Paleomagnetism of Silurian and Devonian rocks from Central and Northern Kazakhstan - comparison with South Kazakhstan directions.

Central and Northern Asia consist of a set of continental blocks, which were amalgamated and subsequently welded to Europe during Carboniferous and Permian times. Within this mosaic, Kazakhstan takes a central position. Understanding the tectonic evolution of this area is the key to understanding the tectonic processes to the east of the Urals as well as for the understanding of the geological history of Eurasia as a whole during the Paleozoic. Here we present the results of a detailed paleomagnetic study of Silurian and Devonian rocks from the Teniz depression (Central Kazakhstan) and the Chingiz Range (Northern Kazakhstan). Overprint components were isolated for Lower Silurian redbeds of Northern Kazakhstan(D=352.2\Grad, I=+87.2\Grad, k=20.3, α _{95}=6.4\Grad, 11 sites) 2),Lower Silurian redbeds from the Chingiz Range(D=193.0\Grad, I=-65.2\Grad, k=25.2, α _{95}=15.5\Grad, 5 sites) Middle - Upper Devonian redbeds from Central Kazakhstan (D=005.0\Grad, I=+62.6\Grad, k=37.9, α _{95}=5.9\Grad, 10 sites. Both, prefolding directions published elsewhere and secondary magnetizations, presented here identified in rocks covering in time the Ordovician to Carboniferous indicate counterclockwise rotations of Southern Kazakhstan with respect to Baltica (80\Grad) and to Siberia (140\Grad) since Ordovician times. These rotations are not in accord with tectonic models based on the existence of the Kipchak arc (Sengör et al., 1993, Sengör and Natal'in, 1996) during the Paleozoic. These models postulate the existence of an Early Paleozoic volcanic arc, dconnecting Baltica in the South to Siberia in the North with the edges of the arc being tied to these two cratons. The suggested geometry of the original arc puts very strict kinematic constraints on relative motions and rotations of individual blocks within this framewor. In particular it implies, that since the Early Devonian, southern Kazakhstan has experienced clockwise rotation of about 90° relatively to Baltica and about 30\Grad clockwise rotation with respect to Siberia (Sengör and Natal'in, 1996). This is in strong contrast to the observed paleomagnetic results indicating counterclockwise rotation.of a consolidated Kazakhstan independent of both Baltica and Siberia.