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

Abstract

Abstract. We have inverted the sea surface total magnetic field measurements of the Reykjanes Ridge, between 57øN and 63øN, 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 fastspreading 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 ø 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 59ø10'N and 60ø50'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 60ø50'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 57ø50'N, 58ø05'N, and 59ø12'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. IntroductionOur understanding of the structure of mid-ocean ridges and the process by which melt upwells and solidifies to form a new crust has improved steadily over the last two decades.

Temporal and spatial cyclicity of accretion at slow-spreading ridges-evidence from the Reykjanes Ridge

“…This central anomaly magnetic high (CAMH), which is superimposed on the normal polarity Brunhes anomaly, marks the focus of most recent extrusive activity. Lee & Searle (2000) state that the CAMH is not continuous along‐axis on a small scale (<10 km). Instead discrete highs and lows in intensity are associated with AVR‐scale features, which appear to be associated with either the age of extrusives and/or variations in layer 2A thickness (assumed to be the magnetic source layer in common with most studies—e.g.…”

“…Lee & Searle (2000) show that no more than 50 per cent of the observed short‐wavelength anomaly can be attributed to on‐axis layer 2A thickness variations (see also Smallwood et al 1995; Navin et al 1998; Topping 2002). As oxidation of titanomagnetite results in decay of magnetization intensities with age, the shorter‐wavelength anomalies can be assumed to reflect local age variations of the extrusive layer.…”

“…The results presented in this paper are based upon the analysis and interpretation of geophysical data acquired during a number of research cruises undertaken between 1990 and 1998 (syntax: cruise number/year— RRS Charles Darwin CD81/93, R/V Maurice Ewing EW9008/90, RRS Charles Darwin CD87/94 and RRS Discovery D235c/98—Parson 1993; Searle et al 1994b; Sinha et al 1994; Peirce & Sinha 1998, respectively). The bathymetry data from the first three of these forms the basis of the compilation by Keeton et al (1997), while the corresponding magnetic data form the basis of the compilation of Lee & Searle (2000).…”

“…Combined interpretation of the RAMESSES results (Sinha et al 1998) suggests that this AVR is at the young to adolescent stage. However, its low magnetization, combined with a number of local highs over small topographic features, led Lee & Searle (2000) to suggest that this AVR may be older and undergoing late‐stage volcanism. The long length of this AVR, its broad convex shape, and the presence of cratered conical and flat‐topped volcanoes, suggest a more mature phase of the cycle and that the recent extrusive volcanism is overprinting faulting on the AVR giving it a younger appearance.…”

Propagating rift model for the V‐shaped ridges south of Iceland

“…The active A propagator is continuing south near 59°N, where the axial ridge changes to an axial valley, and the active A′ propagator tip is near 61.7°N. Two of these tips correlate with high‐amplitude magnetic anomalies, and the 3rd is in a data gap [ Searle et al , 1998; Lee and Searle , 2000]. The large outward facing conjugate VSR scarps such as Vogt's A and E scarps are pseudofaults formed symmetrically at their corresponding propagating rift tips.…”