We present paleomagnetic results obtained from samples collected in the summer of 1989 along a 1000 km traverse between Yecheng (Xinjiang) and Shiquanhe (Tibet) in the western Qinghai‐Xizang Plateau (along ∼80°E longitude). During this field trip, 480 paleomagnetic cores in Jurassic to Tertiary limestones and sandstones were drilled at 49 sites; 400 were measured. Isothermal remanent magnetization analysis indicates the existence of high coercivity minerals, and thermal, rather than alternating field demagnetization, was found to be more efficient in isolating paleomagnetic directions. The characteristic magnetization has rather low unblocking temperatures (250–560°C) in most limestone samples and higher unblocking temperatures (400°C–680°C) in the sandstone samples. About half of the measured samples show the present geomagnetic field or aberrant directions. The remainder display stable multicomponent magnetizations. The stable characteristic magnetizations indicate (1) a probable Jurassic remagnetization; (2) a Cretaceous (120‐80 Ma) magnetization which passes a fold test, yielding a mean pole at 66.2°N, 245.0°E (A95=5.1°, N=14 sites); and (3) a large scatter of Tertiary directions, which may be due to tectonic problems and/or remagnetization in the sampling zone. The Cretaceous paleodirections from Aksaichin and Longmuco (11 sites on the Qiangtang (North Tibet) block) are very similar to those from Shiquanhe (three sites on the Lhasa (South Tibet) block), separated by 300 km. This suggests that the North and South Tibet blocks formed a single unit at least as early as the Cretaceous. Comparing our results with previous paleomagnetic work on the Tibetan plateau, it is found that the paleolatitude of 10.6°±5.1° obtained from this study for a reference at Domar (33.75°N, 80.4°E) is close to that (9.6°±2.8°) obtained by Achache et al. (1984) and Lin and Watts (1988) from the Takena formation (110‐100 Ma) in the Lhasa region (30°N, 91°E). In addition, it appears that the southern margin of the Tibetan blocks (i.e., Eurasia) occupied an E‐W trending position at about 7°±6°N latitude. Paleodeclination differences between the western and eastern Tibetan blocks suggest internal deformations of the plateau at different scales (from 1 to 1000 km). According to differences in paleomagnetic declinations, the arcuate shape of the blocks would have been partly acquired after the collision.

We conducted a paleomagnetic study on the Early Permian volcanic and sedimentary rocks, and the Neoproterozoic mafic dikes in the Yili Block, NW China. Magnetite and hematite were proven to be the principal magnetic remanence carriers. Demagnetizations revealed stable characteristic remanence magnetizations with a sole reversed magnetic polarity. The magnetic remanence of only the Early Permian strata turned out to be primary based on positive fold tests; meanwhile, the magnetic remanence age of the mafic dikes is ambiguous. Accordingly, the first Early Permian paleomagnetic pole for the Yili Block is calculated at λ = 81.5°N, φ = 256.5°E, N = 11, and A 95 = 10.9°. Comparisons of this new pole with published ones from the Yili, Tarim, and South Junggar blocks provide new quantitative constraints on late Paleozoic kinematic evolution of the SW Central Asian Orogenic Belt: (1) Between the Yili and Tarim blocks, significant relative movement took place along major strike-slip faults during the Late Carboniferous to Early Permian (580 ± 290 km) and the Early to Late Permian (585 ± 340 km), and the displacement rate increased from the Late Carboniferous to Early Permian (~19.3 ± 9.7 mm/yr) to the Early to Late Permian (~29.3 ± 17.0 mm/yr); (2) a significant relative rotation of 28.3°± 18.3°in the Late Permian, and a lateral displacement of 630 ± 295 km after the Late Permian occurred between the Yili and South Junggar blocks. The significant strike-slip movements played an important role in the formation of the Central Asian Orogenic Belt and should be considered with great attention in tectonic and paleogeographic reconstructions. ZHU ET AL.

In order to better understand the late Paleozoic tectonic evolution of the southwestern Central Asian Orogenic Belt (CAOB), we carried out structural and geochronological studies on the poorly investigated Xiaergou and Wulasitai shear zones around and in the Chinese Central Tianshan. The Xiaergou shear zone is the connecting segment between the North Tianshan Fault and Main Tianshan Shear Zone along the northern margin of the Yili-Central Tianshan blocks, it strikes NW-SE with a width of ~3-5 km and shows predominant dextral kinematics. Zircon U-Pb ages of pre-and syn-kinematic granitic dykes within the Xiaergou shear zone indicate that the dextral shearing was active at ~312-295 Ma. The Wulasitai shear zone is a high-strain belt occurring in the interior of the Central Tianshan block, it extends NW-SE for more than 40 km with variable widths of ~1-5 km, steep mylonitic foliations and sub-horizontal stretching lineation are well developed and various kinematic indicators suggest prevailing sinistral shearing. New biotite 40 Ar/ 39 Ar ages of two meta-sedimentary rocks, together with the published metamorphic zircon ages constrain the timing of the sinistral shearing at ~312-301 Ma. Our new results combined with the previous studies reveal that the dextral strike-slip shear zones framing the Central Tianshan formed almost simultaneously in the latest Carboniferous (~310 Ma) and lasted until the middle to late Permian. They resulted from the eastward tectonic wedging and relative rotations between continental blocks in the SW CAOB. The sinistral shearing of the Wulasitai shear zone within the Central Tianshan was likely generated due to differential eastward motions of the northern and southern parts of the Central Tianshan.

In order to better understand the kinematics and geodynamics of the late Paleozoic to early Mesozoic transition from accretionary to intracontinental tectonics in the southwest Central Asian Orogenic Belt, we performed a paleomagnetic study on Lower Permian strata in the Turpan‐Hami Basin, NW China. Magnetite and hematite are shown to be the principal magnetic remanence carriers. A typical sedimentary fabric that has not been modified by postdepositional deformation is recognized via magnetic fabric study. Stable characteristic remanent magnetizations, all of reverse magnetic polarity, are revealed by step demagnetizations. Remanence age is determined to be of Early Permian affinity, based on a fold test, remanence behavior of minerals, and geochronologic data. With inclination shallowing corrections, the first Early Permian paleomagnetic pole for the Turpan‐Hami Block is acquired as λ = 75.7°N, φ = 276.3°E, A95 = 5.7°, and N = 10 sites. Comparisons of this new pole with published ones from neighboring cratonic blocks provide kinematic constraints on the tectonic transition and subsequent intracontinental tectonics of this region. The Turpan‐Hami and Yili blocks have kept their relative positions since the Early Permian. Significant latitudinal movement of 14.7° ± 7.2° occurred between the Turpan‐Hami and South Junggar blocks and could be mainly accommodated by late Paleozoic crustal shortening of at least ~530 km across an intra‐arc basin in the Bogda belt. The Turpan‐Hami Block has experienced large‐magnitude relative rotations of −41.1° ± 7.1° and 21.2° ± 9.4° with respect to Tarim and South Junggar since the Early Permian, respectively, corresponding to dominant intracontinental tectonics characterized by large‐magnitude strike‐slip displacements along megashear zones.