Proterozoic tectonic overprinting of Archaean gneisses in Nuussuaq, West Greenland

Abstract

Archaean orthogneisses and supracrustal rocks in eastern Nuussuaq in the southern part of the Rinkian orogen preserve much indirect evidence of Proterozoic reworking. In northern Nuussuaq, a marble occurrence which can be correlated with the Proterozoic Mârmorilik Formation of the Uummannaq district, is interleaved and intensely folded with Archaean orthogneisses. Dykes and sills of presumed Proterozoic age in southern Nuussuaq pre-date two major shear zones. Furthermore, published K-Ar and ⁴⁰Ar-³⁹Ar hornblende ages indicate that metamorphic temperatures of at least c. 550°C were reached in large parts of Nuussuaq during the Proterozoic. Although available data are not sufficient to firmly establish the tectonic evolution of Nuussuaq, it can be shown that development of a regional flat-lying structure was succeeded first by upright folding and development of a major NW–SE-trending shear zone during crustal shortening, and subsequently by formation of an ENE–WSW-trending shear zone during crustal extension. A Rinkian detachment zone across Nuussuaq, similar to those known in the Uummannaq district, may link Proterozoic marble occurrences south of the peninsula with the marbles known in northern Nuussuaq and in the Uummannaq district. Observations suggest that geological structures and lithologies can be traced from north to south across the fjord Torsukattak, into the Ataa area which is hardly affected by Proterozoic deformation. This is used to infer that a major crustal boundary structure along the fjord (previously assumed to form the northern boundary of the Burwell terrane in West Greenland) does not exist.