Structures and age at the tectonic contact between the Kvitenut and Revseggi nappes, Hardanger-Ryfylke Nappe Complex

Abstract

The Hardangervidda-Ryfylke Nappe Complex is the southernmost slice of the Scandinavian Caledonides, traditionally assigned as Middle Allochthon. Overlying these rocks are the Holmasjø Allochthon, the Dyrskard, Kvitenut and Revseggi nappes. The relationships between structures, metamorphism and radiometric ages show that thrusting of the Kvitenut onto the Dyrskard Nappe had occurred already during the Sveconorwegian orogeny (c. 1000 Ma). The relationship between the nappes in Seljestadjuvet is, however, more enigmatic. The established nappe tectonostratigraphy must be changed as the unit initially described as the bottom of the Kvitenut Nappe has been re-defined as a separate tectonic lens of local significance in the Seljestad area. The protolith age of the tectonic lens (1522 Ma) reflects a Telemarkian origin, unlike the Kvitenut Nappe (1615 Ma). The Revseggi Nappe is the uppermost tectonostratigraphic unit of the Hardangervidda-Ryfylke Nappe Complex. It consists mainly of gneisses and pelitic metasediments with veins of pegmatites, quartz, and with mafic- and felsic intrusives. The Revseggi Nappe displays a striking lithological and structural contrast compared to the Kvitenut Nappe below it. The contact between the Kvitenut and Revseggi Nappe involves tens of metres with mylonite gneiss, and a central (2-5 meters) ultramylonite with associated shear indicators, which proves the contact to be tectonic rather than depositional. Titanites from the ultramylonite reflect that peak metamorphism between the Kvitenut and Revseggi nappes occurred at 334 Ma. Previous work has dated pegmatite in the Revseggi Nappe to 434 Ma and 450 Ma. The finding of a pegmatite with an age of 447 Ma indicates that there where two instructional events. A late Ordovician (c. 450 Ma), and a middle Silurian (434 Ma).