Paleozoic to Cenozoic deformation along the Altyn Tagh fault in the Altun Shan massif area, eastern Qilian Shan, northeastern Tibet, China

The Altyn Tagh range shows evidence of a long history of deformation and metamorphism. Apart from the Cenozoic evolution largely associated with propagation of the Altyn Tagh fault, the area shows evidence of a Paleozoic orogeny. Mylonitic schist and granite from the northern Altyn Tagh range in the area between Subei, Lenghu, and the summit of Altun Shan show microscopic evidence of complex deformation that can be correlated with regional schistosity and folding. Ductile deformation started at ∼350-400 ° C and was followed by an episode with a strong horizontal shear component at a maximum temperature of 300 °C. It is very difficult to assess whether this represents a continuum of deformation or two separate episodes. U/Th/Pb microprobe dating of monazite, Rb/Sr dating, and 40Ar/39Ar dating and modeling of cooling from micas and feldspar indicate a series of 280-230 Ma ages that weakly support a possible late Paleozoic-early Mesozoic event. This is tentatively linked either with suturing of the Qiantang and Kunlun blocks farther to the south, or accretionary tectonism farther to the north or east in the Qilian Shan and Bei Shan ranges. The most clearly identified deformation took place at 150 ± 10 Ma and could correspond to the whole series of deformation observed, which would then be interpreted as a continuum. Previously unrecognized, this significant event is probably associated with strike-slip movements along a fault that broadly coincides with the currently active trace of the Altyn Tagh fault, but interpretations concerning its origin remain speculative. Cenozoic brittle deformation overprints the entire area. The lack of a clear Cenozoic thermal and/or tectonogeochronological signature in the analyzed samples is likely due to the small amount of Cenozoic exhumation, which agrees well with the proposed distribution of shortening between pure strike-slip along the Altyn Tagh fault and thrusting and mountain building on linked thrusts in the Qilian Shan.