INVESTIGATING THE STRUCTURE, KINEMATICS, AND TIMING OF EXHUMATION OF THE ROAN WINDOW, NORWAY

Abstract

The Western Gneiss Region (WGR) is the deepest exposed structural unit of the Caledonides along the south-central Norwegian coastline and is divided into two sections, north and south, by the steep, strike-slip Møre-Trøndelag shear zone (MTSZ). The southern WGR contains high-pressure and ultrahigh-pressure terranes that underwent eclogite/granulite and amphibolite facies metamorphism at 425-400 Ma. To the north of the MTSZ, rocks belonging to the WGR record a similar age range and a maximum pressure of ~1.5 GPa in the same lithologies as exposed in the southern WGR. Extensional faulting during exhumation of both the southern and northern WGR has created tectonic windows, exposing deep levels of the WGR. This thesis presents the results of structural and kinematic analysis combined with geochronology on the lowermost exposed basement unit of the northern Western Gneiss Region, Norway, exposed on the peninsula of Roan. The predominant lithologies on Roan are migmatitic hornblende +/- garnet gneisses that host interfolded mafic layers and pods, some of which record high-pressure (HP) granulite facies assemblages. U-Pb dating of zircon in various leucosome and pegmatite structural sites document that crystallization and deformation of Roan rocks were coeval with that of ultrahigh-pressure (UHP) migmatite and eclogite in the southern WGR (410-400 Ma). In this study, migmatite and gneiss were examined to determine the kinematics of deformation and the timing of exhumation, with special attention to the timing of shear/strain localization in relation to melt-present deformation. A variety of methods link field and microstructural data and observations to regional structures and establish the regional history of the exhumation of this segment of continental crust. We present field-based kinematic measurements along transects that highlight transitions in lithologies, structures, and strain intensities from the core to the periphery of the dome. To establish kinematic geometry, Electron Backscatter Diffraction (EBSD) was performed on selected samples to examine grain shape, distribution, and crystallographic orientations. X-Ray Computed Tomography (XRCT) provided 3D information on the distribution of the mineral phases and provided data for the analysis of shape fabrics. Crosscutting relations among various generations of fabrics and leucosomes, complemented with U/Pb zircon, titanite, and rutile geochronology, were analyzed to establish the timing of melt production and emplacement relative to deformation events. We argue that the shear zones surrounding Roan acted in concert and collectively participated in exhumation. The orientation of the shear zones and their deformational fabrics are consistent with wrench-dominated transtension in which wrench motion and normal motion dominated along the NW and SW bounding structures, respectively. This kinematic framework resulted in horizontal contraction (sub-vertical shear zone) at the margin and vertical shortening (sub-horizontal shear zone) in the core, with top to the south extension along the southern margin.