Inclined transpression at the toe of an arcuate thrust: an example from the Precambrian ‘Mylonite Zone’ of the Sveconorwegian orogen

The ‘Mylonite Zone’ (MZ) forms a major, arcuate terrane boundary in the Precambrian Sveconorwegian orogen of SW Scandinavia. SE-directed thrusting along this curvilinear shear zone emplaced the higher-grade Idefjorden Terrane to the west onto the lower-grade Eastern Segment terrane to the east. Detailed structural characterization of the MZ mylonites in two different localities (Va¨rmlandsna¨s and Bua peninsulas) reveals a complex three-dimensional strain pattern. Inclined transpression is inferred on the basis of coexisting (and broadly coeval) foliation- parallel oblique shearing (resolvable in a strike-slip and dip-slip component) and acrossfoliation shortening. The former accommodated the transpressive component of the MZ, and its kinematics is either sinistral or dextral depending on the local strike of the MZ with respect to the regional thrust shortening vector. The latter led to pure-shear shortening perpendicular to the thrust sheet and subsequent lateral extrusion parallel to the mylonitic foliation via the development of antithetic displacements. No significant strain partitioning is observed at the meso-scale and strain is thus truly triclinic. The example described is an outstanding case of triclinic deformation, confirms theoretical analyses of complex strain models and adds valuable natural field constraints to our knowledge of deformation in the crust.