Thrust‐related very low grade metamorphism in the marginal part of an orogenic wedge, Scandinavian Caledonides

Abstract

The pattern and characteristics of thrust‐related very low grade metamorphism in the marginal part of an orogenic wedge have been investigated by combining a clay mineral crystallinity survey with detailed structural mapping of the thin‐skinned foreland thrust belt along the external part of the Scandinavian Caledonides. This external part is composed of late Neoproterozoic to Ordovician sedimentary sequences of the autochthonous cover and Lower Allochthon, which are overlain by the higher Caledonian nappes (Middle and Upper Allochthonous units). Stages of the Scandian phase of thrust wedge development are described which are related to the very low grade metamorphic history. The initial stage involved the emplacement of cooled nappes belonging to the Middle and Upper Allochthons, with very low grade peak metamorphic conditions attained within the underlying Lower Allochthon (cover) sediments as they were progressively buried and deformed beneath the thrust wedge. During this initial emplacement the isotherms are considered to have been undisturbed and dipping parallel to the wedge surface. The following stages of wedge development consisted of extensive post‐metamorphic imbrication of the underthrusted cover sediments, with a transition from basal accretion and uplift at the rear, to accretion and forward propagation at the wedge's toe. During accretion into the wedge, the externally dipping isograd surfaces were extensively displaced from deeper levels toward higher tectonic horizons. The last stage of wedge development considered here was characterized by late out‐of‐sequence thrusting with enhanced (epizonal) metamorphic grades developed in the vicinity of the fault zones, which either resulted from further displacement of the isograds toward higher levels, or from localized heating via intense fluid activity. Overall, the pattern of metamorphic grade, fabric relationships, and physical calculations of heat transfer based on the geometry of the thrust wedge, suggest that neither inverted temperature gradients nor shear heating were likely causes of the metamorphism in this flat‐lying part of the orogenic wedge. The description of inverted very low grade metamorphic isograds in other marginal parts of the Scandinavian Caledonides, which have been previously attributed to either the rapid emplacement of hot thrust nappes, or the effects of dissipative shear heating, are discussed in terms of variations in both the critical wedge geometry and its controlling boundary conditions.