Paleornagnetic constraints on the paleogeography and oroclinal bending of the Devonian volcanic arc in Kazakhstan

摘要

A prominent feature of the central part of the Ural-Mongol orogenic belt is a series of concentric horse-shoe shaped volcanic arcs, with the youngest arc on the inside. This structure was long-suspected to be an orocline, but unequivocal evidence for this was lacking, mainly because paleomagnetic results of suitable age from this area remained sparse, but also because their interpretation was not straightforward due to a long history of deformations associated with the protracted late Paleozoic assembly of Asia. Our paleomagnetic study of Middle Devonian basaltic and andesitic flows in southeastern Kazakhstan revealed two main components of magnetization. The primary nature of a high temperature magnetization (tilt corrected Dec=286.5, Inc=46.4, alpha(95) = 7.8, k = 29.2, N = 13 sites) is supported by the presence of antipodal directions and a baked-contact test. We also isolated a post-folding overprint with an in situ mean direction Dec= 134.9, Inc=-43.0 (alpha(95) = 4.9, k =71.6, N = 13 sites). The age of this overprint can be estimated as Early Permian with a high degree of confidence. The declination of the overprint is seen to be deflected counter-clockwise by 100 +/- 6 degrees relative to the 290-Ma reference direction, indicating that the studied locality, similar to many other localities in the region, was affected by late-orogenic rotations. We use the overprint's deflection to correct the declination of the primary Devonian magnetization for these late-orogenic block-rotations. Declinations from other Silurian and Devonian paleomagnetic results in the subduction-related Devonian volcanic arc of Kazakhstan have been corrected for such rotations wherever the latter are reasonably well documented. Using corrected declinations as passive markers we restored the trend of the volcanic belt to its Devonian configuration. Our analysis indicates that the presently curved belt was nearly straight and NW-SE trending. This similar to 1500 km long volcanic belt characterized the northeastern margin of a landmass in today's central Kazakhstan where subduction occurred towards the southwest. Oroclinal bending of this arc took place in the interval between the Middle Devonian and the Late Permian. (C) 2007 Elsevier B.V. All rights reserved.