Arctic geophysical review - a summary

Abstract

Seismic refraction experiments, magnetotelluric soundings and analyses of the magnetic anomaly field indicate tha1 the continental shelves bordering the Arctic Ocean are covered by sedimentary sequences that are 2 km to over 10 km thick. Similar data indicate that the Chukchi Plateau is a continental fragment composed largely of sedimentary material. The presence of thick sedimentary loads may be responsible for the free-air gravity anomaly highs and the low-level seismicity along parts of the North American polar margin. The Arctic abyssal plains are covered by a 0.5 to 6 km thick subhorizontally bedded sedimentary carpet. The seismically active Nansen-Gakkel Ridge is composed of a series of narrow peaks and troughs with up to 2.5 km of relief. Heat flow values over the ridge are typical of actively accreting margins but the associated magnetic anomaly amplitudes are less than expected. The magnetic anomaly pattern over the Greenland end of the ridge indicates that spreading rates have been slow ( <10 nun/a) over the last 20 Ma with little evidence of transform offsets greater than a few kilometres. Seismic reflection results and magnetotelluric soundings together with heat flow and magnetic anomaly data suggest that the Lomonosov Ridge is composed of stratified sedimentary units. Magnetic anomalies over the Alpha Ridge are of high amplitude and the sediment-corrected relief of the ridge is 2900 ± 500 m. The Arctic Ocean basin originated during Early Cretaceous time when the Arctic-Alaska plate (northern Alaska and the Chukotsk Peninsula) rotated 70 degrees anticlockwise away from Arctic Canada and thereby opened the Amerasia Basin. This major rotation resulted from the compression between the Arctic-Alaska and Eurasia plates that was a consequence of the early opening of the North Atlantic about a pole in northern Greenland. The margins of the Arctic-Alaska plate developed along existing zones of weakness. This compressional regime could have produced the Alpha and Mendeleev Ridges by sea-floor buckling or by subduction. Alternatively, the ridges could be continental fragments or volcanic accumulations left in the wake of the rotating Arctic-Alaska plate. The present relief of the Alpha Ridge precludes it being a spreading centre during Late Cretaceous time. At this time sediments began to accumulate within the Amerasia Basin. In Late Cretaceous-Early Tertiary time the pole of rotation for the opening of the North Atlantic moved into northeastern Siberia. The Lomonosov Ridge is a fault block of continental crust rifted from the polar shelf of Eurasia as a new accreting margin developed. Sea-floor spreading activity has been intermittent, and the Nansen-Gakkel Ridge is the recent location of the accreting margin.