Cenozoic tilting history of the south slope of the Altyn Tagh as revealed by seismic profiling: Implications for the kinematics of the Altyn Tagh fault bounding the northern margin of the Tibetan Plateau

The Altyn Tagh fault (ATF) plays a significant role in the northward growth of the Tibetan Plateau, but its Cenozoic kinematics and related structural response in adjacent basins remain debated. In this study, we identified a transition zone between the ATF and the Qaidam Basin interior and termed it the Altyn Slope, based on a dense network of two- and three-dimensional seismic reflection profiles and isopach maps. Tilted by a series of E-W–trending transpressional faults that constitute the positive flower structure of the ATF, the present Altyn Slope is characterized by a southeast-dipping slope with its undulating southeastern boundary with peaks coincidentally located at the major anticlinal belts in the basin. We propose a method for restoring the Cenozoic tilting history of the Altyn Slope during different time periods by identifying growth-strata geometry from the recent isopach maps. The results show that the Altyn Slope began to form in the late Eocene (ca. 40 Ma) and continued to expand until the mid-Miocene (ca. 15 Ma) albeit with a temporally developing shape. However, the Altyn Slope shrank toward the ATF and underwent significant NE-SW–directed folding since the mid-Miocene (ca. 15 Ma), resulting in formation of undulations of its southeastern boundary. We thus infer that the left-slip motion on the ATF is divided into two distinct stages: during the first stage, ca. 40–15 Ma, the ATF was activated with slow slip rate, and most transpressional stress was converted to vertical strain, raising the Altyn Slope instead of producing strike-slip motion. During the second stage, since ca. 15 Ma, faster sinistral strike-slip motion on the ATF took place, releasing the stress beneath the Altyn Slope and inducing intense NE-SW–directed shortening within the Northern Tibetan Plateau.