Structural evolution of the Yushu-Nangqian region and its relationship to syncollisional igneous activity east-central Tibet

Field mapping, geochronological analyses, and cross section construction reveal a protracted deformation history and a minimum of 61 km of Cenozoic NE-SW shortening (in present coordinates) across the Yushu-Nangqian thrust belt in northern Tibet. Cenozoic contraction started prior to 51 Ma and was followed first by northwest-striking right-slip faulting and later by northwest-striking left-slip faulting. Renewed NE-SW contraction is expressed by folding of Neogene strata and thrusting, which again was followed by northwest-striking left-slip faults. Late Neogene deformation is expressed by local north-striking normal faults. Shortening across the Yushu-Nangqian belt appears to be accommodated by thin-skinned thrusting, which raises the question of how the deformation was accommodated in the lower crustal levels. To resolve this problem, we perform geo-chemical analysis of igneous rocks dated as 51-49 and 38-37 Ma. The rocks exhibit geochemical signatures characteristic of subduction, which implies that coeval crustal thickening in northeastern Tibet was most likely induced by continental subduction.