THE KAZAKHSTAN OROCLINE (CENTRAL ASIA). PART 2: EVIDENCE FOR RELATIVE ROTATIONS

Models for the amalgamation of the Kazakhstan part of the Ural-Mongol Belt range between two end-members. One model implies that the now-telescoped elements of the area were originally part of one long arc (the Kipchak arc proposed by Sengor and colleagues). This model involves large rotations of some of the elements. The other model involves successive accretion of independent and separate smaller arc fragments, not necessarily accompanied by large-scale rotations. The area today shows a strongly curved structure. Oroclinal bending of this structure is supported by three lines of evidence. (1) Paleomagnetic directions in the NE (Chingiz) arm of the belt show declinations in the southeastern and southwestern quadrants for what are interpreted to be normal-polarity primary early and mid-Paleozoic magnetizations, as discussed in the preceding presentation. This indicates that large rotations occurred after the youngest (Devonian) of these magnetizations. In contrast, the SW arm of the belt in the North Tien Shan reveals early and mid-Paleozoic magnetizations, also interpreted to be normal-polarity, which are primarily northerly. Thus the southwestern arm did not experience large rotations. (2) Devonian directions recorded by us and by Grishin et al. (Geotectonics, vol. 31, pp. 65-75 [1997]) in the NE arm show south-southwesterly normal-polarity declinations, whereas in the NW arm east-southeasterly declinations have been observed. The change in declinations (from SSW to ESE) correlates with the change in strike (from SE to NE). (3) Our new Ordovician results from the NE (Chingiz) and SW (North Tien Shan) arms of the curved belt reveal declinations that are about 180 degrees apart, when taking the shallowly upward inclinations of both the magnetizations as being of similar paleolatitude and the same polarity. To first approximation, this is in good agreement with the Kipchak Arc model proposed by Sengor & Natal'in [1996]. By Permian time, the rotations appear to have been largely completed.