Multiple reactivation and strain localization along a Proterozoic orogen-scale deformation zone: The Kongsberg-Telemark boundary in southern Norway revisited

Abstract

Structural analysis defines a multiphase Sveconorwegian tectonic evolution for the boundary zone between the Kongsberg and Telemark lithotectonic units in S Norway, referred to as the Kongsberg-Telemark Boundary Zone (KTBZ). This large-scale weakness zone developed predominantly within and at the margin of a c. 110km long granitic belt, the intrusion of which is dated between 1170±11 and 1146±5Ma by U–Pb SIMS zircon geochronology. The oldest KTBZ ductile fabric formed during the Sveconorwegian orogenic cycle (c. 1140–900Ma) as a penetrative top-to-the-W shear fabric, which was subsequently reactivated selectively by sinistral transpression that formed characteristic mylonitic shear zones within the granitic belt. Later folding affected the area at the northern end of the Kongsberg lithotectonic unit. Analysis of the subregional foliation trajectories unravels the occurrence of a large-scale fold structure, the “Norefjell-Hønefoss Fold”. All these structures are in turn cut by late-Sveconorwegian, E-dipping shear zones and normal faults, which accommodated a distinct phase of exhumation of the Telemark lithotectonic unit in the footwall of the KTBZ. This extensional detachment widens toward the north, where it might have controlled the emplacement of the late-orogenic Flå granite. Since late Sveconorwegian times, the KTBZ was repeatedly reactivated in a brittle fashion forming complex fault patterns, extensive quartz vein networks and leading to the generation of the so-called “Store Rivningsbreksje”, a 100km long brittle fault zone that follows the trend of the KTBZ and that locally juxtaposes blocks with different ductile precursor histories. The newly established deformation history helps to refine existing models for the orogenic evolution of the central Sveconorwegian orogen. The characterization of the Norefjell-Hønefoss fold structure provides a new perspective on Sveconorwegian geometries and fabrics in the area. The reactivation history established for the KTBZ helps to better understand the dynamics of long-lived weakness zones of Precambrian origin in general.