Features of the Canadian polar margin

Abstract

Along the Canadian “passive” polar margin, magnetic and gravity anomalies provide distinctive markers of tectonic features formed as a result of rifting. Interesting changes are noted between areas where gravity highs correspond to magnetic lows and areas where the potential fields are in phase. Offshore, earthquake epicentres correlate mainly with linear negative magnetic anomalies in the vicinity of major free-air gravity highs. The elliptical gravity highs appear to mark sedimentary depocentres along the margin. Seismic activity focussed along the linear, magnetic lows in the vicinity of the gravity highs indicates active tectonic adjustment along rift margin structures, possibly triggered by loading effects of the depocentres. A zone of negative magnetic anomalies, collectively named the Polar Shelf Magnetic Anomaly (PSMA), helps delineate the regional rift geometry of the Canadian polar margin from north of Ellesmere Island to the Beaufort Sea. Parts of the PSMA appear similar to magnetic lows observed along the Atlantic passive margin off North America. From (1) changes in margin strike, (2) changes in the character and relationship between potential field anomalies and (3) the pattern of extensional features across the northern Sverdrup Basin, it is suggested that margin rifting varied along strike. Two principal controlling factors were pre-existing basement features and the development of the Alpha Ridge. The Beaufort Sea is bisected by a central linear magnetic low. The crust of the western Beaufort Sea, between the central low and Alaska, has different structural (magnetic) and dynamic (seismicity) properties compared to the crust to the east. Positive magnetic trends in the western Beaufort Sea are conformable with the regional arcuate tectonic trends of the northeastern Alaska to western Yukon terrane and the adjacent offshore. The regional rift geometry of the Canadian polar margin and the east-west differences in the nature of the crust beneath the Beaufort Sea are constraints to be considered in tectonic reconstructions.