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Abstract

Geophysical data from Fletcher's Ice Island (Station T-3) for the period mid-1962 to mid-1970 show that, during this time, the ice station traversed the Chukchi Rise; portions of the Alpha Cordillera and Mendeleyev Ridge; and the Chukchi, Mendeleyev, and Canada plains. The geophysical findings, together with pertinent observations from older investigations, support the suggestion of earlier investigators that the Alpha Cordillera is an inactive center of sea-floor spreading. Several fractures are shown to cut the Mendeleyev Ridge and Alpha Cordillera, and many other closely spaced fractures are suggested by topographic, magnetic, and gravity trends. These fractures appear to parallel the 142°W meridian. Seismic reflection profiles show a buried topography similar to that of the Mid-Atlantic Ridge. Offsets in the apparent axial rift suggest that the fractures are traces of transform faults. The angular relation between the Mendeleyev Ridge and the Alpha Cordillera appears to result from a southerly displacement of the cordillera crest along numerous en echelon transform faults. Magnetic anomalies are con-sistent with the sea-floor-spreading hypothesis. A crustal gravity model based on a continuous 600-km-long gravity and bathymétrie profile and on one unreversed refraction measurement from Station Alpha shows the observed gravity to be consistent with a section of East Pacific Rise type; a 5-km-thick oceanic layer overlies 27 km of “anomalous” mantle (ρ = 3.15).

A proposed history for the Amerasian basin since late Precambrian time suggests that the basin was affected by spreading at least once in the Paleozoic and again in the late Mesozoic and early Tertiary. The early Paleozoic episode is thought to be related to the opening and closing of a proto-Atlantic ocean and the development of the Appalachian-Caledonian orogen. It is concluded that the oceanic crust beneath the Beaufort Sea is Permo-Carbon-iferous or older.

Seismic reflection profiles show more than 2 km of sedimentary rocks beneath the Mendeleyev and Canada plains, but no basement reflections have been recorded. Pronounced reflectors may represent major climatic or depositional changes. The sedimentary cover on the Alpha Cordillera and the Mendeleyev Ridge ranges from several hundred meters to more than 1 km. Sedimentary ridges (sand waves) up to 55 m high are abundant on the crestai plateau of the Alpha Cordillera; they appear to be the result of currents which transported sediment across the ridge from northwest to southeast. This process is presently inactive, and may have terminated with the initiation of continental glaciation, perhaps as early as late Miocene time. Similar sedimentary features 700 m beneath the Mendeleyev plain suggest a strong bottom circulation in the past. A zone of bottom erosion along the Mendeleyev Ridge flank may reflect a circulation of water through Cooperation Gap, a trough which appears to cross the ridge. Two buried channels extending to subbottom depths of 700 m were observed between the Mendeleyev fracture zone and the Mendeleyev plain.

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