Cretaceous and Cenozoic spreading of the northern Atlantic basins steps via transform faults to the Arctic Ocean, where simple spreading of the Eurasia basin appears highly probable, and more complex opening of the polar half of the Amerasia basin by spreading of Alpha and Mendeleyev Ridges appears likely. The Alaskan half of Amerasia basin may have opened behind counterclockwise-rotating Alaska, as proposed by S. W. Carey; this accounts for many features, including the provenance and northern source of clastic upper Paleozoic and lower Mesozoic sediments of northern Alaska.

As no young subduction zones are evident around the Arctic Ocean, these spreading motions must be matched by continental deformation and transform faults in Alaska and northeastern Siberia. A transform fault from the Eurasia basin may cross the East Siberian shelf, displacing the New Siberian Islands from Taimyr and separating Wrangel Island and northeastern Chukotka from mainland Siberia. Further deformation is absorbed by clockwise oroclinal rotation of the Verkhoyansk geosyncline south of this fault.

Reversing these motions indicates that the late Paleozoic and early Mesozoic Verkhoyansk, Wrangel, and Brooks geosynclinal terranes were parts of a continuous continental shelf, facing the open Pacific Ocean. The Laurasian continent fringed by this shelf was an aggregate of North American, European, and Siberian plates that had collided in Paleozoic time as Caledonian and Uralian oceanic plates vanished beneath them.

The stability of the Verkhoyansk-Brooks shelf ended when Jurassic subduction inaugurated conveyor-belt accretion at the Pacific continental margin, and magmatism above Benioff zones. Lena River and northern Alaska foreland basins, superimposed on the old continental shelf, received sediments from the new Pacific mountain systems concurrently with thin-skinned overthrusting. In middle Cretaceous time, the Verkhoyansk belt was wrapped into a compound orocline. Later Cretaceous and Cenozoic subduction produced successively the Okhotsk-Chukotsk, Kamchatka-Koryak, and Kuril-Aleutian systems. Central and southern Alaska may consist largely of debris (including continental fragments and island arcs) swept in since Triassic time on oceanic plates, and of successor-basin deposits and Benioff-z ne magmatic rocks.

End_of_Article - Last_Page 2484------------