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

Abstract

The Seve Nappe Complex in the Scandinavian Caledonides contains the fragmented late Precambrian continent–ocean transition between Baltica and the Iapetus Ocean. This passive margin was fragmented and thrust eastwards over the Baltic Shield during Caledonian orogenesis. The individual thrust sheets in the Seve Nappe Complex went through different P–T–t evolutions, resulting 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 (subduction and subsequent uplift) processes in the thrust sheets of the Seve Nappe Complex. Even though it has been transported several hundred kilometres, only the margins of the eastern part of the Sarektjåkkå Nappe are affected by penetrative Caledonian deformation. This part of the Sarektjåkkå Nappe is dominated by pristine tholeiitic dykes and cross-bedded sandstones. The dykes are 608±1 Ma old and make up 70–80% of the nappe. Widely spaced thin shear zones of the Ruopsok fault system represent the only Caledonian penetrative deformation in the interior of the nappe. Previously published Ar–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 muscovite from a cross-laminated metapsammite in the Sarektjåkkå Nappe gave well-defined ages of 428.5±3.6 and 432.4±3.8 Ma, respectively. Muscovite 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 unknown time, presumably at the emplacement of the dyke swarm. No subsequent excess argon contamination can be detected. A likely candidate for the degassing event is the emplacement of the dykes at 608 Ma. The interior of the nappe, and thus the entire nappe complex, cooled below ∼ 350 °C at around 430 Ma. Cooling from more than 500 °C at c. 470 Ma to 350 °C at c. 430 Ma suggests an average cooling rate of [les ] 4 °C/Ma. A prolonged period of slow cooling (≈exhumation?) following the initial, rapid uplift of the eclogite-bearing nappes and Early Ordovician construction of the Seve Nappe Complex is suggested.