Magnetic susceptibility ellipsoids in Nagssugtoqidian and Archaean rocks in South-East Greenland.

Abstract

Measurements of magnetic susceptibility have been carried out on Precambrian rocks in south-east Greenland in the Nagssugtoqidian mobile belt from Ammassalik northwards to its boundary with the Archaean craton, and slightly beyond. Directions of maximum susceptibility are the best defined, and are as follows: Ammassalik: Declination = 3º, Inclination = 40º,α95 = 7º; Nagssugtoqidian/Archaean “boundary”: Declination = 311º, Inclination = 62º, α95 = 10º; area enclosing post-tectonic plutons: Declination = 194º, Inclination = 87º, α95 = 17º. The boundary is invisible to the directions of maximum susceptibility. A shear zone near the boundary has been studied in detail. The maximum directions of the samples are tightly grouped and lie in the plane of the zone, whilst the intermediate directions rotate about the maximum direction as the zone is approached, until they lie in its plane. Such rotation is widespread in the boundary area. A plate tectonic explanation for the maxima from the boundary and from Ammassalik is proposed as follows: the maximum direction from the boundary is attributed to subduction and collision of the Archaean plate arriving from the north-east, followed by a vertical component imprinted by the emplacement of the plutons. The maximum direction at Ammassalik is due to overriding Archaean crust coming from the north. Anisotropy of magnetic susceptibility is useful in detecting shear zones and rock fabric when these are not apparent in the field or hand specimen.