Search

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. [Copyright &y& Elsevier]

The tectonic and paleogeographic evolution of the Ural-Mongol belt between the cratons of Baltica, Siberia, and Tarim is key to the formation of the Eurasian composite continent during Paleozoic time, but the views on this complicated process remain disparate and sometimes controversial. A study of three volcanic formations of mid-Silurian, Lower to Middle Devonian, and Middle Devonian age from the southwestern boundary of the Chingiz Range (NE Kazakhstan) yields what are interpreted as primary paleomagnetic directions that help clarify the evolution of the belt. A single-polarity characteristic component in mid-Silurian andesites yields a positive intraformational conglomerate test, whereas dual-polarity prefolding components are isolated from the two Devonian collections. Post-folding, reversed-polarity overprint directions have also been isolated and are likely of Permo-Triassic age. These new data can be evaluated together with previously published paleomagnetic results from Paleozoic rocks in the Chingiz Range, and allow us to establish with confidence the polarity of each result, and hence to determine the hemisphere in which the area was located at a given time. We conclude that NE Kazakhstan was steadily moving northward, albeit with variable velocity, crossing the equator in Silurian time. These new paleomagnetic data from the Chingiz Range also agree with and reinforce the hypothesis that the strongly curved volcanic belts of Kazakhstan underwent oroclinal bending between Middle Devonian and Middle Permian time. A comparison of the Chingiz paleolatitudes with those of Siberia shows, insofar as the sparse data allow, similarities between the northward motion of the Chingiz unit and that of Siberia, which imposes important constraints on the evolving paleogeography of the Ural-Mongol belt. [ABSTRACT FROM AUTHOR]