Older crust underlies Iceland

The oldest rocks outcropping in northwest Iceland are similar to 16 Myr old and in east Iceland similar to 13 Myr. The full plate spreading rate in this region during the Cenozoic has been similar to 2 cm a(-1), and thus these rocks are expected to be separated by similar to 290 km. They are, however, similar to 500 km apart. The conclusion is inescapable that an expanse of older crust similar to 210 km wide underlies Iceland, submerged beneath younger lavas. This conclusion is independent of any considerations regarding spreading ridge migrations, jumps, the simultaneous existence of multiple active ridges, three-dimensionality, or subsidence of the lava pile. Such complexities bear on the distribution and age of the older crust, but not on its existence or its width. If it is entirely oceanic its maximum age is most likely 26-37 Ma. It is at least 150 km in north-south extent, but may taper and extend beneath south Iceland. Part of it might be continental-a southerly extension of the Jan Mayen microcontinent. This older crust contributes significantly to crustal thickness beneath Iceland and the similar to 40 km local thickness measured seismically is thus probably an overestimate of present-day steady-state crustal production at Iceland.