CRUSTAL ACCRETION AT THE REYKJANES RIDGE, 61-DEGREES-62-DEGREES-N

We report results of a seismic, gravity, and magnetic survey pf the Re ykjanes Ridge spreading center at 61 degrees-62 degrees N, about 600 k m from the center of the Iceland mantle plume. Anomalously shallow wat er on the ridge crest enabled us to record seismic refractions on a 2. 4 km hydrophone streamer. The velocity within layer 2A is 2.4 +/- 0.3 km s(-1), and its mean thickness is 400 +/- 100 m. The velocity at the base of layer 2A is 3.3 +/- 0.3 km s(-1) on the ridge axis, increasin g with crustal age to similar to 4.0 kms(-1) at 1.5 Ma and similar to 4.5 km s(-1) at 5 Ma. Assuming that seismic layer 2A on the ridge axis Is also the extrusive layer, i.e., the magnetic source layer, we have successfully modeled the variations in amplitude of the magnetic fiel d. The best magnetic model includes enhanced magnetization within laye r 2A at the sites of recent volcanic activity as independently recogni zed in side-scan sonar data. We also present a full crustal seismic mo del, based on wide-angle seismic recordings on digital ocean bottom hy drophones and disposable sonobuoys. The seismic model is complemented by gravity modeling, which further suggests that the ridge crest is in isostatic equilibrium. The zero age crust. is 10.0 km thick, while cr ust of age 5 Ma is 7.8 km thick. These crustal thicknesses are greater than those of normal oceanic crust, which we attribute to the presenc e of anomalously hot asthenospheric mantle beneath the spreading cente r. We suggest that the variation in thickness between 0 Ma and 5 Ma cr ust is caused by temporal variation in the plume-fed asthenospheric te mperature beneath the Reykjanes Ridge.