Abstract The two branches of the East African Rift System (EARS) are believed to have initiated diachronously. However, a growing body of work continues to suggest the onset of rifting in the Western Branch occurred in the Paleogene, coeval to the Eastern Branch. Due to a lack of pre-Miocene stratigraphy, attempts to resolve the geological history of the Western Branch must study the uplift and erosional histories of the modern rift topography. In this study, the rock uplift history of the Rwenzori Mountains, Western Uganda, is resolved to better our understanding of the tectonic history of the Western Branch of the EAR. Through the application of low-temperature thermochronology, χ-mapping and the modelling of river profiles, we show that rock uplift of the Rwenzori dates back to the Oligocene, with thermal history models suggesting uplift induced exhumation may date back as far as the Eocene. This provides tangible evidence that extension began in the region in the Paleogene, coeval with the Eastern Branch, and not the late Neogene. These results have broad implications for the tectonic evolution of the entire East African Rift System and suggest our current understanding of the region's rift history remains incomplete.

中文翻译：

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东非裂谷西支的古近纪起始：乌干达西部鲁文佐里山脉的隆升历史

摘要 东非裂谷系统（EARS）的两个分支被认为是历时兴起的。然而，越来越多的工作继续表明西部分支的裂谷发生在古近纪，与东部分支同时期。由于缺乏前中新世地层学，试图解决西支的地质历史必须研究现代裂谷地形的抬升和侵蚀历史。在这项研究中，解决了乌干达西部鲁文佐里山脉的岩石隆起历史，以更好地了解 EAR 西部分支的构造历史。通过应用低温热年代学、χ-mapping 和河流剖面建模，我们表明 Rwenzori 的岩石隆升可以追溯到渐新世，热历史模型表明抬升引起的剥脱可能可以追溯到始新世。这提供了切实的证据，表明扩张始于古近纪地区，与东部分支同时期，而不是新近纪晚期。这些结果对整个东非裂谷系统的构造演化具有广泛的影响，并表明我们目前对该地区裂谷历史的理解仍然不完整。