SILURIAN PALEOGEOGRAPHY OF ARMORICA - NEW PALEOMAGNETIC DATA FROM CENTRAL BOHEMIA

Due to the lack of reliable paleomagnetic data from Armorica for mid-O rdovician to Late Devonian times, the drift history for this tectonic element is poorly understood. In order to help rectify this problem, a paleomagnetic study of Upper Silurian volcanic, intrusive and sedimen tary rocks from the Barrandian Basin of the Central Bohemian Massif (C zech Republic) has been carried out. This basin comprises a sequence o f unmetamorphosed lower Paleozoic rocks, the folding of which has an u ppermost age limit of Early Carboniferous. Stepwise thermal demagnetiz ation of the samples collected (182 samples from 21 sites), and princi pal component analysis of both great circle and stable endpoint data r eveal a total of three directions of magnetization. The first (labelle d A) is in the direction of the present-day local Earth's magnetic fie ld (D/I=360-degrees/67-degrees) and is thus interpreted as being of re cent origin. The other two directions, termed B and C, comprise the in termediate and high blocking temperature directions respectively. The overall in situ mean direction obtained for B, identified between 150- degrees and 320-degrees-C, is 195-degrees/-8-degrees (D/I), k=18.5, al pha95=13.2-degrees (eight sites). It fails the fold test and yields an in situ paleo-south pole position of 42-degrees-S, 354-degrees-E. Thi s coincides with the Late Carboniferous segment of the apparent polar wander (APW) path for stable Europe and is considered to be a secondar y overprint of this age. The third magnetization (termed C) is identif ied between 320-degrees and 600-degrees-C, gives an overall mean direc tion of 208-degrees/-40-degrees, k=96.8, alpha95=5.7-degrees (eight si tes), after bedding correction. It passes fold and reversal tests on b oth local and regional levels and is interpreted as representative of the Late Silurian paleofield direction in the Bohemian Massif; it yiel ds a paleo-pole position of 55-degrees-S, 325-degrees-E. Assuming a no rmal polarity, the inclination value obtained translates into paleolat itudes of 23-degrees-S for this part of Armorica and implies a positio n adjacent to the southern margin of Baltica. This therefore indicates closure of the intervening ocean by Late Silurian times. However, ado pting this polarity also implies large-scale (up to 140-degrees) antic lockwise rotations of the Bohemian Massif prior to deformation in the Early Carboniferous.