Nadir Halim

The north‐striking Tancheng‐Lujiang (Tan‐Lu) fault is a conspicuous and controversial feature of the eastern Asian landscape. Near the southeast extremity of the fault in Anhui Province, we collected paleomagnetic samples at 17 Middle Triassic (T2) and 10 Upper Cretaceous (K2) to lower Cenozoic (E1) sites. T2 remanent magnetizations are interpreted as primary in two of three areas. The three areas are rotated 37° to 137° counterclockwise with respect to the South China Block (SCB) reference direction. K2‐E1 remanent magnetization directions pass regional fold and reversals tests and are not rotated with respect to surrounding areas. Counterclockwise rotation of T2 strata therefore ended before K2 and is attributed to left lateral shear acting along Tan‐Lu during the North China Block (NCB)‐SCB collision. In Shandong Province, 700 km north of the Anhui sites, four areas containing 33 Upper Jurassic (J3) and Cretaceous sites have negligible declination differences, except for one which has dispersed directions. The fold test is inconclusive for this latter area and positive for the other three. Regional concordance of the J3‐E1 paleomagnetic data (including paleolatitudes) together with observed deformation patterns suggest that an extensional regime prevailed in the Late Cretaceous and Cenozoic. Euler pole positions that constrain the North‐South China collision and account for Tan‐Lu motion suggest at least 500 km of sinistral shear took place along the fault, and either (1) subduction and related ultrahigh pressure (UHP) metamorphism occurred near the present location of the Qinling‐Dabieshan and Sulu UHP belts while Tan‐Lu acted as a transform fault that connected the two subduction zones, or (2) Tan‐Lu and Sulu were parts of the same transform fault system and no UHP rocks formed in situ at Sulu. In either case, UHP rocks originally exhumed near Dabieshan could have been transported by plate capture toward Sulu along Tan‐Lu. After North and South China impacted near Dabieshan, the Tan‐Lu fault grew within the SCB as the Dabieshan corner indented the SCB, causing folds in SCB cover rocks to conform to the NCB margin. Late Cretaceous to Cenozoic reactivation of Tan‐Lu, with both right lateral strike‐slip and normal fault motion, occurred as the SCB extruded east relative to the NCB under the influence of the India‐Asia collision.

We present new palaeomagnetic results from the Transbakal area (SE Siberia), from the Mongol-Okhotsk suture zone, the boundary between the Amuria and Siberia blocks. In order to better constrain the time of closure of the Mongol-Okhotsk Ocean in the Mesozoic, we collected 532 rock samples at 68 sites in six localities of basalts, trachy-basalts and andesites, from both sides of the Mongol-Okhotsk suture: at Unda river (J 3 ; 51.7 N, 117.4 E), Kremljevka peak (K 1 ; 51.8 N, 117.5 E) and Torey lakes (K 1 ; 50.1 N, 115.9 E) on the southern side of the suture, and at Monostoy river (J 1 ; 51.1 N, 106.8 E), Ingoda river (K 1 ; 51.2 N, 112.2 E) and Bichura town (K 1 ; 50.6 N, 107.6 E) on the northern side. Progressive thermal demagnetization enabled us to resolve low (LTC) and high (HTC) temperature components of magnetization at most sites. Jurassic palaeopoles computed from the HTCs show a large discrepancy with respect to the Apparent Polar Wander Path of Eurasia, which we interpret in terms of 1700-2700 km of post-Late Jurassic northward movement of Amuria with respect to Siberia. Although geological data suggest a middle Jurassic closure of the Mongol-Okhotsk Ocean in the west Trans-Baikal region, our data give evidence of a large remaining palaeolatitude difference between the Amuria and Siberia blocks. In contrast, Early Cretaceous sites cluster remarkably well along a small-circle, which is centred on the average site location. This implies the absence of post-Early Cretaceous northward motion of Amuria relative to Siberia, and demonstrates the pre-Early Cretaceous closure of the Mongol-Okhotsk Ocean. Finally, we interpret the very large tectonic rotations about local vertical axes, evidenced by the small-circle distribution of poles, as arising both from collision processes and from left-lateral shear movement along the suture zone, due to the eastward extrusion of Amuria under the effect of the collision of India into Asia.

In order to better understand the tectonic evolution of central Asia under the influence of the India‐Asia collision, we carried out a paleomagnetic study of 1500 cores from 106 sites along the Altyn Tagh fault, in the Qaidam and Tarim basins, and on the Tibetan plateau. Samples were mainly collected from Jurassic to Neogene siltstones and sandstones. In most cases stepwise thermal demagnetization unblocks low and high temperature components carried by magnetite and hematite. Low temperature components are north and down directed and lie close to the recent geomagnetic field. High temperature components from 10 of 13 age/locality groups pass fold and/or reversal tests and likely represent primary remanent magnetizations. The ten overall mean directions display a complex pattern of vertical‐axis block rotations that are compatible with a tectonic model of clockwise rotation of the Qaidam Basin and concomitant left‐lateral slip on the Altyn Tagh fault. Two of the ten localities are rotated significantly counterclockwise; they lie adjacent to the Altyn Tagh fault zone, consistent with the idea that left‐lateral strike‐slip motion occurred along it. The age of counterclockwise rotation near the eastern extremity of the fault was dated as younger than 19 Ma. Three widely spread areas within the Qaidam Basin exhibit similar and significant clockwise rotations, on the order of 20°, with respect to the North China Block, Tarim and Eurasia. The mean of the three values is thought to represent the total rotation of Qaidam. Because the youngest rocks displaying clockwise rotations are Oligocene, the main phase of Qaidam Basin rotation, and hence shear on the Altyn Tagh fault, took place after or near the end of the Oligocene (∼24 Ma). Upper Neogene strata located on the Qaidam Basin are not significantly rotated, thus tectonic deformation acting since the Upper Neogene (∼5 Ma) is not resolvable by paleomagnetic methods. Given a 20° ± 5° clockwise rotation of the Qaidam Basin with respect to the Tarim Basin, the maximum left‐lateral displacement on the Altyn Tagh fault since 24 Ma is 500 ± 130 km.

We present new paleomagnetic results obtained at 39 sampling sites from five sections of Tertiary red bed formations: two Eocene formations from the Qiangtang block of Tibet (Xialaxiu locality; 32.8°N, 96.6°E) and the Xining basin of Qaidam (Xining locality; 36.5°N, 102.0°E) and three Neogene formations from the Xining basin (Jungong locality; 34.7°N, 100.7°E) and the Kunlun block (Tuoluo lake and West Yushu localities; 35.3°N, 98.6°E and 33.2°N, 96.7°E, respectively). Thermal demagnetization of the rocks isolated a high‐temperature component that we interpret as the primary magnetization in four localities. The paleopoles lie at 52.6°N/352°E ( dp / dm = 6.0°/10.7°) for Xialaxiu, 61.6°N/211.3°E ( dp / dm = 9.7°/16.1°) for Xining, 66.0°N/228.6°E ( dp / dm = 3.6°/6.9°) for Jungong, and 53.9°N/205.4°E ( dp / dm = 5.6°/10.0°) for West Yushu. As in previous studies of Tertiary formations from Asia, the inclinations we obtained are shallower (by 18° to 26°) than the magnetic field computed from the Eurasian apparent polar wander path (APWP) at 10 and 20 Ma for Neogene rocks and at 40 and 60 Ma for Eocene rocks. On the basis of a compilation of Eocene data from the South China Block, Tibet, central Asia and Kyrgyzstan, we conclude that this inclination anomaly reflects erroneous predictions of positions of the Siberian craton when based on the APWP of Eurasia. The main reason for this discrepancy might be nonrigid behavior of the Eurasian plate in the Tertiary. Combination of this with intracontinental shortening of Asia under the penetration of India provides a full explanation for the anomaly. Verification of this new interpretation of the “inclination anomaly” will require new geologic and paleomagnetic data from the northern parts of these remote regions in Mongolia and Siberia.

We present the results of a paleomagnetic study of 360 cores, drilled at three different areas from the Tibetan Plateau: 13 sites from Lower Cretaceous red beds in the Xining‐Lanzhou basin, around Xining and Lanzhou cities, southwest of the Qilian mountains (36.2°N, 103.5°E); 13 sites from Cretaceous red beds in the Kunlun block, near Maqin (34.5°N, 100.1°); and 9 sites from Paleocene/lower Eocene red beds in the Qiangtang block, near Fenghuoshan (34.5°N, 92.8°E). Thermal demagnetization of the samples allowed us to isolate a high‐temperature component which passes both positive reversal and fold tests for all formations. The corresponding paleopoles lie at 50.3°N, 195.5°E (A 95 = 4.6°) for Xining‐Lanzhou, 80.1°N, 281.2°E (dp = 7.8°, dm = 12.7°) for Maqin, and 62.6°N, 210.5°E (dp = 3.9°, dm = 6.8°) for Fenghuoshan. We discuss these in the frame of a new paleogeographic reconstruction of the Cretaceous paleoposition of the blocks forming the Asian mosaic. We conclude that the Xining‐Lanzhou area could not be part of the North China Block but rather was associated with the Tarim‐Qaidam assemblage. Paleomagnetic data argue in favor of a Qaidam‐Kunlun‐Tarim‐Junggar assemblage in the Cretaceous, significantly to the south of its current position with respect to the Asian continent (Siberia, Mongolia and North China). The large N‐S convergence (800±500 km) implied since the Cretaceous appears to be far larger than could be absorbed in the Altay range to the north and Qilian Shan to the east (of the order of 300 km). Part of this motion could have occurred along a large left‐lateral strike slip fault system, which may connect with the Mongol‐Okhotsk suture to the northeast. Verifying this hypothesis will require new geologic and paleomagnetic data from these remote regions.