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The upper–middle Yellow River flows through the Fenwei graben, a structure resulting from extensional tectonism that was formed and repeatedly extended during the Cenozoic. The drainage system within this graben was formerly isolated from the lower reaches of the Yellow River system by the Xiaoshan mountains, an actively growing ∼ NW–SE trending range. The modern course of the Yellow River takes it through this range along the Sanmen gorge, the formation of which was of great significance in that it initiated through-going drainage between the upper–middle and lower reaches of the system. The timing of this event, which was clearly a critical point in the evolution of the Yellow River, can be established by dating the terraces in the gorge. Intermittent deepening of this gorge by the Yellow River from a high-level planation surface capping the mountain range has resulted in the formation of five terraces. Magnetostratigraphic records from aeolian deposits accumulated on these surfaces provide a geochronological sequence for this geomorphic archive, in which the ages of the planation surface and of terraces T5, T4, T3, T2, and T1 have been determined as ∼3.63 Ma, ∼1.24 Ma, ∼0.86 Ma, ∼0.62 Ma, ∼129 ka, and ∼12 ka, respectively. Under the constraint of this chronological framework, a model for landscape evolution is proposed here. Uplift of the inner Fenwei graben and of the surrounding mountain ranges led to dissection of the 3.63 Ma old planation surface in conjunction with the formation of the Sanmen gorge. Drainage of the lake previously occupying the basin would have promoted incision into the fluvio-lacustrine graben sediments; indeed, gorge formation through the Xiaoshan may have been initiated or intensified by lake overflow. The ages obtained for the planation surface and uppermost terrace suggest that the formation of the Sanmen gorge and the initiation of the through-going eastward drainage of the Yellow River occurred between 3.63 and 1.24 Ma. Before the start of gorge entrenchment, the products of erosion in the modern upper catchment of the Yellow River were unable to reach the sea. The dramatic increase in deposition rates in the Bohai Gulf (at the mouth of the modern Yellow River in the East China Sea), ∼1.0 Ma ago, thus resulted from the initiation of an integral (enlarged) Yellow River catchment drainage through the Sanmen gorge; it does not imply an increase in erosion rates at that time. [ABSTRACT FROM AUTHOR]

In North China, the Ordos block located northeast of the Tibetan Plateau is regarded as a stable craton surrounded by the Yinchuan graben to the west, the Hetao graben to the north, and the Fenwei graben to the southeast. The major tectonic structure around this block in the Cenozoic reflects intensive extension as a response to the eastward extrusion of the Tibetan Plateau. Intermittent downcutting by the Yellow River, starting from the planation surface level on the Ordos block and penetrating the bedrock, resulted in the formation of the Jinshaan gorge. A series of well-preserved fluvial terraces was identified along this gorge, providing new insight into the far-field effect of the eastward extrusion of the Tibetan Plateau. Magnetostratigraphic records from the planation surface and uppermost terrace suggest that this gorge has been entrenched over its total length in the period from 3.7 to 1.2 Ma. A preliminary explanation is proposed here for the morphological evolution of the Yellow River in the Jinshaan gorge. On the basis of terrace correlation between seventeen geomorphic sections, multiwave headward erosion initiated by the Yellow River along this gorge has been reconstructed. This process may be attributed to the lowering of the base level (the Fenwei graben). This graben has been formed during the Cenozoic as the Tibetan Plateau extruded eastwards resulting in accelerated ground subsidence. A combined approach of magnetostratigraphy, pedostratigraphy, and optically stimulated luminescence (OSL) was adopted to establish a new geochronology for terraces in the gorge at 1.2 Ma, 1.1 Ma, 787 ka, 412 ka, 245 ka, 128 ka, 29 ka, and 11 ka. These ages provide an excellent framework for calculating the extension within the Fenwei graben. The average incision rate appears to be enhanced significantly since 412 ka, indicating a rapid extension within the Fenwei graben during this time. [ABSTRACT FROM AUTHOR]

Abstract: The Yellow River (Huang He) downstream of the Jinshaan Canyon displays well-preserved fluvial terrace sequences below an extensive planation surface at the northeastern Ordos Plateau. Despite a series of well developed geomorphic surfaces, the landscape evolution of the plateau is largely unconstrained. The Jinshaan Canyon, formed by the deeply incising Yellow River through the Ordos Plateau, provides keys to understanding the landscape evolution of this Plateau with respect to the nearby northeastern margin of the Tibetan Plateau. This paper presents two sets of magnetostratigraphic results derived from the aeolian deposits (Red Clay and loess) on the planation surface and the uppermost Yellow River terrace in the middle part of the Jinshaan Canyon. Magnetostratigraphic analysis reveals that the planation surface and the uppermost Yellow River terrace formed respectively at circa 3.7Ma and circa 1.2Ma ago. The Ordos Plateau is the product of a period of planation culminating just before circa 3.7Ma and then was interrupted by an episode of uplift, which may be correlated with the accelerated growth of the Tibetan Plateau in the Miocene–Pliocene. The drainage system in the Jinshaan Canyon was re-organized around 3.7Ma. No correlation seems to exist between an older drainage existing prior to circa 3.7Ma and the present Yellow River network in the Jinshaan Canyon, even though sedimentary and tectonic evidence suggests that the drainage in the middle reach of the Yellow River formed in the late Miocene-early Pliocene. The Yellow River has developed its rectangular course around the Ordos Plateau only since circa 3.7Ma. [Copyright &y& Elsevier]