Fluvial terraces and their implications for Weihe River valley evolution in the Sanyangchuan Basin

Fluvial terraces are important geomorphic markers for modern valley development. When coupled with numeric ages, terraces can provide abundant information about tectonic, climatic, paleohydrological and the paleoenvironmental changes. On the basis of the paleomagnetic, electron spin resonance (ESR) and optically stimulated luminescence (OSL) dating, in addition to an investigation of local loess-paleosol sequences, we confirmed that 13 fluvial terraces were formed, and then preserved, along the course of the Upper Weihe River in the Sanyangchuan Basin over the past 1.2 Ma. Analyses of the characteristics and genesis of these terraces indicate that they resulted from the response of this particular river system to climate change over an orbital scale. These changes can further be placed within the context of local and regional tectonic uplift, and represent an alternation between lateral migration and vertical incision, dependent upon the predominance of climatic and tectonic controls during different periods. Most of the terraces are strikingly similar in that they have several meters of paleosols which have developed directly on top of fluvial deposits located on the terrace treads, suggesting that the abandonment of terraces due to river incision occurred during the transitions from glacial to interglacial climates. The temporal and spatial differences in the distribution patterns of terraces located on either side of the river valley indicate that a tectonic inversion occurred in Sanyangchuan Basin at similar to 0.62 Ma, and that this was characterized by a transition from overall uplift to depression induced by fault activity. Synthesized studies of the Basin's terraces indicate that formation of the modern valley of the Upper Weihe River may have begun in the late Early Pleistocene between 1.4-1.2 Ma.