Southwestward tilting of the Ordos Loess Plateau, central China: topographic response to India-Asia convergence deduced from drainage systems

The Ordos Loess Plateau with its iconic fluvial incision pattern represents an uplifted but internally stable plateau crustal block on the eastern fringe of the Tibetan Plateau. The Ordos Loess Plateau deeply incised river landscapes and hence its inaccessibility helped to protect ancient China from invading nomads from the north. The Ordos Block is internally free of seismicity but its boundaries feature severe high-magnitude earthquakes. Due to the ongoing India-Asia convergence, the northeastward expansion of the Tibetan Plateau leading to the eastward lateral extrusion of fault-bounded blocks. The Ordos Loess Plateau is part of one of these blocks and is still affected by lateral eastward motion along crustal scale faults and large surface uplift from Late Miocene to present. In this study, we investigated the effect of fault activity on the morphological evolution of the Ordos Loess Plateau. To quantify the effect of uplift gradients on the drainage systems, we investigated topographic patterns and landform metrics through field surveys and topographic analysis based on digital elevation models. Field surveys show that the southern boundary of the Ordos Loess Plateau to the Weihe Graben is still tectonically active (evidence for faulting in quaternary sediments). We found that the drainage is consistently directed towards the Weihe Graben in the southeast. Fluvial channels are in a state of morphological disequilibrium, with steep channel segments towards the Weihe Graben and meandering low gradient rivers in the central Ordos Loess Plateau. Over substantial portions, the shape of the longitudinal channel profile in the Ordos Loess Plateau is straight and deviates from usual graded longitudinal channel profiles. We further found that the degree of erosion and plateau incision is pronounced in the eastern part of the Ordos Loess Plateau, while the southwestern part is less incised. The drainage network indicated that the drainage basins are tilted toward the Liupanshan Mountains overthrust in the southwest. We conclude that the far-field influence of the Cenozoic uplift of the Tibetan Plateau activated the southwestern and southern boundary faults around the Ordos Loess Plateau. The drainage systems reorganized to a principal southern flow direction and thereby progressively incised in the Ordos Loess Plateau, causing severe soil erosion.