A gravity and magnetic anomaly study of the extinct Aegir Ridge, Norwegian Sea

Abstract

In 1990 the U.S. Naval Research Laboratory mapped the extinct Aegir Ridge in the Norwegian Sea with SeaMARC II side‐scan sonar, Hydrosweep multibeam, seismics, gravity, and magnetics onboard R/V Maurice Ewing. We analyzed the 92 center beam bathymetry, gravity, and magnetic profiles in this study to examine slow spreading ridge processes and cessation of spreading about 25 Ma. In bathymetric expression the rift valley deepens and narrows southward, while the rift mountain summits become higher. South of ∼67°N, the western rift mountains and flanks stand higher than the eastern mountains and flanks. Free‐air anomalies along the ridge axis show undulating intermediate wavelength (∼100 km) anomalies superimposed on a regional decrease of the rift valley negative from ∼0 mGal in the north to −55 mGal in the south. After removal of sediment effects, results from three‐dimensional gravity modeling imply a variation of crustal thickness. A new rotation pole for the movement of the Jan Mayen microplate relative to the Eurasia plate from ∼50 Ma to 25 Ma was found at 64°34′N, 13°W, based on magnetic lineations and inferred discontinuity traces. A plot of angular separation between significant anomaly peaks versus absolute age suggests an opening rate of ∼2.0°/m.y. (>36 Ma) followed by gradual deceleration to 0 at 25 Ma. Full spreading rate between chrons C13n and C21n ranged from ∼13 mm/yr in the north to 8 mm/yr in the south.