EXTENSIONAL DEFORMATION ALONG THE LATE PRECAMBRIAN-CAMBRIAN BALTOSCANDIAN PASSIVE MARGIN - THE SAREKTJAKKA NAPPE, SWEDISH CALEDONIDES

The structural evolution of a part of the late Precambrian Baltoscandian passive margin just before the inception of seafloor spreading is described, recording the change from deformation by faulting to dominantly magmatic extension of the crust. The allochthon of the Scandinavian Caledonides contains the imbricated passive margin of continental Baltica overlain by various exotic terranes. The Sarektjakka Nappe in the Seve Nappe Complex, which contains the outer parts of Baltica's passive margin, consists of sedimentary rocks, occurring as screens between Vendian (573 +/- 74 Ma) diabase dykes. These dykes constitute 70-80% of the nappe and locally form sheeted dyke complexes. The Sarektjakka Nappe largely escaped penetrative Caledonian deformation and preserves igneous, metamorphic and structural elements that are linked to the evolution of a pre-Caledonian rift to a passive continental margin. Extensional deformation before dyke emplacement is recorded by normal faults, pull-apart structures and folds. Unconformities, dykes affected by brittle deformation, and fluidization of sediments during dyke emplacement indicate close relations between the deposition of sediments, extensional deformation and dyke emplacement. The Sarektjakka Nappe is compared with other parts of the Baltica's passive margin and its tectonic evolution is discussed.