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Journal of the Geological Society

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Mantled gneiss antiforms and fold nappes in the Rinkian belt, West Greenland: diapiric structures or structures formed in a thrust system?

¹ School of Geological Sciences, Kingston Polytechnic, Kingston upon Thames KT1 2EE, UK
²Institut voor Aardwetenschappen, Rijksuniversiteit Utrecht, Budapestlaan 4, 3508 TA Utrecht, The Netherlands

In Lower Proterozoic metasedimentary cover rocks of Svartenhuk Halvø, and in their reactivated Archaean basement of tonalitic gneisses, features of the structure and metamorphism are identified which are critical to the understanding of the kinematics and dynamics of the orogenic evolution of the Rinkian belt. These features are as follows: (1) a major ductile shear zone at the gneiss–sediment contact which has a normal-slip displacement and dips steeply NE away from the gneisses in its present orientation, (2) a fan in the main cleavage in the metasediments, in which cleavage steepens over a distance of 15 km from a 30° dip SW towards the contact through the vertical, finally coming to lie parallel with the contact, (3) the existence of a major synform in the sediments near the gneiss–sediment contact which is overprinted by the main cleavage, (4) the orientation and restricted distribution of later crenulation cleavage, and (5) a high temperature–low pressure metamorphic facies series, with thin metamorphic zones close to the basement, reaching anatexis at the gneiss–sediment contact.

The features which relate to the pattern of strain are compared with those to be expected: (1) as a result of the ascent of a diapiric ridge of gneiss into the cover sediments, and (2) as a result of thrusting to the NE, followed by SW-directed backthrusting. It is concluded that gneisses on Svartenhuk Halvø rose diapirically into the cover sediments. This has implications for the origin of flat lying gneiss-cored nappes in areas adjacent to Svertenhuk Halvø. These may respresent, (1) the spreading lobes of mature diapirs, or (2) diapiric antiforms amplified during a subhorizontal shear regime.