Formation Of The Western Xuefengshan Salient And The Capture And Reversal Of The Middle Yangtze River, Eastern Sichuan Basin, China

The landscape in eastern Sichuan Basin does not show tectonically active, but theMesozoic bedrock suffered from intensive erosion/denudation, and very few Cenozoicdeposit remained even in low-relief area. Two Mesozoic arc-shape fold thrust belts,Western Xuefengshan and Southern Dabashan, build the original margins of easternSichuan Basin. Modern river system, made up of Yangtze river and its large tributaries,dominate the landscape evolution as erosion agency. This thesis focus on the two sub-jects, one is the formation of the western Xuefengshan salient, the other is the captureand reversal of the Middle Yangtze river and the ?uvial landscape evolution in easternSichuan Basin.Compared with classical trend-line patterns of the salients worldwide, the westernXuefengshan salient shows asymmetry of the two limbs. The northern limb includelarge-scale NE-trending curved folds and converge with the SE-trending curved fold ofthe southern Dabashan. While the southern limb protrude further than the northern limbwithout large-scale curved folds. Also in the northern limb zone are many superposedfolds, which indicate the at least two phase compression of the western Xuefengshansalient. In Phase I (properly from Middle or Late Jurassic to Early Cretaceous), thethrusts of both western Xuefengshan and southern Dabashan compose a conjunct struc-tural system with large-scale arc-shape folds. The conjunct structural process is alsosupported by 2D elastic finite element model using ABAQUSTM. In Phase II (properlyin late Early Cretaceous or Late Cretaceous), the southern Dabashan became tectonicquiescence while the western Xuefengshan still propagated westwardly. Subsequentthrusting lead to straight folds near HuaYing Shan and small-scale curved folds super-posing on the first deformation west of QiYue Shan.The local relief of the eastern Sichuan Basin are mostly controlled by bedrock ero-sion of the Middle Yangtze river system. Lithology and original tectonic topographyalso in?uence the distribution of the local relief. From Chongqing to Yichang, revealedby swath profiles and river buffer profile, the Middle Yangtze stem ?ow reversely to thetopography gradient. Based on Hack's law, the logarithm relationship of the drainage area and distance suggests the continuous capture events in Middle Yangtze stem. Thecon?uence angle of the tributaries also suggests the main stem was captured and re-versed in upper reach of the Middle Yangtze river. The Paleo-Yangtze river had twobranches divide by QiYue Shan near Fengjie, the west branch ?owing westwardly andthe east branch ?owing eastwardly. Driving by the increased rate of distal base-level fallof the Jianghan-Dongting basin, the east branch grew headward and captured the westbranch gradually. The mean normal steepness index (ksn), minimal erosion volume inunit area (Rva) and mean slope angle ( Sˉ) are calculated in 33 side-tributary basins ofthe Middle Yangtze River. The three parameters quantitatively re?ect that the erosionincrease gradually along the Middle Yangtze stem from Chongqing to Yichang. Thelongitude profiles in five side-tributary basins with Jurassic substrate are also extractedand analyzed using detachment-limited erosion model. Most profiles are characterizedby knickpoints in side-tributary basins of the upper and middle reaches of the MiddleYangtze river. However, in side-tributary basins of the lower reach the profiles show noknickpoints. As the reversal of the Middle Yangtze stem caused by the incision pulsefrom distal base-level fall, the tributaries will also respond to the base-level fall in themain stem. Thus the tributaries will adjust toward a new equilibrium state, or be cap-tured to the main stem. The tributaries in the middle and upper reaches were capturedsuccessively, and now in the transient state adjusting to the new base-level. The stylesof the knickpoints mainly depend on the structure of the substrate and lithologies.