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Evolution of the Norwegian-Greenland Sea

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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 -1 BO Figure 1. Tracks of R/V Vema during sum- men of 1966, 1969, 1970, 1972, and 1973 in Norwegian-Greenland Sea. 70 BO * 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 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 970 TALWANI AND ELDHOLM 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). 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 EVOLUTION OF THE NORWEGIAN-GREENLAND SEA 971 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 un- usually 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.
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