Primary Carboniferous and Permian paleomagnetic results from the Yili Block (NW China) and their geodynamic implications on the evolution of Chinese Tianshan Belt.
Résumé
In order to better understand the tectonic role of the Yili Block on the Paleozoic evolution of the Chinese Tianshan Belt, we performed a primary paleomagnetic study on Carboniferous and Permian rocks from different areas in the Yili Block, NW of China. More than 320 sedimentary and volcanic samples were collected from 39 sites. Except for the Ordovician samples from the Axi area showing a weak and unstable magnetic remanence, the majority of this collection present characteristic remanent magnetization carried by magnetite and hematite. Though positive fold test has been observed on the part of Early Carboniferous rocks, a general remagnetisation of these rocks has been identified due to Late Carboniferous magmatism in this area. Moreover, all Early and Late Carboniferous samples of Zhaosu and Xinyuan areas located in the interior of the Yili Block yield stable and coherent magnetic directions with exhaustively reversed magnetic polarity. The Late Carboniferous is considered as the remanent age since these rocks are covered or intruded by synchronous magmatic rocks of the Yili arc, which lasted until to –310 Ma. The C2 paleomagnetic pole is therefore calculated at 68.6°N, 290.6°E with !95=6.4° and n=15. The Late Carboniferous rocks from Yuxi section located close to deformation zone present a coherent magnetic inclination but significant different declination with respect to other areas and are suspected to have probably experienced a local rotation. Although no fold test can be performed due to the monoclinal bedding, stable magnetic components are isolated from Late Permian (P2) red beds of the Gongliu area with also a solo reversed magnetic polarity, and the P2 paleomagnetic pole of the Yili Block has been, therefore, calculated from the characteristic remanent magnetization: 79.7°N, 172.0°E with !95= 11.3° and n=5. Comparison of the C2 and P2 paleomagnetic poles of the Yili Block with coeval poles of Junggar, Tarim and Siberia indicates (1) no significant relative motion between the Yili and Junggar blocks since the Late Carboniferous, (2) no significant or weak latitudinal relative motion occurred since the Late Carboniferous among these blocks, but (3) the 46.2°±15.1° and the 31.6°±15.1° counterclockwise rotations of the Yili-Junggar blocks with respect to Tarim and Siberia took place during C2 to P2. These rotations are accommodated by the Permian dextral strike-slip faults along the northern and southern sides of Tianshan Belt and sinistral strike-slip faulting along the Erqishi Fault of Altay Belt, resulting in about 1000 km and 600 km lateral displacements in the Tianshan and Altay belts, respectively.