Abstract Understanding the mechanisms of fan incision/aggradation provides key insights into the dynamics of fan evolution and hazardous fan-forming processes. This paper focuses on the discrepancy in fan evolution for two nearby valleys of different catchment areas along the Bailong River. Specifically, we study fan evolution in the small-sized CJB valley (watershed area being 1.1 km2) using sedimentary analyses and 14C dating. Sedimentary logging of seven exposed profiles indicates that mudflows and debris flows are the primary fan-forming processes. Seven samples were taken from paleosols developed in mudflow sediments, and the humin fraction was extracted for 14C dating. These ages constrain the fan aggradation period to between 10 - 4.9 cal kyr BP, and then the incision period occurred after 4.9 cal kyr BP. As the mudflow sediments may contain organic matter from hillslope legacies, the fan aggradation period may be later than the 14C ages defined in this study. In any case, the time of fan incision/aggradation in CJB is younger than that of the GLP valley (watershed area being 20 km2) where fan aggradation occurred in 21.7 - 7 ka and incision occurred afterward. The fan aggradation period defined by the 14C ages in CJB is consistent with an alluvial fan of similar thickness in the southeastern Tibetan Plateau and two other fans along the Bailong River. This consistency may suggest a plausible climatic control on fan evolution for small-sized tributary valleys, while the inconsistency with the larger GLP valley may suggest different climate-response regimes for tributary valleys of different sizes. More research on similar types of alluvial fans and cross-validation of different dating methods is needed.

中文翻译：

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青藏高原东缘白龙江不同规模支流河谷扇形加成切割的不同时期

摘要 了解扇形切割/聚集的机制提供了对扇形演化和危险扇形形成过程动力学的关键见解。本文重点研究了沿白龙河不同流域的两个邻近河谷在扇演化上的差异。具体而言，我们使用沉积分析和 14C 测年研究了小型 CJB 河谷（流域面积为 1.1 平方公里）的扇演化。七个暴露剖面的沉积记录表明泥流和泥石流是主要的扇形形成过程。从泥流沉积物中发育的古土壤中采集了 7 个样品，提取了腐殖质部分进行 14C 测年。这些年龄将扇形聚集期限制在 10 - 4.9 cal kyr BP 之间，然后切口期发生在 4.9 cal kyr BP 之后。由于泥流沉积物中可能含有来自山坡遗产的有机质，扇形加成期可能晚于本研究定义的 14C 时代。在任何情况下，CJB 扇切入/加积的时间都比 GLP 河谷（流域面积为 20 km2）更年轻，其中扇加成发生在 21.7 - 7 ka 之后发生切入。CJB 14C时代定义的扇加成期与青藏高原东南部一个厚度相似的冲积扇和白龙河沿岸的另外两个扇相一致。这种一致性可能表明对小型支流河谷扇演化的气候控制似乎是可信的，而与较大的 GLP 河谷的不一致可能表明不同规模的支流河谷具有不同的气候响应机制。