Paleomagnetism indicates no Neogene vertical axis rotations of the northeastern Tibetan Plateau

Paleomagnetic data were obtained from 108 paleomagnetic sites collected in Cretaceous to Tertiary red beds from seven localities distributed in three general regions adjacent to the Altyn Tagh fault at the northern edge of the Tibetan Plateau. In the Hexi corridor, 12 sites in Oligocene strata at Yaoquanzi (39.97degreesN; 97.68degreesE) yield a mean paleomagnetic direction (I = 33.0degrees; D = 8.6degrees, alpha(95) = 6.0degrees) with concordant declination, and 36 sites in Early Cretaceous mudstones of the Longshou Shan (39.09degreesN; 100.50degreesE) provide a concordant paleomagnetic direction (I = 53.4degrees; D = 6.8degrees, alpha(95) = 3.8degrees). Across the Nan Shan fold-thrust belt, Miocene paleomagnetic directions from 39 sites distributed among four localities have a concordant mean declination (I = 40.6degrees; D = 7.2degrees, alpha(95) = 5.8degrees). In the Altyn Tagh range, 21 sites in Oligocene strata at Xorkoli (38.93degreesN; 91.43degreesE) yield a paleomagnetic direction with concordant declination (I = 49.7degrees; D = 5.0degrees, alpha (9)5 = 5.4degrees). These results combined with existing regional paleomagnetic data indicate that (1) the Hexi corridor along with the North China block has not undergone tectonic vertical axis rotation since at least Early Cretaceous time and is separated from the Xining-Lanzhou basin and eastern Tibet by an important post-Early Cretaceous tectonic boundary; (2) the Nan Shan fold-thrust belt and the Qaidam Basin have not experienced wholesale vertical axis rotation during Neogene time; and (3) absence of vertical axis rotations in areas adjacent to the Altyn Tagh fault indicates that sinistral shear strain between the Tarim Basin and the northern Tibetan Plateau is concentrated on the fault. These results are consistent with Asian tectonic models that combine distributed lithospheric deformation and thickening with narrow and weak shear zones.