Evolution of the Norwegian-Greenland Sea

MANIK TALWANI Department of Geological Sciences and Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 OLAV ELDHOLM* Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964

ABSTRACT Litvin, 1964, 1965; Johnson and Eckhoff, 1966; Johnson and Heezen, 1967; Vogt and others, 1970). A geological-geophysical Geological and geophysical data collected aboard R/V Verna dur- exploration of the Norwegian-Greenland Sea was carried out ing five summer cruises in the period 1966 to 1973 have been used aboard R/V Vema during the summers of 1966,1969, 1970, 1972, to investigate the geological history and evolution of the and 1973. The Vema tracks are shown in Figure 1. A primary ob- Norwegian-Greenland Sea. These data were combined with earlier jective of this exploration was the investigation of the geological data to establish the location of spreading axes (active as well as history and evolution of the Norwegian-Greenland Sea. To do so, it extinct), the age of the ocean floor from magnetic anomalies, and is necessary to identify the geological features that are related to the the locations and azimuths of fracture zones. The details of the process of sea-floor spreading. Thus, first, we need to know the lo- spreading history are then established quantitatively in terms of cations of the axes of the present spreading-ridge crest as well as the poles and rates of rotation. Reconstructions have been made to lo- location of extinct spreading axes. Second, we must know the loca- cate the relative positions of Norway and Greenland at various tions and azimuths of the fracture zones to define the direction of times since the opening, and the implications of these reconstruc- spreading. Third, we must determine, as precisely as possible, the tions are discussed here. boundaries of the oceanic crust — that is, the location of the lines of initial rifting, as well as the boundaries of any continental areas INTRODUCTION lying within the Norwegian-Greenland Sea. Fourth, we need to identify the magnetic lineations and thereby, by using a reversal The Norwegian-Greenland Sea has been the subject of several chronology, the age of the ocean crust asssociated with these linea- earlier surveys and investigations (Nansen, 1904; Stocks, 1950; tions. If these features can be determined, it is possible to describe

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Figure 1. Tracks of R/V Vema during sum- men of 1966, 1969, 1970, 1972, and 1973 in Norwegian-Greenland Sea.

* Present address: Department of Geology, University of Oslo, Oslo, Norway.

Geological Society of America Bulletin, v. 88, p. 969-999, 20 figs., July 1977, Doc. no. 70708.

969

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the evolution of the Norwegian-Greenland Sea in quantitative track by a notation such as 2704/2935. This indicates a location at terms. 2,935 mi along track on Vema cruise 27, leg 4. We have used our own data as well as the observations of previ- The term "Iceland Plateau" has been used by different authors in ous investigators to identify as many of the features listed above as different ways. Quite often, Iceland Plateau has been used for a possible, and we illustrate these with representative geophysical large area including Iceland, the Iceland-Jan Mayen Ridge, the Jan profiles. In this paper we denote a particular location along a ship's Mayen Ridge, and so forth. Purely for the sake of convenience, we

10° 5° 0° 5° 10° Figure 2. Physiographic and major structural features in Norwegian-Greenland Sea. Profiles I through VI of Figures 4A and 4B are located on this map. Earthquake epicenters are taken from Husebye and others (1975).

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refer only to the area lying between the Iceland-Jan Mayen Ridge Iceland-Greenland Ridge (which connects Iceland to Greenland). and the Jan Mayen Ridge as the Iceland Plateau in this paper. Except for a thicker cover of sediments, one would expect it to be similar to the Iceland-Faeroe Ridge. MID-OCEANIC RIDGE The Iceland-Jan Mayen Ridge (Kolbeinsey Ridge) is also an unusually shallow segment of the mid-oceanic ridge. Earlier studies From south to north, the mid-oceanic ridge consists of the fol- include detailed topographic, magnetic, and gravity surveys from lowing segments (Fig. 2): (1) Reykjanes Ridge, (2) Iceland, (3) Iceland to lat 70°N by Meyer and others (1972) and profiles of Iceland-Jan Mayen Ridge (also known as Kolbeinsey Ridge), (4) magnetics, topography, and seismic reflection between lat 69° and Mohns Ridge, and (5) Knipovich Ridge. 70°N by Johnson and others (1972). Vema seismic reflection data The Reykjanes Ridge southwest of Iceland has been studied ex- have also been discussed by Eldholm and Windisch (1974). The tensively (Ulrich, 1960; Heirtzler and others, 1966; Talwani and western flank is buried under terrigenous sediment derived from others, 1971; Fleischer, 1971; Herron and Talwani, 1972; Vogt Greenland. The axial relief is subdued near Iceland but pro- and Avery, 1974). North of about lat 58°N the Reykjanes Ridge is gressively increases northward. North of lat 67°N the axial magne- distinctive from the remainder of the North Atlantic mid-oceanic tic anomaly is clearly developed. Between lat 67°N and the Spar ridge in that it is unusually shallow but without an axial rift valley Fracture Zone at lat 69°N, the axial anomaly lies over an elevated and has a strikingly well-developed symmetrical magnetic anomaly feature at the axis. This is similar to the northern part of the Reyk- pattern. Prominent identified magnetic lineations lying in the area janes Ridge, where the axial rift is also absent. north of lat 60°N and east of long 30°W are shown in Figure 3. As North of the Spar Fracture Zone the ridge axis is offset to the Iceland and the Iceland-Faeroe Ridge are approached from the east. Between the Spar Fracture Zone and the fracture zone at lat south, the magnetic anomalies are less and less well developed. 70.5°N, a shallow but distinct axial rift is developed, in contrast However, the axial anomaly continues uninterruped into the Reyk- with the segment lying south of the Spar Fracture Zone. Thus, these janes Peninsula. Relative to anomaly 5, the axial anomaly is pro- two contiguous sections of the mid-oceanic ridge, which otherwise gressively shifted to the east as the Reykjanes Peninsula is ap- are similar, differ in this important property of whether there is a proached, implying a jump in the axis or the occurrence of asym- rift or a horst at the ridge crest. metric spreading (Talwani and others, 1971). The older anomalies Our identification of magnetic anomaly profiles to anomaly 5 is (19 to 24) associated with the Reykjanes Ridge are well developed. similar to that of Meyer and others (1972). In particular, we note On the east side, anomaly 24 lies close to Hatton Bank, and on the that anomaly 5 continues north without any offsets, even though west side it lies near the base of the slope off Greenland (Herron the Spar Fracture Zone offsets the crest at lat 69°N (Fig. 5), but the and Talwani, 1972). detailed pattern is not shown in this study. Our data show that Iceland as a part of the mid-oceanic ridge system has also been anomaly 5 appears to continue without any offset even north of the discussed widely in the literature. Recent estimates indicate a fracture zone at lat 70.5°N. Thus, the ridge axis was offset after maximum age of 20 m.y. (Dagley and others, 1967; Moorbath and anomaly 5 time. For the segment north of the Spar Fracture Zone, others, 1968) for Icelandic rocks. While the neovolcanic zone in Meyer and others (1972) have identified the time of the shift of the eastern Iceland is generally considered to be the principal center of ridge axis as 3 m.y. ago. Before and after the shift the spreading was spreading at the present time, Saemundsson (1974) and Palmason essentially symmetrical. Johnson and others (1972) correctly iden- (1974) have inferred that this spreading center has been active only tified anomaly 5 on the east side, but they did not detect the shift in for about the past 3 or 4 m.y. Before that time the western axis was the ridge axis and their identification of anomaly 5 on the west side the axis of spreading. Prior to the existence of Iceland, the corre- appears to be in error. If the correct anomaly 5 is used in computing sponding section of the mid-oceanic ridge formed what are now the spreading rates, there does not appear to be any serious asymmetry Iceland-Faeroe and Iceland-Greenland Ridges. in spreading rates. The Iceland-Faeroe Ridge is a smooth flat-topped relatively shal- Johnson and others (1972) also correlated a sequence of low ridge (with its crest at about 400 m) that connects Iceland with anomalies lying east of anomaly 5 between lat 69° and 70°N. They the Faeroe Islands. If Norway and Greenland have moved apart to considered a prominent minimum (one that we tentatively identify form the Norwegian Sea, with sea-floor spreading extending from as lying just west of the western anomaly 6 profile V, shown in Fig. the Mohns Ridge to the Reykjanes Ridge (and beyond), the 4B; for the nomenclature and ages of anomalies between 5 and 6, Iceland-Greenland Ridge, Iceland, and the Iceland-Faeroe Ridge see Blakely, 1974, and Chase and others, 1970) as the axis of an must also have come into existence as a result of the same basic earlier period of spreading. We believe that their suggestion about process. The crest of the ridge is nearly bare of sediments. Observed the existence of an intermediate extinct axis where spreading took high-amplitude short-wavelength magnetic anomalies suggest the place between the time of spreading in the Norway Basin and the presence of basalt flows at shallow depths. The northeast and time of spreading on the Iceland-Jan Mayen Ridge is correct, but southwest flanks are both covered by sediments; the northeast flank we locate this axis some distance to the east of where they do (M. has a smoother and thicker sediment cover than the southwest Chapman and M. Talwani, in prep.). In Figure 5, the intermediate flank. It is possible that fracture zones underlie both flanks. area of spreading is seen as lying between two prominent topo- Magnetic anomaly lineations of the kind typically associated with graphic features on the Iceland Plateau. These features can also be sea-floor spreading have not been identified. The situation may be seen in profile V, Figure 4B. Some distance east of the eastern topo- similar to that in Iceland, where a large outpouring of basalt in a graphic feature lies the Jan Mayen Ridge, and some distance west wide zone of injection generally obscures the typical striped pattern of the western topographic feature lies the Iceland—Jan Mayen of magnetic anomalies. Ridge. Chapman and Talwani's (in prep.) identification of Fleischer and others (1974) described the topography and anomalies suggests that spreading in this intermediate area started magnetic and gravity anomalies over the Iceland-Faeroe Ridge in at anomaly 6A time and stopped during anomaly 5D time. We call detail. Bott and others (1971) discussed refraction measurements this proposed extinct axis of the spreading the Iceland Plateau axis. on the Iceland-Faeroe Ridge, but layering beneath layer 2 does not It does not have a distinctive topographic expression. appear to be well determined. In gross crustal structure, the The Mohns Ridge, which lies between the Jan Mayen Ridge and Iceland-Faeroe Ridge appears to be similar to Iceland, with a the Greenland-Senja Fracture Zone, is symmetrically situated be- thicker crust than is normal for ocean basins and an "anomalous tween Greenland and Norway and, as we shall discuss below, has mantle" with lower than normal seismic velocities. an identifiable magnetic pattern associated with sea-floor spreading Very little geophysical work has been carried out over the since early Tertiary time. Johnson and Heezen (1967) showed that

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a rift valley existed on most of their crossings of the Mohns Ridge. The Knipovich Ridge, as demonstrated in profiles I and II in Fig- On the basis of about ten almost evenly spaced crossings controlled ure 4, has a very prominent central rift. The relief of the eastern by satellite navigation, it appears that north of lat 71.5°N a single flank of the ridge is almost completely buried by sediments, and its rift valley is continuous through the entire length of the Mohns presence is indicated by the reflection profiler data as well as by Ridge. Profiles III and IV in Figure 4 are profiles over this ridge. The gravity variations. The topographic relief of the mountains on the magnetic anomalies, especially those between 19 and 24, are clearly west side appears to be more prominent because they are not com- developed. The rough basement topography and the increase in the pletely buried under sediments. The rift valley appears to be con- thickness of the overlying sediments as the margins are approached tinuous throughout the length of the Knipovich Ridge. A small but are clearly documented. persistent axial magnetic anomaly is seen over the rift valley in all

30° 25° 20

Figure 3. Identified magnetic lineations in Norwegian-Greenland Sea. For details, see Figures 5 and 9.

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the profiles except the one at lat 75.6°N. At its northern end, the ture zone the appearance of a trough. The eastern segment of the Knipovich Ridge is very close to the continental shelf off Svalbard. Jan Mayen Fracture Zone (Fig. 7) has a distinctly different trend, We have investigated the relationship between the rift valley in which varies in azimuth from about 130° at its western end to the Mohns Ridge and the rift valley in the Knipovich Ridge. By about 150° at its eastern end. This segment is generally also charac- means of several closely spaced profiles run on Vema cruise 28, we terized by a southwest-facing escarpment, which reflects the differ- have established continuity between these two valleys. As with ence in basement elevation between the Lofoten and Norway Ba- many other features in the Norwegian-Greenland Sea, the free-air sins. The eastern segment approximately marks the southwest gravity anomaly (Fig. 6, taken from Talwani and Gr0nlie, 1976) boundary of the Vering Plateau. was very useful in mapping trends and establishing the continuity It is possible that the eastern segment is not completely continu- of the rift valley. ous and that during a short period of time the ridge axis was not discontinous but rather had a sharp bend in it. If that were the case, FRACTURE ZONES the corresponding magnetic anomalies would not be offset by the fracture zone but would have sharp bends in this area. Detailed Vicinity of Iceland surveys are necessary to examine this in greater detail. The fact that the two segments of the Jan Mayen Fracture Zone Primarily on the basis of offsets in the epicentral belt and the have different azimuths is of great importance. The more northerly presence of microearthquake activity, fracture zones have been trend of the eastern segment is attributed to a more northerly mo- proposed near the north (Tj0rnes Fracture Zone) and south coasts tion of Greenland relative to Norway in early Tertiary time than at of Iceland (Sykes, 1965; Ward, 1971). We have been able to de- present. Both segments are clearly characterized by a negative lineate a linear gravity low on two closely spaced north-south pro- free-air anomaly, the axis of which is closely parallel to the fracture files north of Akureyri and a third profile somewhat farther to the zone. We also note that there is an overlap between the two seg- east. The location of the Tj0rnes Fracture Zone (although not the ments between long 1° and 3°W. control) is shown in Figure 5. This gravity low has also been noticed by Palmason (1974), and it appears to continue eastward Greenland-Senja into a deformed zone mapped in Iceland and described in detail by Saemundsson (1974). Our shipboard data indicate that this frac- Johnson and Eckhoff (1966) mapped a well-defined linear ridge ture zone does not continue much farther to the west than the posi- in the Greenland Sea which they named the Greenland Fracture tion shown in Figure 5, an observation which has also been sub- Zone. There were several puzzling aspects about this fracture zone. stantiated by measurements made by Fleischer (1971). The most important was the absence of a complementary feature on the eastern side of the Mohns Ridge. Whereas the western part Northern and Southern Boundaries of Iceland-Faeroe Ridge of the Greenland Fracture Zone has a well-defined northwest- southeast azimuth, its trend close to the ridge axis and the nature of The Iceland-Faeroe Ridge is a northwest-trending feature with the junction of the fracture zone and the ridge axis had not been sharp changes in basement depth along its northeast and southwest established. margins. If we assume that the strike of the margins indicates the We have been able to define a complementary fracture zone east direction of motion of Greenland away from Norway, we can use of the ridge crest by means of gravity measurements. Deeply buried this assumption to test the direction of "flow lines" predicted by fracture zones can be detected by gravity and seismic methods. our model for the evolution of the Norwegian Sea. However, in this case we were not able to determine depths to basement by routine seismic reflection profiling or by sonobuoy re- Spar fraction methods, apparently because of the large basement depth. However, gravity measurements reveal the presence of a large Johnson and Heezen (1967) discovered the Spar Fracture Zone, well-defined positive anomaly on the continental slope, the axis of which contributes to a small offset of the Iceland—Jan Mayen Ridge which makes a small angle with the bathymetric contours. Profiles near lat 69°N. As discussed earlier, this fracture zone does not dis- across the western margin of the Barents shelf, two of which are turb the magnetic anomaly pattern older than anomaly T (see shown in Figure 8, show this gravity high clearly. By making iso- nomenclature of Talwani and others, 1971) and came into exis- static corrections we have been able to demonstrate that the posi- tence by an eastward shift of the ridge about 3 m.y. ago. tive anomaly does not arise out of any isostatic "edge effects." Be- cause of the symmetrical location of the gravity high and the Green- At Lat 70.5°N land Fracture Zone relative to the ridge axis, we believe that the gravity high reflects the presence of a fracture zone, for which we From limited data we have deduced that a similar fracture zone had earlier proposed the name Senja Fracture Zone (Talwani and further offsets the Iceland—Jan Mayen Ridge to the east at lat Eldholm, 1972). The usual topographic relief generally associated 70.5°N. Anomaly 5 is not offset by this fracture zone. The distance with a fracture zone may be hidden by the deep burial under sedi- of the ridge crest from the anomaly indicates that this fracture zone ments. The question may be raised as to why the Tjornes and Jan was created by a shift of the axis not earlier than 4 m.y. ago. Mayen Fracture Zones have negative gravity anomalies, whereas the Greenland and Senja Fracture Zones have positive anomalies. Jan Mayen The gravity anomalies in this context mainly indicate the presence of basement relief, and the question therefore really is why some Sykes (1965) attributed an east-trending pattern of earthquakes fracture zones are associated with troughs and others with ridges. near the island of Jan Mayen to a major fracture zone that offsets As discussed fully below, the Jan Mayen Fracture Zone is a more the ridge crest. Johnson and Heezen (1967) found that the Jan typical fracture zone associated with an offset ridge crest, whereas Mayen Fracture Zone trends northwest between the crest of the the Greenland-Senja Fracture Zone is associated with the sliding of Iceland—Jan Mayen and Mohns Ridges. Greenland past Svalbard, which might have some bearing on the Our investigation has shown that the Jan Mayen Fracture Zone difference in the basement relief in the two cases. consists of two distinct segments. The western segment, lying ap- Because the Greenland Fracture Zone is also well defined by proximately between long 15° and 2°W has a trend of about 110° gravity anomalies, we made a gravity survey (Fig. 6) to determine (Fig. 9) and is characterized by a linear northeast-facing escarpment the trend of the Greenland-Senja Fracture Zone near the ridge axis that runs its entire length. North of Jan Mayen, a south-facing es- and also the nature of the intersection of the Greenland-Senja Frac- carpment at the southern end of the Mohns Ridge gives this fracture Zone with this axis. We found that there is a clear change to a

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Figure 4A. Composite profiles showing total intensity magnetic data (IGRF removed), isostatic gravity anomalies (two-dimensional Airy type with depth of compensation 30 km), and reflecaon profiler data (black = basement). For location of profiles, see Figures 2, 5, and 9.

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less northerly azimuth in the Greenland Fracture Zone at about lat ment high on the outer part of the Varing Plateau, where it termi- 75°N, long 2°E. However, farther east we are unable to determine nates in the steep V0ring Plateau escarpment. (Recent multichannel any clear evidence of the ridgeward continuation of the fracture seismic reflection data acquired by K. Hinz, 1975, personal com- zone. The continuation of the rift valley in the Mohns Ridge to the mun., shows that in some areas on the outer Vering Plateau the Knipovich Ridge, the absence of earthquake epicenters off the ridge smooth layer that we [Talwani and Eldholm, 1972] had identified crest (Fig. 6), and the lack of definition of the Greenland-Senja as acoustic basement is actually underlain by reflectors that could Fracture Zone in the vicinity of the crest all lead to the inference be sedimentary, and "true" basement lies somewhat deeper. In that no transform fault exists at the intersection of the Mohns and other areas, basement detected by the two methods lies at the iden- Knipovich Ridges at the present time. tical depth. Hinz's data show that the situation may be more complicated than we believed earlier, but it does not, in our opinion, Spitsbergen change the conclusion that oceanic basement underlies the outer Varing Plateau.) Further landward a thick section of sediments We have been able to map only the extreme southeastern end of exists. Similarly, the basement in the Norway Basin can be followed the Spitsbergen Fracture Zone at the northern termination of the landward to a buried basement high lying under the lower conti- Knipovich rift. This fracture zone has been defined primarily by nental slope and terminating in the Faeroe-Shetland escarpment. earthquake epicenters. The study of focal mechanisms (Horsfield These escarpments are characterized by gradients in the isostatic and Maton, 1970; Conant, 1972) confirms the presence of a trans- gravity anomaly and mark the seaward boundary of magnetic quiet form fault connecting the Knipovich Ridge to the Nansen Ridge in zones. We have also suggested that the oldest sediments landward the Arctic. Both the earthquake data (Sykes, 1965; Vogt and of the escarpment predate the rifting between Norway and Green- others, 1970; Husebye and others, 1975) and the bottom topog- land. We believe that these escarpments lie at the boundary be- raphy (Johnson and Eckhoff, 1966) indicate that the fault is com- tween oceanic and continental crust and mark the site of initial rift- posed of several en echelon segments. ing. (As has been noted in the literature, similar escarpments within an oceanic area can be associated with hiatuses in spreading, and so forth). We are implicitly ignoring here initial relative motion be- MARGINAL ESCARPMENTS AND LOCATION OF tween Norway and Greenland, which probably caused stretching INITIAL RIFTING and downfaulting of the continental crust in the area of the margin but without extrusion of the highly magnetized rocks associated In reconstruction of continental drift, the usual practice has been with sea-floor spreading. to close drifted continents to a given bathymetric contour. As has We have investigated the Greenland continental margin to look often been pointed out, features, such as the V0ring Plateau, which for similar features. A basement high exists between about lat 75° distort the bathymetric contours pose a problem in such recon- and 76°N on the lower continental slope. Ice conditions prevented structions. These are special cases of the basic question of whether us from making a thorough investigation of this basement high a given bathymetric contour does, in fact, in all cases define the line landward, but by analogy with the Vering Plateau escarpment we of initial rifting. believe that this basement high also terminates landward in an es- Talwani and Eldholm (1972) have described how the basement carpment for which we propose the name Greenland escarpment. in the Lofoten Basin can be followed landward to a buried base- Note in profile III in Figure 4 that a prominent high in the isostatic

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Figure 4B. Profiles as in Figure 4A. Layer indicated by vertical lines is acoustically opaque (Eldholm and Windisch, 1974).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/7/969/3429960/i0016-7606-88-7-969.pdf by guest on 02 October 2021 Figure 5. Area between Jan Mayen Fracture Zone and Iceland-Faeroe Ridge. This area can be divided into four parts: (I) Norway Basin, roughly within 1,500-fm contour and having an extinct axis; (2) Jan Mayen Ridge region (east and west boundaries indicated by dot-dash lines); (3) region produced hy spreading about extinct Iceland Plateau axis; between roughly lat. 69° and 70°N, this region is bounded by topographic ridges or escarpments denoted by heavy dashed lines; spreading took place from just before anomaly 6 time to just before anomaly 5 time; (4) now-active Iceland-Jan Mayen Ridge. Spreading about roughly present axis has gone on since anomaly 5 time. Circled numbers 1 through 6 are basement ridges mapped from seismic reflection (Fig. 14) and isostatic gravity (Fig. 12) profiles. Western ridges 4, 5, and 6 interrupt opaque layer (Fig. 14) and have progressively greater elevation northward, finally coalescing to form Jan Mayen Ridge block. Three eastern ridges (1, 2, and 3) are covered or formed by opaque layer as seen in seismic reflection profiles (Eldholm and Windisch, 1974). In our interpretation, area of these eastern ridges, which lie between southern part of Jan Mayen Ridge area and area of prominent anomalies of Norway Basin, is oceanic; it was formed by sea-floor spreading in complementary fashion to fan-shaped lineations of Norway Basin. This map also serves as index for profiles across Jan Mayen Fracture Zone (Fig. 7), over extinct axis of Norway Basin (Figs. 11, 12, 13) and over Jan Mayen Ridge (Fig. 14). Dots below track lines indicate extent of reflection profiles in Figure 14.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/7/969/3429960/i0016-7606-88-7-969.pdf by guest on 02 October 2021 Figure 6. Free-air gravity map, contour interval 25 mgal (Talwani and Grenlie, 1977), showing connection between Mohns Ridge rift and Knipovich Ridge rift. Note tendency of earthquake epicenters near bend to lie east of rift valley. Note also change in trend of Greenland Frac- ture Zone at about long 2°E and of Senja Fracture Zone at about long 14°E. Dotted lines indicate track control. Patterned areas indicate gravity greater than 25 mgal. Large dots = earthquake epicenters.

6 |o 2° 3° 4' 10° 11° 12° 13° 14° 15° 16° I7

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/7/969/3429960/i0016-7606-88-7-969.pdf by guest on 02 October 2021 ! 0 50 100km. i 1 i Figure 7. Reflection profiler sections across eastern segment of Jan Mayen Fracture Zone. Stipple = basement. Note clear change in basement elevation across fracture zone.

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gravity anomaly exists seaward of the escarpment in the same way damentally different under the Faeroes block from the Iceland- as a maximum occurs seaward of the Varing Plateau escarpment. Faeroe Ridge crust. Also, a negative magnetic anomaly is associated with the Greenland Bott and others (1974) also described seismic results from a mul- escarpment, as with the Vering Plateau escarpment. We have inter- tinational experiment performed in 1972. Crustal velocities in the preted the latter as being caused by the edge of the highly magnetic range 5.9 to 6.2 km/s were obtained. Mantle velocities were some- oceanic basement, which acquired its magnetization during a what uncertain, but time-term analysis yielded a crustal thickness period of reverse polarity (Talwani and Eldholm, 1974). of about 30 km. These crustal velocities and thicknesses are indeed The Varing Plateau escarpment has been followed as far north as usually considered indicative of a continental crust; however, we lat 68.5°N. Farther north there exists a narrow steep continental note that the crustal velocity is not greatly different from the veloc- slope, and the magnetic anomalies associated with sea-floor spread- ity of layer 3 in Iceland (an average velocity of 6.35 km/s has been ing lie just seaward of the well-defined base of the slope. We there- given by Palmason, 1970) and would be even closer to the average fore assume that the base of the slope represents here the site of ini- velocity of the entire crust in Iceland. Russian investigators (I. tial rifting. Similarly, south of the Greenland escarpment, magnetic Kosminskaya, 1975, personal commun.) have found no significant anomalies are present close to the base of a steep and narrow slope. difference in the crust between the Faeroes block and the Iceland- We have again made the assumption that the base of the slope de- Faeroe Ridge. They have examined the same data used by Bott and scribes the line of initial rifting. others (1974). It has proved more difficult to define the line of initial rifting Bott and others (1974) found a significant change in crustal vel- elsewhere along the western margin of the Greenland Sea. We have ocities close to the eastern edge of the Faeroes block. Velocities of generally assumed that the 500-fm bathymetric contour at the base 5.3 to 5.5 km/s were found, and although detailed interpretations of the continental slope defines this boundary. Our postulated are not available yet to determine whether these represent a very boundary on the western margin of the Norwegian-Greenland Sea local situation, one might speculate that they indicate an important is indicated in Figure 3. boundary east of the Faeroes block. We (Talwani and Eldholm, 1972) considered the ocean- We have discussed the ocean-continent boundary in the vicinity continent boundary in the vicinity of the Faeroes block (the block of the Faeroes to emphasize the uncertainty that exists regarding in this context being defined by the 200-m contour; see Fig. 18 A) to the exact location of the boundary. We will return to this point be along the western margin of the Faeroe-Shetland channel, that is when we make the predrift reconstruction. the eastern margin of the Faeroes block. Bott and others (1971, 1974) and Fleischer and others (1974), on the other hand, consid- MAGNETIC ANOMALIES AND AGE OF OCEAN FLOOR ered the Faeroes block to be continental in origin and the boundary to be along an escarpment on the western margin of this block. The Mohns Ridge question is whether the early Tertiary lavas of the Faeroes lie on hidden continental crust or on newly formed oceanic crust. On either side of the Mohns Ridge the magnetic anomalies, The Bouguer gravity profile from the Iceland-Faeroe Ridge to the especially the long-wavelength anomalies 19 through 24, are gen- Faeroes block has a steep gradient in it near the western edge of the erally well defined (see Figs. 9, 10). These anomalies along a profile Faeroes block (Fig. 18B). Bott and others (1971) and Fleischer and in the Lofoten Basin were first identified by Avery and others others (1974) attributed this gradient to a deep discontinuity that (1968). In addition to the large axial anomaly, anomalies 5, 6, 7, separates the oceanic from the continental crust. Bott and others and 13 are also relatively well identified. Several interesting features (1971) emphasized the importance of the deep discontinuity be- can be mentioned in connection with the anomalies associated with cause of the coincidence of its location with the bathymetric es- the Mohns Ridge. In the northeast part of the Lofoten Basin the carpment to the west of the Faeroes block. When examined in de- anomaly amplitudes progressively decrease as the Senja Fracture tail, however, the derived discontinuity along Bott's profile is at F' Zone is approached. Over the western part of the Vetring Plateau, about 15 km east of the top of the escarpment (F' would move even the magnetic anomalies have high amplitudes and are somewhat farther east if Bott's profile did not pass through an area of the difficult to identify. However, we believe we are able to identify Iceland-Faeroe Ridge with unusually high Bouguer anomalies, anomaly 23 almost as far south as the Jan Mayen Fracture Zone. greater than 100 mgal). Fleischer's profile is perhaps more repre- We have identified the anomalies continuously from the axis to sentative: the density discontinuity is about 35 km east of the top of anomaly 23 on either side. However, the margins on either side are the escarpment. High-amplitude short-wavelength magnetic not quite parallel to anomaly 23. There is more distance between anomalies coincident with the escarpment argue against the edge of anomaly 23 and the margins in the southeast and northwest cor- the Faeroes block having been built outward to the northwest by ners than there is in the northeast corner of the Norwegian Sea off sedimentary processes. Lofoten or in the southwest corner off Greenland. The Bouguer gravity field on the Faeroes block can be thought of as consisting of two elements. One is the very localized circular low Norway Basin and Problem of Extinct Axis that Fleischer and others (1974) ascribed to rocks with a density contrast of -0.25 g/cm3, extending to 10-km depth. These rocks In order to explain the asymmetric position of the now-active give rise to the steep gradient in the Bouguer anomaly in the west- Iceland-Jan Mayen Ridge, Johnson and Heezen (1967) suggested ern part of the Faeroes block (see Fig. 18B, top) but appear to be the possibility that an earlier spreading axis may have existed in the unrelated to the bathymetric escarpment. The other element is the Norway Basin but had shifted to a position west of the Jan Mayen difference of about 40 mgal in the Bouguer anomaly between the Ridge. This earlier spreading center would have been associated Iceland-Faeroe Ridge and the Faeroes block. This is related to the with a now-extinct axis. Vogt and others (1970) examined two overall isostatic balance between the Faeroes block and the magnetic profiles that appeared to be symmetrical, and they gave Iceland-Faeroe Ridge. In the model of Fleischer and others (1974), an age corresponding to anomaly 16 for the axis of symmetry, this balance is provided with a small density difference of 0.10 whereas Le Pichon and others (1971), using the same profiles, g/cm 3 in the top part of a crust. It could equally well have been suggested an age corresponding to anomaly 20. Both studies in- provided with a still smaller density difference in the entire crust, or ferred that the spreading in the Norway Basin and Labrador Sea a thicker crust with no density difference, or a change in the density stopped at the same time. of the upper mantle, or a combination of all these factors. None of The locations of ships' tracks in the Norway Basin are shown in these models, however, necessarily indicates that the crust is fun- Figure 5, and the projected profiles of residual magnetics, gravity

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Figure 9. Magnetic lineations on Mohns Ridge. Ship tracks on which anomalies are observed are also shown. Vertical-rule pattern = Jan Mayen Ridge area.

and topography are shown in Figures 11, 12, and 13, The presence wide near the southern end of the extinct axis. During the time of of a continuous valley is indicated in the topographic profiles, but the creation of the zone with the fan-shaped pattern, the rate of the topographic relief tends to be buried toward the north, where spreading was not constant along the length of the extinct axis. the presence of a structural valley is best indicated by the gravity This could be accounted for either by having a pole of opening very profiles, which indicate a relative minimum of about 80 mgal. An close to the southern end of the extinct axis or by a simultaneous axis of symmetry in the magnetic profiles lies over this valley, which opening along some other axis in such a way that the total amount we therefore judge to be the extinct spreading axis. This axis lies of new crust generated at both axes remains constant. The spread- over a well-defined continuous rift, not over a "zone of seamounts" ing history of both the Reykjanes and Mohns Ridges is known well as inferred by earlier authors. We have tentatively identified enough to preclude a pole of opening at any time in or near the anomalies 20 through 23 on either side of the extinct axis (Figs. 5, Norway Basin. Therefore, in order to explain the fan-shaped zone 11). Beyond anomaly 23 on the west side lies an area where the of anomalies, it is necessary to invoke the existence of a contem- magnetic signature is relatively smooth. The westward end of poraneous axis of spreading that was active for the limited time anomaly 23 is also associated with an isc static gravity maximum (probably between anomaly 7 time and anomaly 20 time) during indicated by lineation C in Figures 5 and 12. The magnetic which the fan-shaped zone was created. anomalies lying between the extinct axis and anomaly 20 are not Anomaly 23 on the west side defines the limit of the Norway Ba- parallel, as magnetic anomalies generated by sea-floor spreading sin. It is likely, therefore, that anomalies generated between the usually are; rather they form a fan-shaped pattern, which is nearly time of opening of the Norwegian Sea and the time of anomaly 23 300 km wide at its northern end but decreases to about 150 km were generated in the eastern part of the Norwegian Basin adjacent

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500 400 300 200 100 O 100 200 300 400 500 NW —i u i 1 1 I 1 SE

2703 3336

Figure 10. Magnetic anomaly (total intensity, IGRF removed) profiles across Mohns Ridge, projected along azimuth of 145°. Computed model with Heirtzler and others (1968) time scale modified for post-anomaly 5 reversal chronology by Talwani and others (1971). See Figure 9 for location of profiles.

to the Faeroe-Shetland escarpment and that the axis jumped to the tinuous geophysical profiles over it, as well as by shooting several "extinct axis" just prior to the time of anomaly 23. In other words, sonobuoy seismic refraction profiles over the ridge. A number of we infer that an earlier short-lived extinct spreading axis exists just seismic reflection profiles arranged from north to south are shown west of the Faeroe-Shetland escarpment. in Figure 14. Near its northern end the Jan Mayen Ridge is developed as a single blocklike feature. Seismic refraction and reflection JAN MA YEN RIDGE data show that under a layer of flat-lying sediments about 100 m thick, a sedimentary section dips to the east (Fig. 15). The sedimen- Because of the asymmetrical location of the Iceland—Jan Mayen tary velocities are similar to the velocities found on the continental Ridge, the absence of large magnetic anomalies, and the presence of margin off Norway east of the Faeroe-Shetland and Vtfring Plateau thick sediments on the Jan Mayen Ridge, Johnson and Heezen escarpments, whereas the seismic section contrasts significantly (1967) and Johnson and others (1971, 1972) suggested that it with seismic data from the surrounding oceanic areas. We consider might be a continental fragment — a part of Greenland that was this strong evidence of the continental nature of the Jan Mayen separated by a westward jump of the ridge axis. However, these au- Ridge. thors left the possibility open that the Jan Mayen Ridge is underlain We next look into the southward continuation of the Jan Mayen by oceanic crust and originally occupied a position on the continen- Ridge. Topographically, the blocklike feature appears to divide tal rise off Greenland. into several ridges. We have also used the isostatic gravity profiles We have studied the Jan Mayen Ridge by obtaining several con- (Fig. 12) to trace the continuity of these ridges. The isostatic gravity

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lineations numbered 1 through 6 in "Figure 12 are also indicated on opaque layer, indicated by vertical lines. (The opaque layer in this the seismic reflection profiles (Fig. 14). It appears that the isostatic area has been described by Eldholm and Windisch, 1974.) Thus, highs 4, 5, and 6 merge northward and form the Jan Mayen Ridge the continental ridge that appears as a single block in the northern block. It is important to note that the topographic highs associated part continues as fragments plunging to the south. In the four with lineations 4, 5, and 6 appear to interrupt an acoustically southern profiles shown, no prominent topographic highs exist in the area where lineations 4, 5, and 6 might be expected to continue. However, note that just west of lineation 3 in the southernmost MAGNETICS profile, buried peaks are present and are associated with a gap in 23'22? 21 ? 20 3010 3010 52!15 5385

2304 3970

1-800

•0 EXTINCT! AXIS KM (approx.) -800 Figure 11. Projected magnetic (total intensity, IGRF removed) profiles across Norway Basin and southern part of Jan Mayen Ridge area. Identification of anomalies younger than anomaly 20 in Norway Basin is uncertain. Location of extinct axis, as well as ridges 1 through 6, is based on gravity data. Bars with circles = regions of strong gravity gradients on western side of Jan Mayen Ridge area. Dot-dash lines = western and eastern boundaries of Jan Mayen Ridge region. Triangles in Iceland Plateau area indicate topographic escarpments.

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the opaque layer. Thus, we can view the Jan Mayen Ridge as a that we have used in the continental reconstruction is indicated by prominent topographic feature in the north but appearing only as a dot-dash lines in Figure 5. The same dot-dash lines are also drawn fragmented and buried structure associated with interruptions in through the various geophysical profiles that cross this feature the opaque layer to the south. The term "ridge" is not quite appro- (Figs. 11, 12, 13). priate for the entire feature and is retained only to indicate the ap- parent continuity of the structures. SEISMIC REFRACTION DATA The southward continuation of the Jan Mayen Ridge has been difficult to determine because the magnetic quiet zone that charac- Seismic refraction profiles that obtained deep crustal information terizes the Jan Mayen Ridge in the north becomes more disturbed have been described by Ewing and Ewing (1959) and by Hinz and as one proceeds southward. The extent of the Jan Mayen Ridge Moe (1971). Ewing and Ewing's measurements revealed the presence of an unconsolidated layer of sediments above a basement with an average velocity of 5.2 km/s. At an average depth of 7 km GRAVITY below sea level, they found another layer in which the velocities are in the range of 6.97 to 8.04 km/s, with an average of 7.5 km/s. This

3010 607

2703 K 13881 '

r40 0

-40 KM (approx.)

Figure 12. Projected isostatic gravity profiles across Norway Basin and southern part of Jan Mayen Ridge area. Symbols as in Figure 11.

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corresponds to the velocity generally associated with the anomal- proximity of the Jan Mayen Fracture Zone, which may indeed be ous mantle that is typically associated with mid-ocean ridges. associated with the presence of ultramafic rocks, the existence of The location of the three profiles by Hinz and Moe (1971) are complicated structures does not disprove the formation of the shown in Figure 5. Profile Hinz III, which was shot parallel to the Norway Basin by sea-floor spreading. strike of the well-developed set of magnetic lineations east of the Profile Hinz I straddles the V0ring Plateau escarpment. Because Jan Mayen Ridge, exhibits typical oceanic structure. Profile Hinz II the western end of this profile is on top of the basement high but was shot in an area where the basement relief is rough and the very close to its eastern edge, we believe that only the structure de- magnetic anomalies are poorly developed. Hinz and Moe (1971) termined at the eastern end of the profile can be considered reliable. postulated a complicated structure envisaging the presence of very This structure is not typically oceanic, and it has unusually high high-velocity blocks rising close to the ocean bottom along part of velocities at shallow crustal depth compared with a typical conti- the profile. Because this profile is located in an area judged unusual nental model. It might indicate subsided continental crust that has because of the presence of the subdued magnetic anomalies and the undergone massive intrusions associated with stretching and downfaulting. A large number of seismic refraction stations with sonobuoys TOPOGRAPHY

2703 912

2703 1368

•0

2000 EXTINCT KM (approx.) 4000

Figure 13. Projected topographic profiles across Norway Basin and southern part of Jan Mayen Ridge area. Symbols as in Figure 11.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/7/969/3429960/i0016-7606-88-7-969.pdf by guest on 02 October 2021 Figure 14. Tracings of seismic reflection profiler records across Jan Mayen Ridge. Blocklike ridge at northern end (detailed seismic refraction data shown in Fig. IS) is fragmented into ridges numbered 4, 5, and 6, which lie at a lower elevation to south. Important as- pect of ridges is that they are not covered by opaque layer (defined WA by seismic reflection) as are ridges 1, 2, and 3 to east. See Figure 5 for location of profiles.

WEST EAST

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108 107 106 105 104 118 E w i i i i 0 i

2 Figure 15. Seismic refraction results obtained on northern part of Jan Mayen Ridge (sonobuoy refraction data were obtained on Vema cruise 29). is Great thickness of sediments and layer velocity are similar to those obtained on Norwegian continental margin. 4

5 5.86

6 r I 6.71 0 50 100 KM

were obtained during the Vema cruises. These profiles, which sel- RECONSTRUCTION OF NORWEGIAN-GREENLAND SEA dom exceed 15 km in length and were shot with airguns, sometimes with the addition of a small amount of explosives, generally give Determination of Rotations That Described Opening of information only for the shallow part of the crust. Norwegian-Greenland Sea The sonobuoy data have been discussed by us (Talwani and Eld- holm, 1972) for the continental margin off Norway, by Eldholm We have summarized our information and assumptions about and Ewing (1971) and Eldholm and Talwani (1977) for the Barents the axes of the mid-oceanic ridge, the azimuths and locations of shelf area, and by Eldholm and Windisch (1974) for other parts of fracture zones, the location of the initial lines of rifting, and the age the Norwegian-Greenland Sea. In summary, these results show that data for the ocean floor from magnetic lineations. In order to de- on the landward side of the boundaries that we have assumed scribe the opening of the Norwegian-Greenland Sea in quantitative define the lines of initial rifting, there exist large thicknesses of sed- fashion, it is useful to express the information about ages of the iments in the velocity range 1.8 to 4.7 km/s. Velocities within this ocean floor and the azimuths of the fracture zones in terms of poles range have been measured on the Norwegian margin, on the Ba- and rates of opening. If the poles and rates are correctly chosen and rents shelf, on the Jan Mayen Ridge, and on the Greenland margin. if the hypothesis of rigid plates is valid (ignoring any motions as- Seaward of the line of initial rifting the sedimentary thickness sociated with the large number of intraplate earthquakes in Fig. 2), seldom exceeds 2 km, and basement velocity is slightly higher than we can compute synthetic isochrons that correspond to the deter- 5 km/s. Velocities in the range 4.0 to 4.5 km/s are determined under mined ages of the ocean floor and the azimuths of synthetic flow the Iceland Plateau and the Iceland-Faeroe Ridge. These are as- lines that correspond to the azimuths of the observed fracture sociated with basement because of the association with magnetic zones. anomalies ascribed to shallow sources. These velocities are similar We use the terms "finite rotation" poles and "difference" poles to those obtained for flood basalts on Iceland and in the Faeroe Is- in the same manner as Pitman and Talwani (1972), and making as- lands (Palmason, 1965). sumptions similar to theirs, we have obtained the instantaneous poles of rotation for the motion since anomaly 5 time, and for the GRAVITY DATA rotations between the times of anomaly 5 and 13, anomaly 13 and 21, and anomaly 21 and 23. Gravity surveys have been reported over the Iceland-Faeroe For the total opening since anomaly 5 time, we have adopted the Ridge (Fleischer, 1971; Fleischer and others, 1974; Bott and others, value of lat 68°N, long 137°E for the pole, and we have assumed 1971) and over the Iceland—Jan Mayen Ridge (Meyer and others, 2.50° for the total opening, as given by the Pitman and Talwani 1972). Talwani and Granlie (1976) have utilized the data collected (1972) study of sea-floor spreading in the North Atlantic. The aboard Vema as well as all other available data to construct a anomalies 5 (Fig. 16, open circles) on the western side of the Reyk- free-air anomaly map for the Norwegian-Greenland Sea, contoured janes, Iceland—Jan Mayen, and Mohns Ridges have been rotated at an interval of 25 mgal (Fig. 6 shows part of this map). eastward around this pole, and the rotated positions are plotted as We have found it useful to construct isostatic anomaly profiles closed circles (Fig. 16), which should be compared with the ob- for aid in interpretation of other geophysical data. Because the re- served anomalies 5 (open circles) on the east side. The degree of fit ality of approximate isostatic equilibrium can be considered estab- is good enough to demonstrate that the opening of the North At- lished, the isostatic anomalies are very useful, especially in marginal lantic from the Azores-Gibraltar Ridge to the Spitsbergen Fracture areas, because they eliminate topographic effects. The isostatic Zone can be described by a single rotation around the pole given by anomalies have not been computed in the conventional manner, Pitman and Talwani (1972). but have been computed for profiles assuming two-dimensionality When we similarly rotated anomaly 13 on the west side of the and Airy compensation at a depth of 30 km (see Talwani and Eld- Reykjanes Ridge and Mohns Ridge to the east side, we found a holm, 1972, for instance). This simplification introduces some very noticeable divergence of these rotated lineations to the anomaly 13 long wavelength errors, which are unimportant for examination of identified on the east side. Proof that this divergence cannot be at- local features for which the isostatic anomaly profiles are utilized in tributed to an opening of the Labrador Sea prior to anomaly 13 this study. time is supplied by the identification of anomaly 13 as an uninter-

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Figure 16. Magnetic anomaly locations. Circles = anomaly 5, squares = anomaly 13, triangles vertex up = anomaly 21, triangles vertex down = anomaly 23. Asterisk = anomaly 6 in Iceland Plateau area. Using total opening poles (Table 1), anomaly locations on west side are rotated to east side and are indicated by solid circles, squares, and triangles. For generating isochrons and flow lines, difference poles in Table 1 were used. For Mohns and Reyk- janes Ridges, isochrons and flow lines were obtained from present to time of opening. For Iceland-Jan Mayen Ridge, they were obtained from present to 13 m.y. ago. Synthetic isochrons about extinct Iceland Plateau axis were generated for interval 18 to 21 m.y. ago. For isochrons and flow lines in Norway Basin, we have taken time interval from 27 m.y. to time of opening. Note that although total space occupied by synthetic isochrons in Norway Basin shows no overlaps or gaps relative to margins, the observed anomalies, such as anomaly 21, cut across synthetic isochrons. Complementary zone of spreading east of southern part of Jan Mayen Ridge area may explain this discrepancy. Thicker lines along margins mark edge of continental shelf. Thinner lines that pass through escarpments are postulated to be lines of opening. IJMRA = Iceland-Jan Mayen Ridge axis; TFZ = Tjernes Fracture Zone; FSE = Faeroe- Shetland escarpment; VPE = Vering Plateau escarpment. IPA = extinct Iceland plateau axis.

rupted Iineation on the west flank of the Reykjanes Ridge (Kristof- rotated anomaly 13 by open squares and the anomalies on the west fersen and Talwani, 1977). By trial and error, we were able to side rotated to the east as filled-in squares. The pole of opening is modify the pole position given by Pitman and Talwani (1972) for lat 68°N, long 129.9°E, and the amount of rotation is 7.78°. The the North Atlantic north of the Azores-Gibraltar Ridge such that it total opening pole since anomaly 13 time is less than 300 km away satisfied the North Atlantic data as well as the Reykjanes Ridge and to the west from the pole of total opening since anomaly 5 time. Mohns Ridge data. The change of the position of the pole is only We have also determined the total pole of rotation since anomaly about 350 km to the north. In Figure 16 we have indicated the un- 23 time. The determination of this pole is more difficult, because,

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due to the opening of the Labrador Sea, we cannot make the as- of opening, the finite rotation between anomaly 23 time and the sumption that the opening between Greenland and Europe is the time of the initial break was estimated at 0.98°. From these finite same as the opening between North America and Europe. Thus, the difference rotations, the total opening since anomaly 21 time and matching of lineations at a large range of distances from the open- since the time of the initial break were calculated and are listed in ing pole, that is, all the way from the Mohns Ridge to the Azores- Table 1. Finite difference poles are listed in Table 2. Gibraltar Ridge, cannot be used to reduce the ambiguity in its posi- The specific anomalies 5, 13, 21 and 23 have been picked simply tion. Furthermore, there are no sharp bends in the anomaly 23 because they are prominent and easy to identify. The choice of lineation which could be used to reduce the ambiguity in determin- anomalies should not be taken to imply discontinuities in spreading ing the total pole of opening since anomaly 23 time. However, two at exactly the times corresponding to these anomalies. other constraints can be used. One is that the vector difference between the Norwegian Sea and North Atlantic opening should equal Synthetic Flow Lines and Isochrons the Labrador Sea opening since anomaly 23 time. The second is that the pole deduced for the opening during the time between Using the instantaneous poles and rates of opening, and the pres- anomaly 13 and 23 for the Norwegian Sea opening predicts flow ent mid-oceanic ridge axis of the Norwegian-Greenland Sea, we lines that should match the fracture zones in the Norwegian Sea. computed synthetic flow lines and isochrons (Fig. 16). Fracture Unfortunately, the two constraints are, in this case, not completely zones and other lineations are also plotted in Figure 16. We present compatible. In other words, if we select a pole so that the fracture this figure not only to describe the evolution of the Norwegian Sea zone azimuths are matched, the opining in the Labrador Sea is to a first approximation, but also more importantly, to point out larger than the observed opening. If we select a pole so that the the discrepancies between the synthetic constructions and the ac- Labrador Sea opening is duplicated precisely, the calculated flow tual features. Some of these discrepancies arise from shifts in the lines in the Norwegian Sea have azimuths that are much more eas- position of the ridge axis. We have attempted to take some of these terly than the observed azimuths of the fracture zones. The dis- shifts into account in constructing Figure 16, which shows the vari- crepancies are not large and do not argue against a first-order as- ous stages in the opening of the Norwegian Sea, but other shifts sumption of the rigidity of the plates involved. We have chosen a have not been considered. Other discrepancies arise from possible pole of total opening since anomaly 23 time that satisfies the frac- errors in the positions of the derived poles and in details of our ture zone azimuths in the Norwegian Sea. The coordinates of this model. pole are listed in Table 1. Finite difference poles between anomaly Clearly, the fracture zones in the Norwegian Sea exhibit two dis- 13 and 23 time were calculated in the two frames of reference, one tinct azimuths. One azimuth is that of the Greenland and Senja fixed to Greenland and the other to Norway. We further assumed Fracture Zones as well as of the eastern segment of the Jan Mayen that these difference poles not only give the instantaneous rotation Fracture Zone. This corresponds in time from opening to about the during the time interval between anomalies 13 and 23 but also give time of anomaly 13. The second azimuth is more easterly and is it back to the time of initial opening. The finite difference rotation best developed in the western segment of the Jan Mayen Fracture between anomaly 13 and 23 time is 3.72°. By matching anomaly 21 Zone. The change in azimuth corresponds to a stop in spreading in on either side, it was determined that the rotation is 2.48° between the Labrador Sea. Originally Greenland was moving in a more anomaly 13 and 21 time and 1.24° between anomaly 21 and 23 northerly direction relative to Europe, but when Greenland became time. Similarly, by matching what are believed to be the initial lines a part of the American plate it assumed the more westerly motion of that plate relative to Europe. TABLE 1. POLES OF TOTAL OPENING BETWEEN The flow lines for the period later than anomaly 13 time match GREENLAND AND NORWAY the western segment of the Jan Mayen Fracture Zone azimuth quite well. However, the synthetic flow lines corresponding to the time Anomaly Time Lat Long Opening between anomaly 13 and 23, although considerably steeper than C C (m.y. B.P.) ( N) ( E) n the post—anomaly 13 flow lines, are somewhat less steep than the Senja Fracture Zone and the eastern segment of the Jan Mayen 5 10 68 137 2.5 Fracture Zone. We could obtain a somewhat better match by 13 38 68 129.9 7.78 21 53 52.4 125.9 8.79 changing the position of the total opening poles for anomalies 21 23 58 46 125.0 9.52 and 23. However, then the implied opening of the Labrador Sea Initial opening 41.7 124.5 10.15 since anomaly 23 time becomes even larger and disagrees with the observed opening even more than it does with the poles we have Note: Heirtzler's time scale used here may give ages that are too used, and the fit of the observed flow lines with the Greenland Frac- great by 5 to 10 m.y. in early Tertiary time. ture Zone becomes poorer.

TABLE 2. FINITE DIFFERENCE POLES

Time Reference frame fixed to Norway Reference frame fixed to Greenland Lat Long Rotation Lat Long Rotation ce) n CE) n Present to anomaly 5 0 to 10 m.y. ago 68.0CN 137.0 2.50 68.0°N 137.0 -2.50 Anomaly 5 to anomaly 13 10 to 38 m.y. ago 67.8CN 126.6 5.26 68.0CN 126.5 -5.26 Anomaly 13 to anomaly 21 38 to 58 m.y. ago 5.74CS 124.91 2.48 5.29CS 117.42 -2.48 Anomaly 21 to anomaly 23 53 to 58 m.y. ago 5.74CS 124.91 1.24 5.2 9CS 117.42 -1.24 Anomaly 23 to initial opening 58 m.y. ago to initial opening 5.74CS 124.91 0.98 5.29CS 117.42 -0.98 Note: Westward rotations considered positive.

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Mohns Ridge areas of continental crust. The southern end of the Jan Mayen Ridge, which appears to consist of slivers of buried continental We note that for the later lineations (anomalies 5 and 13) the crust, might be an indication of such a situation. synthetic isochrons match the observed lineations. The match is There is no indication of anomalies older than anomaly 23 on the also good for anomalies 21 and 23, except at the north and south west side of the Norway Basin even in the northern area (Fig. 5). ends, where the synthetic lineations curve off slightly to the west This suggests that an earlier axis of spreading existed in the basin while the observed lineations do not. Thus it seems that originally up to shortly before anomaly 23 time and that the anomalies gen- the Mohns Ridge did not curve westward at its northern end to erated during this time lie adjacent to the Faeroe-Shetland escarp- join the Knipovich Ridge, but sometime before anomaly 13 time it ment. Near anomaly 23 time the axis of spreading shifted to the shifted slightly at its northern end to do so. At its southern end, not extinct axis of the Norway Basin. only do the anomalies prior to anomaly 13 time lie east of the isochrons, but the Mohns Ridge appears to have extended farther Knipovich Ridge south. Thus, subsequent to anomaly 13 time, when the axis south of the Jan Mayen Ridge shifted west, the Jan Mayen Fracture Zone The continuation of the rift valley at the axis of the Mohns Ridge shifted to a more northerly location, thus truncating part of the into the Knipovich Ridge, the presence of a continuous rift valley, Mohns Ridge at its southern end. and an axial anomaly all suggest that the spreading axis is indeed the Knipovich rift at the present time. The synthetic isochrons Iceland-Jan Mayen Ridge and Norway Basin clearly show that the major part of the opening of the Greenland Sea took place subsequent to anomaly 13 time. The flow lines for The area south of the Jan Mayen Fracture Zone is considerably the earlier period of opening are more nearly parallel to the bound- more complex than the area north of it. In constructing the synthe- ary lines, suggesting that from the time of early opening to the time tic isochrons, we can use the present active axis on the Iceland-Jan of anomaly 13, Greenland slid against the Barents shelf and Sval- Mayen Ridge as well as the two earlier axes that are now extinct, bard with little or no opening of the Greenland Sea. one lying in the Iceland Plateau between the Iceland—Jan Mayen We also notice that as we go to the north the Knipovich ridge Ridge and the Jan Mayen Ridge (the intermediate axis) and the axis is progressively closer to the eastern boundary of the Green- other in the Norway Basin. Chapman and Talwani (in prep.) have land Sea, suggesting that the ridge axis has migrated or shifted to identified the anomalies on either side of the intermediate axis, and the east. One or several shifts in axis could help explain the poorly a date corresponding to anomaly 5D (18 m.y. ago) can be assigned developed magnetic anomaly in this region. The proximity to the for the time this axis became extinct. From the identification of the shelf edge in the north may argue for a recent shift in axial position. anomalies as well as from geometrical considerations, it seems reasonable to assign periods of spreading of 0 to 13 m.y. ago to the Progressive Opening of Norwegian Sea present active axis, 18 to 24 m.y. ago to the intermediate axis and prior to 27 m.y. ago (anomaly 7 time) to the axis in the Norway Figure 17 depicts the various stages in the opening of the Basin. The gaps 13 to 18 m.y. ago and 24 to 27 m.y. ago are not Norwegian Sea. In constructing this figure we have used the rota- accounted for in terms of ocean floor generated by spreading. If the tions listed in Table 1 and considered various shifts in the ridge axis boundaries chosen for the reconstruction are not in error, the gaps as discussed. in spreading have to be accounted for by phenomena such as At anomaly 5 time, only a small part of the Iceland-Jan Mayen stretching of the crust prior to opening of each shifted center of Ridge had not yet come into existence. The position of the ridge spreading. axis in the southern part of Iceland has been deduced by rotating The synthetic isochron for anomaly 5 time for the southern seg- anomaly 5 on the shelf west of the Reykjanes Peninsula eastward. ment of the now-active Iceland—Jan Mayen Ridge agrees well with We have also used the suggestion of Palmason (1974) and the observed anomalies. North of the Spar Fracture Zone the Saemundsson (1974) that the present ridge axis in eastern Iceland anomalies lie slightly west of the isochrons, and north of the frac- has been active only for the past 3 or 4 m.y., and, before that, the ture zone at lat 70.5°N the anomalies lie an even larger distance western axis lying roughly between the Reykjanes Peninsula and west of the synthetic isochrons. This discrepancy is explained by the axis of the Iceland—Jan Mayen Ridge was active. Note in Figure shifts in the axis of the Iceland-Jan Mayen Ridge, as discussed ear- 17 that Iceland lies entirely in the stippled zone, suggesting that Ice- lier. land came into existence subsequent to anomaly 7 time (27 m.y. Isochrons corresponding to anomaly 6 agree well with the ob- ago). The presence of anomaly 5 west of the Reykjanes Peninsula served anomalies in the intermediate axis region. These anomalies (see Fig. 16) and the derived position of the ridge axis at anomaly 5 have not yet been identified in the region farther to the south. time (Fig. 17) suggest that up to anomaly 5 time, the Reykjanes In the Norway Basin the synthetic isochrons have been generated Ridge continued farther north than it does at present. Since anom- with the assumption that the extinct axis was active between the aly 5 time, Iceland appears to have grown southward at the expense time of opening and anomaly 7 time. There seems to be approxi- of the Reykjanes Ridge with, as noted earlier, an accompanying mately the right amount of space in the Norway Basin for this se- eastward shift in the ridge axis in the vicinity of the Reykjanes quence of anomalies. However, except near the axis, the observed Peninsula. lineations (only anomaly 21 is shown in Fig. 16) diverge greatly In reconstructing back to anomaly 7 time, we have made the as- from the synthetic isochrons, especially toward the southern end of sumption that the Norway Basin axis had become extinct before the area. We have discussed the fan-shaped sequence of anomalies this and that the spreading subsequent to anomaly 7 time took earlier. Perhaps the simplest geometrical way to accommodate the place west of the Jan Mayen Ridge. In this reconstruction a small missing sea floor is to invoke another spreading center that was ac- overlap is observed between Greenland and the Jan Mayen Ridge. tive simultaneously, so that the total amount of sea floor older than Subsequent to anomaly 7 time the ridge axis jumped from the anomaly 7 could have been generated. It seems likely that sea floor Norway Basin to west of the Jan Mayen Ridge. The positions of the generated by this other spreading center lies in the area between the ridge axis before and after the shift are shown in Figure 17. In the Jan Mayen Ridge and gravity lineation C (Fig. 5). We cannot, on reconstruction to anomaly 7 time, we have made the assumption the other hand, rule out the possibility that this complementary that there was a similar shift in the ridge axis from the eastern part area of spreading lies between Greenland and the Iceland—Jan of the Iceland-Faeroe Ridge to the western part. This assumption Mayen Ridge between about lat 67.5° and 69°N. It is also possible was made for the following reasons. Unless the Iceland-Faeroe that a much more complicated pattern of sea-floor spreading took Ridge has been entirely developed by very strongly asymmetrical place, with a number of spreading centers interspersed between spreading, it is likely that a westward shift of the active axis has

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taken place at some time in its history, because the present active constructions (Bullard and others, 1965; Bott and Watts, 1971; axis in Iceland lies asymmetrically close to Greenland. The north- Pitman and Talwani, 1972; Vann, 1974) includes the matching of ern and southern boundaries of the Iceland-Faeroe Ridge are paral- the oldest identified magnetic lineations and consideration of the lel to the flow lines for the time prior to anomaly 13, suggesting continent-ocean boundary on either side of the Mohns Ridge on the that this feature was generated at an early time. Iceland, however, basis of geophysically defined structures. was generated after a shift of the axis. It is also likely that the dif- The major area of overlap is the Greenland-Svalbard fit as well as ference in elevation between the Iceland-Faeroe Ridge and Iceland the Greenland—Jan Mayen Ridge fit. We do not have a simple exis at least in part related to a jump in the ridge axis, when one con- planation for these overlaps. We do note that while spreading from siders the relationship of age of ocean floor to its elevation (Sclater a single axis has continued since anomaly 23 time in the Reykjanes and others, 1971). and Mohns Ridges areas, the area of the Greenland Sea and the The position of the ridge axis before the shift is estimated to lie area between the Jan Mayen Fracture Zone and Iceland are differ- midway between the two ocean-continent boundaries on either ent. In the Greenland Sea, prior to anomaly 13 time, a shear motion side. This axis is nearly colinear with the extinct axis in the Norway was taking place, with Greenland sliding past Svalbard, and a Basin. If the axis of the Mohns Ridge was parallel at this time to splitting apart motion took place only after anomaly 13 time. South anomaly 23 on the east side, there would be only a small displace- of the Jan Mayen Fracture Zone, shifts of axis have taken place. In ment across the Jan Mayen Fracture Zone between the Mohns all these cases new spreading centers were developed since anomaly Ridge axis and the extinct axis in the Norway Basin. Thus, prior to 23 time within areas of continental crust. If stretching of the pre- anomaly 7 time the ridge axis had its largest offset at the northern existing crust takes place prior to opening at each new spreading end of the Reykjanes Ridge. The shift at anomaly 7 time made the center, then stretching of the continental crust has taken place con- ridge axis nearly continuous from south of the Reykjanes Ridge tiguous to these areas, whereas no stretching of the continental through Iceland and the Iceland Plateau, with the biggest offset crust contiguous to the Reykjanes and Mohns Ridge has occurred now occurring at the Jan Mayen Fracture Zone. The shift in the since anomaly 23 time. A reconstruction based primarily on match- ridge axis also coincides with the beginning of the growth of Ice- ing magnetic lineation 23 in the Reykjanes and Mohns Ridges areas land and the Iceland Plateau. It may be argued that the age of the will, therefore, be expected to show an overlap of the Greenland- proposed shift is greater by about 10 m.y. than the age of the oldest Svalbard fit as well as the Greenland-Jan Mayen Ridge fit. rocks formed in Iceland. Two explanations can be given for this: The situation relevant to the Faeroes block is shown in Figure (1) the oldest rocks are not exposed at the surface, and (2) in the 18 A. The western boundary rotated through the pole of total open- reconstruction we have assumed a constant rate of spreading being shows an overlap relative to the bathymetric escarpment tween anomaly 13 time and anomaly 5 time, whereas Figure 19 northwest of the Faeroes block but a gap relative to the line of shows that this is not true in detail; this assumption has the effect of opening used in this paper (Fig. 17). Since the exact position of the making the shift in ridge axis appear earlier than it actually is. western boundary itself is not known exactly and some small over- For the reconstruction to anomaly 13 and earlier times, we have laps and gaps in reconstructions may be unavoidable in any event, considered the Jan Mayen Ridge attached to Greenland and rotated it is not possible through the procedure of rotating to decide which the two together to close the Norway Basin. The most important is the best choice of the ocean-continent boundary in the Faeroes feature of the reconstruction to anomaly 13 time is that this recon- area. We have presented arguments earlier why the gravity and struction essentially closes the Greenland Sea. Thus, the Greenland seismic data cannot be used to prove conclusively that the Faeroes Sea only came into existence subsequent to about 38 m.y. ago. are continental in origin. The reader may question whether we are Prior to this time a land bridge existed between Svalbard and applying different criteria to the southern part of the Jan Mayen Greenland. The Greenland-Senja Fracture Zone formed the sheared Ridge than we are to the Faeroes block in determining continental- northern margin of the Norwegian Sea from the time of opening to ity. In Figure 19 the contiguity of the reconstructed position of the anomaly 13 time. The sliding of Greenland against Svalbard is Jan Mayen Ridge to the Faeroes block might indeed suggest a con- documented by evidence of transcurrent motion in western Sval- tinuity of structures. However, we feel that the great difference in bard (Harland, 1969; Lowell, 1972) during early Tertiary time. magnetic anomalies between the Jan Mayen Ridge as a whole and With an appropriate strike of the boundaries involved, a small the Faeroes block and the difference in elevation between the compressive component of the motion could also be ascribed to the southern part of the Jan Mayen Ridge and the Faeroes block argue relative plate motions. against similar origins for the two regions. Therefore, our preferred The location of the spreading axis in the Iceland-Faeroe Ridge at solution is to consider the Faeroes block oceanic, with a boundary anomaly 13 time is estimated as lying midway between the ocean- somewhere through its eastern margin. We do not, however, rule continent boundary on either side. If this estimate is correct, there is out the possibility of a much more complicated origin and structure a major offset in the ridge axis at the northern end of the Reykjanes for either the Faeroes block or for the southern part of the Jan Ridge. Notice also that, in effect, the Reykjanes Ridge extended Mayen Ridge. farther north than it does at present. After the shift of the ridge axis Figure 19 shows the geological features at the time of separation near anomaly 7 time, Iceland came into existence and grew south- of Greenland from Eurasia. Because the nature of the Faeroes block ward at the expense of the Reykjanes Ridge. is uncertain we have shown it with a dashed line. In this recon- The reconstruction to anomaly 23 time shows an overlap in the struction the Caledonian front in Greenland does not quite line up Svalbard-Greenland area. The overlap is, however, small, not ex- with the Caledonian front in Scotland. As Vann (1974) has pointed ceeding about 50 km at any point. out, this may not be an objection to the reconstruction. Because the Lewisian of Scotland appears to continue along a belt that is RECONSTRUCTION TO TIME OF OPENING characterized by high gravity and magnetic anomalies and which passes through the Precambrian rocks of Lofoten in Norway (Tal- Figure 17 (top left) shows a reconstruction to the time of open- wani and Eldholm, 1972), it seems reasonable to infer that the ing. The relative positions of the assumed continent-ocean bound- western half of the Caledonian Mountain belt that lies between aries on the two sides indicate gaps as well as areas of overlap. The Lofoten Island and Greenland continues to the southwest and lies fit is good in the Vering Plateau-Greenland escarpment area, and also between Scotland and Greenland. Therefore, one should not there is also a reasonably good fit of the Jan Mayen Ridge against try to match the Caledonian front in Scotland with the Caledonian the Vetring Plateau to the northeast and the Faeroe-Shetland es- front in Greenland. carpment to the southeast. The major difference from earlier re- The limit of Mesozoic rocks appears to be symmetrical relative to

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PROPOSED CONTINENT-OCEAN BOUNDARIES Free-Air Anomaly 25— Bouguer Anomaly

Bathymétrie contour 1000 — in meters Bathymétrie Escarpment

Figure 18A. Gravity anomalies and proposed continent-ocean boundaries over Faeroes block. Bouguer anomaly map is adapted from Fleischer and others (1974).

the boundaries of the Caledonian front. The Mesozoic basin continues north from the North Sea and, in this reconstruction, continues through the Greenland shelf to the Barents shelf. This suggests that major Mesozoic basins might underlie the Greenland shelf north of Scoresby Sound, which points toward the importance of this area for oil exploration. Note that the Mesozoic basin has been defined in a general way. Just as grabens and platforms exist in the North Sea and on the Norwegian margins and the Barents shelf, they should be expected to exist in the shelf off Greenland. We do not have any data at present to outline their details there. Note that the line of initial opening lies well within the Mesozoic basin. The time of formation of the Mesozoic grabens, however, does not coincide with the time of opening but predates it by a large interval. We note that there is a similarly large interval between the time of formation of the Triassic grabens along the east coast of North America and the subsequent initiation of sea-floor spreading that separated North America from Africa. The contiguity of the early Tertiary basalts is obvious. Also note that although not specifically depicted in Figure 19, the magnetic

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quiet zone matches up across the line of opening. Landward of the AREAS OF UNUSUAL ELEVATION Varing Plateau escarpment, as also for the northern part of the Jan Mayen Ridge, the magnetic quiet zone is very undisturbed. How- We can consider the question of areas of unusual elevation at ever, landward of the Faeroe-Shetland escarpment, as with the two scales: a regional one of hundreds or thousands of kilometres matching point on the southern part of the Jan Mayen Ridge, the and a more local one of a few tens of kilometres. magnetic quiet zone is more disturbed. On a regional scale, the areas of the Norwegian Sea, Reykjanes Ridge, Iceland-Jan Mayen Ridge, Iceland, and Iceland Plateau are SPREADING RATES AT MOHNS RIDGE unusually high relative to other mid-oceanic ridge areas of compar- able age. Where the age of the ocean floor is precisely known, the Figure 20 gives the spreading rates (measured along the direction application of age versus depth curves (Sclater and others, 1971) of spreading) for the central part of the Mohns Ridge. There is a gives a quantitative method of computing depth anomalies. The steady decrease in spreading from the time of opening to about the depth anomalies calculated in this region (Vogt and Avery, 1974; time of anomaly 7. There is a slight increase in the rate of spreading Cochran and Talwani, 1977) show some interesting charac- between anomaly 7 and anomaly 6 time, a decrease after anomaly 6 teristics. For sea floor older than anomaly 7, depth anomalies time, and finally a marked increase after anomaly 5 time. These greater than 1,000 m are confined to two areas: (1) the Iceland- rates are subject to the errors that exist in the Heirtzler and others Faeroe Ridge, where they have the highest values, and (2) the Reyk- (1968) time scale and also to the errors in picking the anomaly lo- janes Ridge. Depth anomalies are much smaller in the Norway Ba- cations. Anomaly 7 is particularly difficult to pick on the Mohns sin. After the shift of the ridge axis at anomaly 7 time, the depth Ridge as well as on the Reykjanes Ridge; the corresponding spread- anomalies (with values larger than 1,000 m) persisted in the ing rates must be treated with caution. Accepting these rates, a Iceland-Faeroe and Reykjanes Ridges areas. But, in addition, they slowing of spreading is indicated near anomaly 7 time. These re- extend to a third area consisting of the Iceland Plateau, the sults also argue against a steady increase of spreading since anom- Iceland-Jan Mayen Ridge, and the southern end of the Mohns aly 7 time. The time between anomaly 6 and 5 appears to have been Ridge. one of slower spreading. On a regional scale the large elevation anomalies thus appear to

BOUGUER ANOMALY o 10 20 30 40 50 60 70 80 90 100 110 120 130 KM -h J—1—L~r~'—'—> A '—'—'—'—'—L 400 200 h o Meter Contour Fleischer and others 1974

Figure 18B. Bott and others (1971) have proposed that ocean-continent boundary coincides with bathymétrie escarpment at northwest edge of Faeroes block and that Faeroes block is continental. However, inspection of gravity profiles of Fleischer and others (1974), shows that steep -Aî=-0.25g/cm^ Aî=-0.10 g/cm' gradient is caused by shallow local low-density 3 rocks that do not necessarily extend throughout : — - 5 î= 2.85 g/cm î0 2.95 g/cm = U 2 70 g/cm . crust and whose seaward boundary lies at F and F' a considerable distance southeast of escarp- 10- ment (Fig. 18A). We believe that a continent- V7777777777777777777777777777777777777777777777\ 3 3 ocean boundary may well lie near eastern margin A t = 0.35 g /cm î = 3.30 g/cm of Faeroes block (see text for further discussion).

BOUGUER ANOMALY Bott and others (1971) h Profile D-

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persist over a long time — over the Iceland and Iceland-Faeroe We thus believe that if the unusual elevation of areas considered Ridge system as well as over the Reykjanes Ridge since the initia- above is to be attributed to deep mantle plumes of limited extent, tion of spreading and over the Iceland—Jan Mayen Ridge-Iceland then the plumes must (1) be fixed to the spreading ridge axis, (2) be Plateau area since anomaly 7 time. able to appear, disappear, and reappear in several areas within the Areas of unusually high elevation are also present on the local past 60 m.y., (3) be able to increase and decrease in their activity, scale of a few tens of kilometres. We note that these areas appear to and (4) be able to operate in such a way that they have different be associated with initiation of sea-floor spreading or with shifts in effects on elevation on different sides around them (comparing the the spreading axis. The high elevations associated with the base- elevation of the Norway Basin and the Reykjanes Ridge). Added to ment on the seaward side of the Varing Plateau, Greenland, and these complexities is the fact that in studies such as those by Mins- Faeroe-Shetland escarpments are examples of this. In profile V in ter and others (1974), in which a mantle plume is left fixed in a Figure 4, the area of spreading between the Jan Mayen Ridge and frame of reference containing other hot spots, the plume gives rise, the Iceland-Jan Mayen Ridge clearly lies between two basement on the overlying moving plate, to a trace that has a strike quite dif- highs that may be associated with the shift of the ridge axis from ferent from the observed strike of the Iceland-Faeroe Ridge. (Mins- the Norway Basin to the Iceland Plateau around anomaly 7 time. ter and others' results apply only for the past 10 m.y., but even dur- The unusual elevation of the basement at the time of early opening this time there is no indication of a nearly north-south trace of ing is not a phenomenon limited to the Norwegian Sea, but it is an Icelandic plume.) most easily seen here because of the recent date of the opening. Many of these difficulties are resolved if instead of imagining a Gravity highs coincident with the oldest magnetic anomalies west deep mantle plume of limited extent that moves around and whose of South Africa (Eldholm and Talwani, 1973) and south of Aus- activity increases and decreases, we appeal to a hot spot of much tralia (Konig and Talwani, 1977) point to the same phenomenon. larger areal extent and postulate that whenever the spreading axis The existence of areas of unusual elevation has been attributed to lies within the hot spot, the resulting ridge elevation is unusually hot spots (Wilson, 1965) or plumes originating deep in the mantle high. Such a hot Spot would, at the time of opening of the Norwe- (Morgan, 1971; Vogt and Avery, 1974; Schilling and Noe- gian Sea, underlie the ridge axis in the Faeroes, Reykjanes Ridge, Nygaard, 1974). If plumes are of limited extent, some severe con- and Davis Strait (west of Greenland) areas as well as under the straints are imposed on their geometry and on their activity if the areas of early Tertiary volcanic activity of the Brito-Arctic prov- features of unusual elevation that we have discussed above are to be ince: the northern Scotland, eastern Greenland, Disko Island (west attributed to them. The north and south boundaries of the of Greenland) areas. If this hot spot is kept fixed relative to the Re- Iceland-Faeroe Ridge are roughly parallel to the flow lines gener- ykjanes Ridge axis, we can see (Fig. 20) that as the Norwegian Sea ated prior to anomaly 7 time (Fig. 16). Therefore if this ridge origi- opens, the ridge crest is high where it lies over the hot spot. In par- nated from a plume of limited extent, this plume must have been ticular, at anomaly 7 time the shift of the ridge axis from the deep stationary relative to the axis of spreading. Second, if the high ele- Norway Basin to the shallow Iceland Plateau would be associated vation of basement on the seaward side of the V0ring Plateau es- with a jump of the axis from a position off the hot spot to a posi- carpment and the northern section of the Faeroe-Shetland escarp- tion on the hot spot. (Fixing the hot spot to the Reykjanes Ridge is ment are ascribed to plume activity, then this activity would have certainly arbitrary, but it is not essential to the argument, and a been short-lived, because depth anomalies seaward of these areas small motion relative to the ridge could yield the same result.) are small. Third, after anomaly 7 time still another plume would We noted above the association of areas of locally (at a scale of a have to start north of Iceland to explain the high elevation of the few tens of kilometres) high elevation with the initiation of sea- Iceland Plateau and Iceland—Jan Mayen Ridge. Schilling and floor spreading or with shift in the spreading axis. The question Noe-Nygaard (1974) have presented a model in which chemical arises whether these local highs are associated with a "pulse of variations in the basalts are associated with variations in the plume mantle plume convection" prior to opening (Vogt and Avery, activity. This model suggests decrease in plume activity from 58 to 1974; Schilling and Noe-Nygaard, 1974), or whether transient 52 m.y. ago in the Faeroe Islands and employs results presented by O'Nions and Pankhurst (1973) to suggest decrease in plume activity since 13 m.y. ago in eastern Iceland. Vogt and Avery (1974), on the other hand, suggested, on the basis of the variation in depth anomalies in the Reykjanes Ridge area, that an increase in plume activity has taken place since 15 or 20 m.y. ago. <

Figure 19. Reconstruction prior to time of Tertiary opening of Norwe- gian Sea. Limit of Mesozoic rocks in Greenland is taken from Haller (1971). Stipple = Mesozoic basins. Contours for Mesozoic basin on Barents shelf (Eldholm and Talwani, 1977) and off Norway (Talwani and Eldholm, 1972) are drawn on assumption that velocities greater than 2.5 km/s represent pre-Tertiary rocks. Boundary of Nordland ridge has been defined by Rennevik and others (1975), and those of Viking and Central grabens and other features in North Sea are taken from Naylor and others (1974). Ba- sins on Shetland-Hebrides shelf have been defined by Bott and Watts (1970) and by McQuillin and Binns (1973). Extent of Rockall Basin Mesozoic sediments (based on seismic velocity data) are after Roberts (1974). Jan Mayen Ridge is called a ridge because of its present configuration after drift. In reconstructed position it should be considered simply part of Mesozoic basin. High gravity and magnetic belt (crosshatched) from Scotland to Lofoten has been defined by Talwani and Eldholm (1972). Ages for Precambrian rocks have been taken primarily from Moorbath (1975). Black areas = Tertiary volcanic rock. Faeroes block is outlined by dashed lines to indicate uncer- Figure 20. Spreading rates for Mohns Ridge measured along flow lines. tainty of its existence at time of opening. Diagonal ruled areas = structural Heirtzler's time scale used here may give ages that are too great by 5 to 10 highs in North Sea. m.y. in early Tertiary time.

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phenomena associated with the opening of a new spreading center others, 1976). The ages of oldest sediments determined from the not necessarily connected with the increase or decrease of plume fossils and the radiometric ages of underlying basement rocks are activity are involved. The transient phenomena could be connected compared with the ages predicted by this study (Table 3). with the chemistry of early opening basalt produced below the Two sites (336 and 352) were drilled on the Iceland-Faeroe continent being split apart (Schilling, 1971), or with thermal con- Ridge. Basement was reached at site 336. The fossils in the oldest ditions at the time of opening which may produce increased mag- sediments that were recovered had an age range of late to middle matic activity. It is also possible that early oceanic sea-floor basalts Eocene. Since magnetic lineations have not been identified on the are "welded" to the continent and dc> not undergo the appropriate Iceland-Faeroe Ridge, a precise comparison of ages is difficult. The amount of subsidence. Finally, where the two effects coincide faunal and radiometric ages are within the range of ages corre- —that is, a new spreading axis is opened on top of a hot spot sponding to magnetic anomalies 7 and 24 that is predicted by this —there appears to have been greatly increased activity — for study. Site 336 is on the north side of the Iceland-Faeroe Ridge; site example, in the Faeroes region, with plateau basalt eruptions on the 352 is on the south side. The middle Oligocene faunas at the two contiguous continental areas of Greenland and Scotland. sites are very different, suggesting the presence of an emerged Iceland-Faeroe Ridge that blocked the connection between the DRILLING RESULTS North Atlantic and the Norwegian Sea at least until middle Oligocene time. Results from drilling on Glomar Challenger Leg 38 generally At site 337, which was located in the Norway Basin on the rift substantiate the conclusions of this study (Talwani, Udintsev and mountains about 20 km east of the extinct axis, there is a discrep-

TABLE 3. FAUNAL, RADIOMETRIC, AND MAGNETIC AGES, GLOMAR CHALLENGER LEG 38 SITES

Site Lat Long Area Age range Radiometric Magnetic Corresponding (°N) determined from age lineation age1 from fauna in oldest (m.y.) observed or Heirtzler and datable sediment predicted by others time scale recovered this study (m.y.)

336 63°21.06' 07°47.27'W Iceland-Faeroe Ridge on Late Eocene-middle 40-43 Between 7 Between 27 and northeast side; about 120 km Eocene, 38-49 m.y. and 24 60 north-northeast of Faeroe Islands 337 64°52.30' 05°20.51'W Norway Basin; about 20 km Middle Oligocene-late 18-25 4 m.y. older 31 east of extinct axis Eocene, 29-43 m.y. than 7 338 67°47.11' 05°23.26'E Outer Varing Plateau; about Early Eocene, 49-53 46.6 Between 24 Between 60 and 25 km northwest of Varing m.y. and 25 63 Plateau escarpment 339 67°12.65' 06°17.05'E Inner Varing Plateau; about Eocene (diapiric Basement 340 67°12.47' 06°18.38'E 50 km southeast of Varing activity) not reached Plateau escarpment 341 67°20.10' 06°06.64'E Inner Varing Plateau; about Middle Miocene Basement 25 km southeast of Varing not reached Plateau escarpment 342 67°57.04' 04°56.02'E Outer Varing Plateau; about Early Miocene" 44 Between 24 Between 60 and 50 km northwest of Varing and 25 63 Plateau escarpment 343 68°42.91' 05°45.73'E Base of Varing Plateau Early Eocene, 49-53 28.5 23 58 m.y. 344 76°08.98' 07°52.52'E Knipovich Ridge; about 20 Pliocene-late Miocene 3" Younger than Less than 10 km east of rift axis 3-10 m.y. 5 345 69°50.23' 01°14.26'W Southern Lofoten Basin Early Oligocene-late 27 Between 13 Between 38 and Eocene, 34—43 m.y. and 20 49 346 69°53.35' 08°41.14'W Jan Mayen Ridge; near Oligocene to late Basement 347 69°52.31' 08°41.80"W western end of main block (and middle?) Eocene not reached 348 68°30.18' 12°27.72'W Iceland Plateau; about 20 Early Miocene- 18.8 6 21 km east of extinct axis Oligocene, 16-37 m.y. 349 69°12.41' 08°05.80'W Jan M.iyen Ridge Oligocene to late Basement (and middle?) Eocene not reached 350 67°03.34' 08°17.68'W Southern extension of Jan Late Eocene, 38-43 40-44 Between 7 Between 27 and Mayen Ridge, on ridge 1 of m.y. and 20 49 Figures 3, 5, 14 352 63°38.97' 12°28.26'W Iceland-Faeroe Ridge; on Middle Oligocene Basement southeast side; about 70 not reached km eas: of Iceland shelf edge

Note: See Talwani, Udintsev, and others (1976). * Heirtzler's time scale gives ages that are probably too great in early Tertiary time by 5 to 10 m.y. * Assuming a half-spreading rate of 0.5 cm/yr. " Eocene and Oligocene sediments are missing from seismic reflection record. *1 Intruded sill is younger than overlying sediments.

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ancy between the radiometrically determined basement age and the tor represents an important unconformity. Above the unconformity paleontologically determined age of the overlying sediments. A best the sediments are of middle Oligocene age and younger. Below the estimate is made for the age of the basement rocks at this site and unconformity, the sediments that were recovered range in age from after an allowance is made for its distance from the extinct axis, an early Oligocene to late (and middle?) Eocene. Basement rocks were age of 25 m.y. is estimated for the extinct axis which agrees with not recovered. Therefore, the drilling results do not demonstrate the model developed in this study. conclusively that the Jan Mayen Ridge is a continental fragment; Sites 338 and 342 lie on the outer Voring Plateau in an area be- however, the large thickness of sediments (deduced seismically) tween the anomaly 24 lineation and the Vering Plateau escarp- below the Eocene sediments recovered by the drill certainly suggest ment. There is a complication at site 342 because the oldest sedi- the formation of the Jan Mayen Ridge prior to the initiation of the ments are missing, but basement ages at both sites and the age of Norwegian Sea. the overlying sediment at site 338 give an early Eocene age. There is Site 348 was drilled in the Iceland Plateau on magnetic lineation a further complication because recent multichannel seismic results 6 east of the extinct axis. The radiometric age of 18.8 m.y. obtained in the area between anomaly 24 lineation and the escarpment show for basement agrees well with the age of 21 m.y., corresponding to dipping reflectors below the acoustic basement, which was shown the age of the magnetic lineation. to be basalt by the drilling results. In our view, the layers corre- Site 350 lies on the morphological Jan Mayen Ridge but to the sponding to the reflectors, as well as the overlying basalt layer that south of its main block. We have suggested that the area of site 350 was drilled into, were all emplaced within a few million years of the was probably created by sea-floor spreading contemporaneously initiation of the opening of the Norwegian Sea; that is, the outer with the development of the Norway Basin (to account for the fan- V0ring Plateau was formed after the opening of the Norwegian Sea. shaped pattern of anomalies in the Norway Basin). The late Eocene To prove this conclusively, one needs to drill through the basalt age determined both radiometrically from basement rocks and into the underlying reflectors and further into "true" basement paleontologically from the overlying sediments is not in conflict below them. with our suggestion. Site 343 at the foot of the Voring Plateau is located on anomaly At sites 348 and 350 the layer penetrated by the drill is the 23 lineation. The age of overlying sediments is early Eocene, but opaque layer. The unusually smooth surface of the opaque layer the radiometric age is anomalously young. had raised the question of whether it was basement. The drilling re- At sites 339 and 340, located on diapiric bodies on the inner V0r- sults showed that the layer was formed of basaltic rocks. ing Plateau, Eocene and Miocene fauna was recovered at very shallow depths and confirmed the presence of diapiric activity. At site CONCLUSIONS AND INFERENCES 341, also on the inner Varing Plateau, middle Miocene sediment was recovered at a penetration depth of 456 m. Since a large thick- The opening of the Norwegian-Greenland Sea started between ness of sediments (determined seismically) lies below this depth, the anomaly 24 and 25 time (between 60 and 63 m.y. ago on the results do not contradict the prediction of a pre-Tertiary age for the Heirtzler time scale). Anomaly 24 is found in the Norwegian Sea, oldest sediments on the inner Varing Plateau. but anomaly 25 is not. The negative anomaly just seaward of the Site 344 was located on the Knipovich Ridge about 20 km east of Varing Plateau escarpment is an edge anomaly formed by the initi- the rift axis. The drill reached an intruded sill; the radiometric age ation of opening during a period of negative polarity. There is some of 3 m.y. is younger than the faunal age range (Pliocene—late evidence that the Heirtzler time scale gives early Tertiary ages that Miocene) of the overlying sediments. are too great by 5 to 10 m.y. The opening of the Norwegian- Site 345 lies near the southern margin of the Lofoten Basin. We Greenland Sea may therefore be younger by this amount. have stated earlier that no jump in the Mohns Ridge axis has taken From the time of opening to about anomaly 13 time (38 m.y. place since anomaly 24 time, but that at a time close to anomaly 7 ago), the Labrador Sea was also opening and the motion of Green- time the spreading axis in the Norway Basin jumped westward. G. land was northwest relative to Eurasia. Because of this nortwesterly Granlie and M. Talwani (in prep.) suggest that, in fact, the jumping azimuth, only the Norwegian Sea opened up, while Greenland slid of the ridge axis was more complicated. The jump of the spreading past the Barents shelf and Svalbard in transcurrent fashion. Evi- axis actually took place in two steps. North of the northern seg- dence of early transcurrent motions in the area of west Svalbard ment of the Jan Mayen Fracture Zone, no ridge-axis jump oc- have been noted by many authors, including Harland (1969). Thus curred, and spreading has continued since the opening of the the Greenland Sea did not open until after 38 m.y. ago. There was a Norwegian Sea. In the area that lies between the two segments of land connection between Svalbard and Greenland up to this time. the Jan Mayen Fracture Zone, the spreading axis jumped at nearly Since anomaly 13 time, the opening of the Norwegian-Greenland anomaly 13 time. Thus, between anomaly 7 time and anomaly 13 Sea is described by the same poles and angular rates of opening as time, there were three segments of the axis of the mid-oceanic ridge the North Atlantic, north of the Azores-Gibraltar Ridge — that is, in the area. One segment lay north of the northern segment of the the entire opening is described by the separation of the Eurasian Jan Mayen Fracture Zone, a second segment lay between the two and North American plates. During this motion, Greenland moved segments of the Jan Mayen Fracture Zone, and a third segment in a nearly westerly direction relative to Norway. Although the coincided with the extinct axis of the Norway Basin. Three magne- rates of opening vary substantially during this time (Fig. 20), there tic lineations, 21, 22, and 23, have been tentatively identified in the is almost no change in spreading direction (Fig. 16). area west of site 345. They belong to the intermediate area among The fracture zone azimuths are much steeper during the early the three mentioned above. Site 345 must also belong to this inter- period of opening than during the time subsequent to anomaly 13. mediate area and, estimating roughly from these tentatively iden- Since anomaly 23 time the opening has been generally steady tified lineations to the west of it, the age of site 345 lies somewhere (without ridge-axis jumps) along the Mohns and Reykjanes Ridges, between anomaly 13 time and anomaly 20 time, that is between 38 although there are minor exceptions to this. A single rift valley is and 49 m.y. ago. This date agrees with the age determined for the continuous at the present time through the length of the Mohns fossils in the oldest sediments recovered — late Eocene to early Ridge (north of lat 71°31'N) and continues into the Knipovich Oligocene — although the date 27 m.y. determined radio- Ridge axis. metrically from basalt samples appears to be anomalously low. The northern part of the Knipovich Ridge axis has probably mi- At sites 346, 347, and 349, located on the main block of the Jan grated eastward since its inception at about anomaly 13 time. The Mayen Ridge, drilling penetrated the major reflector lying between magnetic anomaly pattern is not very clear for the Knipovich layers with velocity 1.8 km/s and 2.2 km/s (Figs. 14,15). This reflec- Ridge.

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The area lying south of the Jan Mayen Fracture Zone has had a servatory and the Norwegian Geotraverse Committee. Charles complicated development. After the opening there was a probable Windisch, Philip Rabinowitz, and Yngve Kristoffersen contributed westward shift in the axis at anomaly 23 time to the extinct axis in significantly to the success of the field program. We are especially the Norway Basin. The extinct axis was active until about anomaly grateful to Professor K. Heier, chairman of the Norwegian Geo- 7 time, but the fan-shaped pattern of magnetic anomalies generated traverse Committee, for his help. Talwani is also grateful to the during part of this time demands an additional complementary Guggenheim and Fulbright Foundations for fellowships that sup- spreading axis. We suspect that this latter axis lay between the ported his stay in Oslo in 1974, during which much of the writing southern part of the Jan Mayen Ridge area and the Norway Basin. of this paper was done. We thank Ulrich Fleischer, Jean-Guy Schil- Subsequent to anomaly 7 time, the axis of spreading jumped ling, Anthony Watts, and Charles Windisch for critical reviews of west, separating the Jan Mayen Ridge from Greenland. This new the manuscript. Valerie Ewing assisted in the compilation of Figure axis of spreading generated a suite of anomalies on the Iceland 19. Plateau but jumped again before anomaly 5 time to generate the Support for this project came from U.S. Office of Naval Research now-active Iceland—Jan Mayen Ridge. Thus, the Iceland Plateau Contracts Nonr 266(48), Nonr 266(79), N00014-67-A-0108- has been created by sea-floor spreading. 0004, and N00014-75-C-0210 and from National Science Foun- We believe that the Greenland-Iceland-Faeroe ridge has been dation Grants GA-1434, GP-5392, GA-17731, GA-27281, and generated like the rest of the Norwegian Sea, by the moving apart DES-71-00214-A07. of Greenland and Norway. It is likely that the ridge axis on the Iceland-Faeroe Ridge shifted westward and that the formation of Iceland dates from this shift about 27 m.y. ago. REFERENCES CITED Structurally, the Jan Mayen Ridge continues southwest, rather than south. Its blocklike structure is replaced southward by a series Avery, O. E., Burton, G. D., and Heirtzler, J. R., 1968, An aeromagnetic of ridges that lie between areas defined seismically by an acousti- survey of the Norwegian Sea: Jour. Geophys. Research, v. 73, cally opaque layer. Farther south, these ridges are replaced by what p. 4583-4600. appear to be buried structures seen under interruptions in the Blakely, R. J., 1974, Geomagnetic reversals and crustal spreading rates dur- acoustically opaque layer. There is a suggestion here of the conti- ing the Miocene: Jour. Geophys. Research, v. 79, p. 2979-2986. nental mass being fragmented and buried to the south. Bott, M.H.P., and Watts, A. 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MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 4, 1975 Naylor, D., Pegrum, D., Rees, G., and Whiteman, A., 1974, The North Sea REVISED MANUSCRIPT RECEIVED OCTOBER 8, 1976 trough system: Norwegian Jour. Oil and Gas Matters, v. 2, p. 1-5. MANUSCRIPT ACCEPTED NOVEMBER 23, 1976 O'Nions, R. K., and Pankhurst, R. J., 1973, Secular variation in the Sr- LAMONT-DOHERTY GEOLOGICAL OBSERVATORY CONTRIBUTION No. 2486

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