Abstract In this paper, we provide insights into the eruptive history and volcano-tectonic evolution of the Paka volcanic complex in a revised stratigraphic framework. We integrate all available 40Ar/39Ar radiometric dates along with field observations, remote sensing data, and the analysis of surface and cutting samples from a 2552 m deep borehole (PK-01). Chemical analyses distinguish basalt, hawaiite, mugearite, benmorite, and trachyte rock type in Paka. It also indicates that the eruptive products are genetically related by fractional crystallization. Major element mass balance and Rhyolite-MELTS models are consistent with the trachyte being generated by 74–83% fractional crystallization of Paka basalt. Before the growth of the volcano, volcanic activity was characterised by plateau fissure eruptions at 582–405 ka. Eruptions directly related to the Paka edifice are divided into four different volcanic phase sequences. Sequences 1–4 span the periods 390–278 ka, 247–205 ka, 160–36 ka, and 36–8 ka. The Pre-Paka plateau eruption products outcrop at the base of the volcano flanks and are overlain by edifice-forming trachyte. Initial caldera-related subsidence is expressed by arcuate structures to the west and southeast parts of the volcano. These initial collapses are associated with tuff and pyroclastic eruptions estimated to have occurred at ∼38 ka. Following these explosive eruptions, a ∼1.5 by ∼1.6 km main caldera developed. The lithological information from the drilled geothermal exploration borehole PK-01 indicates a minimum thickness of ∼2200 m for the extrusive lava sequences, with the uppermost 1050 m of the strata being directly related to eruptions from the Paka volcano (Phases 1–4). We estimate a minimum total bulk volume of ∼50 km3 for the volcanic material erupted from Paka during the last ∼390 ka. This translates to an average eruption rate of ∼1.2 × 10−4 km3/yr. The associated advected magmatic heat supported an ‘excess’ heat flux ranging between 110 and 138 mW/m2 or about twice present day average continental flux (57 mW/m2) and about 1.3 times the background values (90 mW/m2) in geothermal areas. The apparent high heat flux implies a magma-driven geothermal system, where convection of the hydrothermal fluid is occurring above the heat source. Structural mapping revealed normal faults, strike-slip faults, lineaments, and an array of eruptive vents and domes as the main volcano-tectonic structures. We infer at least four faulting and fault reactivation events that have occurred during Paka geological history. The structural orientations in the area are dominated by NNE and NW with subordinate N-S and NNW striking structures. Evidence suggests that the intersection of NNE and NW trending structures may have had a major influence on the volcanism in the area.

中文翻译：

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肯尼亚北部裂谷帕卡火山群的喷发历史和火山构造演化：对地热热源的洞察

摘要 在本文中，我们提供了对修订后的地层框架中帕卡火山复合体的喷发历史和火山构造演化的见解。我们将所有可用的 40Ar/39Ar 辐射测量数据与实地观测、遥感数据以及来自 2552 m 深钻孔 (PK-01) 的地表和切割样品的分析相结合。化学分析可区分帕卡的玄武岩、夏威夷岩、穆格瑞特、膨润土和粗面岩类型。它还表明喷发产物通过分步结晶与遗传相关。主要元素质量平衡和流纹岩-熔体模型与帕卡玄武岩 74-83% 分步结晶所产生的粗面岩一致。在火山生长之前，火山活动的特点是在 582-405 ka 的高原裂缝喷发。与帕卡大厦直接相关的喷发分为四个不同的火山相序列。层序 1-4 跨越 390-278 ka、247-205 ka、160-36 ka 和 36-8 ka 时期。Pre-Paka 高原喷发产物在火山两侧的底部露头，并被形成建筑物的粗面覆盖。火山口西部和东南部的拱形结构表现出与火山口有关的初始沉降。这些最初的坍塌与估计发生在 38 ka 的凝灰岩和火山碎屑喷发有关。在这些爆炸性喷发之后，形成了约 1.5 x 约 1.6 公里的主火山口。钻探地热勘探钻孔 PK-01 的岩性信息表明喷出熔岩层序的最小厚度约为 2200 m，最上面 1050 m 的地层与帕卡火山的喷发直接相关（阶段 1-4）。我们估计在过去的 390 ka 期间从 Paka 喷发的火山物质的最小总体积为 50 km3。这意味着平均喷发率为 ~1.2 × 10−4 km3/yr。相关的平流岩浆热支持了 110 到 138 mW/m2 之间的“过量”热通量，或大约两倍于当今平均大陆通量 (57 mW/m2) 和大约 1.3 倍的地热区域背景值 (90 mW/m2) . 明显的高热通量意味着岩浆驱动的地热系统，其中热液的对流发生在热源上方。构造图揭示了正断层、走滑断层、线状构造以及一系列喷发喷口和圆顶作为主要的火山构造结构。我们推断在 Paka 地质历史期间至少发生了四个断层和断层再激活事件。区内构造方位以NNE和NW为主，次要构造为NS和NNW显着构造。有证据表明，NNE 和 NW 向结构的交叉点可能对该地区的火山活动产生了重大影响。