High-rate flexure of the East Greenland volcanic margin: constraints from super 40Ar/ super 39Ar dating of basaltic dykes

In the North Atlantic domain, the SE Greenland volcanic margin developed in response to continental break-up at 57-54 Ma. Progressive tilting and intrusion of dykes reflect a major tectonic seaward-dipping flexing of the continental crust. We report eight new ages of tilted (pre-flexure) and vertical (post- flexure) dykes, determined by super 40Ar/ super 39Ar incremental heating experiments. Despite strong excess argon on plagioclase, measured super 40Ar/ super 39Ar plateau and mini-plateau ages of dykes from detailed step-heating experiments on mineral separates appear reliable and a systematic difference of ages is found between tilted and vertical dykes. In both studied areas-Kap Wandel (66 20'N) and Kap Gustav Holm (66 40'N)-tilted dykes show ages of 54-55 Ma whereas vertical dykes are younger with ages of 51 Ma. Therefore, it appears that the crustal flexing of the volcanic passive margin is a short-duration event ( < 2.9 Ma). These new dates and estimates of finite extensional strain in the studied areas suggest a lower boundary for extensional strain rates as high as 7+ /-2x10 super -15 s super -1. This value is, however, a clear underestimation as it is difficult to estimate the full finite extension in the studied area. It suggests that the continental lithosphere at volcanic passive margins is softened by high thermal gradients.