Thermal history of thrust sheets in an orogenic wedge: Ar-40/Ar-39 data from the polymetamorphic Seve Nappe Complex, northern Swedish Caledonides

The Seve Nappe Complex in the Scandinavian Caledonides contains the fragmen ted late Precambrian continent-ocean transition between Baltica and the Iap etus Ocean. This passive margin was fragmented and thrust eastwards over th e Baltic Shield during Caledonian orogenesis. The individual thrust sheets in the Seve Nappe Complex went through different P-T-t evolutions, resultin g in dramatic metamorphic contrasts: eclogite-bearing nappes are juxtaposed with nappes showing no evidence of Caledonian deformation or metamorphism in their interiors. Strain localization to the marginal parts of the thrust sheets left records of both pre-orogenic (rift) and early orogenic (subduc tion and subsequent uplift) processes in the thrust sheets of the Seve Napp e Complex. Even though it has been transported several hundred kilometres, only the margins of the eastern part of the Sarektjakka Nappe are affected by penetrative Caledonian deformation. This part of the Sarektjakka Nappe i s dominated by pristine tholeiitic dykes and cross-bedded sandstones. The d ykes are 608 +/- 1 Ma old and make up 70-80 % of the nappe. Widely spaced t hin shear zones of the Ruopsok fault system represent the only Caledonian p enetrative deformation in the interior of the nappe. Previously published A r-Ar dates indicate cooling below the closure temperature of hornblende at c. 470 Ma, but numerous ages have been recorded. Ar dating of biotite and m uscovite from a cross-laminated metapsammite in the Sarektjakka Nappe gave well-defined ages of 428.5 +/- 3.6 and 432.4 +/- 3.8 Ma, respectively. Musc ovite from a shear zone in the Ruopsok Fault System gave 428.2 +/- 4.0 Ma, whereas hornblende from the same locality did not yield interpretable data. The results indicate that these rocks were completely degassed at some unk nown time, presumably at the emplacement of the dyke swarm. No subsequent e xcess argon contamination can be detected. A likely candidate for the degas sing event is the emplacement of the dykes at 608 Ma. The interior of the n appe, and thus the entire nappe complex, cooled below similar to 350 degree s C at around 430 Ma. Cooling from more than 500 degrees C at c. 470 Ma to 350 degrees C at c. 430 Ma suggests an average cooling rate of less than or equal to 4 degrees C/Ma. A prolonged period of slow cooling (approximate t o exhumation?) following the initial, rapid uplift of the eclogite-bearing nappes and Early Ordovician construction of the Seve Nappe Complex is sugge sted.