Two key switches in regional stress field during multi-stage deformation in the Carboniferous-Triassic southernmost Altaids (Beishan, NW China): Response to orocline-related roll-back processes

The orogenic architecture of the Altaids of Central Asia was created by multiple large-scale slab roll-back and oroclinal bending. However, no regional structural deformation related to roll-back processes has been described. In this paper, we report a structural study of the Beishan orogenic collage in the southernmost Altaids, which is located in the southern wing of the Tuva-Mongol Orocline. Our new field mapping and structural analysis integrated with an electron backscatter diffraction study, paleontology, U-Pb dating, Ar-39-Ar-40 dating, together with published isotopic ages enables us to construct a detailed deformation-time sequence: During D-1 times many thrusts were propagated northwards. In D-2 there was ductile sinistral shearing at 336-326 Ma. In D-3 times there was top-to-W/WNW ductile thrusting at 303-289 Ma. Two phases of folding were defined as D-4 and D-5. Three stages of extensional events (E-1-E-3) separately occurred during D-1-D-5. Two switches of the regional stress field were identified in the Carboniferous to Early Permian (D-1-E-1-D-2-D-3-E-2) and Late Permian to Early Triassic (D-4-E-3-D-5). These two switches in the stress field were associated with formation of bimodal volcanic rocks, and an extensional interarc basin with deposition of Permian-Triassic sediments, which can be related to two stages of roll-back of the subduction zone on the Paleo-Asian oceanic margin. We demonstrate for the first time that two key stress field switches were responses to the formation of the Tuva-Mongol Orocline.