A Discussion on the evolution of the Precambrian crust - Recumbent folding and flat-lying structure in the Precambrian of northern West Greenland

Abstract

The Precambrian of West Greenland, north of 69° 30' is part of a lower Proterozoic orogenic complex characterized by low dips and the lack of any clear regional trend. Well-preserved metasedimentary cover rocks occur within this complex. The gneisses of the complex are regarded as belonging to a reactivated basement. In one area, the Umanak—Rinks Isbrae area, a structural contour map has been prepared for a marker level near the base of the metasedimentary cover. This map shows a relatively simple pattern of domes; little or no crustal shortening has taken place since the deposition of the cover sediments. However, the structure of the gneiss below is characterized by isoclinal recumbent folds. There are metabasite bodies in the gneiss. These were emplaced into rocks that were already migmatitic gneiss at the time of basite intrusion. The metabasites were affected by the recumbent folding, so the recumbent folds must have attained their present form at a relatively late stage in the structural development of the gneiss. The recumbent folds and flat-lying structure in the gneiss are interpreted as being due at least partly to lateral adjustments in the gneiss infrastructure during the metamorphism and doming of the younger metasedimentary suprastructure. Crustal shortening has not played an important part in the structural development.