PALEOMAGNETISM APPLIED TO VERTICAL-AXIS TECTONIC ROTATIONS AND DEPOSITIONAL HISTORY OF TERTIARY STRATA ALONG THE NORTHERN MARGIN OF THE TIBETAN PLATEAU

Cenozoic deformation of northern Tibet is expressed by the development of the 1200km-long left-slip Altyn Tagh fault (ATF) and the kinematically linked western Kunlun and Nan Shan - Qilian Shan thrust belts at its two ends. More than 4300 paleomagnetic samples from 780 sedimentary horizons on both sides of the ATF were collected from predominantly Tertiary red sedimentary strata to document the Cenozoic vertical-axis rotational and depositional histories. Arcuate structures branching south of the ATF suggest oroclinal bending. For example, the northwestern part of the Arka Tagh (and the Tula syncline) trends N85E, approximately parallel to the ATF, while the southeastern part trends N125E. Fold and reversal tests suggest a primary origin for paleomagnetic directions from nine sites in the eastern half of the syncline and 41 sites in the western half of the syncline. However, the observed difference in paleomagnetic declination between the two halves of the Tula syncline is far less than the ~40 degree difference predicted by oroclinal bending. Instead the arc shape of the syncline is the original configuration produced by transport above an arcuate thrust ramp. Concordant paleomagnetic declinations in the northern Qaidam basin further suggest crustal displacement between the Tarim Basin and northern Tibet is concentrated along the ATF. Within the Nan Shan - Qilian Shan thrust belt, no systematic rotation of regional scale was observed. Additionally, paleomagnetic data from the Longshou Shan indicate no Cenozoic vertical-axis rotation of the Hexi corridor. A major conclusion is that documenting the absence of vertical-axis rotations is critical to regional tectonic syntheses and deciphering the crustal mechanics of thrust belt curvature. Our magnetostratigraphic studies of Tertiary continental sedimentary sections adjacent to ATF illustrate the challenges posed by sparse independent age constraints. Integration of field mapping, sedimentology, magnetostratigraphy, sandstone petrology, and fission-track dating of detrital apatites in the Tertiary strata was required to achieve geochronologic calibration. Our results suggest that crustal thickening in the western Kunlun Shan, central Altyn Tagh range, and the Nan Shan of northern Tibet initiated at or prior to 30-25 Ma.