ACCRETION AND EXHUMATION AT A VARISCAN ACTIVE MARGIN, RECORDED IN THESAXOTHURINGIAN FLYSCH

The Saxothuringian flysch basin, on the north flank of the Central Eur opean Variscides, was fed and eventually overthrust by the northwester n, active margin of the Tepla-Barrandian terrane. Clast spectra, miner al composition and isotopic ages of detrital mica and zircon have been analyzed in order to constrain accretion and exhumation of rocks in t he orogenic wedge. The earliest elastic sediments preserved are of ear ly Famennian age (ca. 370Ma). They are exposed immediately to the NW o f the suture, and belong to the par-autochthon of the foreland. Beside s ultramafic (?ophiolite) material, these rocks contain clasts derived from Early Paleozoic continental slope sediments, originally deposite d at the NW margin of the Saxothuringian basin. These findings, togeth er with the paleogeographic position of the Famennian clastics debris on the northwestern passive margin, indicate that the Saxothuringian n arrow ocean had been closed by that time. Microprobe analyses of detri tal hornblendes suggest derivation from the ''Randamphibolit'' unit, n ow present in the middle part of the Saxothuringian allochthon (Munchb erg nappes). Detrital zircons of metamorphic rocks formed a little ear lier (ca. 380 Ma) indicate rapid recycling at the tectonic front. The middle part of the flysch sequence (ca. early to middle Vistan), both in the par-autochthon and in the allochthon, contains abundant clasts of Paleozoic rocks derived from the northwestern slope and rise, toget her with debris of Cadomian basement, 500-Ma granitoids and 380 Ma (ea rly Variscan) crystalline rocks. All of these source rocks were still available in the youngest part of the flysch (c. middle to late Visean ), but some clasts record, in addition, accretion of the northwestern shelf. Our findings permit deduction of minimum rates of tectonic shor tening well in excess of 10-30mm per year, and rates of exhumation of ca. 3 mm/a, and possibly more.