Multiscale lithospheric buckling dominates the Cenozoic subsidence and deformation of the Qaidam Basin: A new model for the growth of the northern Tibetan Plateau

How the present-day northern Tibetan Plateau was involved in the plateau formation process in context of the Cenozoic India - Eurasia collision has been disputed for decades. As the largest sedimentary basin within the Tibetan Plateau and enclosed by high mountains such as the Qiman Tagh - East Kunlun Shan and the Qilian Shan in the Cenozoic, the Qaidam Basin preserves a continuous and complete record that is key to addressing this issue. Herein, we review the sedimentary and structural features of the Qaidam Basin in the Cenozoic, as well as their relationship with surrounding orogenic belts. The review suggests that the subsidence and in-filling of the Qaidam Basin are closely linked to the uplifting and exhumation of the bordering orogenic belts during the Cenozoic. Therefore, the tectonic subsidence pattern of the Qaidam Basin could be applied to investigate the deformation mechanism of the northern Tibetan Plateau. Guided by this, we implement two-dimensional (2-D) subsidence analysis and related numerical modeling to investigate the subsidence and deformation mechanisms of the Cenozoic Qaidam Basin. The tectonic-driven subsidence across the Qaidam Basin has occurred since the deposition of the Lulehe Formation, the lowermost Cenozoic stratum in the basin, and is largest (several kilometers) near the center but decreases gradually toward the margins. This subsidence pattern is best explained by lithospheric buckling with a wavelength of -200 km, which is likely controlled by the strength of entire lithosphere. Structural observations further reveal that parallel folds with a peak wavelength of -20 km prevail in the basin interior, and are likely caused by upper-crustal buckling controlled by the strength of upper crust. These observations suggest that the northern Tibetan Plateau initially responded to the India - Eurasia collision by long-wavelength lithospheric buckling in Eocene to Oligocene. As a result, the Qaidam Basin tectonically subsided as a synclinal depression. Destruction of the lithospheric buckling folds occurred under continued horizontal force, and may generate major bounding reverse faults of the Qaidam Basin, leading to strain localization and thus rapid crustal thickening in the surrounding mountains since the middle Miocene. At the same time, shorter-wavelength upper-crustal buckling folds developed in the interior of the Qaidam Basin. Our findings portray a multiscale buckling mechanism, which is controlled by a strong upper crust and uppermost mantle with a weaker lower crust beneath the Qaidam Basin, and suggest that horizontal contraction plays a dominant role in driving the tectonic evolution of the entire northern Tibetan Plateau.