SIBERIAN ORIGIN OF PALEOZOIC ROCKS OF ARCTIC ALASKA-CHUKOTKA AND SOME SOUTHERN ALASKAN TERRANES: EVIDENCE FROM MEGAFOSSIL AND PALEOMAGNETIC DATA

New data for the Alexander terrane of Southeast Alaska, the Farewell terrane of Southwest Alaska, and the Arctic Alaska-Chukotka superterrane indicate a Siberian origin distinct and separate from the northern Cordillera through at least Triassic time.

Arctic Alaska-Chukotka superterrane: Central Brooks Range Middle Cambrian trilobites are strictly of Siberian affinity, as are Upper Ordovician trilobites and other fauna from the Shublik Mountains, Baird Mountains, and Seward Peninsula (York Mtns.). Middle Devonian (Givetian) stringocephalid brachiopods from Arctic Alaska are close to or identical to stringocephalids from northeast Russia and the Alexander terrane. Brooks Range Upper Triassic bivalves and brachiopods from Arctic Alaska show closer affinities to northeast Siberia than to northwest Laurentia.

Farewell and Alexander terranes: Farewell terrane Middle Cambrian trilobites are strictly of Siberian aspect. An algal barrier reef defining the Farewell terrane outer margin during Late Silurian time contains aphrosalpingid sponges known also from the Alexander terrane, Ural Mountains, and Kuznetsk Basin. Farewell terrane Lower Devonian brachiopods share many species with Siberia and none with North American cratonic rocks, and compare closely with coeval faunas from the Alexander terrane, northeast Russia (Kolyma) and Taimyr. The Farewell terrane Middle Devonian contains a Eurasian-like fauna recognized in the Alexander terrane and Arctic Alaska.

The Alexander terrane contains 11 poles of mainly Ordovician to Triassic age that show low latitudes, mostly 10°-20°, that cannot be matched with Baltica or Laurentia during most of the Paleozoic since these continents varied in latitude during this time span. For Kolyma-Omolon there is one Late Devonian pole of recent vintage (Kolesov and Stone 2002) compatible with a location near Siberia and the Alexander terrane; other poles are older Russian data that differ from the Alexander data. Review of paleomagnetic data suggests the Alexander and Kolyma-Omolon terranes were closely related to each other and Siberia. Both likely rifted from Siberia in the Late Devonian, then separated from each other by Permian time (~280 Ma) before accretion to the Siberian Platform (Kolyma-Omolon) and to North America (Alexander terrane) by the end of the Jurassic.