Influence of a rigid backstop on the flow pattern during thrusting of the supracrustal Balsfjord Series of the North Norwegian Caledonides

Abstract

The Caledonian evolution of the low–medium grade Balsfjord Series (Troms, Northern Norway) is characterized by four sets of deformation structures that developed progressively during burial and exhumation from underneath the high-grade Nakkedal and Tromsø nappe complexes. The Balsfjord Series commonly displays top-to-the SSE–SE directed movement during the development of the main metamorphic fabric with local folding at different evolutionary stages. However, we document a structural horizon within the Balsfjord Series showing a strong reorientation of the regional stretching lineation from NW–SE to E–W and finally NE–SW. This rotation occurred during exhumation and is interpreted as a lateral escape mechanism resulting from thrusting of the rheologically soft Balsfjord Series against the rigid backstop of the Lyngen Magmatic Complex and the Mauken Window basement. U–Pb dating of titanite from shear zones within the West Troms Basement Complex yielded an age of 429 ± 5 Ma interpreted as the age of thrusting of the overlying Balsfjord Series. To account for all field relationships with the overlying and underlying units, we interpret the tectonic evolution of the Balsfjord Series as a result of an interaction of two sutures active at ca. 430 Ma.