Constraints on the mechanism of the Altyn Tagh fault from paleomagnetism.

摘要

First, paleomagnetic data were obtained from Tertiary red beds at two localities separated by several hundred kilometers within the Qaidam Basin. When compared with equivalent-age expected directions for Eurasia, the mean paleomagnetic directions indicate no Neogene vertical-axis rotation of the Qaidam Basin or the Altyn Tagh fault. The Qaidam Basin may act as an indentor translating without rotation toward the Sino-Korean craton.; Second, the observed paleomagnetic direction from Miocene red beds in the central Tarim Basin indicates 30.8° ± 5.5° flattening of inclination and 15.3° ± 6.7° clockwise vertical-axis rotation. Anisotropy of magnetic susceptibility measurements indicate a rock-magnetic (depositional or compaction shallowed) origin for the inclination flattening. Local deformation is the preferred interpretation for the vertical-axis rotation. Third, paleomagnetic data were obtained along the arcuate Tula syncline to test whether its strike curvature is due to oroclinal bending. The observed 13.3° ± 8.8° declination difference between the two halves of the Tula syncline is far less than the ∼40° difference predicted by 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 Qaidam Basin, this result indicates that crustal displacement between the Tarim Basin and the Tibetan Plateau is accommodated by strike-slip motion on the Altyn Tagh fault rather than distributed shear within the northern Tibetan Plateau.; Finally, we present results from red beds in the Altun Shan, the Qilian Shan/Nan Shan fold-thrust-belt and the Hexi corridor. In the Altun Shan, results from Miocene beds indicate no significant vertical-axis rotation confirming that the Altyn Tagh fault has not rotated and that sinistral shear strain is concentrated on the fault. Paleomagnetic results from Nan Shan/Qilian Shan fold-thrust-belt indicate no rotation since 30 Ma contrasting with prediction from previous kinematic models. This result indicates that there is no transrotation of the belt associated with motion on the Altyn Tagh fault. In the Hexi corridor, the observed mean paleomagnetic direction is concordant with the expected direction at 110 Ma providing a new local paleomagnetic reference for the Sino-Korean craton in agreement with published reference paleomagnetic poles for Eurasia.