Concentration of crustal displacement along a weak Altyn Tagh fault: Evidence from paleomagnetism of the northern Tibetan Plateau

[ 1] Like many regional structures in the northern Tibetan Plateau, the Tula syncline changes strike by -40degrees from NW-SE to nearly EW as it approaches the Altyn Tagh fault from the south. To test whether this strike curvature is due to oroclinal bending, we analyzed paleomagnetic samples from 109 sites collected from Late Jurassic to Paleogene red sedimentary strata of the Tula syncline. Fold and reversal tests suggest a primary origin for the characteristic remanent magnetization from nine sites in the eastern half and 41 sites in the western half of the syncline. The observed 13.3degrees+/- 8.8degrees declination difference between the two halves of the Tula syncline is far less than the -40degrees difference predicted for oroclinal bending. Instead the arc shape of the syncline is an original configuration produced by transport above an arcuate thrust ramp. Along with paleomagnetic data from the northern Qaidam Basin, these results from the Tula syncline indicate that crustal displacement between the Tarim Basin and the northern Tibetan Plateau is accommodated by strike-slip motion on the Altyn Tagh fault rather than distributed shear within the northern Tibetan Plateau.