Palaeomagnetism of Ordovician and Silurian rocks from the Chu-Yili and Kendyktas mountains, south Kazakhstan

Despite significant progress during the last decade, the overwhelming majority of the details of the crustal evolution of Kazakhstan remain enigmatic. The central question of whether Kazakhstan amalgamated during the Ordovician and has acted as a coherent microplate since or whether Kazakhstan is the result of continuous accretion of terranes and arcs along a major subduction system throughout the Palaeozoic is still unanswered. Defining structural units and reconstructing their interaction relative to each other and with respect to Siberia and Baltica will improve our understanding of continent formation in Central Asia. Addressing this problem, a detailed palaeomagnetic study of the Palaeozoic formations in the Chu-Yili and Kendyktas mountains, south Kazakhstan was undertaken. Detailed stepwise thermal demagnetization experiments reveal the presence of a post-folding component of magnetization with intermediate to high unblocking temperatures (component A) in almost all samples. The in situ mean directions identified in 31 sites range in declination between 150 degrees and 260 degrees and show intermediate and upward pointing inclinations; based on the mean inclination a late Palaeozoic to early Mesozoic age is assumed for this secondary component of magnetization. After removal of component A a second component of magnetization (component B) was identified in red coloured argillites of early Arenigian age (Kendyktas Mountains) as well as in sediments of Early Silurian and Late Silurian to Early Devonian age (Chu-Yili Mountains). Component B passes the fold test (Enkin 2003) on the 95 per cent confidence level and yields a bedding-corrected site mean direction of D = 9.2 degrees, I = -16.9 degrees (five sites, k = 26.9, alpha(95) = 15.0 degrees) for the Lower Ordovician unit and D = 346.9 degrees, I = +23.8 degrees (eight sites, k = 20.8, alpha(95) = 12.4 degrees) for the Silurian and Lower Devonian units. Based on the age of angular unconformities, the timing of regional deformation in the Agalatas area can be constrained to be Late Ordovician in age and Middle Devonian in the Chu-Yili area. Based on the mean inclination, a southerly palaeolatitude of 8.6 degrees-8.7 degrees/+7.6 degrees can be derived for the study area during the Early to Late Ordovician and a northerly palaeolatitude of 12.4 degrees + 7.7 degrees/-6.6 degrees for the Silurian to Early Devonian. The observed mean declinations for the post-folding component A are significantly different from the expected reference directions for the Permian based on either the apparent polar wander path for Baltica or Siberia and suggest counter-clockwise rotations of Kazakhstan with respect to both during Devonian to Permian time.