Thermochronological and structural study of exhumation of the Høybakken Detachment, Western Gneiss Region, Norway

Abstract

The Caledonian orogeny in Norway was formed through a tectonic process that buried crustal rocks to ultrahigh pressures during the middle Paleozoic. During the exhumation of these ultrahigh pressure rocks and the collapse of the Caledonide mountain range, numerous Devonian sedimentary basins were formed on top of the high-grade metamorphic basement. The study of these basins’ structures and ages allows for an understanding of how high-grade orogenic rocks move up through the crust to be exposed at the surface. This thesis focuses on investigating the Høybakken Detachment, a large-scale extensional fault active in the Devonian, and the Bjugn Basin, a conglomerate and sandstone filled basin overlying the Høybakken Detachment. Faults and fractures deforming the basin sedimentary rocks as well as the underlying crystalline basement were studied in the field, along with quartz microstructures in thin section. These structural results provide an estimate for the paleostress conditions that formed the Bjugn Basin, as well as the different structures produced from the interaction of regional normal and strike-slip faults. Additionally, using Ti-in-quartz thermometry, temperatures for the formation of quartz microstructures were calculated. Thermometry results revealed a wide range of temperatures preserved in quartz deformed by the Høybakken Detachment both structurally above and below the detachment. Depending on the thermometry calibration used, the temperature range is from 485-696°C or 357-534°C. Quartz temperatures were combined with ages obtained from 40Ar-39Ar dating of hornblende, muscovite, and biotite to link mineral Ar closure temperatures with the formation temperatures of quartz microstructures. 40Ar-39Ar ages were acquired at different structural levels, from conglomerate clasts to pegmatites intruded within granulite-facies rocks. Thermochronological results showed that the deepest structural level of the crust was not decoupled from the upper crust. However, the detachment footwall did not move towards the surface as a single unit, as exhumation of the footwall near a strike-slip section of the Høybakken Detachment took place later in the Devonian.