TEAMWORK OF SHEAR ZONES DURING EXHUMATION, THE ROAN WINDOW, NORWAY

Regional studies of P-T conditions, including melt crystallization, structures, and localized shear zones inform the mechanisms of orogenic exhumation. The Roan peninsula, Norway contains HP granulites and migmatites of the northern Western Gneiss Region, the deepest structural level of this part of the Caledonian orogen. Roan is an elongate dome in which a series of shear zones were active, likely in concert. Two main high-strain zones, one sub-horizontal in the core of the dome, and the other sub-vertical along the NW margin, are separated by a domain of less deformed, crenulated gneiss with leucosome veins occupying the folds’ axial surfaces. U-Pb zircon ages from intensely deformed leucosomes fall between 410-404 Ma (Gordon et al., 2016; this study); axial plane leucosomes in crenulated gneiss and cross-cutting pegmatite dikes that are locally sheared record U-Pb ages between 400-396 Ma. This suggests that melt crystallized over 10-15 My and accompanied deformation, likely during exhumation and progressive cooling of the terrane, when strain localized within shear zones. A third shear zone, defined by a shallowly S-dipping mylonitic fabric, is exposed along the SW margin of the Roan peninsula. The plagioclase-hornblende mylonitic gneiss is well exposed along a 25 m road cut that displays m-scale mylonitic folds with hinges parallel to the stretching lineation; sense of shear is top to the S-SW. A U-Pb zircon crystallization age of ~407 Ma was obtained from a leucocratic mylonitic gneiss. The age of mylonitization is not known, but this shear zone is kinematically consistent with the sinistral shear along the NW-trending, steep bounding shear zone. We interpret the three shear zones to have worked “as a team”, with shallowly dipping shear zones allowing exhumation of HP granulites in the core of the dome and the sinistral shear zone bounding the dome to the NW accommodating exhumation in a pull-apart structure.