Paleomagnetism of the Arka Tagh South of the Altyn Tagh Fault: Implications for Intracontinental Deformation Processes in Asia

The Altyn Tagh Fault (ATF) is a 1200km long left-lateral strike-slip fault bounding the Tarim Basin to the north from the Tibetan Plateau and the Qaidam Basin to the south. Determining the crustal mechanics of this fault and adjacent regions is critical to the understanding the formation of the Tibetan Plateau in particular and intracontinental deformation processes in general. Arcuate structures branching off the south side of the ATF systematically have a horsetail pattern. These structures extend up to 200 km south of the ATF and may be oroclines resulting from drag folding along the fault. Bending of these structures by up to 90 degrees would imply distributed shear south of the ATF that in turn would indicate an additional 100 km displacement between northern Tibet and the Tarim Basin. To determine whether these structures have experienced vertical-axis rotations, geological mapping and paleomagnetic studies were carried out in Jurassic to Eocene age red sedimentary rocks within the Arka Tagh, an arcuate range extending 40 km south of the ATF. The northwestern part of the Arka Tagh trends N85°E, approximately parallel the ATF while the southeastern part trends N125°E. If the curvature of the range resulted from oroclinal bending, the northwestern part should have recorded 40° counterclockwise rotation with respect to the southeastern part. Out of 109 sites sampled, 53 sites covering both parts of the structure yielded interpretable Characteristic Remanent Magnetization (ChRM) carried by hematite. The site-mean ChRM directions from both the northeastern and the southeastern regions cluster following tilt correction and pass the reversal test indicating a primary origin for the magnetization. Comparing the mean ChRM directions for the southeastern part (I=38.7° ; D=19.4° ; a95=8.7° ) to the northwestern part (I=26.8° ; D=9.1° ; a95=7.4° ) indicates no significant relative vertical-axis rotation (RažDR = 10.3° až16.1°). Thus oroclinal bending is not the primary reason for the present curvature of the Arka Tagh (and possibly other arcuate structures south of the ATF). Our preferred interpretation is that these arcuate structures resulted from thrust displacement above curved faults, thus requiring no vertical axis rotations.