A palaeomagnetic study of the coast-parallel Jurassic dyke swarm in southern Greenland

Results are reported from palaeomagnetic samples collected in two traverses across the coast-parallel dyke swarm of southern Greenland. This swarm probably resulted as the consequence of initial rifting between Greenland and Labrador, and a reversal of magnetisation has been found which is correlated on the basis of KAr age determinations (～168 m.y.) with the Mateke event of the Middle Jurassic (Bajocian). All of fifteen sites show significant grouping of directions after a.f. cleaning; three have anomalous directions of magnetisation while the remainder (nine normal, three reversed) give a combined mean direction of D = 336°, I = 66° (α₉₅ = 4.6°) with a palaeomagnetic pole at 191°E, 72°N. The dykes exhibit the same corelation between polarity and deuteric oxidation state as that found in Tertiary volcanics. There is a systematic change in magnetisation across the dyke swarm in south Greenland from normal to anomalous to reversed directions; this is interpreted as due to lateral migration of the response to the regional stress field with time. The pole position lies in the vicinity of Jurassic poles from North America after closing the Labrador Sea according to the reconstruction of Bullard, Everett and Smith, but the scatter of these latter poles precludes a confirmation of this reconstruction for Middle Jurassic and earlier times.