Abstract The relationships between volcanic activity and tectonics at the southernmost termination of the Main Ethiopian Rift (MER), East Africa, still represent a debated problem in the MER evolution. New constraints on the timing, evolution and characteristics of the poorly documented volcanic activity of the Dilo and Mega volcanic fields (VF), near the Kenya-Ethiopia border are here presented and discussed. The new data delineate the occurrence of two distinct groups of volcanic rocks: 1) Pliocene subalkaline basalts, observed only in the Dilo VF, forming a lava basement faulted during a significant rifting phase; 2) Quaternary alkaline basalts, occurring in the two volcanic fields as pyroclastic products and lava flows issued from monogenetic edifices and covering the rift-related faults. 40Ar/39Ar dating constrains the emplacement time of the large basal lava plateau to ~3.7 Ma, whereas the youngest volcanic activity characterising the two areas dates back to 134 ka (Dilo VF) to as recent as the Holocene (Mega VF). Volcanic activity developed along tectonic lineaments independent from those of the rift. No direct relations are observed between the Pliocene, roughly N-S-trending major boundary faults of the Ririba rift and the NE-SW-oriented structural trend characteristic of the Quaternary volcanic activity. We speculate that this change in structural trend may be the expression of (1) inherited crustal structures affecting the distribution of the recent volcanic vents, and (2) a local stress field controlled by differences in crustal thickness, following a major episode of reorganization of extensional structures in the region due to rift propagation and abandonment.

中文翻译：

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里里巴裂谷的近期火山构造活动与埃塞俄比亚南部裂谷的演化

摘要 东非埃塞俄比亚主裂谷（MER）最南端的火山活动与构造之间的关系仍然是 MER 演化过程中的一个有争议的问题。此处介绍并讨论了对靠近肯尼亚-埃塞俄比亚边界的 Dilo 和 Mega 火山区 (VF) 的记录不足的火山活动的时间、演变和特征的新限制。新数据描述了两组不同火山岩的出现：1) 上新世亚碱性玄武岩，仅在 Dilo VF 中观察到，形成了在显着裂谷阶段断层的熔岩基底；2）第四纪碱性玄武岩，作为火山碎屑产物和熔岩流产于两个火山区，从单生建筑物中喷出，覆盖与裂谷相关的断层。40Ar/39Ar 测年将大型基底熔岩高原的就位时间限制在~3.7 Ma，而表征这两个区域的最年轻的火山活动可追溯到 134 ka (Dilo VF) 到最近的全新世 (Mega VF)。火山活动沿着独立于裂谷的构造线发展。在上新世，里里巴裂谷的大致 NS 向主要边界断层与第四纪火山活动的 NE-SW 向构造趋势特征之间没有观察到直接关系。我们推测这种结构趋势的变化可能是（1）继承地壳结构影响近期火山喷口分布的表现，以及（2）地壳厚度差异控制的局部应力场，