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When analyzing the nature of Oligocene events reflected in sedimentation types and volcanism in the NW Pacific, as well as the Aleutian island arc and in regions of the Asian framing, two events call our attention.
The first is the completion of volcanism on vast areas at the boundary between the Late Eocene and Oligocene that was expressed in cessation of island-arc ash input to oceanic sediments (Obruchev Rise, BH 883, 884), interruption of volcanic eruptions in the Aleutian island arc [2], as well as on Kamchatka eastern peninsulas and in the southern Olyutorsk zone of the Koryak Highland.
Simultaneous cessation of volcanism prior to the Oligocene was not accompanied by folding. Volcanism has no longer reactivated within the western folded framing and after the Oligocene break it again showed up since the Miocene in the eastern part of the Aleutian arc.
The second conspicuous event that showed up since the Early Oligocene was the drift of exotic terrigenous material mainly of the continental genesis and remains of diatom forms not typical of oceanic environments to sediments that formed on the Obruchev Rise (Meidji drift body) [1]. Furthermore, Buyanovsk conglomerates on the Bering Island formed in the Late Eocene. Pebbles are found in the conglomerates, and later horizons comprise exotic psammitic material. Similar conditions are known in sedimentary melange of the Karaginski Island [3]. Adjacent areas of Asia cannot serves as sources of continental psephitic and psammitic material.
The system of currents, which formed since the earliest Oligocene due to global cooling, determined the continental material drift from the eastern part of the Bering Sea and has not changed until the present time [4]. It follows from these inferences that the morphological structure of uplifts and depressions that existed since the Oligocene retained similarity as well. Although, neither the formation of a new current system nor limited variations in morphostructures could account for the occurrence of continental clastic materials in Oligocene deposits of the Karaginski and Bering islands and, the more so, had effect on volcanic processes. It can be assumed that two Oligocene events not related to one another---cessation of volcanism and continental material drift from the east at the retaining similar morphostructure in the NW Pacific---are determined by the lateral motion of lithospheric plates.
1. Gladenkov A. Yu. Transactions of Geological Institute, vol. 571, Moscow, GEOS, 2007, 294 p. 2. Jicha B. R., Scholl D. W., Singer B. S. and Yogodzinski // Geology, 2006, vol. 34, no. 8, pp. 661-664. 3. Ledneva G. V., Garver J. I., Shapiro M. N., Lederer J., Brandon M. T., and Hollocher K. T. // Russian Journal of Earth Sciences, 2004, vol. 6, no. 2, pp. 1-28. 4. Scholl D. V., Stevenson M. A., and Rea D. K. // From Greenhouse to Icehouse, Columbia Univ. Press, New York, 2003, pp. 119-153. RFBS grants nos. 08-05-00539, 08-05-00019;
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