Abstract

The mid-Atlantic Ridge and its northern extension across the Arctic, the Gakkel Ridge, form the eastern boundary of the North American continental plate. The position of the western boundary separating North America from Asia is less certain and has been sought in the Bering Sea region and in Yakutia, Northeast USSR.

Geologic similarities between Alaska and easternmost USSR indicate that these two continental areas have been connected since the Paleozoic and probably since the Precambrian, and that the western plate boundary does not pass through the Bering Strait.

The trend of earthquake epicenters aligned along the Gakkel Ridge continues inland along the Cherskiy system of mountain ranges and depressions. High-angle faulting and volcanism in the Moma-Zyryansk Basin along the east side of the Cherskiy region seem to be a landward extension of the seafloor-spreading tectonics on the Gakkel Ridge. Modern seismic activity, faulting, and volcanism in Yakutia may represent incipient rifting of the Eurasian continent, a situation similar to that in the Gulf of California.

In Yakutia earlier compressional features of late Paleozoic and especially Mesozoic age on trend with the Arctic Ocean opening contrast strongly with the modern tensional movements. Mesozoic rocks of the Verkhoyansk geosyncline on the west are faulted against a Paleozoic geosyncline in the Cherskiy Mountains that passes eastward into platform deposits of the Kolymski massif. Most of the faults are large thrusts that not only bring Paleozoic rocks against Mesozoic rocks, including synorogenic granite masses, but also displace sedimentary facies in the Paleozoic rocks. High-pressure metamorphic minerals are developed along these faults in wide zones of breccia and mylonite.

The geologic history of Yakutia indicates that it was the site of a major seaway that separated the Siberian platform from the Kolymski massif during the early Paleozoic and then again in the early Mesozoic. Deposits of geosynclinal proportions developed along the margins of these subcontinents and were deformed into foldbelts as the seaways closed between the leading edges of the Eurasian and North American continental plates. The boundary between the Cherskiy and Verkhoyansk foldbelts marks the suture of the collision of these plates in Early Cretaceous time.

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