Metamorphic zoning and behaviour of an underthrusting continental plate
Summary, in English
Lower continental plates of collision zones are normally inaccessible and their properties inferred by geophysical surveying. By contrast, the eastern margin of the deeply eroded 1-billion-year-old Sveconorwegian Orogen in Scandinavia exposes an underthrusting lower continental plate that records the metamorphic evolution and behaviour during collisional orogeny. A 200 km wide structurally coherent belt of the mid- to upper levels of the underthrusting lower plate, composed of Proterozoic igneous protoliths, exposes the gradual transition from unmetamorphosed to high-P granulite facies. Metamorphic assemblages in Fe–Ti-rich metagabbro in a continuous ~120 km wide transect across the lower continental plate define a regional metamorphic zoning by eight zones. Significant metamorphic recrystallization is first recorded at conditions of lower to middle amphibolite facies (550–600°C), progressing successively towards the contact with the upper plate to high-P granulite facies at ~850°C and 10–11 kbar, the latter corresponding to ~40 km depth. An eclogite-bearing tectonic terrane, detached from the leading edge of the underthrusting plate and extruded back along the interface with the overlying crust, occurs just beneath the lithotectonic contact of the overriding plate. The gradual metamorphic transition demonstrates that, with the exception of the eclogite-bearing terrane, the underthrusting plate remained essentially structurally coherent despite highly ductile deformation and partial melting. The metamorphic rocks record that, by and large, the underthrusting plate underwent short-lived metamorphic recrystallization and deformation that started first at an advanced stage of collision, at c. 980–970 Ma; this is in contrast to the eclogite-bearing terrane which records more protracted metamorphic recrystallization both through prograde metamorphism and decompression. The investigated section is the first described in situ exposure of a coherent underthrusting lower continental plate, and defines a metamorphic field gradient at higher temperatures and P/T than the classical Barrovian style. The exposed section provides potential for increasing understanding of the thermal evolution, rheology, petrophysical properties and distribution of volatiles in an elsewhere inaccessible part of Earth's collisional systems. The data demonstrate that the underthrusting lower continental plate differs in metamorphic character from other parts of orogenic systems in that the crust is relatively dry, competent and responds by significant metamorphic recrystallization first at elevated temperatures (middle amphibolite facies) and that metamorphic recrystallization is largely coupled to deformation that facilitates the entry of hydrous fluids into the crust.