E. A. Gusev
All-Russian Research Institute of Geology and Mineral Resources of the World Ocean (VNIIOkeanologiya), St. Petersburg
S. I. Shkarubo
Marine Arctic Geological Exploration Expedition (MAGE), Murmansk, Russia
The Knipovich Ridge and surrounding area has long been a focus of interdisciplinary and specialized surveys performed by international marine expeditions. The global scientific community has sound reasons for scrutinizing this region because the Svalbard Archipelago and its adjacent Norwegian-Greenland Basin are key features to unraveling the tectonics and evolution of the western Arctic sector and the development of structural links between the North Atlantic and Arctic Ocean in Late Cenozoic time. Most researchers admit a spreading origin for the Knipovich Ridge in the Norwegian-Greenland Basin. The presence of a well-developed rift valley, modern seismicity beneath the ridge, and its sign-variable magnetic field, all seem to suggest that it is a regular segment of the global mid-oceanic ridge system. Many «anomalous» features of this structure, however, do not fit in the traditional concepts and remain as yet to be elucidated.
Then, a seismostratigraphic analysis of multichannel seismic profiles was performed. In order to refine the structural pattern of sedimentary cover in the Norwegian-Greenland deep-sea basin with due regard for seismostratigraphic schemes of various workers [Baturin, 1986, 1992; Faleide et al., 1996; Hinz and Schluter, 1978; Savostin and Baturin, 1986; Shkarubo, 1999], seismic horizons were correlated. In contrast to previous studies, our stratigraphic tie of key reflectors is based, among other things, on deep-sea drilling data from the Fram Straight.
In the crestal zone of the Knipovich Ridge, distinctive for its heterogeneous structure, and in which reflectors are hard to trace continuously, sedimentary cover was subdivided by means of identifying the "structural styles'' of individual stratigraphic assemblages. In small isolated basins, seismostratigraphic assemblages were established on the basis of a number of distinctive features, such as seismic signature carrying indirect information on the sedimentary facies, degree and style of deformation in sedimentary piles, and relations among stratigraphic assemblages and between these assemblages and acoustic basement. The use of such methodological approaches is justified by the lack of direct geological observations in the numerous isolated basins and ponds, on terraces, etc., whose sedimentary cover requires dating prerequisite to paleotectonic reconstructions.
Deep-sea drilling in the southern Norwegian-Greenland Sea (Voring Plateau, Lofoten and Norwegian basins, Icelandic Plateau) [Talwani and Udintsev, 1976] has elucidated the main evolutionary phases of the oceanic basin. In a number of localities, the Paleocene-Eocene sedimentary cover was laid down in shallow-water environments, and it is only the Miocene and Pliocene to Quaternary strata, unconformably overlying this cover, that can be labeled with confidence as bathyal. The marked hiatuses displayed by the sequences penetrated by deep-sea drilling led some workers to infer that, over some part of the present-day Norwegian-Greenland Sea, Early Oligocene to Middle Eocene times were characterized by uplifting and even by continental environments [Rudich, 1983].
According to the plate tectonics, the southern part of the Norwegian-Greenland Sea was incepted in the epoch of M24 magnetic anomaly (56-58 Ma) [Talwani and Eldholm, 1977]. However, the presence of chemiñally anomalous basalts and the temporal mismatch between the cored basalts and magnetic anomalies called for tectonic models other than plate-tectonic ones [Rudich, 1983; Udintsev, 1982]. These models invoke taphrogeny, flood magmatism, oceanization, and rifting as leading ocean-forming processes. Besides, the mid-oceanic ridges were assumed to have formed at a concluding phase of formation of the Norwegian-Greenland oceanic basin [Rudich, 1983].
Sediments deposited in the system of oceanic margin basins rimming the continental shelves are chiefly Cenozoic in age. Note also that the sharp thickening of Upper Cretaceous strata on the western Barents margin [Gabrielsen et al., 1990] and on the Voring Plateau [Sigmond, 1992] toward the modern oceanic basin suggests a pre-Cenozoic inception age for the system of oceanic margin basins located south of the Senja Fracture Zone. The dikes and sills recovered by deep-sea drilling from oceanic basement sections alongside volcanics and postdating the overlying sediments [Talwani and Udintsev, 1976] testify to magmatism that continued. Relationships like this are characteristic of the transitional zone between the continent and the Norwegian-Greenland Sea basins, in which sedimentary sequences grade into lavas and tuffs pertaining to oceanic crust [Klitin, 1983, 1988]. Such structures, detected by seismic surveys on the east Greenland margin, are termed "pseudo-scarps'' [Larsen, 1990].
The Paleogene diatom assemblages from Hole 908, located northwest of the Knipovich Ridge, on the aseismic Hovgaard Ridge, display epiphytic forms indicative of low-salinity neritic and nearshore environments. Further evidence for nearshore paleoenvironment is furnished by the finds of epipelic (growing on soft sediment) and epipsammic (growing on sandy bottom) epibenthic diatom taxocoenoses. This conclusion draws on the analysis of modern habitats of the genera Paralia, Diploneis, Cocconeis, Grammatophora, Rhaphoneis, etc. [Scherer and Koch, 1996]. The Paleogene diatom assemblages from Hole 908 also indicate a great sedimentation rate in a shelf environment in water depths of a few hundred meters. Rather common are finds of freshwater diatoms representative of paralic swamps and marshes (acidophilic diatoms of the genera Eunotia and Pinnularia) [Scherer and Koch, 1996]. Some of the forms encountered in Hole 908 have been reported from Western Siberian Oligocene strata, in which they are also suggestive of a low-salinity habitat in a nearshore setting. The benthic foraminiferal assemblages from Paleogene sediments in the northern Norwegian-Greenland Basin point to a somewhat deeper shelf environment [Ostermann and Spiegler, 1996]. During Miocene time, the Fram Strait was distinguished by great depositional rates, and it was an isolated, relatively deep-water basin, as suggested by the studies of agglutinated benthic foraminifers. This sedimentation environment favored a longer lasting preservation of agglutinated benthic foraminifer assemblages than in the North Atlantic, where they disappeared much earlier [Ostermann and Spiegler, 1996]. The fact that the bottom water was isolated from the rest of the North Atlantic is further supported by the absence of calcareous faunas from Hole 909.
The above circumstances imply an isolated nature for the northern Norwegian-Greenland Sea and an unusually dynamic formation mode of the deep-sea basin, with great subsidence rates in Miocene and, especially, Pliocene to Quaternary times.
In order to identify the structural and genetic type of the Knipovich Ridge and reconstruct the tectonic and geodynamic processes responsible for the present-day morphostructure of the crestal zone and rift valley of the ridge, one should primarily consider the general architecture of the deep in the northern Norwegian-Greenland Basin.
The plate tectonic formation model for the Norwegian-Greenland Basin [Talwani and Eldholm, 1977] proposes that its northern part began to open at 36 Ma (M13 magnetic anomaly), as the North American and Eurasian plates split up giving rise to an oceanic rift. This tectonic scenario implies that the Oligocene deposits, coeval to the initial phase of the oceanic basin opening, must be confined to oceanic margin basins at the opposite continental margins, and their overlying Pliocene to Quaternary strata must be spread more widely, reaching as far as the ridge zone.
The outline of the deep-sea basin is marked by the faulted continental-slope flexure zone, in which continental basement is cut by a system of listric faults and is overlain by a sedimentary wedge. Near the continental slope break and further outboard, the structure of sedimentary cover on the continental margin exhibits elongate, en echelon oceanic margin basins.
The distributional pattern and seismic signature of rock assemblages in the Norwegian-Greenland Sea basin suggest a dramatic change in depositional environment at the Miocene /Pliocene boundary.
The chain of the highest summits in the crestal zone of the ridge is associated with M3 magnetic anomaly. The axial anomaly is well-developed in the northern segment of the Knipovich Ridge alone. The Knipovich Ridge volcanic rocks are relatively enriched in Na, Si, and K and depleted in Fe [Neumann and Schilling, 1984; Sushchevskaya et al., 1997]. In the rift valley, a present-day hydrothermal activity is documented [Poroshina et al., 1998].
The gap in stratigraphic record at the ridge axis due to nondeposition or erosion suggests uplifting of this seafloor region. Judging from the seismically portrayed syndepositional deformations in sedimentary cover, such as horizons that "ride up'' and sedimentary assemblages that pinch out toward basement highs, it was in pre-Late Miocene time that this area was involved in uplifting. To a first approximation, the uplifting was comparable in magnitude to the elevation, as great as 0.5 to 1.0 km, of the ridge crestal zone above the contiguous Boreas Abyssal Plain. Simultaneously, the chain of the highest volcanic summits took shape, which makes up the modern submarine ridge and bounds the rift valley.
The magnitude of post-Oligocene horizontal extension in the northern part of the Knipovich Ridge can be quantified approximately by adding up horizontal plane projections of the areas devoid of Oligocene rocks, which corresponds roughly to the rift valley width (ca. 20 km) and to the sum total of horizontal throw on the normal faults (up to 1.5 km). Structural peculiarities of the crestal zone of the Knipovich Ridge suggest a domal uplift and its subsequent splitting as a leading formative mechanism.
The trend of lineaments, which is expressed topographically and emphasized by the structural features of basaltic basement, reflects both parallel extensional features and discordant structural elements, such as faults and volcanic (extrusive) topographic forms. The fault pattern on the western flank apparently mimics an older one, which is discernible in the magnetic anomaly fabric [Olesen et al., 1997] and trends NE-SW.
Our detailed study of bathymetry, single- and multichannel seismic profiles, and modern seismicity for the Knipovich Ridge reveals its discordant attitude with respect to the surrounding features and points to its genesis due to recent, superimposed tectonic processes. Although currently the Knipovich Ridge is an active spreading center with a conspicuous rift valley, numerous active submarine volcanoes, hydrothermal manifestations, and a modern seismicity, it exhibits no constant addition of new seafloor. The extension processes are cyclic, with pulses of dramatically intensified tectonic and magmatic activity alternating with protracted quiescent periods. No synchroneity exists between the extension pulses that occur in various segments of the ridge’s rift zone and which involve normal or listric faulting and emplacement of basaltic extrusions.
The Knipovich Ridge took shape in the deep eastern part of the Norwegian-Greenland Basin in the immediate vicinity of the western Svalbard margin in Miocene time. This corollary is based on our analysis of seismic sections correlated with faunally characterized stratigraphies at deep sea drilling sites.
The above suggests that the Knipovich Ridge has features that are not normal in typical mid-oceanic ridges. Rather, this is a young oceanic rift that came into being in Miocene time but, thus far, has not fully developed as a mid-oceanic ridge structurally.
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