Crustal magnetization of the Reykjanes Ridge and implications for its along-axis variability and the formation of axial volcanic ridges

Abstract

We have inverted the sea surface total magnetic field measurements of the Reykjanes Ridge, between 57 degrees N and 63 degrees N, for crustal magnetization intensity assuming a uniformly thick magnetic source layer and direction of magnetization corresponding to the geocentric axial dipole field. This section of the north Mid-Atlantic Ridge (10 mm/yr half spreading rate) is unique in that it lies within the influence of the Iceland mantle plume, which causes it to spread obliquely and in the north to resemble morphologically a fast-spreading ridge. On a regional scale the axial magnetization anomalies of the Reykjanes Ridge can be divided into three regimes from south to north. These regimes correspond to sections of the ridge with different magmatic and tectonic styles which are believed to represent the effects of a pulsing plume beneath Iceland. The axis south of 59 degrees 10'N has the highest magnetization anomaly with the greatest variability and exhibits the characteristics of a slow-spreading ridge unaffected by a plume. The axis between 59 degrees 10'N and 60 degrees 50'N has the lowest magnetization anomaly on average and is where we presume the present day plume front is located. The absence of large fault scarps and low magnetization are consistent with the idea of a midcrustal magma reservoir, where mixing causes the magma to become less highly fractionated and produces less magnetic lavas. Finally, a moderate increase in the magnetization anomaly and degree of faulting occurs north of 60 degrees 50'N, signaling a gradual recovery in the crustal production and underlying structure after the passage of the plume front. On the local scale, with a few exceptions, the loci of high magnetization anomalies coincide with the axial volcanic ridges. Notable lateral offsets in axial magnetization and decay in magnitude were found at 57 degrees 50'N, 58 degrees 05'N, and 59 degrees 12'N. Comparisons with seismic layer 2A show that variations in the magnetic source layer thickness may account for as much as half of the observed variability in the along-axis magnetization. There are also indications that the relative age of the extrusives and the degree of faulting and fissuring may be factors affecting the magnetization intensity of volcanic edifices on the seafloor.