Origin of the marine magnetic quiet zones in the Labrador and Greenland Seas

Abstract

The central part of the northern Labrador Sea is a magnetic quiet zone, and is flanked by regions exhibiting well developed linear magnetic anomalies older than anomaly 24. The quiet zone dies out progressively to the south, where it becomes possible to correlate anomalies between adjacent profiles. A 45 degree change in spreading direction at anomaly 25 time was accompanied by a major jump in ridge position and orientation. As a consequence of this reorganisation, spreading in the northern Labrador Sea next occurred within a rift that was oriented at 45 degrees to the spreading direction, while to the south spreading occurred within in a rift that was orientated at 90 degrees to the spreading direction. Obliquity of spreading changed, between these limits, progressively along the ridge. The quiet zone may be present to the north because the oblique northern geometry resulted in a fragmented ridge composed of many small-offset transform faults joining many short spreading ridge segments. Each magnetic source block produced by magnetisation of sea floor at these small ridge segments will be surrounded by similar small blocks that have opposite polarity, so that none can be resolved at the sea surface. Supporting evidence comes from multi-channel seismic profiles across the Labrador Sea, which show that the basement is more textured within the quiet zone than outside, suggesting the presence of numerous small fracture zones in the quiet zone.

A magnetic quiet zone is present in the northern Greenland Sea between margins that are oblique to the spreading direction. In contrast, there are clear lineated magnetic patterns in adjacent areas to north and south where the margins are orthogonal to the spreading direction. This quiet zone may also be due to the geometry of spreading.