The role of the Kazakhstan orocline in the late Paleozoic amalgamation of Eurasia

Abstract

The Kazakhstan orocline, a horseshoe-shaped belt with volcanic arcs of Devonian (external) and late Paleozoic (internal) age, is thought to have formed as a result of convergence between the cratons of Siberia, Baltica and Tarim leading to the amalgamation of Eurasia. Paleomagnetic and geologic data indicate that in the Middle Devonian the arc, which is now strongly curved, was nearly straight near the northwest–southeast trending volcanic margin of a Kazakhstania continent. To constrain the age of oroclinal bending we conducted a paleomagnetic study of Late Carboniferous to Late Permian subduction-related volcanics from the middle (NW) and north-eastern (NE) limbs of the orocline. Our new results indicate that the rotation of the middle arm of the orocline was essentially completed by the earliest Permian, while the NE arm probably was still ∼ 50° short of its final orientation with respect to Baltica. The rotation of, or rotation within, the NE arm was completed by the Late Permian. The paleomagnetic data constraining the timing and rotation patterns lead us to propose the following scenario for the bending of the Kazakhstan orocline. The orogenic deformation scenario began in the Late Devonian when an initial collision with Tarim pinned Kazakhstania's southern corner, while a dextral shear motion and a considerable clockwise rotation of Siberia dragged its northern end. Relative convergence between Siberia and Tarim caused initial buckling of the Kazakhstania continental element trapped between them, subdividing the belt into three (SW, NW, NE) segments. Continued subduction under the established limbs of the orocline with an estimated outward-directed subduction velocity of well less than 1 cm/yr gradually led to closure of the intervening Junggar–Balkhash oceanic basin and tightening of the orocline.