THE KAZAKHSTAN OROCLINE (CENTRAL ASIA). PART 1: PALEOMAGNETISM AND PALEOLATITUDINAL MOTION

A major feature of Eurasia's tectonic pattern is the Ural-Mongol belt, UMB. Its evolution remains highly controversial as evidenced by disparate UMB palinspastic models. Kazakhstan, in the central part of the UMB, is a key to understanding the history of the entire belt. As a first approximation, the geological structure of Kazakhstan can be regarded as a large-scale, highly curved belt or almost closed loop, and the evolution of this structure is of crucial importance. Analysis of geological data shows that this loop comprises subduction-related complexes ranging in age from at least the Middle Ordovician to the Late Permian. An age-progressive inward shift of volcanic activity persisted for this interval. However, the geometry of this nearly circular arc would be incompatible with subduction, unless oroclinal bending has occurred. To test this, we performed paleomagnetic studies of Middle Cambrian through Late Permian rocks from the Chingiz (CH) and North Tien Shan (NTS) areas at the NE and SW arms of the loop, respectively. After removal of a ubiquitous late Paleozoic overprint, presumably primary magnetizations were isolated from Upper Cambrian, Lower Ordovician, Lower Silurian, Middle Devonian and Upper Permian rocks in the Chingiz area, and from Upper Ordovician, Upper Silurian, Lower and Middle Carboniferous, and Permian rocks in the North Tien Shan. Assuming unidirectional motion of the CH and NTS through the Paleozoic, and interpreting the changing inclination patterns as reflecting steady northward displacements, we find that the northern arm of the loop underwent large-scale rotations (up to 180 degrees), whereas the southern arm did not. This suggests oroclinal bending during the late Paleozoic, presumably upon amalgamation with Baltica and Siberia. To first approximation, this is in agreement with the model of the UMB evolution as part of the Kipchak Arc proposed by Sengor & Natal'in [1996].