Bathymetry of the northeast Atlantic: Mid-Atlantic ridge to southwest Europe

Deep-Sea Research, 1975, Vol. 22, pp. 791 to 810. Pergamon Press. Printed in Great Britain.

Bathymetry of the northeast Atlantic: Mid-Atlantic Ridge to southwest Europe A . S. LAUGHTON,* D . G . ROBERTS* a n d R. GRAVES*

(Received 16 April 1975; accepted 19 June 1975) Abstraet--A new bathymetric chart is presented of the northeast Atlantic between 32 and 50°N and extending westwards to include the Mid-Atlantic Ridge. The chart includes areas of sea-floor spreading, transform faulting, compression and past subduction and illustrates the complexity of the resulting topography. The features are related where possible to current theories of the evolution of the oceanic crust and continental margin.

INTRODUCTION

As A basis for geophysical and oceanographic studies during the last 15 years, the Institute of Oceanographic Sciences has prepared contour charts of the ocean floor in the northeast Atlantic from soundings collected by the Hydrographic Departments in U.K., France and Germany, supplemented by whatever additional data has been available. These collections, made under the auspices of the International Hydrographic Bureau for GEBCO,~ are at a scale of 1 : 1 million in various latitudinal bands, and the contour charts have been initially drawn at the same scale. Because of the steadily increasing supply of data, the charts have been repeatedly revised and have therefore been available to scientists only as unpublished manuscript charts..However, the data are now dense enough and the morphology of the northeast Atlantic is sufficiently well established to publish a combination of these charts at a reduced scale. COVERAGE OF THE CHARTS The area for which contoured charts have been prepared at 1 : 1 million scale stretches from Iceland to the Canary Islands, and from western Europe and northwest Africa to the Mid-Atlantic Ridge. The area has been divided into four sheets of matching Mercator projection with a scale of 1 : 2.4 million at 41°N. The sheet boundaries have been chosen so that each sheet is a distinct and

useful geographical unit with a sufficient overlap to minimize problems of working near chart boundaries. The set of four charts has been titled 'Bathymetry of the northeast Atlantic' with subtitles as follows: Sheet 1 (47-64°N, 12-37°W)'Reykjanes Ridge and Rockall Plateau' Sheet 2 (47-64°N, 6°E-18°W) 'Continental margin around the British Isles' Sheet 3 (32-50°N, 0-31°W) 'Mid-Atlantic Ridge to southwest Europe' Sheet 4 (11-34°N, 6-37°W) 'Continental margin of northwest Africa and Canary Basin'. This paper presents the first of these sheets to be published (Sheet 3),:~ chosen because it covers the area of main current interest in the U.K. The depth units used are corrected fathoms, since these were the units in which the basic data has been compiled by the U.K. Hydrographic Department. Contours are drawn at 100 fathom§ intervals both at sea and on land so that a direct comparison of relief can be made. PREPARATION OF CONTOURS Contours have been drawn at the 1 : 1 million scale using, as a principal source, the Collected *Institute of Oceanographic Sciences, Wormley, Godalming, Surrey, England. tGeneral Bathymetric Chart of the Oceans. ~Chart is in wallet attached to inside back cover of this issue. §1 fathom = 1.8288 m.

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A.S. LAUG['-ITON,D. G. ROBERTS and R. GRAVES

Oceanic Sounding Sheets 28, 29, 41, 42, 43, 59, 60, 61, 79, 80 and 81 supplied by the Hydrographer of the Navy (U.K.). These are in fathoms corrected according to MATrHEWS' tables (1939) HD 282, and contained data up to October 1971. Additional data were used from the GEBCO collections of the Deutsches Hydrographisches Institut, Service Hydrographique et Ocranographique de la Marine Fran~aise and the U.S. Naval Oceanographic Office. These collections, however, did not contain many of the recent cruises of several oceanographic organizations so that those data were obtained directly, either in plotted form or in computer compatible form. The following organizations kindly made unpublished data available: Department of Geodesy and Geophysics, Cambridge University, U.K. National Institute of Oceanography (now the Institute of Oceanographic Sciences), U.K. Centre National de l'Exploitation des Oceans, France. Lamont-Doherty Geological Observatory, U.S.A. Woods Hole Oceanographic Institution, U.S.A. University of Rhode Island, U.S.A. Bedford Institute, Canada. The position of each sounding used from these sources is indicated as a dot on the chart. Positions of soundings on the continental shelf away from the shelf edge have been omitted. In addition, some areas have been surveyed in greater detail by various ships using anchored buoys or specially installed electronic navigation aids. These areas are enclosed by boxes and the tracks are not shown. The sources of these data are listed in the margin of the chart. The philosophy behind the contouring procedure has been expounded by LAUGHTOtq,ROBERTS and GRAVES (1973). In short, any information or background experience that is available has been applied in order to interpolate the contours between sounding lines and hence to lead to the nearest approach to the true morphology. Where a detailed survey has shown the texture of the relief in one area, a similar texture has been drawn

in similar physiographic regions. For instance the 45°N survey on the Mid-Atlantic Ridge (AUMENTO, LONCAI~-EVlCand Ross, 1971) shows a typical length/breadth ratio of the ridges neighbouring the median valley, and a similar ratio has been used elsewhere on the Ridge even where the tracks are more widely spaced. A knowledge of geological processes, such as the ponding of sediments in abyssal plains, and the passage of gravity controlled turbidity currents down canyons, enabled correlations to be made which would not otherwise be apparent. Often soundings that are in error in depth or in position have been adjusted or rejected on such geological grounds. In a few areas, such as the Gloria Fault Zone, the use of long-range side-scan sonar (RusBY, 1970) has enabled interpolations between so unding lines to be made with considerable assurance and the texture to be shown clearly. Passage tracks with this sonar have determined morphological trend lines. On land, the 100 fathom (600 ft) contours have been derived by interpolation of the i : 500 thousand series 1404 maps published by D Survey, War Office and Air Ministry (edition I-GSGS). The coastline has also been derived from these maps, except for the oceanic islands where they have been obtained from the relevant Admiralty charts. PRODUCTION OF THE CHARTS The cartographic work in preparing the chart has been done by the Experimental Cartography Unit of the Natural Environment Research Council, using computer data banking of the contours and sounding positions, and subsequent automatic cartography (BICKMOrte, 1969, 1971). Hand-drawn contours on the compilation sheets at ascale of 1 : 1 million or 1 : ½ million were digitized and stored as labelled streams of coordinates. After computer processing, these were plotted on photographic material at l : 2-4 million on a GEAGRAPH flat bed plotter using a light spot projector. The position of all soundings used in the contouring (about i00,000 excluding those within the survey boxes) were digitized separately and plotted automatically, together

Bathymetry of the northeast Atlantic: Mid-Atlantic Ridge to southwest Europe

with the grid and border. Names, contour labels, legends and title box were prepared manually. The chart (Plate I) is available, unfolded, from the Hydrographic Department, Ministry of Defence (Navy) and is listed in the Admiralty List of Charts as C6568. NOMENCLATURE OF UNDERSEAS FEATURES

With the increasing studies of the ocean floor it is important that the sea-floor features acquire names that are widely known and used. The principles of nomenclature have been discussed periodically (WISEMAN and OVEY, 1953; GEBCO Sub-Committees on nomenclature of forms of oceanic relief and on proper geographic names; UNITED STATES BOARD ON GEOGRAPHIC NAMES,

1971). The list of generic terms given in the 2nd edition of Undersea Features (USBGN, 1971) has, with one or two exceptions, been adopted for the northeast Atlantic Charts. The term 'abyssal plain' is preferred to 'plain' since it has been clearly defined in the literature (HEEZEN and LAUGHTON, 1963) and has an important association with the geological process by which it is formed. The term 'table-mount' or 'guyot' has not been used for seamounts with a more or less flat top since it is difficult to determine how smooth and flat the top should be, or whether isolated peaks on an otherwise flat top disqualify the term. Many seamounts are inadequately surveyed to define the shape of the top. In some cases, the generic names of features already described in the literature have been altered to conform with the criteria listed in Undersea Features (e.g. Cantabria Seamount has been changed to Cantabria Knoll since it does not rise more than 500 fathoms above the neighbouring sea-floor). The proper names of features have been derived from Undersea Features (UNITED STATES BOARD ON GEOGRAPHIC NAMES, 1971) and from the punished literature cited in the later description of the features of the chart. Several new names were felt necessary to describe major features revealed by the new bathymetry and the origin of these is also discussed later. A policy for allocating proper names is presented in Undersea

793

Features and the new names conform in general with this policy. It is important, however, that features which have been studied in detail for scientific or surveying purposes should acquire the names given to them by those who have studied them, since the major purpose of a proper name is to allow easy and unambiguous reference to it. The policy of allocating names associated with nearby land features is only tenable with features that are large compared to their distance from the land; other~,ise new features might be discovered nearer to land that would be more appropriately named after the land. In due course, more and more proper names will be given to the sea-bed features and these need to be able to stand on their own as distinctive, associative and even sometimes amusing names. A gazetteer of England would be the poorer without Piddletrenthide or Ashby-de-la-Zouche. GENERAL TECTONIC SETTING

The greater part of the chart 'Mid-Atlantic Ridge to southwest Europe' (Plate I) comprises the oceanic part of the Eurasian lithospheric plate. To the west it abuts the American plate along the line of the median valley of the MidAtlantic Ridge, and to the south it abuts the African plate along the Azores-Gibraltar plate boundary. At the junction of these two plate boundaries, the Azores complex has resulted from an additional spreading centre at a migrating triple junction (KRAUSE and WATKINS, 1970; MCKENZIE, 1972 ; LAUGHTONand WHITMARSH, 1974). The bathymetric chart permits the real shape of the sea-floor to be related to the concepts of plate tectonics yet reveals major features that are not obviously related to present-day plate boundaries. In this paper, the major bathymetric features will be described, and related to what is known of their origin, but detailed interpretations of their evolution require consideration of all available geological and geophysical data, much of which is outside the scope of this paper. HE.EZEN, THARP and EWING (1959) described in great detail the major features of the North Atlantic and developed a still viable system of classification of features. The physiographic

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provinces which they defined provide a valuable starting point for describing the physiography, but there are many cases where the morphology does not easily fit into any one of their categories. In particular, the continental rise is difficult to map precisely owing to the very small change of gradient between the outer edge and an abyssal plain. Further, the division of Mid-Atlantic into five regions parallel to the median valley is difficult to support on present data. However, most of the major classifications remain valid and the increase since 1959 in sounding data enables the boundaries between the main provinces to be more accurately delineated. The statistics of relative relief, slope and topographic texture in

the North Atlantic have been presented in detail by HOLCOMBEand HEEZEN(I970) and will not be treated in this paper. Figure 1 summarizes the boundaries to the main physiographic regions and both major and minor topographic trends derived from the bathymetric chart and relates them to key magnetic anomalies, earthquake epicentres and processes of sedimentation related to the continental margin. MID-ATLANTIC RIDGE The chart includes the crestal region and the whole east flank of the Mid-Atlantic Ridge (Fig. 2). It is not possible to define an eastern boundary to the Ridge because of the progressive

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burial of older crust by sediments. SCLATERand FRANCIIETEAU(t970) have shown that the topographic profile of a m i d , o e ~ n ridge is determined by the accretion and cooling history of the oceanic lithosphere. SCI.AT~R, ANt)F.RSON and BELL (1971) have compared theoretical and observed ridge profiles in the Pacific, Indian and Atlantic oceans and concluded that the best match to the topography is achieved by assuming a I00 km thick lithosphere and a basal temperature of 1475°C. For the North Atlantic north of 45°N the sparse data did not fit their model which assumed a depth to basement of 6000 m at infinite age. HAIGH (1973) examined E-W profiles between 43 and 61°N and concluded that a thinner lithosphere and lower basal temperature provided a better fit, and further, that these >c

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parameters varied along the axis of the ridge, the lithosph~'e decreasing in thickness and increasing in basal temperature towards the Icelandic hot spot, Haigh's derivation of the profiles, however, differed substantially from those of Sclater. Anderson and Bell. To compare the observed profiles at 40, 43, 46 and 49°N with theoretical ones, calculations were made using the equations of SCLATER. AND~SO~ and BELL (1971) but assuming (a) Haigh's lithospheric thickness of 80 km and ~ basal temperature of 1200°C (derived from iai~ quoted temperatures at 65 kin), (b) a lithospheric thickness of 100 km and a basal temperature of 1475°C. Spreading rates have been derived from a new magnetic anomaly contour chart (Jones and Roberts, personal commurdcation). These can be i~

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Bathymetry of the northeast Atlantic: Mid-Atlantic Ridge to southwest Europe approximated to three phases of spreading (a) 0 to 9 my, 1.5 cm yr -1 (b) 9 to 60 my, 1.0 cm yr -1 (c) 60 to 75 my, 2-5 cm yr -1, consistent with those identified by Wlt.HAMS and MCKENZIE (1971), Pn'MAN and TAt.'a'ANI (1972) and WILt.IAMS(1975). The comparisons showed that the parameters ha assumption (b), (Fig. 3) fitted the observed profiles of 46 and 49°N in shape and elevation of the ridge. A depth to basement of 5500 m at infinite age was necessary to fit the topography, explaining the anomalous North Atlantic point at 45°N in Fig. 2c of Sclater et al. However, major deviations from the theoretical curve are seen in profiles at 40 and 43°N. At 40ON, the observed profile is strongly influenced both east and west of the spreading centre by the Azores complex, where there is a triple junction with the Terceira spreading centre (KRAUSE and WATKINS, 1970; LAUGNTON and WHITMARSH, 1974). The anomalously shallow Azores plateau may reflect a higher temperature of the lithosphere. However, SCLATER,LAWYER and PARSONS (1975) have mapped the residual elevation anomaly associated with the Azores triple junction in the central North Atlantic and have related it to an anomaly in the smoothed free air gravity field. They suggest that the anomalies are due to dynamic forces associated with flow in the upper mantle raising the rigid lithospheric plate. At 43°N, the profile crosses the anomalously shallow regions associated with King's Trough (MATTHEWS, LAUGHTON, PUGH, JONES, SUNDERr a N , , TAKIN and BACON, 1969) and the AzoresBiscay Rise, which are also revealed by topographic profiles along the isochrons (Fig. 4). These features are not explicable by the simple cooling history of the Eurasian plate and will be discussed later. The profiles in Fig. 3 include the western half of the ridge so that its symmetry can be assessed. The associated theoretical curves are based on the spreading rate history established for the eastern half, and have not been fitted to magnetic anomalies west of the crest. A notable feature on the 43°N profile is the swell 600 km west of the crest which matches that around King's Trough. The swell trends southwestwards from Altair

797

Seamount and can be seen on Atlantic charts published in 1971 by the Institute of Oceanology, U.S.S.R., and also by UCHUPI (1971). The median valley can be located with little ambiguity throughout the length of the ridge north of Azores, but south of 40°N, the sounding control is insufficient. Recent data by Krause and McGregor (personal communication), however, suggest that the spreading axes between the east and west Azores islands is typified by a ridge and not a valley. The continuity of the valley is in some places disrupted by fracture zones frequently marked by a local increase in the depth. Elsewhere segments of the median valley end to be replaced by other en echelon segments without any transverse fracture zone. The two median valleys at a given latitude are separated by a ridge. One example is seen at 43.6°N, another is quoted by LITV~N, MAROVA, RUDENrCOand UDINTSEV (1972) just north of the Kurchatov Fracture Zone, and yet a third has been unambiguously mapped at latitude 36.5°N in the FAMOUS* area of the MidAtlantic Ridge. The mean crestal depth of the Mid-Atlantic Ridge does not appreciably change north of 42°N, but is of course shallower near the Azores. However, there is a steady increase in the maximum depth in the median valley from 1600 fathoms at 42°N to 2100 fathoms at 49°N. The median valley has been surveyed at 47°N (HILt., 1960) and in considerable detail between 45 and 46°N (AUMENTO, LONCAREVlC and Ross, 1971; BHATTACHARYYAand Ross, 1972). The texture of the mountainous topography associated with sea-floor spreading in the crestal area is preserved on the ocean crust as it is carried away from the ridge and reflects the trend of the spreading axis at the time the crust was formed in the same way that the magnetic anomalies do. Even though subsequent tectonic events may cut across and displace them, the original sea-floor spreading trends can often be detected by close survey (e.g. the northward fingering ridges on the north side of King's Trough). *French-American Mid-Ocean Study.

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As the crust becomes older, sedimentation fills the valleys preferentially, especially in turbidite areas, and only the crests of the ridges are seen. An example is Swallow Bank and associated abyssal hills in the Iberian Abyssal Plain at 41°15'N, 14°30'W. Further evidence of relict ridge crest topography was obtained in the side-scan sonar survey at 38°N, 20°W (LAuGHTON and WHITMARSH, 1974), which showed relict ridges trending exactly parallel to the magnetic

anomalies. Relict ridge trends can therefore be used as an indication of the trend of the sea-floor spreading axis even in areas where magnetic anomalies are insufficiently mapped or where periods of nonreversal leave no anomalies. They can be rapidly established by single traverses of a long range side-scan sonar. Figure 1 shows all the trends of the minor relict ridges most of which show good correlation with the magnetic anomalies. Regions

Bathymetry of the northeast Atlantic: Mid-Atlantic Ridge to southwest Europe

where the correlation is poor or absent may be due in all but a few places to poor contouring or poor control.

FRACTURE ZONES

The East Azores Fracture Zone is the major E - W feature in the mapped area. East of 24°W it is the active part of a transform fault connecting part of the spreading of the Mid-Atlantic Ridge north of the Azores with the relative plate movements of Eurasia and Africa in the Mediterranean and further east. Between 24 and 15°W, it is principally strike-slip and shows a clearly defined E-W morphology (LAUGHTON and WHITMARSH, 1974) especially in the region of the Gloria Fault Zone (LAUGHTON, WHITMARSH, RUSBY, SOMERS, REVlE, MCCARTNEY and NAFE, 1972). East of 15°W, the relative motion between the Eurasian and African plates becomes more compressive, resulting in the complex topography of the Madeira-Tore Rise and the Horseshoe Seamounts (LE PICHON, BONNIN and PAUTOT, 1970; PURDY, in press). West of 24°W, the plate boundary joins the Mid-Atlantic Ridge spreading centre through the active Terceira spreading centre trending W N W ESE. The East Azores Fracture Zone continues westward along 37°N as the inactive relic of an earlier phase in the development of the Azores triple junction (Krause and McGregor, personal communication). Although it is aligned with the fracture zone at 37°N at the ridge crest, it is separated from it by the ridge extending westsouthwest from Acor and Princesse Alice Banks (LAUGHTON and WHITMARSH,1974). Apart from the East Azores Fracture Zone, there are two main groups of fracture zones. The more northerly group between 50 and 47°N is associated with the left lateral offsets in the MidAtlantic Ridge axis south of the major CharlieGibbs Fracture Zone at 52°N. Between 47 and 51°N these have been mapped by JOHNSON and VOGT (1973) in sufficient detail "to allow contour representation without recourse to geologically or physiographically biased interpretation". Combined with the other data, this survey shows two

799

E-W fracture zones at about 49½ and 48½°N that can be recognized as far east as 20 to 21°W. These are here named the Faraday Fracture Zone (49½°N) after the Faraday Seamount at 43°30'N, 28°30'W, and to pair with this, the Maxwell Fracture Zone (48½°N). Other E-W fracture zones in the vicinity are smaller and have not been named. West of the ridge axis, the fracture zones continue in a westnorthwest direction at least as far as 31°W and probably considerably further. Within 200 km of the ridge axis the east and west halves of these fracture zones make a broad V, which JOHNSON and VOGT (1973) have interpreted as resulting from a southward asthenosphere flow from the Iceland hot spot. However, there is some evidence that the Charlie-Gibbs Fracture Zone would act as a barrier to any such flow (Schilling, personal communication) so that an alternative explanation may be necessary. These fracture zones do not appreciably offset some of the prominent magnetic anomalies nor the axis of the median valley, which appears to have developed obliquely to the spreading directions defined by the fracture zones. JOHNSON and VOGT (1973) point out the alternating segments of oblique and normal rifts during the last 10 to 20 my but do not offer an explanation. South of the Azores the chart shows the second group of E - W fracture zones. Although the sounding control is much poorer in this region, prominent ridges and scarps can be traced and correlated with offsets of identifiable magnetic anomalies (LAUGHTONand WHITMARSH, 1974) and hence can be described as fracture zones. The Mid-Atlantic Ridge at this latitude is offset by a series of right-lateral transform faults, the largest of which gives rise to the Oceanographer Fracture Zone at 35°N. However, it is not possible to correlate unambiguously the ridge transform faults with the fracture zones mapped between 20 and 30°W, owing to a lack of good data. There is some indication that one of these fracture zones crosses the Madeira-Tore Rise at 35.7°N offsetting the group of Mesozoic magnetic anomalies which lie along the Rise (LAUGHTON and WHITMARSH, 1974).

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A.S. LAUGHTON, D. G. ROBERTS and R. GRAVES

Several other smaller fracture zones cross the crestal region of the Mid-Atlantic Ridge but are not traceable for more than a few tens of kilometres. The Kurchatov Fracture Zone (LITVIN, MAROVA, RUDENKO and UDtNTSEV, t972) at 40°30'N offsets the median valley by about 30 km but is traceable only for 140 km to the east. Although not shown on the new chart, the fracture zone also extends to the westnorthwest, giving a broad V similar to the fracture zones north of 47°N. It has been suggested by HEEZEN and THARP (1968) that the Mid-Atlantic Ridge between the Azores and 50°N is characterized by a series of fracture zones spaced by 100 km and trending 285 °, one of which extends into King's Trough. From the data contoured in the chart now presented, there is no evidence for an extension of King's Trough across the ridge axis nor for a series of fracture zones of this trend on the eastern side of the ridge, nor would it be possible to contour the data in this way. THE AZORES P L A T E A U

parallel to the Mid-Atlantic Ridge axis. South of Pico Island, the Santa Maria Ridge (KRAUSE, 1965) crosses N-S magnetic anomalies (LAuGHION and WHITMARSH, 1974). North of Terceira at 40½°N, and northeast of San Miguel between 41°N 25°W and 38½°N 22°W there are several other ridges which cross the main trend of the magnetic anomalies. These features are not apparently related to Mid-Atlantic Ridge fracture zones, which trend E-W, and therefore can best be ascribed to faulting associated with the stress field resulting from accretion at the Terceira Rift plate boundary. A prominent feature of the Azores complex is the ridge running westsouthwest from Pico through Acor and Princesse Alice Bank. The ridge is well defined between two scarps of over 500 fathoms and there are many shoal regions of about 200 fathoms with smooth rounded tops, suggesting that the ridge may at one time have been subjected to wave erosion and subsequently subsided. It can be traced for at least 300 miles approximately parallel to the now step-like axis of the Mid-Atlantic Ridge (LAUGHTON and WHITMARSH, 1974), and may have evolved during a phase of extreme oblique spreading. A similar but less well-mapped ridge is found on the other side of the spreading axis, running as far north as the island of Corvo.

The term Azores Plateau has often been used to describe the shoal area between 24 and 30°W from which rises the East Azores Islands. However, far from being fiat topped, it is characterized by a series of highs (islands and shoals) and deeps. Krause and McGregor (personal communication) present a bathymetric chart of the region between KING'S TROUGH 25 and 29° based on data more recent than is First mapped in 1965 (LAUGHTON, 1965) and included in Plate I. Along the Terceira ;spreading later studied in greater detail on a number of axis (which the above authors believe to have occasions, King's Trough is a major feature of" shifted some tens of kilometres southward during the oceanic part of the Eurasian plate. It consists the last 2 my) the islands alternate with a series of of a series of parallel or subparallel troughs deeps which are associated with mid-oceanic ridge totalling 250 miles linked en echelon by cois and type rifting, in contrast to the late development of flanked by ridges, giving rise to a relative relief the central vent volcanism of the islands. Within of over 1000 fathoms. As has been pointed out the zone of spreading from the Terceira spreading above, it lies on a regional swell on the flanks of axis, which can best be delineated by the magnetic the Mid-Atlantic Ridge. The whole feature is anomaly pattern and which is outlined in Fig. 1, parallel to the Terceira spreading axis and to the the topographic trends are primarily WNW-ESE, many minor lineations lying between King's parallel to the axis. Trough and the Azores. However, there are topographic features of Peake and Freen deeps, which lie at the this trend well away from the Terceira Rift which southeast end of King's Trough, are thought to lie in regions where the magnetic anomalies are .~h a v e originated from compressional overthrusting,

Bathymetry of the northeast Atlantic: Mid-Atlantic Ridge to southwest Europe

followed by phase changes in the thickened basaltic crust and subsequent formation of an anomalously high density body in the upper mantle (MATTHEWS, LAUGH'I'ON, PUGH, JONES, SUNDERLAND, TAKIN and BACOr~, 1969). A similar mechanism may explain the whole of King's Trough, although evidence supporting the theory is very meagre. Linear magnetic anomalies can be traced close to the marginal ridges and do not appear to be significantly displaced (Jones, personal communication) and so a strike slip or fracture zone origin for the feature is unlikely. LE PICHON a n d SIBUET (1971) and WILLIAMS (1975) have attempted to relate the compressional formation of King's Trough with the Upper Eocene northward movement of the Iberian peninsula which was part of the compressional phase (45 to 38 my ago) of the Pyrenean orogeny. However, the northwest end of King's Trough cuts crust of Middle Oligocene age (and not Late Eocene age as stated by Le Pichon and Sibuet) implying that it was formed at least as late as this. CANN and FUNNELL (1967) concluded from a study of rocks on Palmer Ridge between Peake and Freen Deeps, this was upthrust in the Late Oligocene (26 my ago). THE A Z O R E S - B I S C A Y RISE

Closing the southeast end of King's Trough is the irregular Azores-Biscay Rise (Fig. 5), some 800 km in length and 100 km in width trending SW-NE (LAUGHTON, 1965). The trend of the individual ridges on the crest of the rise conforms to the trend of the regional magnetic anomalies whereas the rise as a whole is cut by the anomalies at an angle of 30° (Fig. 1). The feature appears therefore to be a piece of oceanic crust which has been uplifted by 500 to 1000 fathoms but there has been no significant horizontal translation, since the magnetic anomalies are not displaced. LE PICHON and SmUET (1971) have suggested that the Rise marks a zone of predominantly strike slip movement linking the northward movement of Spain with the compressional formation of King's Trough. However, this is inconsistent with the absence of anomaly displacement. Furthermore, the Azores-Biscay Rise continues southwest

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Fig. 5. Bathymetric profiles showing the Azores-Biscay Rise. Vertical exaggeration 34 : i. (Position of profiles shown in Fig. 1).

beyond King's Trough where no transform movement would be expected on this model. WILLIAMS(1975) has further developed the ideas of Le Pichon and Sibuet relating the inception of the Terceira Rift at 45 my ago, to the formation of King's Trough and the Pyrenean compression phase by the Eocene movement of an Iberian plate. However, the evidence for a link between Iberia and King's Trough via the Azores-Biscay Rise is not convincing and the latter features must remain for the time being unexplained examples of intraplate tectonics. SEAMOUNTS ASSOCIATED W I T H THE BAY OF BISCAY

At the northeastern end of the Azores-Biscay Rise, the dominant topographic trend changes abruptly to E-W. The North and South

802

A. S. LAUGHTON, D. G. ROBERTS and R. GRAVES

Charcot Seamounts form a more or less sym- prominent ENE-WSW trends. Ashton Seamount metrical pair of ridges which may continue (named after the Commanding Officer of the skip eastward into the Bay of Biscay as a series of which surveyed it) is by contrast a circular ridges progressively more deeply buried by peak. sediments. DSDP* Hole 118 was drilled near At the northern end is the unusually shaped such a buried ridge (LAUGHTON,BI~GOREN et al., Tore Seamount, so named because of its toroidal 1972) and Hole 119 was drilled on Cantabria shape. The circular ridge of 80 km diameter Knoll, also trending E-W. North of this line, the encloses an isolated deep of over I000 fathoms Biscay Seamounts trend E-W as one or two relief, the bottom of which is deeper than the ridges. neighbouring abyssal plains owing to the absence The Bay of Biscay has evolved by the anti- of turbidite sediments derived from the land. To clockwise rotation of Spain away from the the west of the feature is another ENE-WSW Armorican continental margin (LE PICHON, trending ridge. 'The unusual shape of Tore BONNIN, FRANCHETEAU a n d SIBLIET, 1971) which Seamount called for further study. Additional has given rise to the magnetic anomalies associ- sounding data were included in the recontoured ated with the Bay. The anomalies associated with chart shown in Fig. 6, and profiles across the the North and South Charcot Seamounts link with feature were constructed from sounding lines the Mid-Atlantic Ridge spreading anomaly (Fig: 7). To the south, the containing ridge is number 31-32 at the northeast end of the Azores- broad and single crested and there can be little Biscay Rise. However, seismic profiles across doubt about its continuity. The north boundary these seamounts show evidence of uplift sugges- consists of a double ridge, the southern part of ting that they are not the dead remains of a which may be an extension northeastwards of the Biscay spreading centre but that they have resulted big ridge to the west. However, the continuity of from subsequent tectonic' activity. The two DSDP this ridge northwest of the central hole cannot be holes, together with other geological and geo- completely established on the present data. physical evidence, indicate that uplift started in There are three possible origins of Tore the Eocene and that the Charcot, Biscay and Seamount which could account for its nearassociated seamounts were uplifted at the same circular form. First, it could be an immense time as the compressional folding of the Pyrenees caldera, about 100 km across its rim. The N-S (LE PICHON and SIBUET, 1971). seismic section across it (Fig. 8) suggests that the inner facing slopes are fault scarps and that there THE M A D E I R A - T O R E RISE AND THE may be a volcanic feature within the caldera on HORSESHOE $EAMOUNTS the south side. But a caldera of this size would Crossing the Eurasian-,adrrican plate boundary require an excessively large emptied magma is the massive Madeira-Tore Rise stretching chamber into which collapse could take place. 1400 km from a point southwest of Madeira to Secondly, it could be the result of meteorite Tore Seamount. The Rise, which is sub-parallel impact. Although many large meteor craters have to the Azores-Biscay Rise, is capped by a series been observed on land, none has yet been of ridges or seamounts, which trend along or described in the ocean. It is not obvious by what obliquely across the axis of the Rise. Dragon features an oceanic meteorite crater could be Seamount is newly named to link with Unicorn recognized, nor what is the role of the water layer Seamount (HEEZEN and THARP, 1968) and was in dissipating the meteor's energy. If the feature is described by LAUGHTON, HILL and ALLAN (1960). a meteorite crater, it is a coincidence that it occurs Similarly, Lion Seamount is a new name for a on the line of the Madeira-Tore Rise. Thirdly, feature that has not been closely studied. Further the feature could have arisen by a chance mixture north are two unnamed peaks, Josephine Seamount and two unnamed ridges which all have *Deep Sea Drilling Project.

Bathymetry of the northeast Atlantic: Mid-Atlantic Ridge to southwest Europe

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of the tectonic trends which typify this compressional part of the Eurasian-African plate boundary. Certainly the north side can be considered as tectonically uplifted and tilted sea-floor with older sediments preserved. The E N E - W S W trend parallels similar tectonic trends of Gorringe Ridge and Coral Patch Seamount. The east side parallels the main trend of the Madeira-Tore Rise. However, the southwest part of the encircling ridge has a trend N W - S E which is not observed elsewhere. On balance the authors support this third explanation but recognize that the true origin could only be established by more data. The Horseshoe Seamounts comprise the seamounts of the centre part of the Madeira-Tore

Rise (Lion and Josephine Seamounts), a northerly group (Hirondelle II, Gettysburg and Ormonde Seamounts) and a southerly group (Unicorn, Ampere and Coral Patch Seamounts). A considerable amount of geological and geophysical field work has been undertaken to determine the tectonic history of the Horseshoe Seamounts. MCKENZlE (1970, 1972) and UDIAS and ARROYO (1972) concluded from fault plane solutions of earthquakes along the African-Eurasian plate boundary that this was in general an area of compression, and Lz PICHON, BONNIN and PAUTOT (1970) presented geophysical and geological evidence for this. FUKAO (1973) analysed the first movements in an earthquake swarm in the Horseshoe abyssal plain and concluded that

804

A.S. LAUGI-rrON,D. G. ROBERTSand R. GRAVES

nental margin is passive except off southwest Iberia where it is crossed by the active .~zoresGibraltar plate boundary (Eurasia-Africa plate boundary). The descriptions passive and active have important structural and geological connotations implying an initial origin by rifting for passive margins and present compression or shear in the active case. Active margins may become passive and also vice versa. The present morphologyof the margin is clearly the expression of its structural and sedimentary history. Our discussion considers the gross morphology in this broad context rather 10000D than by a detailed review of the background geological and geophysical data. For convenience, the margins of the Bay of Biscay, a margin west of Portugal and the active margin off southwest t E' 0 100KM Iberia are discussed separately. The northern (Armorican) and southern (Cantabrian) margins of the Bay of Biscay have a Fig. 7. Bathymetric profiles across Tore Seamount. Vertical exaggeration 15 : I. contrasting morphology that reflects different geological histories. Along the length of the the oceanic crust was being thrust beneath north margin, the complete shelf, slope and rise Gorringe Ridge in a northnorthwest direction. sequence is present. The shelf is broadest in the Gorringe Ridge was drilled by DSDP (Hole 120) Celtic Sea area but is still 150 km wide east of and evidence was found for uplift in the Miocene- France. The entire shelf edge and continental slope Pliocene (RYAN, HS0 et al., 1973). are cut by numerous canyons between 12 and In a region of compression, crustal blocks may 2°W (VANNEY, 1972). The largest development of be depressed, lifted and tilted and the boundary the King Arthur*, Whittardt, Shamrock and between the two major plates may be shifting and Black Mud Canyons in the Celtic shelf area may indeterminate. On the basis of seismic and gravity reflect the continuous passage of sediments data, PtmoY (in press) has suggested that at least transported through them along paths oriented one, and possibly more, mini-plates may be perpendicular to the shelf edge (STroDE, 1963; involved and that Gorringe Ridge may lie on one HADLEY, 1964; I~NYON and STRIDE, 1970). These of these. The evidence for compression is strong canyons have cut back, in Pliocene and Pleistocene and the block topography of the Horseshoe times, through Cretaceous and Lower Tertiary Seamounts and probably also of the Madeira- strata that may have developed as prograding Tore Rise must arise from this. shelf during the early opening of the Bay of Biscay (STRIDE, CURRAY, MOORE and BELDERSON, 1969; THE CONTINENTAL MARGINS MONTADERT, WINNOCK, DELTEIL and GRAU, Continental margins have been classified into 1974). shelf, slope and rise physiographic provinces Unusual features developed along the conti(HEEZEN, THARP and EWING, 1959). The distribu- nental slope and rise include the escarpment tion of these provinces along the Atlantic margin *Newly named after the Celtic legend, following the of Europe is shown in Fig. 1. Continental margins of Celtic names by DAY(1959). have also been divided into passive (aseismic) and usage "['Newly nmned after the late Prof. W. F. Whittard, seismically active types. The European conti- Bristol University,who initiatedstudies of the canyon.

Bathymetry of the northeast Atlantic: Mid-Atlantic Ridge to southwest Europe

bounding the Meriadzek Terrace and the more linear Pendragon and Trevelyan escarpments. The Pendragon escarpment abuts the foot of the Goban Spur which consists of thin prograded sediments overlying a series of tilted and rotated fault blocks. Geological and geophysical studies of the Meriadzek Terrace and Trevelyan escarpment suggests both features are underlain by deeply subsided horsts of continental origin (BAcoN and GRAY, 1971 ; MONTADERT,WINNOCK, DELTEIL and GP.AU, 1974; SMITH and VAN RIESSEN, 1973). The continental rise is about 200 km wide and is best developed between the King Arthur Canyon and Landes Plateau, the result of coalescing deep-sea fans. In addition to the Trevelyan escarpment, the rise is broken by Gascony Knoll consisting of post-Miocene pelagic sediments draped over a tilted and faulted sequence of pre-Miocene sediments (MONTADERT, WINNOCK, DELTEILand GRAU, 1974). Off Aquitaine, between the Armorican and Cantabrian margins, the morphology reflects both the subsurface structure and seaward continuation of the Aquitaine sedimentary basin. The prominent Landes Plateau is thus the surface of the seaward prograding Tertiary sediment fill. The adjoining Cap Ferrat Canyon may owe its location to the underlying Parentis deep Cretaceous Basin and the Cap Breton Canyon follows the line of the North Pyrenean Fault. West of Landes Plateau, Jovellanos Seamount lies within the zone of overthrusting associated with the Iberian margin and is composed of igneous(?) basement. The Cantabrian margin of the Bay of Biscay consists of a comparatively narrow shelf and steep slope rising directly from the Biscay abyssal plain. The rise is absent or only locally developed. The rare canyons (e.g. the Aviles canyon) are apparently related to major shears within the Iberian continent (JuLIVERT, RAMIREZ DEL POZO and TRUYOLS, 1971; BO1LLOT,DUPEUPLE,HENNEQUINMARCHAND, LAMBOY and LEPRETRE, 1973). The irregular slope topography shown in Le Danois Bank Area, and the narrow shelf reflect the thin late Tertiary progradation over the deformed Mesozoic and Lower Tertiary substrate that is due to the limited sediment supply available from the

805

small catchment area of northern Iberia. These broad contrasts in morphology reflect the disparate structural and stratigraphic history of the margins of Biscay. On the Armorican margin initial rifting in the Late Jurassic-Early Cretaceous was followed by subsidence and progradation producing the broad shelf, slope and rise. However, the Cantabrian margin which later became a zone of overthrusting active from Late Cretaceous to Oligocene-Miocene time is seen also in the Pyrenees. The culmination of this deformation may have caused the southerly, asymmetric tilt of Iberia. The structural framework of the now passive Iberian margin is thus young. The kinematics of these different phases of evolution of the Bay of Biscay are not well understood partly because clear transform fault trends are not evident and the relationship of the overthrusting zone to the King's Trough-AzoresBiscay Rise lineament is not known (WILLIAMS, 1975). It is worth noting, however, that the changes in trend of the Armorican margin east of the Goban Spur and Meriadzek Terrace, and in the Iberian margin north of Galicia Bank may indicate transform fault trends. The margin west of Portugal can be considered as two contrasting physiographic areas divided at 41°N. North of this latitude, the shelf is narrow and to the west, beyond a col of 1000 to 1500 fathoms depth, lies the shoal area of Galicia Bank. Geological and geophysical studies of Galicia Bank and Vigo and Oporto Seamounts suggest that they are isolated and subsided continental blocks covered by carbonates and fringed by reefs of Cretaceous or Early Tertiary age (BLACK, HILL, LAUGHTON and MATTHEWS, 1964; PAUTOT, AUZENOZ and LE PICHON, 1970; MONTADERT, WINNOCK, DELTEIL and G ~ u , 1974). The seamounts may represent horsts formed during rifting on which neritic limestones accumulated prior to subsidence in post-Albian time. Beyond these seamounts, the 2500 fathom contour defines the base of the slope. Prominent eastward offsets in the trend of the slope at 43°30'N, 42°30'N and 41°N ma;y indicate early transform faults. If so, a subsequent change in spreading geometry may have taken place since

806

A.N. LAUGHTON,E). G. ROBERTSand R. GRAVES

the offsets are not found in anomaly 32 to the west. South of 41°N the margin is associated with the compressional features of the Azores-Gibraltar plate boundary and there is scattered seismicity. Three prominent canyons are present. The Nazar~ canyon may be located on the line of a prominent NE-SW trending fault (BO1LLOT and MUSELLAC, 1972). The Lisbon canyon fed by the River Tagus, and the S~tubal canyon may also be structurally related to the 100 km offset in the margin that extends west towards the Tore Seamount. It is of some interest to compare the Portuguese and Armorican margins. Both margins were probably formed by rifting in Late Jurassic-Early Cretaceous time. In the Armorican case, the large sediment supply from northwest Europe has permitted growth of a broad shelf, slope and rise. Off Portugal, however, the narrow shelf and slope again reflect limited sediment supply due to the asymmetric drainage pattern of Iberia. It is interesting to note that while Vigo Seamount shows evidence of 1600 fathom of subsidence in post-Early Cretaceous time, evidence of reworked Early Cretaceous Orbitolinae on Galicia Bank (BLACK, HILL, LAuGrrroN and M^rrI-mWS, 1964) suggests rather less subsidence over the same interval. This speculative evidence of a southward tilt may match the regional southerly tilt of Iberia and be related to the Pyrenean orogeny. The margin south of Portugal and west of Gibraltar is crossed by the boundary between the Eurasian and African plates and is characterized by frequent though scattered seismicity. The margin is underlain by the complex Late Tertiary Rif-Betic orogenic belt of North Africa and Southern Iberia. In the western approaches to the Straits of Gibraltar, an upper slope is succeeded by a gentler lower slope that dips oceanward toward the Horseshoe and Seine Abyssal Plains. The lower slope is characterized by curvitinear ridges of tectonic and depositional origin and is cut by erosional channels (HEEZEN and JOHNSON, 1969). In the north, much of the minor relief of the shallow parts of the lower slope is sedimentary in

origin and associated with the deceleration of the Mediterranean undercurrent crossing the area (BELDmtSON and KENYON, 1973). A prominent E-W ridge at 36°10'N 8°20rW is a sediment ridge built of the load dropped by the undercurrent as it leaves the slope (GIESELand SEIBOLD, 1968). Salt structures possibly related to the olistostrome front of the Rif-Betic orogen are also present and control the deeper path of the Mediterranean undercurrent (MELIERES, NESTEROFF and LANCELOT, 1970). The morphology of the E-W slope off southern Portugal may be controlled by the Guadalquivir fault though some canyons (e.g. the Sgo Vincente canyon) appear to be related to other major structural lineaments in Southern Iberia. Off western Morocco the large Rharb Valley is a graben apparently related to the Molasse foredeep of the Rif (LACOMBE. 1955). ABYSSAL PLAINS

Within the area of the chart, there are seven prominent abyssal plains: the interconnected Porcupine, Biscay and Iberia abyssal plains and the separate Tagus, Horseshoe. Seine and Madeira abyssal plains. The Biscay abyssal plain extends northwestward as the Porcupine abyssal plain toward the Rockall Trough and, in the south, is connected with the Iberian plain via the Theta gap interplain channel system. The Porcupine plain has a regional southwestward slope suggesting sediment supply from the Rockall Trough and Porcupine Seabight. RUDDIMAN, BOWLES a n d MOLNIA (1972) have suggested that the Maury mid-ocean canyon winds through the flank topography of the Mid-Atlantic Ridge to merge with the Porcupine plain near 49°N. However, the bathymetric data do not show evidence of a connection although a smalf abyssal plain near 49°N 20°W may represent its termination. An alternative southward continuation of the Maury Channel to join the Iberian abyssal plain at 40°N 15°W is proposed by CHERKIS, FLEMINGand FEDEN (1973). This route. however, is inconsistent with the topography presented in Plate I since it cuts across the Azores-Biscay Rise. The Biscay abyssal plain has a regional

Bathymetry of the northeast Atlantic: Mid-Atlantic Ridge to southwest Europe

southward slope that suggests the greater part of its sediment is derived from the Celtic-Armorican shelf areas of the Bay of Biscay. The turbidites flooring the abyssal plain form a conformable sequence of Early Miocene to Recent age deposited on a tilted and faulted succession deformed during the Pyrenean orogeny (LAUGHTON, BERGGREN et al., 1972). The deepest part of the plain is in the southwest corner where the slope leads toward the Theta Gap. LAUGHTON (1968) showed that the Theta Gap is the site of active erosion, the channels resulting from the passage of turbidity currents from the Biscay to the Iberian plain. Sediments of the Iberian plain are thus partly derived from the Bay of Biscay although turbidity currents are also likely to originate in the Nazar6 and other canyons of western Portugal (HEEZEN, THARP and EWING, 1959; LAUGHTON, 1960; DAVlDSON and KEEN, 1963; HEEZEN and LAUGHTON, 1963; LAUGHTON, 1968; HORN, EWING and EWING, 1972). The Tagus abyssal plain is completely enclosed by the Madeira-Tore Rise and the northern limb of the Horseshoe Seamount group. It is shallower than the Iberian abyssal plain and the mineralogy of the sediments is comparable to sediments in the Tagus estuary (DUPLAIX, NESTEROFFand HEEZEN, 1965), consistent with a downslope transport via the Tagus and S6tubal canyons. The neighbouring Horseshoe abyssal plain is completely enclosed by the Horseshoe seamounts. HORN, EWING and EWING (1972) report pelagic oozes interbedded with graded sands and silts exhibiting a westward decrease in grain size and improvement in sorting. These data suggest that most sediments are supplied by the S~o Vincente and other canyons off southern Portugal. However, THORPE (1972) has observed sediment suspended in Mediterranean outflow water at 36°12.5'N 8°02'W in a submarine valley that may offer a means of channelling such sediment to the abyssal plain. The Horseshoe abyssal plain lies on the AzoresGibraltar plate boundary. One recent earthquake has clearly displaced tile abyssal plain floor (LE PICHON, AUZENDE, PAUTOT, MONTI and FRANCHETEAU, 1971) although the flatness of the plain suggests the topographic effects of such

807

deformation are rapidly smoothed by turbidite sedimentation. Abyssal plains south of the Azores-Gibraltar plate boundary include the Seine and Madeira abyssal plains. HORN, EWlNG and EWING (1972) from an analysis of cores in the Seine plain concluded that the principal source lay to the southeast with a subordinate source to the northeast. This conclusion is consistent with the absence of canyons north of 34 ° compared to their abundance to the south proximal to the higher relief of Morocco. The large Agadir canyon appears to supply much of the sediment to the central part of the Seine abyssal plain where there is a slight shoaling of the sea-floor. The Madeira abyssal plain is situated between the flank topography of the Mid-Atlantic Ridge and the west flank of the Madeira-Tore Rise. The plain is developed in the area of maximum depth and is broken by many small knolls. Sediments comprising the plain may have been derived from the west flank of the Madeira-Tore Rise but in the main are derived from the continental margin of northwest Africa. CONCLUSIONS The principal contribution to an understanding of the ocean floor presented in this paper is the better definition of its morphology. Only when the features are adequately mapped can their origins be understood. From an analysis of the main features of this area of the northeast Atlantic, the following conclusions have been drawn: (1) the region around King's Trough is shallower than expected by a simple theory of cooling of the lithospheric plate. (2) The ridges associated with the accretion process in the axial zone of the spreading centre are preserved in the older crust and define the trend of the zone. (3) King's Trough does not appear to be part of a fracture zone. (4) The Azores-Biscay Rise and King's Trough have resulted from tectonic activity remote l¥om present plate boundaries. There is no topographic evidence for a direct connection with the northward movement of Iberia.

808

A, S. LAtmm'mN,D. G. RoaExrs and R. GRAVES

(5) The origin of Tore Seamount, a near circular ridge of diameter 80 km enclosing a 1000 fathom deep hole, is attributed to a chance configuration of tectonically uplifted ridges associated with the Madeira-Tore Rise. (6) The contrast between morphologies of the Armorican, Cantabrian and Portuguese continental margins reflects the disparate structural and stratigraphic history resulting from differences in sediment supply and tectonic movement.

REFERENCES

DAV[DSONC. and M. J. KEEN (t963) Size analyses of turbidity current sediments. Nature, London, 197, 372-373. DAY A. A. (1959) The continental margin between Brittany and Ireland. Deep-Sea Research, 5. 249--265. DUPLAIX S,, W. D. NESTEROFF and B. C. HEF.ZEN (1965) Minrralogie comparee des srdiments du Tage (Portugal) et de quelques sables profonds de [a plaine abyssale correspondante. Deep-Sea Research, 12, 211-217. FUKAO Y. (1973) Thrust faulting at the lithospheric plate boundary. The Portugal Earthquake of 1969. Earth and Planetary Science Letters, 18, 205-216. Gt~SL W. and E. S~mOLD (1968) Sedimentechogramme von ibero-rnarokkanischen Kontinentalrand. "Meteor" Forschungsergebnisse, Reihe C, 1, 53-75. HADLEY M. L. (1964) The continental margin southwest of the English Channel. Deep-Sea Research, 1 I, 767-779. HAiOH B. I. R. (1973) North Atlantic oceanic topography and lateral variations in the upper mantle.

AuMmcro F., B. D. LONCA~XaCand D. I. ROSS(1971) Hudson geotravers¢: geology of the mid-Atlantic Ridge at 45"N, Philosophical Transactions of the Royal Society of London, A, 2,68, 623-650. BACON M. and F. GRAY(1971) Evidence for crust in the deep ocean derived from continental crust. Geophysical Journal of the Royal Astronomical Nature, London, 229, 331-332. Society, 33, 405--420. BELDRRSON R. H. and N. H. KENYON (1973) B~i- HEEZ_2SNB. C. and G. L. JOHNSON (1969) Mediterforms of the Mediterranean undercurrent observed ranean undercurrent and microphysiography west with side-scan sonar. Sedimentary Geology, 9, of Gibraltar. Bulletin de l'lnstitut oc~anographique, 77-99. Monaco, 67(1382), 95 pp. BrIATTACnARYYAP. J. and D. I. Ross (1972) Mid- H~IZN B. C. and A. S. LAUGrtTON(1963) Abyssal Atlantic Ridge near 45°N, computer interpolation plains, In: The sea, M. N. HXLL, editor, Wiley, and contouring of bathymetry and magnetics. Vol. 3, pp. 312-364. Marine Science Directorate, Department of the H~EZEN B. C. and M. THARP (1968) Physiographic Environment, Ottawa. Marine Science Paper 11, diagram of the North Atlantic Ocean, Geological 9 pp. Society of America Special Paper, 65 (2)~ BICKMORED. P. (1969) Maps for the computer age. HtEZ~N B. C., M. THARP and M. EwtNG (1959) The Geographical Magazine, 41, 221-227. floors of the ocean. I. The North Atlantic. GeoBmKMORED. P. (1971) The work of the Expeximcntal logical Society of America Special Paper, 65, Cartography Unit. Sciences de la terre, Journdes 122 pp. dMtude C.N.R.S., 16, 265-274. M. N, (1960) A median valley of the MidBLACK M., M. N. HILL, A. S. LAUGIrrONand D. H. HILL Atlantic Ridge, Deep-Sea Research, 6, 193-205. MATI~WS (1964) Three non-ma~etic s~xaounts HOLCOMBE T. L. and B. C. H~r.Z~N (1970) Patterns of off the Iberian coast. Quarterly Journal of the relative relief, slope and topographic texture in Geological Society of London, 120, 477-517. the North Atlantic Ocean. Lamont-Doherty BomLor G. and P. MUSF.LLAC(1972) Geologic du Geological Observatory, Technical Report No. 8. plateau continental portugais au Nord du Cap 116 pp. Carveeiro. Structure du Nord et ~lu Sud du canyon de Nazarr. Comptes rendu hebdomadaires des HORN D. R., J. I. EWXUG and M. EWrNG (1972) Graded-bed sequences emplaced by turbidity sd~mces de l'Acad~mie des sciences, Paris, 274 D, currents north of 20*N in the Pacific, Atlantic and 2748-2751. Mediterranean. Sedimentology, 18, 247-275. BOILLOT G., P. A. DUPEUPLE, I. H~rNRQtrn~MARCI-IAND, i . LAMBOY and J. P. Lm~.E'I~E JOHNSON G. L. and P. R. VOOT (1973) Mid-Atlantic (1973) Carte grologique du plateau continental Ridge from 47° to 51°N. Bulletin of the Geological nord-espagnol entre le canyon de Cap Breton et le Society of America, 84, 3443-3462. canyon d'Aviles. Bulletin de la SocidM gdologique JULIVERT M., J. RAMIRF..ZDEL POZO and J. TRUYOLS de France, 7, 367-391. (1971) Le reseau des failles et la couverture postCANNJ. R. and B. M. FUNNELL(1967) Palmer Ridge: hercynienne dons les Asturias. In: Histoire struca section through the upper part of the ocean crust. turale du Golfe de Gascogne, J. DEareR, X. Nature, London, 213, 661-664. LE PlCttON and L. MONTADER'r,editors, Editions CHF.RKISN. Z., H. S. FLEMINOand R. H. FraaEN (1973) Technip, pp. V3-1-V3-34. Morphology and structure of Maury Channel, northeast Atlantic Ocean. Bulletin of the Geological KENYON N. H. and A. H. STRIDE (1970) The tideswept continental shelf sediments between the Society of America, 84, 1601" 1606.

Bathymetry of the northeast Atlantic : Mid-Atlantic Ridge to southwest Europe Shetland Isles and France. Sedimentology, 14, 159173. KRAUSE D. C. (1965) East and west Azores fracture zones in the North Atlantic. In: Proceedings 17th

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