Transition between "Archaean-type" and "modern-type" tectonics: Insights from the Finnish Lapland Granulite Belt

Abstract : This paper discusses the exhumation history of Palaeoproterozoic high-grade units from the Lapland Granulite Belt (northern Finland) through a reappraisal of previous structural and metamorphic data and a new structural analysis. The Lapland Granulite Belt is located between two cratons (Inari Craton and South Lapland Craton). It is marked by a major thrusting event associated with the development of an inverse metamorphic gradient within the underlying Tanaelv and Karasjok Belts. The review of published metamorphic data combined with our new structural analysis suggests that the exhumation history of the Lapland Granulite Belt was accommodated by the development of coeval basal thrusting and early normal-sense shear-zones, and later by distributed erosion. We propose also that the early exhumation stages of the Lapland granulite unit were associated with heating of both the overlying Inari Craton and the underlying South Lapland Craton. The occurrence of coeval thrusts and normal-sense shear-zones combined with the important role of distributed erosion during the final exhumation stages suggest that this Palaeoproterozoic collisional belt may represent an intermediate stage between ultra-hot accretionary orogens involving weak lithospheres and modern orogens involving stiff lithospheres (mixed-hot orogen of Chardon et al., 2009). Our analysis emphasizes that the transition between ancient-type orogens and modern-type ones was gradual in space and time, being firstly dependent on the thermal and rheological state of the sub-continental lithospheric mantle of the lithospheres involved in compression.