STRUCTURE, KINEMATICS, AND STRAIN PARTITIONING OF AN EXHUMED HIGH PRESSURE TERRAIN: THE ROAN WINDOW, NORWAY

High-pressure and high-temperature rocks are commonly exposed in the core of orogens, but the mechanisms by which the rocks were exhumed is poorly known. Roan, an exhumed terrane of the lowermost basement unit of the northern Western Gneiss Region (WGR) of the Caledonian orogen, forms an elongate dome bounded by steeply-dipping NE-SW trending shear zones that are part of the More-Trondelag wrench system. The most abundant lithology of Roan consists of a hbl +/- garnet felsic gneiss with crenulation folds. Gneiss hosting mafic pods and layers – some of which record high-P granulite facies assemblages – are commonly associated with high-strain regions. U-Pb dating of zircon in various textural generations of leucosome and pegmatite documented that crystallization and deformation of Roan rocks was coeval with that of UHP migmatite and eclogite in the southern WGR (410-405 Ma; Gordon et al. 2016). However, the mafic bodies of Roan record a maximum P of ~1.5 GPa and T~800˚ C (Johansson and Möller, 1986). We are evaluating the exhumation mechanisms of Roan to understand why the northern WGR had a different P history that was coeval with the southern UHP terrane.

We obtained field-based kinematic measurements along transects that highlight transitions in lithologies, structures, and strain intensities from the core to the periphery of the dome. We evaluated leucosome orientation, crenulation intensities, stretching lineations, and the relationship between mafic pods and strain partitioning. In the dome core we identified a high-strain zone with a subhorizontal foliation and NE-SW lineation, displaying intense folding and boudinage of mafic pods and migmatite layers. These fabrics transition upward (toward the NW margin of the dome) into a less deformed unit in which crenulation folds systematically display leucosome veins in their axial surfaces that dip toward the NW. The NW margin of Roan dome is defined as a subvertical high-strain zone consistently striking NE-SW; lineations plunge variably within the foliation. Leucosomes in the high-strain region are transposed into the axial planes of tight crenulation folds. We propose that the Roan dome formed in a wrench-dominated system which includes a horizontal contraction (subvertical shear zone) at the margin and vertical shortening (subhorizontal shear zone) in the core.