443 Geotectonics, Vol. 38, No. 6, 2004, pp. 443–458. Translated from Geotektonika, No. 6, 2004, pp. 49–66. Original Russian Text Copyright © 2004 by Piskarev. English Translation Copyright © 2004 by MAIK “Nauka /Interperiodica” (Russia). INTRODUCTION The origin and character of the main geological structures making up the bottom of the Arctic Ocean remain enigmatic. There are several causes for the lack of knowledge about these structures. First, none of the prospect wells drilled in the deep-sea part of this oce- anic basin penetrated into the basement. Second, the geophysical data remain insufficient: deep seismic pro- filing is scarce, aeromagnetic survey was carried out based on a wide-spaced network and with a low accu- racy of tying, and gravity measurements are extremely irregular. At the same time, the available morphological and geological evidence indicate an intricate and pro- longed multistage formation of oceanic structures. An adequate reconstruction of their evolution will be pos- sible only after additional geological–geophysical stud- ies. However, the planning of these works itself requires discussion of the key problems related to the regional geology. Until recently, the Eurasia Basin and Lomonosov Ridge were almost completely ignored in discussions concerning the composition, structure, and geological history of main structures in the deep-sea part of the Arctic Ocean. When discussing the evolution of the region, most of researchers share the view that the Eur- asia Basin was formed owing to slow spreading with the axis localized throughout the entire Cenozoic era in the area of the present-day Gakkel Ridge. This hypoth- esis has remained unchanged since the discovery of the system of linear magnetic anomalies in the Eurasia Basin similar to anomalies of other mid-ocean ridges [10] and the completion of the first cycle of aeromag- netic survey in the Arctic Ocean [26, 29, 30]. The pub- lications of the ICAM III Conference provided addi- tional arguments favoring this hypothesis [20, 27]. It is assumed that the spreading occurred gradually during the entire Cenozoic era, and that its rate, initially approximating 2.0 cm/yr, began to decrease in the Eocene and become equal to 0.5 cm/yr in the Oli- gocene, thus making the Gakkel Ridge the slowest- growing structure among mid-ocean ridges [19]. In the opinion of the previously mentioned authors, the low spreading rate results in the extremely complex mor- phology of the basement beneath the Eurasia Basin. In terms of the concept in question, the Lomonosov Ridge is considered as a fragment of the previously The Basement Structure of the Eurasia Basin and Central Ridges in the Arctic Ocean A. L. Piskarev Institute of the Lithosphere of Marginal Seas, Russian Academy of Sciences, Staromonetnyi per. 22, Moscow, 109180 Russia e-mail: piskarev@ru.ru Received December 26, 2003 Abstract—The study area in the Arctic Ocean comprises the Eurasia Basin, Lomonosov Ridge, Podvodnikov and Makarov basins adjacent to the Amerasia Basin, and a system of the Alpha and Mendeleev ridges. It is widely accepted that slow spreading, the axis of which corresponded to the Gakkel Ridge, was responsible for the formation of the Eurasia Basin during the entire Cenozoic. Thereby the Lomonosov Ridge is considered a fragment of the former Barents–Kara continental margin that separated from it at the Late Cretaceous–Ceno- zoic boundary. However, numerous geophysical and bathymetric data come into direct conflict with this con- cept. Asymmetry in depth distribution, asymmetrical magnetic and gravity anomalies in the eastern part of the Eurasia Basin, and peculiarities in the spatial distribution and thickness of a number of seismic units—these all testify to a more complex geological history of the region. The analysis of geophysical and bathymetric infor- mation is aimed at the reconstruction of the consecutive processes that resulted in the present-day structures of the Eurasia Basin and Lomonosov Ridge. The study included amplitude–frequency analysis of potential field anomalies with application of two-dimensional Fourier spectra, correlation of magnetic anomalies and bottom topography, and model calculations along profiles transecting deep-sea structures. As a result, it has been estab- lished that the Eurasia Basin segment located east of 75° E was formed in the pre-Miocene time, because the spreading axis was localized substantially closer to the Lomonosov Ridge than the present-day Gakkel Ridge. The Gakkel Ridge, 160–200 km wide, is a structure superimposed on the pre-Miocene basement after a jump of the spreading axis. Normal continental crust underlies the Lomonosov Ridge near the Eurasian continental margin, whereas the basement of the central ridge segment is mainly composed of basic igneous rocks. The basement beneath the Alpha–Mendeleev ridge system incorporates bodies of mafic and ultramafic rocks. The upper part of the Alpha Ridge section probably contains flood-basalt members similar in their magnetic prop- erties to flood basalts of Franz Josef Land.