East Greenland's continental crust is little explored compared to the geologically related Norway. As part of a comprehensive geophysical study of east Greenland, the Alfred Wegener Institute for Polar and Marine Research acquired eight seismic refraction profiles between 70°N and 76°N. We present the results of four profiles combined with magnetic and geological data. The data set revealed pronounced differences in the crustal architecture north and south of Kong Oscar Fjord (72°N). The crust north of Kong Oscar Fjord shows three structural units, a Caledonian, a Devonian, and a Mesozoic to Tertiary, whereas only a Caledonian and a Devonian to Tertiary unit can be distinguished in the southern area. A seismic high-velocity layer was discovered in the lower crust north of Kong Oscar Fjord which is absent in the southern area. This high-velocity layer coincides with a pronounced magnetic anomaly related to Tertiary magmatism and is interpreted as Tertiary magmatic underplate. By integrating geophysical and geological data, we showed that a different evolution of the areas north and south of Kong Oscar Fjord started after the Devonian extensional collapse of the Caledonides. Mesozoic extension shifted to the east and preserved Devonian structures in the northern area, whereas the Devonian crust was further thinned and weakened in the southern area. We propose that it allowed an easier ascent of Tertiary melts to the surface than the crust north of Kong Oscar Fjord, where most of the melts got trapped in the lower crust. The model explains the distribution of extrusives and intrusives in east Greenland's Tertiary igneous province. Copyright 1999 by the American Geophysical Union.