Climate-dependent fluvial architecture and processes on a suborbital timescale in areas of rapid tectonic uplift: An example from the NE Tibetan Plateau

Abstract

The substantial tectonic uplift (1000–2500m in a few million years) of the Northeastern Tibetan Plateau (NETP), together with the major climatic changes during the Quaternary, provides an opportunity to study the impact of tectonic and climatic changes on the morphological development and sedimentary architecture of fluvial deposits. The effects of these processes are revealed by a terrace staircase, together with the stratigraphy of each individual terrace, in the confluence zone of the Huang Shui and Yellow Rivers in the NETP, during the late Quaternary. On the basis of morphological mapping and OSL-dating, at least seven strath terraces were identified that formed during the last glacial cycle, which are preserved at locations where tectonic uplift was sufficient to separate them altitudinally from one another. The terraces are composed of stacked fluvial gravels, sands and alluvial loams. The principal result is that we demonstrate that the terraces were formed in response to climatic cycles on a suborbital timescale. For each terrace, the lower coarse-grained sediments (gravel and sand) were deposited during cold periods (such as the LGM, MIS3b, MIS4 and MIS5d) associated with a strong Asian winter monsoon. The aggradation during cold periods was associated with floodplain widening. The river incised slightly during the transitions from cold to warm phases, resulting in the transformation of the previous river plain into a terrace. The coarse grained cold phase deposits are covered by inter-bedded, horizontally-laminated silt and sand (representing flood sediments that often contain reworked soil material), during the (cold to warm) transitional phases. The floodplain accumulation on the terrace continued during the subsequent warm period. The warm periods (such as MIS3a, MIS3c, and MIS5a) of the climatic cycles are associated with a strong Asian summer monsoon. Pronounced incision took place at the subsequent warm–cold transitions. After this warm–cold transition, aeolian loess accumulated on the abandoned terrace without any further fluvial reworking. Our results demonstrate that critical thresholds for fluvial response can be crossed at climatic changes on a suborbital timescale given conditions of accelerated tectonic uplift in the NETP. In addition, based on the OSL ages of different units in the sedimentary sequences of the terraces, we conclude that the durations of terrace aggradations and floodplain widening lasted much longer than the periods of fluvial downcutting.