Crust–mantle transition and Moho model for Iceland and surroundings from seismic, topography, and gravity data

Abstract

An increasing data set exists on the nature and thickness of the Iceland crust. This paper relates topography, i.e., elevation and bathymetry (TOP), Bouguer gravity anomalies (BA) and Moho depths to each other to assess the consequences of the “thick crust model” for Iceland in the context of the North Atlantic. Results of regression of TOP and BA vs. Moho depth are converted into “Airy densities” Δ ρ* (mantle crust density contrasts in the case of ideal Airy isostasy). For Iceland, Δ ρ* is very low (105±10 kg/m 3); for the adjacent continental margins and relicts, it is high and intermediate for the Jan Mayen Ridge. The values are affected by lithosphere cooling and systematic variations of internal crustal structure, and reductions for these effects are applied. For Iceland, the thermal reduction leads to slightly enhanced value of Δ ρ* (112±8 kg/m 3), thus part of the observed topography is due to the cooling effect, while the upper crust reduction reduces the value, implying that part of topography is compensated by internal crustal structure. With both reductions combined, Δ ρ* is 103±10 kg/m 3. The differences between gravity- and topography-derived values are compatible with undulation widths similar to Moho depths (half-wavelengths of order 30 km). Regional variations of Δ ρ* values in Iceland suggest differences in crustal generation and evolution. “Modern” Iceland (MI; surface lava ages ≤3.1 Ma) produced from the currently active and recent axes is remarkably homogeneous while the extinct Skagi–Snaefellsnes zone has extremely low values and the older Westfjords and East Iceland regions have slightly enhanced values. The Iceland–Faeroe Ridge (IFR) resembles Iceland (after thermal reduction) while the Greenland–Iceland Ridge has extremely high values. The Iceland “Moho” and the Iceland “lower crust” are special features of ridge–plume interaction and differ from continental Moho and lower crust. Lower crust, only 100 kg/m 3 less dense than uppermost mantle, is assumed to be a plume-generated transitional layer which, directly above the plume centre, belongs rheologically to the asthenosphere. The regionalized regression results are used to construct a new Moho depth map from the gridded topography file. It compares reasonably to other recent maps. A notable feature is a band of crustal thickening along the SE Iceland shelf edge continuing northward across eastern Iceland to the Jan Mayen Ridge. This and other features suggest an evolution of the North Atlantic where a continental edge splinter carried into eastern and perhaps northern parts of Iceland, beside the Jan Mayen Ridge.