HR: 14:25h
AN: GP43C-04    [Abstracts]
TI: Phanerozoic Tectonic Evolution of the Chukotka-Arctic Alaska Block: Problems of the Rotational Model
AU: * Natal'in, B A
EM: natalin@itu.edu.tr
AF: Istanbul Technical University, Faculty of Mines, Ayazaga, Istanbul, 34469 Turkey
AB: Correlation of tectonostratigraphic units across the Bering Strait suggests that the northern Chukotka including most of the East Siberian Shelf as well as the Brooks Range, Colville Basin, Beaufort Shelf, and Seward Peninsula on the North American side represent a large continental block. The core of this block consists of the Neoproterozoic Bennett-Barrovia block (BBB) that is overlain by the Ordovician-Devonian Novosibirsk carbonate platform. The basement of the block is exposed in the Chukotka Peninsula where orthogneiss yielded Late Proterozoic (650 to 550 Ma) U-Pb ages. These dates are comparable with the age of 699 Ma reported for granites in the Wrangel island. Granites of the same age intrude metasedimentary and metavolcanic rocks in the Hammond subterrane in northern Alaska. In the western part of the BBB, geophysical data imply a presence of a crystalline Precambrian basement at shallow depth beneath the Novosibirsk Archipelago. Weak deformation and consistency of facies of Ordovician-Devonian shelf and lagoon carbonates indicate that the BBB evolved as a rigid structure. A similarity of the Ordovician fossils with Siberia allows an inference about a close location of these two continental entities. The BBB forms a backbone of the Chukotka-Alaska block. Since the early Paleozoic, it started to grow at the expense of subduction-accretion. Ordovician and Silurian oceanic and island arc rocks mark the northern boundary of the BBB. This subduction boundary stopped its development after the late Silurian-early Devonian collision of the BBB and the North America craton. Subduction along the southern boundary of the BBB is recorded by a Devonian-early Carboniferous magmatic arc (granites yielding 360-398 Ma) and an extensional (backarc) basin. Perhaps, this subduction was terminated by a middle Carboniferous collision. The Triassic extension along the same boundary caused formation of the South Anyui ocean and a wide passive continental margin exposed in the northern Chukotka. The early Cretaceous closure of the ocean was followed by longitudinal shortening of the Chukotka-Arctic Alaska block as it is evident from orogen-parallel strike-slip faults and oroclinal bending of structural trends. The Bering Sea orocline accounts for almost double shortening of the original length of the Chukotka-Arctic Alaska block between the Chukotka and Seaward peninsulas. This shortening caused thickening of the crust and its extensional collapse that led to exhumation of metamorphic complexes. The reviewed tectonic structure and history impose constrains on geometry of the Chukotka-Arctic Alaska block. In many tectonic models, its counterclockwise rotation because of the opening of the Canada Basin and the following collision with the mainland of Asia is considered as the primary mechanism of the Cretaceous orogeny. However, the present day length of the BBB stretching from the Alaska-Canada border in the east to the Novosibirsk Archipelago in the west contradicts to this simple scenario. The original length of the BBB must be longer if the Cretaceous longitudinal shortening is restored. To overcome the space problem it is suggested that in the late Mesozoic the BBB moved right laterally along the North American margin and that the opening of the Canada Basin is a consequence of this giant strike-slip motion.
DE: 8110 Continental tectonics--general (0905)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting