The tectono-metamorphic and geochronological history of basement and nappes in the Grotli-Tafjord Area, Western Gneiss Region, Norway

The tectono-metamorphic and geochronological history of basement and nappes in the Grotli-Tafjord Area, Western Gneiss Region, Norway: An investigation of the geological history of the Basement and two nappe units, using structural analysis, geothermobarometry and monazite U-Th-Pb age dating

Haker, Agnes (Utrecht University, Faculty of Geosciences)

Molengraaff Fonds

2005-08

Past research has shown that the Scandinavian Caledonides comprises various units, each with a specific tectono-metamorphic and geochronological history. This has led to the realisation that the Caledonian orogeny was not one single event but comprised distinct, (U)HP tectono-metamorphic episodes. However, more data from all units is needed to reconstruct the complete evolution of the Scandinavian Caledonides. This study seeks to unravel the tectono-metamorphic and geochronological history of basements and nappes in the Grotli-Tafjord area, Western Gneiss Region, southwestern Norway. Field observations have shown that this ~150 km2 area consists of three major tectono-metamorphic units: the Basement-, Nibba-, and Svartegga unit. The geological history of the three units has been investigated, using structural and metamorphic analysis, geothermobarometry and monazite U-Th-Pb age dating. It was found that each unit experienced eclogite-facies HP metamorphism at a specific period in time, interpreted to result from distinct subduction events. The Basement unit is the Baltic (Par)Autochthon and experienced HP metamorphism during the Scandian phase, at ~410-400 Ma. The Basement rocks were subjected to relatively low PT conditions in the study area, with T _ 640ºC. The Nibba unit is correlated with the Jotun-, Risberget-, and Lindås Nappe of the Middle Allochthon. This heterogeneous unit experienced HP metamorphism (T _ 700ºC) at ~425-420 Ma. The Svartegga unit is part of the Upper Allochthon and is correlated with the Blåhø-Surna Nappe and the Jämtland terrane of the Seve Nappe. It experienced peak-metamorphism at ~450 Ma to PT conditions of 770±50ºC and 15.8±0.5 kbar. Following eclogite-facies HP peak-metamorphism the rocks underwent rapid isothermal decompression into the upper amphibolite-facies field, and subsequent slower retrogression to lower amphibolite-facies conditions. A SE-NW, East-dipping, regional transposition foliation throughout the area indicates that the three units have experienced the final part of their exhumational history together. This is interpreted to be the result of the extensional collapse of an overthickened crust. Extension was in an orogen-parallel direction (SW-NE) under ongoing convergence. Slight doming of the Basement under this SE-NW transpressive regime resulted in large-scale upright open folds with fold axes dipping to the ENE. Ductile extensional shear in the area was directed top-to-(S)E and lasted till ~397 Ma.

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Utrecht University

Geoscience Reports

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(c) Agnes Haker