The Gaissa Nappe, Finnmark, North Norway: an example of a deeply eroded external imbricate zone within the Scandinavian Caledonides

Abstract

The Lower Allochthon of the Caledonides of Finnmark, northern Norway, is represented solely by the Gaissa Nappe, which is composed of sub-greenschist facies sedimentary rocks of late Riphean to Tremadoc age. The lithostratigraphic sequence has been shortened by thrusting and folding in an ESE direction. Based on mapping and structural profiling east of Porsangerfjord, the Gaissa Nappe can be divided into four structural segments: the Børsely duplex, developed beneath the Kalak Nappe of the Middle Allochthon, is oblique to an imbricate fan, the Munkavarri imbricate zone, east of which is the Guiverassa duplex zone that is partly covered by the Vuonjalrassa thrust sheet. The sole thrust to the Gaissa Nappe is a flat planar surface which truncates the common N-S folds and associated cleavage in the rocks of the Gaissa Nappe. The Vuonjalrassa-Gaissa thrust cuts down section in the transport direction, possibly as a result of early tectonic downwarping. A balanced cross-section and a hanging-wall diagram have been partially restored, indicating that the metasediments of the trailing edge of the Munkavarri imbricate zone have been displaced by 104 km in their ESE translation direction. Taking the sequence west of Porsangerfjord into consideration, an overall contraction of more than 150 km is possible. In the east, it is argued that the basal Gaissa décollement, formerly thought to die out and pass laterally into an unconformity, extends to the northeast beyond the head of Tanafjord. Folds that occur in front of the sole thrust on the Varanger Peninsula imply the presence of a blind thrust. In an orogenic context, the Gaissa Nappe forms a series of imbricated thrust sheets in the external part to the collision belt, produced during the Finnmarkian orogenic event in late Cambrian to early Ordovician time.