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 Baltoscandi an passive margin just before the inception of seafloor spreading is d escribed, recording the change from deformation by faulting to dominan tly magmatic extension of the crust. The allochthon of the Scandinavia n Caledonides contains the imbricated passive margin of continental Ba ltica overlain by various exotic terranes. The Sarektjakka Nappe in th e Seve Nappe Complex, which contains the outer parts of Baltica's pass ive margin, consists of sedimentary rocks, occurring as screens betwee n 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 preserve s igneous, metamorphic and structural elements that are linked to the evolution of a pre-Caledonian rift to a passive continental margin. Ex tensional deformation before dyke emplacement is recorded by normal fa ults, pull-apart structures and folds. Unconformities, dykes affected by brittle deformation, and fluidization of sediments during dyke empl acement 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 tec tonic evolution is discussed.