Caledonian deformation of the Precambrian basement in southeastern Norway

The Precambrian basement in southeastern Norway consists of the Transscandinavian Igneous Belt (TIB) with ages from 1.86 to 1.66 Ga to the east of the Mjøsa Mylonite Zone. Gothian rocks with an age of 1.64–1.52 Ga occur to the west of this zone together with the highly magnetic 932 Ma old Flå Granite. A map showing the depth to the Precambrian basement in southeastern Norway is based on depth to magnetic basement, gravity models and the position and height of the Cambrian marine transgression in southeastern Norway. The peneplain has a slope facing north– northeast on the western side of Mjøsa and west of the Mjøsa Mylonite Zone. On the eastern side of Mjøsa the slope is facing northwestwards. This basin structure may be primary or partly a result of later subsidence. We propose that the basement depression extending from Lillehammer to the north–northwest represents the southern extremity of a basement structure that hosted the Hedmark Basin. The broad basin can be followed to the north by magnetotelluric surveys to Dovrefjell were it terminates at the late, large, normal fault at the boundary to the Upper Allochthon. Within the zone of deformed basement west of the Mjøsa Mylonite Zone, there are three 3–4 km-deep basins/depressions. The depressions are probably produced by Caledonian thrusting of the basement and are related to the transport of the Atnsjø window and the Osen–Røa Nappe Complex. New data from Åreskutan in Sweden on the thrusting of the Middle Allochthon during Late Silurian and the thrusting of the Lower Allochthon in Early Devonian are in agreement with data from Trollheimen in Norway. The Early Devonian Osen–Røa Nappe is probably related to the out-of-sequence Storli Thrust, and the thrusting involved the sedimentary cover including the Hedmark Basin and the basement below. The distance of transport may have been in the order of 100 km.

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