Arctic Midocean Ridge

Within the deep basin of the Arctic Ocean, several submarine ridges and plateaus rise above the ocean floor (see the bathymetric map in Arctic Ocean). The largest of these, the Gakkel Ridge, is related to the global system of mid-ocean ridges, formed from the rifting and growth of oceanic crust at a plate tectonic boundary.

Exploration of the Arctic Ocean in the 1950s and 1960s by the Russian drifting ice stations and air expeditions led to the discovery of the Lomonosov Ridge in 1951, soon followed by the discovery of two more transoceanic ridges, Gakkel Ridge and the Alpha-Mendeleev Ridge. While the Lomosonov and Alpha ridges are poorly understood but appear to be of continental origin, the Gakkel Ridge is a true mid-oceanic ridge that forms the most northerly segment of a spreading ridge system that runs from Iceland, Jan Mayen, Mohns Ridge, and Knipovich Ridge, to the Gakkel Ridge.

The Gakkel mid-ocean ridge lies about 5 km beneath the Arctic ice cap and open sea, and extends for a length of 1800 km from the Laptev Sea continental shelf to northeast Greenland. The ridge separates the Nansen and Amundsen basins, abyssal plains located at depths of 4000 and 4300 m, respectively. Lomosonov Ridge lies to the east, followed by the Makarov Basin, Alpha-Mendeleev Ridge, then the Canadian Basin, and Chukchi Plateau.

The morphology of the Gakkel Ridge is primarily determined by magmatic and tectonic processes at a spreading mid-ocean ridge. However, Gakkel Ridge is the slowest-spreading ridge in the world—less than 1

cm per year, compared to about 6 cm per year for intermediate spreading ridges elsewhere, and the oceanic crust is anomalously thin, less than 3.3 km compared to the more usual 7 km. Along the axis there is a typical deep rift valley, which is up to 4500 m deep, while the rift walls (although asymmetric) have relief up to 2000 m. The rift floor is uneven with evidence of isolated volcanoes up to 2 km in diameter. Dredging has recovered recent basalt lavas in the east and west rift valley and mantle peridotites (but no basalt) in the central 300 km zone. The rift valley is not offset by transform faults, as the spreading rate is perpendicular to the axis. On the top parts of the Gakkel Ridge, sedimentary cover deepens from a thin covering at the axial crest (the youngest) to 300-400 m depth away from the axis. The rift valley has numerous earthquake epicenters at depths of 0-6 km, with the magnitude of earthquakes approximately M=4.5-6.

Determining the plate boundaries and transition to the continental margin on the outskirts of the ridge is difficult due to abutment of the end of the Gakkel Ridge with the Laptev Sea continental shelf and the indefinite position of the ridge between Greenland and the Yermak Plateau. Bathymetric contours suggest that, at least along 350 km, the termination of the Gakkel Ridge is situated on the continental margins of Eurasia. Increased sedimentation in the region of this margin and equal distribution of sedimentary cover on the slope has resulted in a smooth, concave surface. In this case, it is impossible to establish the boundaries of the continental shelf.

At the other end of the Gakkel Ridge between Spitsbergen and Greenland, where its rift valley reaches a depth of 4300 m, the rift axis shifts 100 km through a series of transform faults over a 50 km segment of ridge to the rift valley of the Knipovich Ridge. Mohns Ridge meets Knipovich Ridge at a highly oblique angle northeast of the Jan Mayen Fracture Zone. Mohns Ridge, also a slow-spreading ridge, has typical ridge features such as a central rift valley 1-2 km lower than the surrounding axial ridge, is seismi-cally active, and has a thin sediment cover that thickens away from the axis. However, it is anomalous, showing spreading centers oblique to the trend of the axis, in en echelon segments of the ridge that might be occupied by transform faults on other ridges. The crustal thickness beneath the ridge is also very low (on average 4 km) compared to typical oceanic crust.

Valery Mit'ko

See also Alpha Ridge; Lomonosov Ridge Further Reading

Coakley, B.J. & J.R. Cochran, "Gravity evidence of very thin crust at the Gakkel Ridge (Arctic Ocean)." Earth and Planetary Science Letters, 162(1/4) (1998): 81

Edwards, M.H. et al., "Evidence of recent volcanic activity on the ultra-slow spreading Gakkel Ridge." Nature, 409 (2001): 808-812

Gorbatskiy, G.V., Physicogeographical Zoning of Arctic. Volume 3, Arctic Basin, Leningrad: Leningrad University Publishing House, 1973 Gramberg, I.S. (editor), Orographic map of Arctic basin.

1:5000000, Helsinki: Karttaneskus, 1995 Jokat, W., O. Ritzmann, M.C. Schmidt-Aursch, S. Drachev & S. Gauger, "Geophysical evidence for reduced melt production on the Arctic ultraslow Gakkel mid-ocean ridge." Nature, 423 (2003): 962-967 Klingelhöfer, F., L. Geli, L. Matias, N. Steinsland & J. Mohr, "Crustal structure of a super-slow spreading centre: a seismic refraction study of Mohns Ridge, 72° N." Geophysical Journal International, 141(2) (2000): 509-526 Michael, P.J., C.H. Langmuir, H.J.B. Dick, J.E. Snow, S.L. Goldstein, D.W. Graham, K. Lehnert, G. Kurras, W. Jokat, R. Mühe & H.N. Edmonds, "Magmatic and amagmatic seafloor generation at the ultraslow-spreading Gakkel Ridge, Arctic Ocean." Nature, 423 (2003): 956-961 Perry, R.K., H.S. Fleming, J.R. Weber, Y. Kristofferson, J.K. Hall, A. Grantz & G.L. Johnson, Bathymetry of the Arctic Ocean; map 1:4,704,075, Washington: Naval Research Laboratory, 1985 Sweeney, J.K, J.R. Weber, & S.M. Blasko, "Continental ridges in the Arctic ocean: Lorex constraints." Tectonophysics, 89 (1982): 217-238 Weber, J.R., Exploring the Arctic Sea Floor in Selected Lorex Contributions, Ottawa, 1985

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