Two-stage strike-slip faulting of the Altyn Tagh Fault revealed by magnetic fabrics in the Qaidam Basin

Abstract

The timing and mechanism of the changes in the stress/strain field direction on the northeastern Tibetan Plateau are controversial, which limits our understanding of the tectonic evolution of the northeastern Tibetan Plateau and the Altyn Tagh Fault (ATF). Here, we conducted a detailed anisotropy of low-field magnetic susceptibility analysis of successive fluvial-lacustrine sediments deposited since the early Oligocene at Eboliang in the middle-western Qaidam Basin. Magnetic fabrics of the upper part (the Qigequan Formation) are predominantly considered to be primary sedimentary magnetic fabrics, while those of the lower-middle parts (the Shangganchaigou to Shizigou Formations) are mainly pre-folding, layer-parallel shortening induced embryonic tectonic fabrics. The magnetic lineation revealed ~50° successive clockwise rotations of the compressional strain orientations from nearly N-S to NE-SW in the middle-western Qaidam Basin during ~15–7 Ma. The nearly N-S compressional event is consistent with the generally N-S-directed India-Asia collision and thus was attributed to its far-field effects, whereas the post-mid-Miocene transition of ~50° clockwise rotations of the compressional strain orientations is consistent with the tectonic deformation in the basin interior. This transition lagged far behind that in the northern Qaidam marginal thrust belt (NQMTB), which began in the early Oligocene. The laterally diachronous transition in compressional strain orientation and deformations in the middle-western and northern Qaidam Basin were most likely due to the two-stage, sinistral strike-slip faulting of the ATF, with confined sinistral shear occurring along the ATF and partially transferred into the NQMTB during the early Oligocene to middle Miocene, while distributed sinistral shear occurred on the northeastern Tibetan Plateau beginning in the middle Miocene. A bidirectional propagation of compressional strain variation and tectonic deformation from the northern Qaidam Basin since the early Oligocene is proposed to reflect the tectonic evolution of the northeastern Tibetan Plateau.