Kinematics of the Høybakken detachment zone and the Møre-Trøndelag Fault Complex, central Norway

The tectonic disintegration of the Caledonian orogen through combined extension, contraction and strike-slip was characterized by spatial and temporal strain partitioning through a period of at least 30 Ma. Early to Mid-Devonian exhumation of the Central Norway basement window was associated with retrograde, top-to-the-SW extensional shearing in the Hoybakken detachment zone, sinistral shearing along the More-Trondelag Fault Complex, and formation of extension-parallel folds. Progressive exhumation led to increasing strain localization and to the transition from ductile to brittle deformation. In the interval between e. 370 and 320 Ma, the Hoybakken detachment fault cut previously folded detachment mylonites, capturing mylonites in its hanging wall. Ar-40/Ar-39 mica and K-feldspar ages indicate a Late Devonian or younger age for the uppermost parts of the adjacent 'Old Red' basin. Gentle folding of this stratigraphic level attests to the continuation of shortening and orogen-oblique extension into Late Devonian-Carboniferous time. Shortening was intensified along strands of the More-Trondelag Fault Complex, as shown by mutually cross-cutting reverse and strike-slip faults. 'Flower structures' may be particularly common in constrictional strain systems where strike-slip faults develop parallel to the principal elongation trend, but normal to the principal axis of shortening.