Confined by the eastern and western boundary faults, Lu Mountain has long been considered a block mountain uplifted due to Mesozoic and Cenozoic crustal deformation in East China. However, the formation and evolution of this block mountain are still debated. In this study, the eastern boundary fault is investigated to confirm the tectonic style of the block mountain. In addition, the burial ages of sediments on the fans of the eastern piedmont are measured by 26Al/10Be dating to evaluate the denudation rate. Field evidence indicates the presence of a reverse fault (Xingzi reverse fault) acting as the eastern boundary fault, which demonstrates that the block mountain is not a horst as once thought but an extrusion structure. Corrected 26Al/10Be burial ages show that the sediments on the high-level fans were deposited at approximately 1.1–1.2 Ma, which indicates denudation rates ranging from 0.033 to 0.082 m/kyr. The vertical displacement along the Xingzi reverse fault is estimated to be at least 1,100 m. The hanging wall could have been eroded to its present position within 13–33 Myr at the above denudation rates. Combining our results with regional geological and geomorphological evidence, we suggest that Lu Mountain was mainly uplifted in the Miocene due to crustal compression deformation, which may have been a response to the movement of the Pacific plate.
The Jinsha River, upstream of the Yangtze River, flows through the southeastern margin of the plateau, providing an opportunity to study the plateau uplift relative to the Sichuan basin through fluvial geomorphology. Eight tributaries of the Jinsha River located at the transition boundary from the plateau to the basin were analyzed for the feature of longitudinal profile and its evolution. The spatial distribution of slope-break knickpoints in the catchment was determined by slope-area analysis and integration analysis. Further, the magnitude and timing of the base level lowering (i.e., uplift of the Tibetan Plateau) were estimated by reconstructing the river longitudinal profile. The results show that there are two levels of slope-break knickpoints, with average elevations of 2865 m and 1523 m, corresponding to base levels relatively lowering of 1400 m and 1100 m, respectively. With reference to the long-term downcutting rate of the Jinsha River, the timings of these two phases base level lowering and uplifts are estimated to be ∼21–24 and ∼7–13 Ma, respectively. These are close to the timing of two uplifts revealed by inversion modeling of river longitudinal profiles. Therefore, the uplift of the studied region on the Tibetan Plateau can be divided into two stages, the early stage (∼20 Ma) uplift from 1500 to 2900 m and the late stage (∼10 Ma) uplift from 2900 to 4000 m, which further supports the staged outside uplift of the Tibetan Plateau.
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