Abstract

A number of planar normal faults displace Tertiary basaltic intrusions in the Traill Ø region of East Greenland. This 'post-magmatic' extension occurs over an along-strike length of >350 km and over an across-strike width of >100 km with fault displacements and fault-block rotations indicating that (3 increases in magnitude eastwards from near unity to β ≈ 1.1. The stretching direction was approximately E-W, orthogonal to the strike of major faults in the region, most of which are reactivated Mesozoic structures. Two periods of Tertiary basaltic magmatism are recognized in the area. ⁴⁰Ar–³⁹Ar step heating suggests that the first event, which gave rise to tholeiitic sills with a composite exposed thickness of c. 1 km, occurred at c. 54 Ma, and the second, which formed smaller volumes of alkaline dykes and two large syenite complexes, occurred at c. 36 Ma. Field observations indicate that 'post-magmatic' extension occurred after 54 Ma and possibly continued after 36 Ma. The tholeiitic intrusions are probably related to a stretching event (β ≤ 1.1) prior to the onset of sea-floor spreading in the Norway Basin, whilst the alkaline basalts and 'post-magmatic' rifting are associated with the separation of the Jan Mayen continental block from the East Greenland margin during the Eocene-Oligocene. Both tholeiitic and alkaline basalts formed by the partial melting of garnet-spinel peridotite with greater degrees of partial melting (8–16%) for the earlier tholeiitic group. Rifting in East Greenland ceased with the development of an oceanic ridge between Jan Mayen and East Greenland in late Oligocene time. Subsequent minor compression, probably the result of ridge-push, is suggested by the development of broad wavelength, low amplitude folds in the Traill Ø region.