Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016 Tectonic and volcanic events at the Jan Mayen Ridge microcontinent S. T. Gudlaugsson, K. Gunnarsson, M. Sand & J. Skogseid SUMMARY: Two main tectonic phases were responsible for the formation of the Jan Mayen Ridge microcontinent: (1) the opening of the Norway Basin in late Palaeocene/early Eocene times, and (2) subsequent rifting within the Greenland margin by which complete separation was achieved in early Miocene times. During the first phase the eastern ridge flank developed as a volcanic passive margin. The initial break-up was associated with flexuring and the formation of sequences of eastward-dipping basalt flows, which are considered equivalent to similar features beneath the Voting and Faeroe-Shetland marginal highs off Norway. Rifting along the Greenland margin during the second phase was accompanied by uplift, listric normal faulting and the formation of large extensional fault blocks. To the W and S of the ridge a flat volcanic marker of probable earliest Miocene age covers the subsided rift and masks the ocean-continent transition. It was formed by a volcanic event of large magnitude, either as submarine lava flows or as a sill complex. In 1985 a detailed marine geophysical survey of the Jan Mayen Ridge area was carried out jointly by the Norwegian Petroleum Directorate and the National Energy Authority of Iceland. A total of 4000 km of multichannel seismic reflection data was obtained (Fig. 1) in addition to gravity, magnetic and sonobuoy measurements. The seismic data in particular have provided important information in terms of the tectonic and volcanic history of the ridge. The main objective of this paper is to describe the nature of the igneous provinces at the Jan Mayen Ridge microcontinent and show how their formation relates to tectonic events at the ridge and the plate tectonic development of the Norwegian Sea. Plate tectonics The Jan Mayen Ridge (see Fig. 1) is a bathymetric ridge complex extending S from the volcanic island of Jan Mayen to about 67~ The main northern ridge block is flat-topped with water depths increasing to about 1000 m in the S. It is separated from the still deeper southern ridge complex by the Jan Mayen Trough. The Jan Mayen Ridge is a crustal fragment which split from the Greenland continental margin by a westward shift in the plate boundary at Oligocene/Miocene time. The evolutionary models for the Jan Mayen Ridge are based on a westward shift of the plate boundary and the observations of Talwani & Eldholm (1977) that the fan-shaped spreading pattern along the Aegir Ridge in the Norway Basin required that complementary spreading must have taken place further W to account for the motion of Greenland relative to Eurasia. Talwani & Eldholm (1977) proposed that the Aegir Ridge was active until about anomaly 7 time and suspected that the complementary crust formed between the southern part of the Jan Mayen Ridge and the Norway Basin. At anomaly 7 time a westward jump to an 'intermediate' axis on the Iceland Plateau occurred. This axis supposedly became extinct when spreading started from the Kolbeinsey Ridge further W just before anomaly 5 time. Alternatively, Vogt et al. (1980) have rejected the existence of an intermediate axis and postulated spreading from the Kolbeinsey Ridge since anomaly 6C time. The three-plate model has been further developed by several investigators (Unternehr 1982; Nunns 1983a, b; Bott 1985). According to Nunns (1983a, b) fan-shaped spreading formed two conjugate wedges of seafloor on either side of the Jan Mayen Ridge during the interval between the formation of anomalies 20 and 7. Similarly, Larsen (1988) suggests northward propagation of the Reykjanes-Kolbeinsey Ridge combined with gradual termination of spreading northwards along the Aegir Ridge during the 20-7 time interval. Structure The first order geological framework was reviewed by Myhre et al. (1984). The ridge is bordered by provinces of basaltic rocks on both sides. A fiat volcanic marker, reflector F, characterizes the western province whereas irregular lava flows and seaward-dipping reflectors are From MORTON,A. C. & PARSON,L. M. (eds), 1988, Early Tertiary Volcanism and the Opening of the NE Atlantic, Geological Society Special Publication No. 39, pp. 85-93. 85 Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016 Ft6.1. The grid of multichannel seismic reflection lines used in this study superimposed on bathymetry. The new survey lines are shown as dotted lines. Contour interval 100 m with contours labelled every 500 m. JM = island of Jan Mayen, JMB = Jan Mayen Basin, J M T = Jan Mayen Trough, SRC = southern ridge complex. Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016 Events at Jan Mayen Ridge mierocontinent found beneath the eastern ridge flank. The ridge is covered by a thick sequence of eastwarddipping sedimentary rocks, above a strong reflecting interface, which in early single-channel seismic records defined the acoustic basement (see Fig. 3, reflector JO). The sedimentary sequence comprises two main units separated by a prominent unconformity (reflector JA; J is prefixed to the commonly-used symbols O and A to indicate the Jan Mayen Ridge). The structural map in Figure 2 is based on the interpretation of existing multichannel seismic reflection profiles including the new data. The seismic database is shown in Figure 1 and line drawings of three seismic profiles crossing the main geological provinces are shown in Figure 3. The Jan Mayen Ridge is strongly affected by normal faulting. The number of fault blocks and the general structural complexity increase southward, as does the depth to the individual fault blocks (see Figs 2 & 3). The main ridge block in the N has a distinct asymmetric structure. The eastern flank dips steeply towards the Norway Basin and is almost undisturbed by faulting whereas the western flank of the ridge is downfaulted towards the Jan Mayen Basin. These faults form a listric fault complex in which individual faults can be seen to sole-out at depth. The ridges comprising the southern ridge complex (Pelton 1985) are also tilted fault blocks. The majority of the blocks, and almost all the large ones, have fault scarps facing W. Where the volcanic marker (reflector F) W of the ridge is absent a number of deep half-grabens are observed between the fault blocks. Two main fault trends are observed at the Jan Mayen Ridge. The trends are also reflected in the bathymetry and intersect at the prominent bend in the ridge at 69~ N of the ridge bend, the faults trend N-S. S of the bend, both on the main ridge block and in the southern ridge complex, the trend is nearly NNE-SSW. Complex structures are observed in the region where these trends intersect. The system of eastward-rotated fault blocks at the Jan Mayen Ridge is interpreted in terms of crustal extension. There is also evidence of a later phase of compression. Reverse faults, similar to those reported by Skogseid & Eldholm (1987), have been identified in the new data, but their detailed correlation has yet to be worked out. Between the eastern and western volcanic provinces there is a seismic window into the deeper crust. The new data show a number of reflecting interfaces beneath reflector JO, some of which are found at depths of 6-7 s two-way time (see Fig. 3). 87 Geophysical investigations and deep-sea drilling have not yet conclusively answered the question of whether the ridge is continental or oceanic in nature. In our opinion the balance of evidence favours a continental crust, probably thinned and modified by rifting processes. The seismic velocity structure (Johansen et al. in press) and the seismic reflector pattern below reflector JO are not compatible with normal oceanic crust. The continent~ocean boundary is difficult to locate for two reasons: (1)the boundary is probably masked in many places by lava flows; and (2) the detailed location of the boundary is to a certain extent a matter of definition. We can, however, place limits on the zone where the crustal transition must occur. The outer limit is marked by the oldest seafloor-spreading anomalies on both sides of the ridge. At the eastern side the inner limit lies at the apex of the wedge of seaward-dipping reflectors. On the western side it follows the scarps of the westernmost fault blocks. Volcanic provinces The distribution of volcanic rocks on either side of the ridge is shown in Figure 2. On the eastern flank we differentiate between a wedge of eastward-dipping reflectors below reflector JO and a younger volcanic overprint. Reflector JO forms the top of the eastward-dipping reflectors. The wedge is most prominent S of the Central Jan Mayen fracture zone, where it is underlain by a sequence of parallel reflectors. On line C (see Fig. 3) this sequence may be traced up-dip towards the boundary fault without change in character, which shows that it predates the faulting and originally continued further W. On line B the same sequence may be continued a short distance beyond the apex of the wedge. The internal reflectors of the overlying wedge converge only to a certain point and then also become parallel. On the basis of these observations we suggest that an equivalent of the E Greenland plateau basalts may underlie the seaward-dipping reflectors and cover the southern part of the ridge. N of the Central Jan Mayen fracture zone the wedge has a different character. Both the dip and the divergence of the reflectors is less prominent and the reflector pattern is more irregular. The mapping of the extent of the dipping reflectors presented here is primarily based on the new survey, and their presence in Figure 2 is only shown where a well-developed dipping and divergent sequence is observed. Reflector JO is overstepped from the E by a Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016 FIG. 2. Structural map of the Jan Mayen Ridge and the surrounding areas. Horizontal ruling = western front of volcanic overprint; diagonal ruling -- area covered by reflector F; stippling = areas of seaward-dipping reflectors; thin continuous lines = bathymetric contours; thick continuous lines = fracture zones; dotted lines = seafloor-spreading anomalies (Skogseid & Eldholm 1987; Vogt et al. 1980; Gr~nlie et al. 1978). JMB = Jan Mayen Basin, JMT = Jan Mayen Trough, C J M F Z = Central Jan Mayen Fracture Zone. Fracture zones from Skogseid & Eldholm (1987). Dashed lines A, B and C show the location of the seismic profiles in Figure 3. Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016 89 Events at Jan Mayen Ridge microcontinent e.e5 ~z < II ~z ..~ .= o ~ ~E e~ ~a M~ e~ u~ ~g ~.~ ; . 7 .~ Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016 9~ S. T. Gudlaugsson et al. strongly reflective horizon characterized by diffractions and both irregular and smooth reflector segments (see Figs 2 & 3). The surface can be traced from opaque oceanic basement westward, where it gradually becomes semi-opaque with irregular structure below before it terminates within the sediments above reflector JO. We interpret this layer as being caused by submarine lava flows, intrusions and pyroclastics interfingering with marine sediments. The western front of this younger volcanic overprint may be traced from the southern ridge complex, with decreasing stratigraphic gap with respect to reflector JO, N to the Central Jan Mayen fracture zone. At this point the front turns westwards and merges with reflector JO (see Fig. 2). W and S of the ridge we have mapped a strong, fiat-lying acoustic basement reflector beneath a thin sedimentary cover. This is the 'opaque horizon' of Eldholm & Windisch (1974) that covers a large part of the Iceland Plateau. The new data allow us to differentiate between an ultra-fiat opaque volcanic marker (denoted F) in the area between the ridge and the oldest seafloorspreading anomaly on the Iceland Plateau and a different more irregular type of basement further W. Reflector F covers the area W of the southern ridge complex locally extending into the Jan Mayen Basin and the Jan Mayen Trough. On the eastern side it terminates abruptly at the fault scarps. The western boundary lies at the foot of an E-facing escarpment associated with the oldest seafloor-spreading anomaly. The height difference between the top of the escarpment and reflector F decreases towards the S. S of about 69~ the relationship between reflector F and the oceanic basement becomes unclear. Here the reflector seems to overstep the basement towards the W, but we cannot verify that it is continuous with the basaltic basement reflector which was drilled at Deep Sea Drilling Project (DSDP) Site 348 (Talwani et al. 1976). We note that W of the escarpment a number of sub-basement reflectors are observed on lines A and B. On line A the reflection pattern is westward-dipping and diverging similar to the pattern of a typical wedge of seaward-dipping reflectors. Reflector F is very strong and normally no reflectors are observed below. The acoustic energy becomes trapped in the water layer and in the thin sedimentary layer. The reflector is not perfectly smooth but exhibits small-scale roughness. As Figure 3 shows, reflector F is composed of a number of offset segments. Because of the poorly developed stratification in the overlying sediments it is difficult to determine the nature of these offsets, but there are indications that some of them are associated with high-angle normal faults. The most striking aspect of the reflector is, however, the extreme flatness of individual segments. In our opinion, the possible interpretations of reflector F are: (1) high impedance sedimentary layer; (2) volcanic ash layer; (3) intrasedimentary sills; and (4)lava flows. In the sedimentary environment of the Norwegian Sea it is unlikely that such a high impedance contrast could occur within a sedimentary succession without the compaction effect of burial and later erosion. Reflector F does not show any evidence of submarine erosion. Volcanic ash may generate highly reflective flat surfaces. An ash layer must either have been pelagically draped over existing topography or redeposited as mass flows. The first mechanism is not viable as there is no draping effect over the blocks and the reflector terminates abruptly against fault blocks and escarpments. It is possible, however, that volcanic ash from an elevated source area close to Iceland may have been transported by turbidity flow northward onto the Iceland Plateau. A regional intrasedimentary sill is also a possible explanation, but seems unlikely from a mechanical viewpoint considering the areal extent of the reflector. However, if the reflector offsets are interpreted as the limits of individual sills and not as normal faults this argument does not apply and reflector F may represent the top of a sill complex. Nonetheless, we favour an interpretation of extensive submarine lava flows as suggested for the ultra-flat opaque reflectors on the Reykjanes Ridge (Vogt & Johnson 1973). On line A (see Fig. 3), which crosses the Jan Mayen Basin just S of the northern termination of reflector F, the layer below the reflector is locally acoustically transparent. A number of reflections are observed beneath it and the reflection pattern suggests a continuity of sedimentary layers beneath the volcanic marker from the main ridge block to a fault block further W which rises above the floor of the Jan Mayen Basin. On seismic lines further S reflections from sedimentary layers and detachment planes in the fault complex at the western margin of the main ridge block are observed to dip under reflector F but are lost a short distance from its edge. In our interpretation, the listric fault complex continues underneath the reflector. A similar relationship is observed in the southern ridge complex where some of the fault blocks plunge beneath it. Since the reflector is fiat-lying and covers the basin fill in the half-grabens between some of the blocks, its formation postdates the block faulting by a considerable interval. It follows that most of the present elevation difference between the Jan Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016 Events at Jan Mayen Ridge microeontinent Mayen Ridge, and the lower areas to the W covered by reflector F, was already established at the time of its formation. The flat unconformity on top of the main ridge block represents the deepest level that wave erosion ever reached since the faulting. Therefore, the reflector was formed under submarine conditions. Whether reflector F is interpreted as a layer of volcanic ash, submarine lava flows or sills, it was formed by a volcanic event of a large magnitude. Timing of volcanic events and evolution of the microcontinent No detailed chronology of events has been available for the Jan Mayen Ridge. The DSDP holes drilled during Leg 38 (Talwani et al. 1976) only sampled part of the sedimentary sequence and the stratigraphy of the ridge has not been well understood. We now propose a chronology for the main tectonic and volcanic events based on seismic stratigraphy and correlation with the geology of the conjugate Greenland and Norwegian margins. We start by examining the age of the oldest well-defined rock sequence at the Jan Mayen Ridge, i.e. the sequence of seaward-dipping reflectors. Comparable sequences at the Norwegian margin are well known (Skogseid & Eldholm, 1987). In the light of their origin as extensive subaerial basaltic lava flows formed during the earliest spreading phase, their position relative to the Jan Mayen Ridge in plate tectonic reconstructions and the symmetry of the wedges on both sides, there can be no doubt that the sequence of seaward-dipping reflectors at the Jan Mayen Ridge has a similar origin and was formed at approximately the same time at the conjugate margin as proposed by Skogseid & Eldholm (1987). They interpret reflector JO as an equivalent to reflector EE of earliest Eocene age at the V~ring margin. Gairaud et al. (1978) divided the cover of Tertiary sediments at the Jan Mayen Ridge into two sequences separated by a prominent unconformity, reflector JA. The unconformity was drilled at DSDP Sites 346, 347 and 349 and proved to represent a hiatus in the early Oligocene (Yalwani et al. 1976). Here, we prefer to divide the sedimentary series covering the Ridge into three sequences numbered 1, 2 and 3 from below (sequence no. 1 being the oldest, see Fig. 3). The base of sequence 3 corresponds to reflector JA. Sequence 1 is mostly parallel-bedded and represents a widespread slope or shelf sequence. 91 It predates the block-faulting at the western margin of the ridge as does the parallel sequence below reflector JO and the wedge of seawarddipping reflectors. Sequence 2 shows a more disturbed sedimentary pattern. On line A the sequence exhibits outbuilding and downlap followed by marine onlap. Over the entire ridge the apparent instability of the sedimentary environment increases upward and the uppermost part of the sequence consists of slumps and other mass-flow deposits. We interpret this as gradual uplift of the ridge flank associated with doming and rifting at the western margin. The uplift of the ridge culminated in subaerial exposure of the main ridge block, a strong erosional phase and the formation of a prominent submarine unconformity on the slope (reflector JA). Sequence 3 formed first by outbuilding of submarine fans and later by passive draping with much reduced sedimentation rates. We now turn to the formation of the fault complex at the western margin of the ridge. Its conjugate part is the Liverpool Land margin, where Larsen (1984) describes a buried rift with an Eocene to early Oligocene graben-fill. He suggests the main tectonic episode of blockfaulting to be early to mid-Eocene and that the rifting accompanied the initial formation of the Norway Basin, inferring that the block-faulting at the Jan Mayen Ridge dates from this time. However, the Jan Mayen Ridge seismic data do not support this model. In fact, the western margin of the Norway Basin is found at the eastern flank of the Jan Mayen Ridge, where it is associated with flexuring and formation of a wedge of seaward-dipping reflectors. No block faulting is observed. The only candidate at the Jan Mayen Ridge for a symmetric counterpart to the landwards-rotated system of fault blocks at the Greenland margin is the system of eastwardrotated listric normal faults at the western margin of the ridge. The two margin types reflect different tectonic events with different thermal and mechanical characteristics and must be separated in time. We associate the block-faulting at the Jan Mayen Ridge with the separation of the ridge from Greenland, thus suggesting a younger age for block-faulting at the Greenland margin. In this connection it is interesting to note that Larsen (1984) finds no evidence for a later tectonic episode at the Liverpool Land margin which might correlate with the separation of the ridge from Greenland. At the Jan Mayen Ridge the minimum time gap between the two events is the interval represented by sedimentary sequence 1. The upper boundary of the sequence represents the earliest possible date for the onset of rifting. Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016 92 S . T . Gudlaugsson et al. Thus, we propose that: (1) Sequence 2 is time equivalent with the Liverpool Land rift basin-fill and is middle Eocene to early Oligocene in age. The deposition of sequence 2 was contemporaneous with the development of the rift beneath the Liverpool Land shelf. On the Jan Mayen side the sequence was deposited outside the rift, on its eastern flank. The sedimentary fill observed on the Greenland side was deposited within the rift during the same period. (2) Sequence 1 at the Jan Mayen Ridge is early Eocene in age, possibly extending into midEocene. It was deposited on the continental slope or shelf of Greenland on top of the seaward-dipping reflectors concurrently with extrusion of submarine lava flows in the Norway Basin. This sequence is probably found within the fault blocks in the Greenland Rift as well as at the Jan Mayen Ridge. Reflector F, the volcanic marker W of the Jan Mayen Ridge, clearly postdates the block-faulting at the western margin. Since it lies fiat on top of the half-graben-fiU in the southern ridge complex and shows no evidence of being affected by movements on the boundary faults, neither there nor W of the main ridge block, the formation of the reflector postdates rifting between Greenland and the Jan Mayen Ridge by a significant time interval. W of the main ridge block it appears that normal seafloor spreading was not established until anomaly 6C time (24 Ma). Line A indicates that the final break-up was associated with the formation of a submarine sequence of westward-dipping reflectors W of the escarpment at anomaly 6C. Examining the western termination of reflector F at the escarpment we find it most likely that the escarpment was formed prior to the emplacement of the reflector. We propose an earliest Miocene age for reflector F. ACKNOWLEDGEMENTS:We wish to thank the Norwegian Petroleum Directorate and the National Energy Authority of Iceland for their permission to publish this paper. S. T. Gudlaugsson and J. Skogseid were supported by a research grant from the Norwegian Petroleum Directorate, and K. Gunnarsson by a grant from the Nordic Council of Ministers. References BOTT, M. H. P. 1985. Plate tectonic evolution of the Icelandic transverse ridge and adjacent regions. Journal of Geophysical Research, 90, 9953-9960. ELDHOLM, O. & WINDISCH, C. C. 1974. Sediment distribution in the Norwegian-Greenland Sea. Bulletin of the Geological Society of America, 8 5 , 1661-1676. GAIRAUD,H., JACQUART,G., AUBERTIN,F. & BEUZART, P. 1978. The Jan Mayen Ridge. Synthesis of geological knowledge and new data. Oceanologica Acta, 1, 335-358. GRONLIE, G., CHAPMAN,M. & TALWANI,M. 1978. jan Mayen Ridge and Iceland Plateau: origin and evolution. Norsk Polarinstitutt Skrifter, 170, 25-48. JOHANSEN,B., ELDHOLM,O., TALWANI,M., STOFFA,P. L, & BUHL,P. (in press). Expanding spread profile at the northern Jan Mayen Ridge. Polar Research. LARSEN, H. C. 1984. Geology of the East Greenland Shelf. In: SPENCER, A. M., et al. (eds) Petroleum Geology of the North European Margin. Graham & Trotman, London, pp. 329-339. - 1988. A multiple and propagating rift model for the NE Atlantic. In: MORTON,A. C. & PARSON,L. M. (eds) Early Tertiary Volcanism and the Opening of the ArE Atlantic. Geological Society of London, Special Publication, 39, pp. 157-158. MYHRE,A. M., ELDHOLM,O. & SONDVOR,E. 1984. The Jan Mayen Ridge: present status. Polar Research, 2, 47-59. NUNNS, A. G. 1983a. Plate tectonic evolution of the Greenland-Scotland Ridge. In: BOTT, i . H. P., SAXOV, S., TALWANI,i . & THIEDE, J. (eds) The Greenland-Scotland Ridge: New Methods and Concepts. Plenum, New York, pp. 11-30. -1983b. The structure and evolution of the Jan Mayen Ridge and surrounding regions. In: WATKINS,J. S. & DRAKE,C. L. (eds) Continental margin geology. Memoir of the American Association of Petroleum Geologists, 34, pp. 193-208. PELTON, C. D. 1985. Geophysical interpretation of the structure and evolution of the Jan Mayen Ridge. Institute of Oceanographic Sciences Report, No. 205, 38 pp. SKOGSEID, J. & ELDHOLM, O. 1987. Early Cenozoic crust at the Norwegian Continental Margin and the conjugate Jan Mayen Ridge. Journal of Geophysical Research, 92, 11471-11491. TALWANI, M, & ELDHOLM,O. 1977. Evolution of the Norwegian-Greenland Sea. Bulletin of the Geological Society of America, 88, 969-999. - - , UDINTSEV, G, et al. 1976. Initial reports of the Deep Sea DrillingProject, US Government Printing Office, Washington, 38, 1256 pp. Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016 Events at Jan Mayen Ridge microcontinent UNTERNEHR, P. 1982. Etude structurale et cinOmatique de ta met de NorvOge et du Groenland. Evolution du microcontinent de Jan Mayen. Thesis. University of Bretagne, 227 pp. VOGT, P. R. & JOHNSON, G. L. 1973. A longitudinal seismic reflection profile of the Reykjanes Ridge: 93 Part II--Implications for the mantle hot spot hypothesis. Earth and Planetary Science Letters, 18, 49-58. & KRISTJANSSON,L. 1980. Morphology and magnetic anomalies north of Iceland. Journal of Geophysics, 47, 67-80. S. T. GUDLAUOSSON& J. SKOGSEID,Department of Geology, University of Oslo, Norway. K. GUNNARSSON,Department of Geology, University of Oslo, Norway and National Energy Authority, Reykjavik, Iceland. M. SAND, Norwegian Petroleum Directorate, Stavanger, Norway.