Geophysical Evidence for Ancient Sea-Floor Spreading from Alpha Cordillera and Mendeleyev Ridge: ABSTRACT

Abstract

This paper presents the geophysical findings from Fletcher's Ice Island (T-3) for the period 1962 to mid-1970. During this time the station traversed the Chukchi Rise, parts of the Alpha Cordillera and Mendeleyev Ridge, and the Chukchi, Mendeleyev, and Canada plains. The findings support the suggestion of earlier investigators that the Alpha Cordillera is a fossil center of sea-floor spreading. Five fractures were observed to cut the Mendeleyev Ridge and the Alpha Cordillera, and many other closely spaced fractures are suggested by topographic, magnetic, and gravity trends. 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 transform faults. The angular relation etween the Mendeleyev Ridge and the Alpha Cordillera appears to result from a southerly displacement of the Alpha Cordillera crest along numerous en echelon transform faults. Magnetic anomalies are consistent with the sea-floor spreading hypothesis, but no spreading dates are available. A broad zone of low amplitude anomalies over the central and southern Canada Plain might represent the Kiaman Magnetic Interval in Permian time or similar quiet intervals in the early Mesozoic. A crustal gravity model based on a 600 km long gravity and bathymetric profile and an unreversed refraction measurement from Station Alpha shows the observed gravity to be consistent with a section of East Pacific Rise type with a 5 km thick oceanic layer overlying 27 km of anomalous (p = 3.15) mantle. The relation of this ridge to the surrounding continental geology is explored. It is suggested that this ridge generated a sea floor which was consumed by marginal trenches along the Lomonosov Ridge (then the northern margin of the European continental block), the northern Alaskan coast, and the Canadian Archipelago. Sea-floor generation and consumption apparently ended with the separation of Greenland from Labrador in late Mesozoic time. The age of the basin is reflected in the large sediment thickness observed. Seismic reflection profiles show more than 2 km of sediment beneath the Mendeleyev and Canada plains, with prominent reflectors suggesting major climatic or depositional changes. Sediment cover on the ridge varies from several hundred meters to more than a kilometer. Sedimentary ridges blank t the crestal plateau of the Alpha Cordillera, apparently the result of currents which transport sediment across the crest from northwest to southeast. This process is presently inactive, and may have terminated with the initiation of continental glaciation as far back as late Miocene time. Similar sedimentary structures 700 m beneath End_Page 2483------------------------------ 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 the 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. End_of_Article - Last_Page 2484------------