in the Palaeozoic,

a subsequent extensional event with formation of the Bay of Biscay oceanic crust during the Mesozoic, and a compressional event with inversion of previous extensional basins and subduction of the Bay of Bis- cay ocean floor beneath the north continental margin of Iberia in the Cenozoic.

Dipping, mid-crustal reflections on profiles ESCIN-3-2 and ESCIN-3-3 that image the inner parts of the continental platform, are interpreted as Variscan features. Younger features imaged mainly at upper crustal levels in profile ESCIN-4, include Mesozoic extensional basins inverted during the Cenozoic (Le Danois and Ribadesella basins). Other upper crustal features are interpreted as recent slope- drape and sag basins in profiles ESCIN-3-1 and ESCIN-3-3, respectively.

Lower crustal reflectivity along the continental margin is well-imaged in profiles ESCIN-3-1, 3-2 and 3-3 and may be seen in a small part of profile ESCIN-4. This lower crustal reflectivity can be fol- lowed until the ocean-continent transition in profile ESCIN-3-1. The age of this reflective lower crust is controversial. In profile ESCIN-3-3 the lower crust contains two reflective bands that could be related to previous, possibly Variscan tectonic processes. The duplication of the LLC could also be produced by subduction processes during the Cenozoic. In profiles ESCIN-3-1 and ESCIN-3-2 the reflectivity

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J. Alvarez-Marrrn et al. / Tectonophysics 264 (1996) 153-174 173

may image shear structures related to ductile crustal stretching during the Mesozoic that has produced the thinning of the continental crust to the formation of the Bay of Biscay. It could also be the remnant, after extension, of an originally thicker reflective lower

crust. A very striking feature in ESCIN marine profiles

is the existence of several, well-imaged, sub-Moho reflections. These reflections are seen beneath the

continental shelf in profiles ESCIN-3-3 and 3-2, and are probably related to the Cenozoic subduction processes. Other structures related to the conver- gence of European and Iberian plates are at the ocean-cont inent transition and include a tectonic ac- cretionary prism imaged in profiles ESCIN-4 and ESCIN-3-1 and a major thrust in profile ESCIN-3-1 that separates the shortened and deformed sediments in the accretionary prism next to the undisturbed, previously extended continental shelf.

The oceanic crust of the Bay of Biscay is also imaged. In profile ESCIN-4 a very thick undis- turbed sedimentary package of about 3 km thick is seen above the oceanic-type basement. The contact between the sediments and the oceanic basement deepens gently towards the south with a correspond- ing thickening of the incoming sediment section of the accretionary prism. In profile ESCIN-3-1, the oceanic Moho is seen at 11.2 s, and about 1.5 km of undisturbed sediments are found above the 2-s-thick oceanic basement.

Acknowledgements

We wish to thank the BIRPS group at Cambridge (UK) for their assistance during the processing, es- pecially J. McBride who is also thanked for his comments on the manuscript. D. Brown is thanked for reviews of the manuscript. J. Alvarez-Marr6n held a post-doc research grant from the Ministry of Education and Science of Spain. The ESCI-N pro- gram was sponsored by the Spanish agencies CICYT (project GEO 90-0660) and FICYT, and STRIDE Program of the EU.

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