A GRAVITY AND MAGNETIC ANOMALY STUDY OF THE EXTINCT AEGIR RIDGE, NORWEGIAN SEA

In 1990 the U.S. Naval Research Laboratory mapped the extinct Aegir Ri dge in the Norwegian Sea with SeaMARC II side-scan sonar, Hydrosweep m ultibeam, seismics, gravity, and magnetics onboard R/V Maurice Ewing. We analyzed the 92 center beam bathymetry, gravity, and magnetic profi les in this study to examine slow spreading ridge processes and cessat ion of spreading about 25 Ma. In bathymetric expression the rift valle y deepens and narrows southward, while the rift mountain summits becom e higher. South of similar to 67 degrees N, the western rift mountains and flanks stand higher than the eastern mountains and flanks. Free-a ir anomalies along the ridge axis show undulating intermediate wavelen gth (similar to 100 km) anomalies superimposed on a regional decrease of the rift valley negative from similar to 0 mGal in the north to -55 mGal in the south. After removal of sediment effects, results from th ree-dimensional gravity modeling imply a variation of crustal thicknes s. A new rotation pole for the movement of the Jan Mayen microplate re lative to the Eurasia plate from similar to 50 Ma to 25 Ma was found a t 64 degrees 34/N, 13 degrees W, based on magnetic lineations and infe rred discontinuity traces. A plot of angular separation between signif icant anomaly peaks versus absolute age suggests an opening rate of si milar to 2.0 degrees/m.y. (>36 Ma) followed by gradual deceleration to 0 at 25 Ma. Full spreading rate between chrons C13n and C21n ranged f rom similar to 13 mm/yr in the north to 8 mm/yr in the south.