A revised magnetostratigraphy for the Mid-Proterozoic Gardar lava succession, South Greenland

Abstract

New palaeomagnetic results from the ca. 1300 Ma Gardar lava succession within the Eriksfjord Formation of South Greenland are presented. This study incorporates stepwise thermal demagnetisation of 256 samples from 66 lava flows from all three lava groups in this formation, accompanied by principal component analysis. The results identify two complete reversals (N → R → N) in the upper lava group and two (R → N and N → R) in the lower group, with a possible palaeoexcursion in the lower group. A minimum of two further reversals are predicted in the time interval represented by the sandstone units intercalated with the lava flows. Both complete reversals in the upper group exhibit inclination asymmetries of 30°. This degree of asymmetry is satisfied by a geomagnetic field model equivalent to a dipole axially offset by 570 km. However, a more complex source is not precluded by the present evidence. Four new pole positions are calculated for the Gardar lavas, two for the upper lava group and one each for the middle and lower groups. The middle lava group shows an anomalous mean direction and pole position, which is unrepresentative of the remainder of the lava succession and is thought to record a virtual geomagnetic pole representative of a transitional field direction. The presence of a non-axial geocentric dipole field means that the use of the Laurentian apparent polar wander path for dating the new poles is not valid.