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Silurian effusive basalts and volcaniclastics compose the Svatý Jan volcanic center, which is located in the northwestern limb of the Prague synform, where three major volcanic phases have been recognized: the first one of early to mid-Wenlock and the last of mid-Ludlow age. Two alkaline basalt dikes of late Wenlock to mid-Ludlow age, respectively tilted to the west and to the northeast, as observed in a 100-m-thick tuff sequence, which represents the second volcanic phase, have been extensively sampled. An anisotropy of the magnetic susceptibility (AMS) study of seventy-nine specimens taken from a 5-m-thick dike (dike1) and thirty-two specimens cored in a 3.5-m-thick dike (dike2) shows two different fabrics, carried mainly by Ti-magnetite and/or magnetite, which are considered to be related to the transtensional opening phase of the dikes. Four components of magnetization, attributed to Middle-Late Silurian (C1), Middle-Late Carboniferous (C2), Cretaceous (B), and Paleocene (D), in agreement with already-published directions for the Bohemian Massif, have been isolated. They are carried by Ti-magnetite for components C1 and C2, hematite and goethite for components B and D. The opening mode, which controlled both dikes, corresponds to a dextral transtensional regime, as deduced from the AMS K1 axis. They may have been opened during several magmatic stages related to different injections during late Wenlock to mid-Ludlow times. The first stage is dominant and controlled by the primary fabric, which is mainly oblate. With a NNW-SSE strike, perpendicular to the shortening direction, this fabric is in agreement with the direction of emplacement of the nappes during the Late Devonian. At that time the nappes emplacement that pre-dates this direction was probably associated with the sinistral closure of the Rheic Ocean, in agreement with post-Givetian folding and faulting, which deformed the synform infill and closed the Barrandian marine sedimentary cycle.

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